Article

High-altitude multi-taskers: bumble bee food plant use broadens along an altitudinal productivity gradient

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Abstract

We use an extensive historical data set on bumble bee host choice collected almost 50 years ago by L. W. Macior (Melanderia 15:1-59, 1974) to examine how resource partitioning by bumble bees varies over a 2,700-m altitudinal gradient at four hierarchical scales: individual, colony, species and community. Bumble bee behavior, resource overlap between castes, and plant-bumble bee networks change with altitude in accordance with tightening temporal constraints on flowering and colony growth in alpine habitats. Individual bees were more likely to collect pollen from multiple sources at high altitude. Between-caste foraging niche overlap increased with altitude. Similarly, alpine forager networks were more highly nested than either subalpine or montane networks due to increased asymmetric specialization. However, interspecific resource partitioning showed a more complex spatial pattern with low niche overlap at intermediate altitude (subalpine) compared to montane (disturbed) and alpine (unproductive) sites. Results suggest that spatial variation in interspecific resource partitioning is driven by a shift in the behavior of long-tongued bumble bees. Long-tongued bumble bees specialized in the subalpine but generalized in montane and alpine zones. Our reanalysis of Macior's data shows that bumble bee behavior varies substantially with altitude influencing plant-bumble bee interaction networks. Results imply that pollination services to alpine host plants will change dramatically as subalpine species with unique foraging strategies move upward under global warming.

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... Owing to such harsh conditions, abundance and richness of consumers are expected to decrease, leading to reduced competition (Ebeling, Klein, & Tscharntke, 2011;Fründ, Linsenmair, & Blüthgen, 2010;MacArthur & Pianka, 1966). This may result in a wider pollinator diet (Hoiss, Krauss, Potts, Roberts, & Steffan-Dewenter, 2012;Miller-Struttmann & Galen, 2014) and thus less structured and specialized networks with increasing elevation. Previous research has shown that specialization of pollinators can vary along productivity gradients (Fontaine, Collin, & Dajoz, 2008;Fründ et al., 2010;Lara-Romero, García, Morente-López, & Iriondo, 2016;Miller-Struttmann & Galen, 2014); despite this, the variation in diet breadth along elevational gradients and its effects on a community scale remain largely unexplored (Benadi, Hovestadt, Poethke, & Blüthgen, 2014). ...
... This may result in a wider pollinator diet (Hoiss, Krauss, Potts, Roberts, & Steffan-Dewenter, 2012;Miller-Struttmann & Galen, 2014) and thus less structured and specialized networks with increasing elevation. Previous research has shown that specialization of pollinators can vary along productivity gradients (Fontaine, Collin, & Dajoz, 2008;Fründ et al., 2010;Lara-Romero, García, Morente-López, & Iriondo, 2016;Miller-Struttmann & Galen, 2014); despite this, the variation in diet breadth along elevational gradients and its effects on a community scale remain largely unexplored (Benadi, Hovestadt, Poethke, & Blüthgen, 2014). ...
... This is actually consistent with physiology-based theories, such as those proposed to explain Bergmann's rule (Classen et al., 2017). Still, the association between body size and resource use, dispersal capacity and pollination efficiency (Chown & Gaston, 2010) might also explain the positive relationship between elevation and insect body size at the community scale, especially when trophic resources decrease with elevation (Miller-Struttmann & Galen, 2014;Ramos-Jiliberto et al., 2010). Pollinator body-size is highly positively correlated with foraging distance and proboscis length in many taxa (Agosta & Janzen, 2005;Byrne, Buchmann, & Spangler, 1988;Casey, May, & Morgan, 1985;Greenleaf, Williams, Winfree, & Kremen, 2007;Levy & Nufio, 2015). ...
Article
Aim To assess whether the reduced nutritional resources available for pollinators due to plant community simplification along an elevational plant‐diversity gradient changes pollinator niche breadth and richness. Additionally, we evaluated how body size and proboscis length of pollinators shifted along the gradient, and whether these changes were related to pollinator niche breadth. Location An elevational gradient (2,350–3,520 m a.s.l.) on the oceanic high‐mountain strato‐volcano of El Teide (Tenerife, Canary Islands). Taxon Flowering plant and pollinator species. Methods We compared quantitative plant–pollinator networks along the plant‐diversity gradient. We calculated a set of niche‐based topological metrics that capture the degree of specialization, niche breadth and niche overlap. Furthermore, we obtained β‐diversity measures and the proportion of replacement and richness components. Results There was an overall decline in species richness of pollinators with increasing elevation. This decline was mainly driven by the loss of species along the elevational gradient, which conformed a nested subset pattern. The whole network showed less specialization, greater connectance and lower modularity towards the summit. At high elevations, pollinators were more generalized and less selective in their flower choice, showing a greater trophic niche breadth compared to pollinators at lower elevations. Mean body size of pollinators increased with elevation, and species body size and proboscis length were positively associated with the number of plant species visited. Main conclusions Overall, results indicated that the elevational gradient filters pollinator species, probably according to their thermal tolerance and ability to exploit a wide range of trophic resources. The finding that pollinators become more generalized and opportunistic at higher elevations is a novel result, which may have implications for new research into how ecological networks vary over environmental gradients. From an applied perspective, our results highlight the importance of considering the spatial variation of species assemblages when aiming to construct functionally reliable interaction networks along environmental gradients.
... Furthermore, the adaptive potential of specialists to optimize foraging dependent on macronutrient requirements (Vaudo, Patch, Mortensen, Tooker, & Grozinger, 2016) is limited. These trade-offs for plants and pollinators point either toward a strong selection pressure on the degree of floral specialization or, alternatively, require a high plasticity to allow for adaptive foraging, depending on the ecological context (Miller-Struttmann & Galen, 2014;Spiesman & Gratton, 2016). ...
... Temperature determines the costs of foraging flights in ectothermic pollinators (Kovac, Stabentheiner, & Brodschneider, 2015) and may thus modulate resource usage strategies in a way that species broaden their dietary spectrum in energy-limited habitats (Miller-Struttmann & Galen, 2014). Restricted foraging times due to persistent mist and/or temperatures below a threshold in which foraging is possible, should equally result in more generalized foraging. ...
... Even within pollinator species, individuals tended to forage more generalized in the highlands than in the lowlands, supporting the concept of energetic constraints. Long-tongued bumble bees species also showed comparable shifts in intraspecific foraging behavior in alpine habitats (Miller-Struttmann & Galen, 2014). The authors suggested that restricted seasonal lengths drive generalized resource usage of bumble bees in cold habitats, which is plausible in temperate, but not in cold tropical mountain habitats that lack distinct seasonality. ...
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Aim: Species differ in their degree of specialization when interacting with other species, with significant consequences for the function and robustness of ecosystems. In order to better estimate such consequences, we need to improve our understanding of the spatial patterns and drivers of specialization in interaction networks. Methods: Here, we used the extensive environmental gradient of Mt. Kilimanjaro (Tanzania, East Africa) to study patterns and drivers of specialization, and robustness of plant-pollinator interactions against simulated species extinction with standardized sampling methods. We studied specialization, network robustness and other network indices of 67 quantitative plant-pollinator networks consisting of 268 observational hours and 4,380 plant-pollinator interactions along a 3.4 km elevational gradient. Using path analysis, we tested whether resource availability, pollinator richness, visitation rates, temperature, and/or area explain average specialization in pollinator communities. We further linked pollinator specialization to different pollinator taxa, and species traits, that is, proboscis length, body size, and species elevational ranges. Results: We found that specialization decreased with increasing elevation at different levels of biological organization. Among all variables, mean annual temperature was the best predictor of average specialization in pollinator communities. Specialization differed between pollinator taxa, but was not related to pollinator traits. Network robustness against simulated species extinctions of both plants and pollinators was lowest in the most specialized interaction networks, that is, in the lowlands. Conclusions: Our study uncovers patterns in plant-pollinator specialization along elevational gradients. Mean annual temperature was closely linked to pollinator specialization. Energetic constraints, caused by short activity timeframes in cold highlands, may force ectothermic species to broaden their dietary spectrum. Alternatively or in addition, accelerated evolutionary rates might facilitate the establishment of specialization under warm climates. Despite the mechanisms behind the patterns have yet to be fully resolved, our data suggest that temperature shifts in the course of climate change may destabilize pollination networks by affecting network architecture.
... Many classic studies have used bumble bees to test hypotheses about competitive interactions and coexistence, for example based on niche differentiation in tongue length and flower use (Inouye 1980;Pyke 1982). Recent work has revisited some of these systems with new results, suggesting that competition interacts with abiotic factors such as elevation to shape community structure in more complex ways than previously thought (Pyke et al. 2012;Miller-Struttmann & Galen 2014). Understanding such interactions is important to predicting the consequences of climate change, but potential effects of variation over time have often by necessity been assessed through spatial proxies, like elevational gradients (Miller-Struttmann & Galen 2014). ...
... Recent work has revisited some of these systems with new results, suggesting that competition interacts with abiotic factors such as elevation to shape community structure in more complex ways than previously thought (Pyke et al. 2012;Miller-Struttmann & Galen 2014). Understanding such interactions is important to predicting the consequences of climate change, but potential effects of variation over time have often by necessity been assessed through spatial proxies, like elevational gradients (Miller-Struttmann & Galen 2014). ...
... Correa & Winemiller 2014). In Colorado, increases in competitor abundance across elevational gradients have been shown to correlate with greater niche separation, as longer tongued Bombus specialise more on flowers with deeper corollas (Pyke et al. 2012;Miller-Struttmann & Galen 2014). Likewise, Brosi & Briggs (2013) found that experimentally removing the most abundant Bombus species from subalpine meadows led to reduced floral fidelity in the remaining Bombus foragers. ...
Article
Time series of abundances are critical for understanding how abiotic factors and species interactions affect population dynamics, but are rarely linked with experiments and also scarce for bee pollinators. This gap is important given concerns about declines in some bee species. I monitored honey bee (Apis mellifera) and bumble bee (Bombus spp.) foragers in coastal California from 1999, when feral A. mellifera populations were low due to Varroa destructor, until 2014. Apis mellifera increased substantially, except between 2006 and 2011, coinciding with declines in managed populations. Increases in A. mellifera strongly correlated with declines in Bombus and reduced diet overlap between them, suggesting resource competition consistent with past experimental results. Lower Bombus numbers also correlated with diminished floral resources. Declines in floral abundances were associated with drought and reduced spring rainfall. These results illustrate how competition with an introduced species may interact with climate to drive local decline of native pollinators.
... The plant has a given quantity of energy that is divided between these two growth modes. [31,32], so that we assume r P to be a decreasing function of the attractiveness α : ...
... We follow the evolution of plant attractiveness using adaptive dynamics techniques [32,33]. Under adaptive dynamics hypotheses (see supplementary material for a full description of the method and hypotheses) we can model the evolution of plan attractiveness and its consequences on species density dynamics, and the feedback of species density on the evolutionary process [33]. ...
... 19, . (doi:10.1111 32. ...
Thesis
Environmental changes affect species but also the interaction connecting them within a community. Their ecological and evolutionary responses partly depend on biological traits affecting the type and strength of these interactions. In this thesis we use models to explore the impact of different environments on resource-consumer communities, either antagonistic (e.g. food webs) or mutualistic (e.g. plant-pollinator webs). We follow the impact of environmental parameters like temperature, abundance, and temporal distribution of interactors on their partner species. We study potential eco-evolutionary feedbacks, following the evolution of traits affecting the interaction like body size, investment in mutualism, or foraging time. Antagonistic interactions seem more stable than mutualistic ones via negative eco-evolutionary feedbacks. The more specialist species are also more fragile, often declining or being less able to evolve with environmental changes. Under specific modelling conditions we observe evolutionary responses previously reported in the literature e.g. decreased body size with warming, disinvestment in the mutualistic interaction when the partner population declines). Explicit modelling of the length and amplitude of the considered environmental perturbations could help explore the stability of the studied communities.
... year-to-year changes, Alarcón et al. 2008). Such an approach can also provide insights on facilitative aggregations of plants (Verdú and Valiente-Banuet 2008;Cranmer et al. 2012), the foraging strategies of insects (Miller-Struttmann and Galen 2014), the importance of phenology (Olesen et al. 2011), plant-pollinator mismatches (Fabina et al. 2010, the effects of invasive plants (Albrecht et al. 2016) and the robustness of complex interactions (Evans et al. 2013). ...
... Montane ecosystems have received low attention from network ecologists even though altitude affects many factors that would influence foraging strategies and thus the communities (Miller-Struttmann and Galen 2014;Watts et al. 2016). Despite that, it is possible to list some recurrent features. ...
Thesis
Several aspects of plant-pollinator interactions are presented in the thesis. It contains a review on the open questions of plant-pollinator interactions from single species to complex networks. The following sections document novel results. Firstly, the conservation of complex pollination networks is addressed through the hierarchy of species’ importance. In addition, the habitat requirements and interactions of a threatened rare pollinator species are explored. In the following chapters, the results from manipulative approaches applied in the field to plant-pollinator interactions are presented. The effect of pollinator’s population decline on pollinators’ foraging for pollen is investigated. Moreover, the way plant species loss impact several aspects of pollinator visitation is presented. Lastly, the impact of species removal on plant-pollinator network topology and on species ability of establishing new interactions is investigated.
... 1541). The reanalysis of Macior's Bombus visitation data from 1966-69 by Struttmann & Galen (2014) found that long-tongued bees had wider foraging niches in the alpine altitudinal zone than in other zones, and that both B. sylvicola and B. balteatus were highly generalized in the alpine. Those characterizations are inconsistent with M-S15, where these alpine species are portrayed as former specialists that have recently become more generalized. ...
... Instead, B. balteatus and B. sylvicola are former generalists, and the changes for each species are nonequivalent. In addition, we think it is unlikely that the differences in niche breadth that M-S15 observed will have lasting impacts on long-tubed alpine plants, as these plants likely evolved with the various selection pressures imposed by highly generalized bumble bee foraging (Miller-Struttmann & Galen 2014). Long-tubed plants are still being visited by B. balteatus, despite a change in tongue length and therefore, the purported " mismatch between shorter-tongued bees and the longer-tubed plants they once pollinated " (p. ...
Article
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Miller-Struttmann et al. (2015) suggest that, in a North American alpine ecosystem, reduced flower abundance due to climate change has driven the evolution of shorter tongues in two bumble bee species. We accept the evidence that tongue length has decreased, but are unconvinced by the adaptive explanation offered. It posits foraging responses and competitive relationships not seen in other studies and interprets phenotypic change as evidence of evolutionary adaptation. By oversimplifying a complex phenomenon, it may exaggerate the potential for bees to quickly adapt to environmental changes.
... Meanwhile, microclimate ranges along altitudinal gradients provide the myriad of conditions required to improve our understanding of the pervasive effects of global change on biodiversity. Exploratory work has only recently started on invertebrate pollination in island and mountain settings (Traveset et al., 2016;Baumann et al., 2021), exposing a higher niche overlap within the species-poor communities of small islands and elevated altitudes (Miller-Struttmann and Galen, 2014;Traveset et al., 2016;Lara-Romero et al., 2019). Yet, no comprehensive research has been done on invertebrate biological control in either setting and only few (largely) observational studies have gauged the abundance and diversity of biological control agents (BCAs). ...
... Altitudinal responses are also greatly modulated by functional traits of target pests and BCAs alike (Péré et al., 2013). In addition to thermal niche space, trophic niche breadth is mediated by altitude and local host or prey communities (Rasmann et al., 2014a); higher degrees of polyphagy have thus been recorded among high-altitude bumblebees (Miller-Struttmann and Galen, 2014). If the "altitude niche breadth hypothesis" also applies to BCAs, this will clearly impact biological control in mountain settings. ...
Article
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For centuries, islands and mountains have incited the interest of naturalists, evolutionary biologists and ecologists. Islands have been the cradle for biogeography and speciation theories, while mountain ranges have informed how population adaptation to thermal floors shapes the distribution of species globally. Islands of varying size and mountains' altitudinal ranges constitute unique "natural laboratories" where one can investigate the effects of species loss or global warming on ecosystem service delivery. Although invertebrate pollination or seed dispersal processes are steadily being examined, biological control research is lagging. While observations of a wider niche breadth among insect pollinators in small (i.e., species-poor) islands or at high (i.e., colder) altitudes likely also hold for biological control agents, such remains to be examined. In this Perspective piece, we draw on published datasets to show that island size alone does not explain biological control outcomes. Instead, one needs to account for species' functional traits, habitat heterogeneity, host community make-up, phenology, site history or even anthropogenic forces. Meanwhile, data from mountain ranges show how parasitism rates of Noctuid moths and Tephritid fruit flies exhibit species-and context-dependent shifts with altitude. Nevertheless, future empirical work in mountain settings could clarify the thermal niche space of individual natural enemy taxa and overall thermal resilience of biological control. We further discuss how global databases can be screened, while ecological theories can be tested, and simulation models defined based upon observational or manipulative assays in either system. Doing so can yield unprecedented insights into the fate of biological control in the Anthropocene and inform ways to reinforce this vital ecosystem service under global environmental change scenarios.
... Pollination networks constitute a sophisticated tool to describe and visualize the structure of interactions among plants and pollinators within a community, allowing comparisons among communities through specific metrics that can be computed. The tool has been employed to document such variation along altitudinal gradients worldwide (Adedoja, Kehinde, & Samways, 2018;Cuartas-Hernandez & Medel, 2015;Hoiss, Krauss, & Steffan-Dewenter, 2015;Lara-Romero, Seguí, Pérez-Delgado, Nogales, & Traveset, 2019;Maglianesi, Blüthgen, Böhning-Gaese, & Schleuning, 2015;Miller-Struttmann & Galen, 2014;Ramos-Jiliberto et al., 2010;, but only once within the wider Mediterranean area: Lara-Romero et al. (2019) for Mt. Teide in Tenerife Island. ...
... In general, larger networks tend to have smaller values of nestedness(Kantsa et al., 2018;, which may also explain the observed trend at intermediate altitudes where networks are the largest along the entire gradient. Like on Mt.Olympus, nestedness in b-p networks was found to increase at high elevations(Miller-Struttmann & Galen, 2014). Being the only studies focusing on b-p networks along an altitudinal gradient, their convergent results imply that communities at high altitudes are more resilient against global warming impacts (cf. ...
Article
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Aim We studied bumblebee diversity and bumblebee pollination networks along the altitudinal gradient of Mt. Olympus, a legendary mountain in Central Greece, also known for its exceptional flora. Location Mt. Olympus, Central Greece. Taxon Bombus (Latreille, 1802). Methods We explored 10 study sites located on the north‐eastern slope of the mountain, from 327 to 2,596 m a. s. l. Bumblebee surveys were carried out on a monthly basis using pan traps (years 2013 and 2014) and random transect observations assisted by hand netting (years 2013, 2014, and 2016); visited flowering plants and their diversity were recorded during the transect observations. Results With a total of 22 recorded bumblebee species and one species complex, Mt. Olympus is one of the richest mountains in Mediterranean Europe regarding bumblebee diversity. Bombus quadricolor was recorded as a new species for Greece, whereas four species were recorded at their southernmost distribution limit, therefore possibly vulnerable to climate change. Species richness of both bumblebees and plants in flower followed a unimodal pattern along the altitudinal gradient, the former peaking at high altitudes (1,900–2,200 m a.s.l.), the latter at lower to intermediate altitudes (500–1,500 m a.s.l.). Bumblebee–plant visitation networks were larger, more diverse and more generalized in the between intermediate altitudes (1,500–1,800 m a.s.l.), while nestedness peaked at low and high altitudes. Main conclusions Our results disclose the differential significance of the altitudinal zones of Mt. Olympus for the conservation of the diversity of bumblebees and their host plants, as well as of the interactions among them. Furthermore, they highlight the importance of this mountain, because of its South‐European location, regarding climate change impacts on the bumblebee fauna of Europe. All in all, they point towards more reinforced conservation measures to be taken including the expansion of the protection status to the entire mountain.
... The change of altitude gradient provides an ideal condition for studying the ecological adaptability of plant growth and reproduction (Dostálek et al., 2018). With the increase of altitude, many environmental variables that regulate plant performance, such as temperature, growth season length and resource availability, will decrease accordingly, which will affect plant reproductive strategies, reproductive success and the relationship between plants and insects (Xu et al., 2017;Miller-Struttmann & Galen, 2014;Ma et al., 2015). It is predicted that if the pollination success of insect pollinators in higher altitude is limited by the lower activity of pollinators, the sex distribution pattern of plants will change (Sena et al., 2014;Dai et al., 2020;Lei et al., 2017). ...
... *Significant differences at P < 0.05; **Significant differences at P < 0.01. corresponding decrease of temperature, growth season length, resource availability, insect richness, diversity and activity with the increase of altitude, as well as the stamen input and number of plants (Xu et al., 2017;Miller-Struttmann & Galen, 2014). Therefore, only by increasing the input of gynoecium, can we ensure the success rate of survival and reproduction. ...
Article
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Abstract In this study, we aimed to explore the effect of altitude on reproductive ingredient and sex allocation of different colors of Anemone obtusiloba in populations. The variations of reproductive ingredient and sex allocation of three colors of Anemone obtusiloba in different four altitudinal gradients at the eastern Qinghai Tibetan Plateau were examined. Our results showed that with the increased altitude, gynoecium/flower, androceium/flower and gynoecium/individual were increased, but androceium/individual, gynoecium number/flower, androceium number/flower, gynoecium number/ individual and androceium number/individual in three flower color of Anemone obtusiloba were decreased. Besides, male allocation and ♂/♀ were also decreased, which showed female-biased sex allocation. Furthermore, in the four altitudinal gradients, the individual size of same color was positively correlated with reproductive ingredient, and altitude had a direct impact on size-dependent reproductive ingredient. However, there were not certain correlations between male allocation, ♂/♀ and individual size in the same color of Anemone obtusiloba. Moreover, in the four altitudinal gradients, the reproductive investment of same color was positively correlated with gynoecium and androceium, but there were not certain correlations between male allocation, ♂/♀ and reproductive investment. In conclusion, altitude had a significant effect on reproductive ingredient and sex allocation in different colors of Anemone obtusiloba. Different colors of Anemone obtusiloba all increased of female function with the increased altitude. However, there were difference in influence mechanism of altitude on size-dependent reproductive ingredient and distribution between female and male function of resource.
... Structural properties of species interaction networks can change along environmental gradients (Maunsell et al., 2015;Miller-Struttmann & Galen, 2014;Morris et al., 2015;Ramos-Jiliberto et al., 2010), but previous analyses generally did not consider the potential variation of ecological determinants with changes in abiotic conditions (but see Sonne et al., 2020). Measures of network variation along environmental gradients, for example, show that the specialization of plant-pollinator networks decreases with increasing latitude (Schleuning et al., 2012) We observed a decrease in the importance of SLA and nitrogen in structuring networks with increasing elevation, in parallel to the decrease in the community mean SLA and nitrogen values at higher elevation reported in the same study area (Pitteloud et al., 2020). ...
... or increasing elevation(Miller-Struttmann & Galen, 2014;Pellissier et al., 2012;Refsnider et al., 2019), a pattern that was directly linked to animal feeding traits or floral morphology. In parallel, studies have been conducted to investigate the mechanisms ruling species interactions, including phylogeny, abundance and functional traits(Dormann et al., 2017), but not the influence of abiotic parameters on network structuration. ...
Article
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Understanding the ecological rules structuring the organization of species interactions is a prerequisite to predicting how ecosystems respond to environmental changes. While the ecological determinants of single networks have been documented, it remains unclear whether network ecological rules are conserved along spatial and environmental gradients. To address this gap, we reconstructed 48 plant–herbivore interaction networks along six elevation gradients in the Central European Alps in Switzerland, using DNA metabarcoding on orthoptera feces. We developed hypotheses on the ecological mechanisms expected to structure interaction networks, based on plant phylogeny, plant abundance, leaf toughness, leaf nitrogen content and plant metabolomics. We show that plant phylogenetic relationships and species abundance have the greatest explanatory power regarding the structure of the ecological networks. Moreover, we found that leaf nitrogen content is a key determinant of interactions in warmer environments, while phenolic compounds and terpenoids are more important in colder environments, suggesting that determinants of species interactions can shift along environmental gradients. With this work, we propose an approach to study the mechanisms that structure the way species interact with each other between bioregions and ecosystems.
... Most studies along environmental gradients focus on biodiversity patterns and only occasionally on biotic interactions. Thus, even the direction of trends in mutualistic network properties along latitudinal gradients is still controversial (Schleuning et al., 2012;Trøjelsgaard & Olesen, 2013), and empirical network analysis along elevational gradients is scarce (Ramos-Jiliberto et al., 2010;Benadi et al., 2014;Miller-Struttmann & Galen, 2014). However, some studies show that environmental filtering increases and competition decreases with elevation, suggesting that realized niches are wider at high elevations (Pellissier et al., 2010;Hoiss et al., 2012). ...
... However, some studies show that environmental filtering increases and competition decreases with elevation, suggesting that realized niches are wider at high elevations (Pellissier et al., 2010;Hoiss et al., 2012). Wider niches and severe abiotic conditions at high elevations implicate that plants and pollinators discriminate less between interaction partners and thus result in less structured networks or a decreasing degree of specialization with increasing elevation (Ramos-Jiliberto et al., 2010;Miller-Struttmann & Galen, 2014). ...
Article
Plant-pollinator interactions are essential for the functioning of terrestrial ecosystems, but are increasingly affected by global change. The risks to such mutualistic interactions from increasing temperature and more frequent extreme climatic events such as drought or advanced snow melt are assumed to depend on network specialization, species richness, local climate and associated parameters such as the amplitude of extreme events. Even though elevational gradients provide valuable model systems for climate change and are accompanied by changes in species richness, responses of plant-pollinator networks to climatic extreme events under different environmental and biotic conditions are currently unknown. Here, we show that elevational climatic gradients, species richness and experimentally simulated extreme events interactively change the structure of mutualistic networks in alpine grasslands. We found that the degree of specialization in plant-pollinator networks (H2') decreased with elevation. Nonetheless, network specialization increased after advanced snow melt at high elevations, whereas changes in network specialization after drought were most pronounced at sites with low species richness. Thus, changes in network specialization after extreme climatic events depended on climatic context and were buffered by high species richness. In our experiment, only generalized plant-pollinator networks changed in their degree of specialization after climatic extreme events. This indicates that contrary to our assumptions, network generalization may not always foster stability of mutualistic interaction networks. © 2015 John Wiley & Sons Ltd.
... For C. unifl orum , Syrphidae were the second most important pollinators; for S. bryoides , Calliphoridae and Sarcophagidae, but also Ichneumonidae were essential. Bumble bees-the most effi cient pollen vectors in alpine and arctic environments ( Galen, 1989 ;Stenström and Bergman, 1998 ;Kasagi and Kudo, 2003 ;Miller-Struttmann and Galen, 2014 ), which may carry more than 20,000 grains on their body ( Escaravage and Wagner, 2004 )-were rarely observed at the study site (but not in the fl owers of the study plants), though they are capable of foraging at temperatures near 0 ° C ( Lundberg, 1980 ). Similarly, FIGURE 4 Number of pollen grains on the stigma, germinated grains, and pollen tubes at the base of the style following natural pollination (left panels) and saturating cross pollination by hand (right panels) in the studied plant species. For natural pollination, the number of ovules and mature seeds per pistil is also given. ...
Article
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Premise of the study: Seed output of high-mountain plants in the uppermost life zones is highly variable. One possible reason might be pollen limitation due to inadequate pollinator services. Methods: We tested this hypothesis for the insect-pollinated species Cerastium uniflorum, Ranunculus glacialis, and Saxifraga bryoides, which have their distribution center in the subnival and nival zone of the European Central Alps. We recorded insect visitors and determined their impact as pollinators. By analyzing pollination success and seed set following natural and saturating hand pollination, the magnitude of quantitative and qualitative pollen limitation was assessed. Key results: Anthomyiid and muscid flies had the highest pollination impact for R. glacialis and S. bryoides and syrphids for C. uniflorum. Natural stigma pollen load was highly variable in individual flowers of all species, but in most cases the number of conspecific pollen grains clearly exceeded the number of ovules to be fertilized. There was also a surplus in germinated pollen grains, whereas the pollen tube to ovule ratio was only sufficient in R. glacialis (2.6 on average) and S. bryoides (1.3), but not in C. uniflorum (0.6). Accordingly, seed to ovule ratio was around 0.8 in R. glacialis, 0.7 in S. bryoides but 0.4 in C. uniflorum. In C. uniflorum, saturating pollination slightly increased seed set. Regression analyses revealed that natural pollination success was more frequently limited by quality than by quantity. Conclusions: Our results do not support the idea of chronic, widespread pollen limitation in the subnival but rather fit into the concept of parental optimism by overinvesting in the number of ovules as an adaptation to variable resource availability.
... [21 , 29,30] [39], or when floral richness is higher [40,41]. However, we need to better understand the fitness consequences of switching floral types; for example, pollens may differ in nutritional quality (e.g. ...
Article
Flowers are ephemeral, yet bees rely on them for food throughout their lives. Floral resource phenology—which can be altered by changes in climate and land-use—is therefore key to bee fitness and community composition. Here, we discuss the interactions between floral resource phenology, bee foraging behaviour, and traits such as diet breadth, sociality, and body size. Recent research on bumble bees has examined behavioural responses to local floral turnover and effects of landscape-scale floral resource phenology on fitness, abundance, and foraging distances. Comparable studies are needed on non-social, pollen-specialist species. We also encourage greater use of information contained in museum collections on bee phenologies and floral hosts to test how phenology has shaped the evolution of bee–plant associations.
... Recent work on bumble bee flight metabolism and respiration [89][90][91][92] and flight biomechanics (e.g., [93][94][95]) dually contributes to our understanding of the basic biology of bumble bees, and also how the metabolic and mechanical challenges associated with movement to higher elevations might be met by upwardly-migrating populations. Under both latitudinal and altitudinal movement scenarios, bumble bees may encounter new challenges as they crowd into refugia, such as greater competition with congenerics and other bee groups as they converge on similar floral resources [96][97][98], or the spread of disease-related microorganisms to new areas and between and within newly-assembled bee communities [99,100]. For example, the humanfacilitated spread of B. terrestris into South America has been linked to both of these phenomena, which appear to be drivers in the decline of the native species B. dahlbomii in this region [99,101,102]. ...
... In these Rocky Mountain field sites, forb reproductive success depends heavily on bumble bee pollination services[35]. We focused on the two main bumble bee species (Bombus balteatus and B. sylvicola) that are year-round residents and predominate above the timberline at our field sites[36][37][38].of the bumble bee: Buzzes predict pollination services ...
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Multiple interacting factors drive recent declines in wild and managed bees, threatening their pollination services. Widespread and intensive monitoring could lead to more effective management of wild and managed bees. However, tracking their dynamic populations is costly. We tested the effectiveness of an inexpensive, noninvasive and passive acoustic survey technique for monitoring bumble bee behavior and pollination services. First, we assessed the relationship between the first harmonic of the flight buzz (characteristic frequency) and pollinator functional traits that influence pollination success using flight cage experiments and a literature search. We analyzed passive acoustic survey data from three locations on Pennsylvania Mountain, Colorado to estimate bumble bee activity. We developed an algorithm based on Computational Auditory Scene Analysis that identified and quantified the number of buzzes recorded in each location. We then compared visual and acoustic estimates of bumble bee activity. Using pollinator exclusion experiments, we tested the power of buzz density to predict pollination services at the landscape scale for two bumble bee pollinated alpine forbs (Trifolium dasyphyllum and T. parryi). We found that the characteristic frequency was correlated with traits known to affect pollination efficacy, explaining 30–52% of variation in body size and tongue length. Buzz density was highly correlated with visual estimates of bumble bee density (r = 0.97), indicating that acoustic signals are predictive of bumble bee activity. Buzz density predicted seed set in two alpine forbs when bumble bees were permitted access to the flowers, but not when they were excluded from visiting. Our results indicate that acoustic signatures of flight can be deciphered to monitor bee activity and pollination services to bumble bee pollinated plants. We propose that applications of this technique could assist scientists and farmers in rapidly detecting and responding to bee population declines.
... Community composition and mutualistic interactions also respond to changes across elevation gradients, which illustrate how environmental stress influences biotic communities [6,7]. Responses of bee-plant interactions across elevation gradients have been explored [8,9]. However, little information is available on bees compare to other pollinator taxonomic groups such as beetles, wasps and flies as regards their response to elevation gradients, especially in Africa. ...
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Interaction networks are sensitive to elevation gradients through changes in local distribution of interacting partners. Here, we use plant-pollinator interaction network metrics to assess the effect of elevation on flowers and flower-visiting insect assemblages on a sentinel mountain used for monitoring climate change in the flower- and insect-rich Cape Floristic Region. We also use these interaction metrics to explain the effect of environmental factors on the interaction networks. We did this over four vegetation zones
... Interestingly, we found that the number of revisits increased with the air temperature. It is known that climate warming may lead to morphological, physiological and functional mismatches between plants and pollinators [35][36][37][38][39][57][58][59][60] as well as to a lower availability of food plants [61][62][63]. The negative impact of increasing temperature on the effectiveness of scent marking, however, has only been studied occasionally and not in the context of plant-pollinator interactions [40,41]. ...
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Background Strawberries are a common crop whose yield success depends on the availability of pollinators. Invasive alien plants, such as Impatiens glandulifera and I. parviflora , are also attractive for bees and hoverflies, respectively, and occur in close proximity to strawberry cultivation areas. The aim of the study was to test whether alien plants may decrease pollination of strawberry cultivation. However, even if the pollinators are abundant, efficiency of their pollination may decrease as a result of revisits of flowers that were already probed. It is addressed by pollinators by scent marking. Moreover, such revisits can be determined by nectar replenishment, which may occur rapidly in nectar-rich flowers. We studied revisits to I. glandulifera by bumblebees and defined the factors that influence the probability of revisits (air temperature; pollinator species; family caste and size; flower area; sun radiation; and time of day). Results We found that the two alien species decreased the number of pollinators visiting strawberries. Apoidea, Bombini and Syrphidae significantly decreased on Fragaria × ananassa when alien Impatiens were present. We also revealed the influence of increasing air temperature on bumblebee foraging, which was particularly significant for female workers. At very high temperatures (> 37°C), bumblebee males revisited probed flowers less often than female workers. Conclusions Our results demonstrate that in experimental conditions attractive alien species decrease pollination of strawberries, which may negatively affect production of this crop. Although the results have not been verified in real-life strawberry fields yet, we recommend that alien plant species that share the same pollinators and occur in close proximity of strawberries are controlled. Moreover, we found that revisits of probed flowers may weaken feeding efficiency of bumblebees. If revisits are not induced by nectar replenishment, then global warming may pose a serious threat to the survival of colonies, which may have consequences also for the plants that attract them, e.g., for strawberries.
... Finally, we evaluated whether ecological specialisation (d' and normalised degree) of Hymenoptera and Diptera species is related to proboscis length and plant composition. We expected that the proboscis length of flower visitors would be positively related to their degree of specialisation, as previous studies on bumblebees (Miller-Struttmann & Galen, 2014) and hummingbirds (Maglianesi et al., 2014;Tinoco et al., 2017) have shown that long-tongued pollinators tend to specialise on flowers with deep nectar tubes, whereas short-tongued pollinators often generalise across flowers with short tube depths. We predicted that plant composition also drives changes in flower visitor specialisation. ...
Article
Morphological trait-matching and species abundance are thought to be the main factors affecting the frequency and strength of mutualistic interactions. However, the relative importance of trait-matching and species abundance in shaping species interactions across environmental gradients remains poorly understood, especially for plant–insect mutualisms involving generalist species. Here, we characterised variation in species and trait composition and the relative importance of trait-matching and species abundance in shaping plant–Hymenoptera and plant–Diptera mutualisms in four meadows across an elevational gradient (2,725–3,910 m) in Yulong Snow Mountain, Southwest China. We also evaluated the effects of morphological traits of flower visitors and plant composition on their foraging specialisation (d’ and normalised degree). There was a high degree of dissimilarity in the composition of Hymenoptera and Diptera visitors and their visited plants between communities. This variation was mainly driven by the spatial replacement of species. Both for plant–Hymenoptera and plant–Diptera networks, trait-matching between nectar tube depth and proboscis length was a stronger predictor of the interactions between temporally co-occurring plants and flower visitors than species abundance. Fourth-corner analyses revealed statistically significant trait-matching between nectar tube depth and proboscis length in plant–Hymenoptera networks at all sites, suggesting that Hymenoptera consistently foraged on plant species with nectar tube depths matching their proboscis lengths. By contrast, significant trait-matching in plant–Diptera networks was only observed at the two lower elevation sites. The species-level specialisation d’ of flower visitors increased significantly as the proboscis length and the difference in nectar tube depth between the plant community and the plants visited by flower visitors increased. Our results highlight that the importance of trait-matching in shaping pairwise interactions and niche partitioning depends on the specific features (e.g. species composition and trait availability) of the plant-pollinator system. For specialised plant-Hymenoptera systems, trait-matching is an important determinant of species interactions, whereas for generalist plant-Diptera systems, trait-matching is relatively unimportant.
... Galen, 2014;Ramos-Jiliberto et al., 2010;. For example,Ramos-Jiliberto et al. (2010) showed that the number and size of modules change with altitude, but modularity remains significant and conserved across altitudes. ...
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Aim Extinctions and coextinctions seriously threaten global plant–pollinator assemblies, and thus a better understanding of the geographic variability in their robustness is urgently required. Although the geographic patterns of species extinction rates are frequently explored, it remains largely unknown how the subsequent coextinction risk of species varies across environments. We hypothesize that the geographic variation of network robustness to extinctions is mediated by modularity – the tendency of a network to be organized in modules of strongly interacting species – because modularity buffers perturbations and varies across environments. Location Global. Time period Current. Major taxa studied Flowering plants and their animal pollinators. Methods Using 79 pollination networks, we first explored the variation of network robustness across geographic and climatic gradients and, second, analysed the role of modularity in explaining the association between robustness and those environmental gradients. We quantified the robustness of taxonomic, functional and phylogenetic diversity of pollinators under simulated coextinctions triggered by specialist‐first, generalist‐first, and random plant removals. Results Only the robustness of phylogenetic diversity under specialist‐first removals showed a global latitudinal trend by which robustness increased towards the tropics on mainlands but increased towards the poles on islands. Generally, robustness was strongly promoted by modularity, and also directly dampened by insularity and precipitation seasonality (PS). Through the mediation of modularity, robustness was indirectly increased by actual evapotranspiration and PS, and decreased by the interaction between PS and insularity. Besides, network size and sampling area affected robustness but did not influence modularity. Main conclusions The indirect environmental effect on robustness via modularity was prevalent, which supports our hypothesis and reveals the importance of network structure in mediating the geographic variation of network robustness. The global pattern of robustness indicates the phylogenetic diversity of pollinators is relatively vulnerable to the loss of specialist plants in tropical islands and high‐latitude mainland compared to other regions.
... Because of these abiotic changes, altitudinal gradients influence variations in species composition and physiognomy structure in tropical forests and in other ecosystems worldwide (Almeida, Mont ufar, & Anthelme, 2013;Gentry, 1988;Guti errez, L opez-Cortes, & Marquet, 1998;Jones, 1992;Leing€ artner, Krauss, & Steffan-Dewenter, 2014;Lieberman, Lieberman, Peralta, & Hartshorn, 1996;Pendry & Proctor, 1996;Proctor, Lee, Langley, Munro, & Nelson, 1988;Sfenthourakis, Anastasiou, & Strutenschi, 2005). In this way, altitude and associated factors may influence insect communities in abundance, species richness and patterns of interaction with plants (see Hodkinson, 2005), as has been observed for bee diversity and behavior (e.g., Hoiss, Krauss, Potts, Roberts, & Steffan-Dewenter, 2012;Miller-Struttmann & Galen, 2014). ...
Article
Euglossini bees are more diverse in tropical and subtropical rainforests in the Neotropics and their distribution may have a strong influence of altitude. These bees are considered important pollinators being able to cross long distances in continuous forests. Euglossini males collect fragrances, mainly from orchids, used as precursors for sex pheromones, a fact that allows the use of synthesized compounds for their capture. The aim of this study was to evaluate the occurrence of Euglossini males in elevated sites of a montane rainforest (from 960 to 1,200 m) in southeastern Brazil (Atlantic Forest). Scent traps were distributed at three locations in different altitudes on one day per month over 19 months. Six traps with different baits were distributed at each location. Overall, the male Euglossini community presented a low number of species (seven) and individuals (110). Moreover, we detected a drastic decay in Euglossini species richness and abundance with increasing altitude, which suggests a limit for the occurrence of Euglossini populations around 1100 m in the study area. This community presented a high dominance degree with a single dominant species (Euglossa annectans). In addition, only one species of perfume orchid was abundant and flowered in the study area during our sampling period. The distribution of this orchid species corresponds to the occurrence of Euglossini males in the study area. Our results indicate that pollinator scarcity may compromise the establishment of perfume orchid populations at higher altitudes in the Atlantic Forest.
... Bumblebees are pollinators of many flowers that have high thermoregulatory abilities and are able to be active even at very low ambient temperatures (Miller-Struttmann et al., 2014;Corbet et al., 1993). Thus, they serve as important pollinators, especially in an alpine ecosystem ( Yu et al., 2012). ...
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Ecology and conservation status of bumblebee species remains poorly understood, especially in rapidly degrading urban ecosystems, which is important considering the role of bumblebees in the pollinations. We collected more than 200 bumblebee (Bombus spp.) specimens under six species in different parts of the Kathmandu valley (Kathmandu, Lalitpur, and Bhaktapur cities) in Nepal. The species of bumblebees were analyzed with their host plant types and the land use change using remote sensing and field observation data. We found that the bumblebees exert strong variation and were significantly affected by the families of the host plants and the nature of flowers (open and closed type) rather than colors and categories (invasive and noninvasive). We underline that the rapid habitat loss by changing land use in the study area can be a potential threat to the conservation of these important pollinators, and thus, need focused habitat conservation efforts.
... Asynchronous migration of (specialist) plant or pollinator mutualists with climate change could limit the pace of migration for the partner species, reduce the fitness of both interacting species, and alter ecoevolutionary dynamics within pollination mutualisms (Gilman et al., 2010). For example, bee diversity in alpine ecosystems in Colorado has increased with the influx of lower elevation bee species over the past 40 years (Miller-Struttmann and Galen, 2014). Additionally, some alpine bee species evolved significantly shorter tongues, which allow these bees to forage on a wider variety of plant species (Miller-Struttmann et al., 2015). ...
... long-term increases in available nesting spaces for mason bees. In addition to Osmia spp., bumblebees (Bombus spp.) were abundant in our study and are often among the most common taxa found at high-elevation sites 31,32 , due in part to their relatively high thermal mass in comparison to other bees 24,25,33 . Our collections provide evidence of shifts in Bombus assemblages following bark beetle outbreak: B. flavifrons was substantially more abundant in post-outbreak stands, whereas several high-elevation specialists were more frequent in non-affected stands including B. balteatus and B. melanopygus. ...
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Landscape-scale bark beetle outbreaks alter forest structure with direct and indirect effects on plants and animals in forest ecosystems. Using alpine spruce forest and a native bee community as a study system, we tested how tree mortality from bark beetles impacts bee foraging habitats and populations. Bees were collected across the growing season (early-, middle-, and late-season) for two years using passive trapping methods, and collections were used to analyze patterns in species abundances and diversity. Three important findings emerged: (1) forest stands that were post-outbreak had 62% higher floral density and 68% more floral species during peak bloom, respectively, than non-affected stands; (2) bee captures were highest early-season (June) and were not strongly affected by bark beetle outbreak; however, mean number of bee species and Shannon-Weiner diversity were significantly higher in post-outbreak stands and this effect was pronounced early in the growing season. Corresponding analysis of β-diversity indicated higher accumulation of bee biodiversity in post-outbreak stands and a turnover in the ratio of Bombus: Osmia; (3) bee captures were linked to variation in foraging habitat, but number of bee species and diversity were more strongly predicted by forest structure. our results provide evidence of increased alpine bee biodiversity in post-outbreak stands and increased availability of floral resources. We conclude that large-scale disturbance from bark beetle outbreaks may drive shifts in pollinator community composition through cascading effects on floral resources, mediated via mortality of overstory trees.
... In the short growing season of alpine environments, floral visits by worker bees drastically increase after mid-July (Kudo 2016). Intensive floral use by worker bees may cause the partitioning of floral resources among bee species, but the pattern of interspecific resource partitioning was not consistent in the previous studies (Goulson 2010;Miller-Struttmann and Galen 2014). ...
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In response to the qualitative and quantitative changes in floral resources, bumble bees flexibly forage multiple plant species throughout the growing season. During the short summer in an alpine ecosystem, the activity of worker bees is maximized in the middle of the season, when the competition for floral resources may be intense. We predicted that the foraging patterns of bumble bees are affected by both relative floral abundance and interactions between bumble bee species. We recorded the floral abundance of individual plant species and the foraging frequency of bumble bees in an alpine meadow during the major flowering period over 3 years. Two bumble bee species were common during the major flowering period. Although they tended to visit abundant floral species, the shorter-tongued species (Bombus hypocrita) showed a more diverse and flexible floral choice than the longer-tongued species (Bombus beaticola). The degree of floral use overlap between two bumble bee species tended to decrease when the foraging density of the longer-tongued species was high. These results indicated that multiple bumble bee species are able to coexist when certain bee species can flexibly change targeting flowers in response to the temporal variations in flowering species and the density of competing bee species. The extent of foraging flexibility is related to the morphological traits of bee species and availability of floral resources.
... Theoretical work on the structurestability relationship of ecological networks suggests a positive association between network resilience to species extinction and structural indices, including connectance and nestedness (Dunne et al., 2002;Lafferty & Kuris, 2009;Memmott et al., 2004). Our empirical analyses along several elevation transects support theoretical expectations showing that networks in cold environments are less specialized and more nested, which presumably enhance network robustness (Burgos et al., 2007;Miller-Struttmann & Galen, 2014). ...
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Aim Ecological gradients are expected to be associated with structural rewiring of species interaction networks. The study of network structures along geographic and ecological gradients, however, remains marginal because documenting species interactions at multiple sites is a methodological challenge. Here, we aimed to study the structural variation in plant–herbivore interaction networks along elevational gradients using molecular metabarcoding. Location European Alps. Taxon Plant and Orthopteran herbivores. Methods We used a standardized DNA metabarcoding method applied to Orthopteran faeces to document the structure of 48 networks of species interactions across six elevational gradients. We examined how structural properties of plant–Orthoptera networks reflecting specialization and robustness vary with elevation. We compared observed variation to null models to account for differences in network size. Results We found an increase in the levels of generality and nestedness with decreasing temperature, and the correlation was stronger than in null models. These relationships corresponded to greater robustness and reduced the importance of specific keystone species in alpine habitats compared to lowland grasslands. Main conclusions In cold environments, plant–herbivore networks are wired in a way that may reinforce the resilience of the system to species extinction. Documenting ecological networks along ecological gradients allows a better understanding of the influence of climate on the structure of ecological communities.
... Various studies suggest that with fewer flower resources, the advantage of a long proboscis, expressed as a specialized phenotype, diminished or was lost, potentially driving the rapid evolution of bees toward a shorter proboscis [77]. On the contrary, the greater and diversified availability of floral morphologies, offered by radiation of the angiosperms, may have led to a greater degree of specialization in various apoid taxa, favoring the evolution of longer mouthparts. ...
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The Mediterranean basin hosts a high diversity of plants and bees, and it is considered one of the world’s biodiversity hotspots. Insect pollination, i.e., pollen transfer from male reproductive structures to conspecific female ones, was classically thought to be a mutualistic relationship that links these two groups of organisms, giving rise to an admirable and complex network of interactions. Although nectar is often involved in mediating these interactions, relatively little is known about modifications in its chemical traits during the evolution of plants. Here, we examine how the current sucrose-dominated floral nectar of most Mediterranean plants could have arisen in the course of evolution of angiosperms. The transition from hexose-rich to sucrose-rich nectar secretion was probably triggered by increasing temperature and aridity during the Cretaceous period, when most angiosperms were radiating. This transition may have opened new ecological niches for new groups of insects that were co-diversifying with angiosperms and for specific nectar-dwelling yeasts that originated later (i.e., Metschnikowiaceae). Our hypothesis embeds recent discoveries in nectar biology, such as the involvement of nectar microbiota and nectar secondary metabolites in shaping interactions with pollinators, and it suggests a complex, multifaceted ecological and evolutionary scenario that we are just beginning to discover.
... Montane ecosystems have received low attention from network ecologists even though altitude affects many factors that would influence foraging strategies and thus the communities (MillerStruttmann andGalen 2014, Watts et al. 2016). Despite that, it is possible to list some recurrent features. ...
Article
Conservation of species is often focused either only on those that are endangered, or on maximising the number recorded on species lists. However, species share space and time with others, thus interacting and building frameworks of relationships that can be unravelled by community-level network analysis. It is these relationships that ultimately drive ecosystem function via the transfer of energy and nutrients. However interactions are rarely considered in conservation planning. Network analysis can be used to detect key species (“hubs”) that play an important role in cohesiveness of networks. We applied this approach to plant-pollinator communities on two montane Northern Apennine grasslands, paying special attention to the modules and the identity of hubs. We performed season-wide sampling and then focused the network analyses on time units consistent with plant phenology. After testing for significance of modules, only some modules were found to be significantly segregated from others. Thus, networks were organized around a structured core of modules with a set of companion species that were not organized into compartments. Using a network approach we obtained a list of important plant and pollinator species, including three Network Hubs of utmost importance, and other hubs of particular biogeographical interest. By having a lot of links and high partner diversity, hubs should convey stability to networks. Due to their role in the networks, taking into account such key species when considering the management of sites could help to preserve the greatest number of interactions and thus support many other species.
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Ecological partnerships, or mutualisms, are globally widespread, sustaining agriculture and biodiversity. Mutualisms evolve through the matching of functional traits between partners, such as tongue length of pollinators and flower tube depth of plants. Long-tongued pollinators specialize on flowers with deep corolla tubes, whereas shorter-tongued pollinators generalize across tube lengths. Losses of functional guilds because of shifts in global climate may disrupt mutualisms and threaten partner species. We found that in two alpine bumble bee species, decreases in tongue length have evolved over 40 years. Co-occurring flowers have not become shallower, nor are small-flowered plants more prolific. We argue that declining floral resources because of warmer summers have favored generalist foraging, leading to a mismatch between shorter-tongued bees and the longer-tubed plants they once pollinated.
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Differences in interaction specializations between nectarivorous birds and plants across continents serve as common examples of evolutionary trajectory specificity. While New World hummingbird-plant networks have been extensively studied and are considered highly specialized, knowledge on the network specialization of their Old World counterparts, sunbirds (Nectariniidae), remains limited. A few studies from tropical Africa indicate that sunbird-plant networks are rather generalized. Unfortunately, these studies are limited to dry seasons and high elevations at the tree line, environments where niche-based hypotheses also often predict lower resource partitioning. In our study, we explored the specialization of sunbird-plant networks and their spatiotemporal variability on Mt. Cameroon (Cameroon). Using a combination of automatic video recordings and personal observations, we constructed eight comprehensive sunbird-plant networks in four forest types at different elevations in both the dry and wet seasons. As reported in previous studies, the montane forest plants, birds and whole networks were highly generalized. Nevertheless, we observed a much higher specialization in forests at lower elevations. Except at the lowest altitude, the wet season was also characterized by higher specialization. While less specialized flowering trees dominated in the dry season networks, more specialized herbs and shrubs were visited by birds during the wet season. As our findings do not support the generally accepted assumption that Old World bird-plant networks are rather generalized, we need further studies to understand the differences in bird-plant specializations on individual continents.
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By specialising on specific resources, species evolve advantageous morphologies to increase the efficiency of nutrient acquisition. However, many specialists face variation in resource availability and composition. Whether specialists respond to these changes depends on the composition of the resource pulses, the cost of foraging on poorly matched resources, and the strength of interspecific competition. We studied hummingbird bill and plant corolla matching during seasonal variation in flower availability and morphology. Using a hierarchical Bayesian model, we accounted for the detectability and spatial overlap of hummingbird-plant interactions. We found that despite seasonal pulses of flowers with short-corollas, hummingbirds consistently foraged on well-matched flowers, leading to low niche overlap. This behaviour suggests that the costs of searching for rare and more specialised resources are lower than the benefit of switching to super-abundant resources. Our results highlight the trade-off between foraging efficiency and interspecific competition, and underline niche partitioning in maintaining tropical diversity.
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Species interactions are fundamental to community dynamics and ecosystem processes. Despite significant progress in describing species interactions, we lack the ability to predict changes in interactions across space and time. We outline a Bayesian approach to separate the probability of species co‐occurrence, interaction and detectability in influencing interaction betadiversity. We use a multi‐year hummingbird–plant time series, divided into training and testing data, to show that including models of detectability and occurrence improves forecasts of mutualistic interactions. We then extend our model to explore interaction betadiversity across two distinct seasons. Despite differences in the observed interactions among seasons, there was no significant change in hummingbird occurrence or interaction frequency between hummingbirds and plants. These results highlight the challenge of inferring the causes of interaction betadiversity when interaction detectability is low. Finally, we highlight potential applications of our model for integrating observations of local interactions with biogeographic and evolutionary histories of co‐occurring species. These advances will provide new insight into the mechanisms that drive variation in patterns of biodiversity.
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Range shifts of mountain organisms toward higher elevations in response to global warming may result in spatial mismatches between plants and their pollinators. Here, we aimed to examine whether bumblebee diversity decreases in a high‐altitude zone, and whether it parallels a decrease in the altitudinal diversity of bumblebee‐visited plants. We surveyed the alpha diversities of flower‐visiting bumblebees and bumblebee‐visited plants along an altitudinal gradient on a Japanese high mountain. Then, we examined whether the alpha diversities of bumblebees and bumblebee‐visited plants could be explained by altitude, or by other factors such as season, surveyed area and flower abundance. We found that a model including only altitude best explained bumblebee diversity, and that flower abundance and plant diversity had considerable value in explaining bumblebee diversity. In contrast, none of the studied factors explained plant diversity. Bumblebee diversity was minimal in the high‐altitude zone (1,900–2,600 m a.s.l.), where the only dominant bumblebee species, Bombus beaticola, visited many species of flowering plants. In contrast, five to seven bumblebee species were distributed in the low‐ (700–1,300 m a.s.l.) and middle‐ (1,300–1,900 m a.s.l.) altitude zones. These results show that plant–pollinator mutualism in high‐altitude zone of a Japanese mountain is asymmetric: many bee‐pollinated plants rely almost exclusively on one bumblebee species (B. beaticola) for pollination. Monitoring future changes in the distribution and abundance of B. beaticola is indispensable for the conservation of alpine plant in Japan.
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Alpine environments are among the habitats most strongly affected by climate change, and consequently their unique plants and pollinators are faced with the challenge of adapting or going extinct. Changes in temperature and precipitation affect snowpack and snowmelt, resulting in changes in the growing season in this environment where plant growth and pollinator activity are constrained to the snow‐free season, which can vary significantly across the landscape if there is significant topographic complexity. As in other ecosystems, the resulting changes in phenology are not uniform among species, creating the potential for altered and new interspecific interactions. New plant and animal species are arriving as lower altitude species move up with warming temperatures, introducing new competitors and generating changes in plant–pollinator interactions. Repeating historical surveys, taking advantage of museum collections, and using new technology will facilitate our understanding of how plants and pollinators are responding to the changing alpine environment.
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Differences in bird-flower interaction specialization across continents serve as a common example of evolutionary trajectory specificity. While New World hummingbird-flower networks have been subject to numerous studies and are considered highly specialized, our knowledge of network specialization for their Old World counterparts, sunbirds (Nectariniidae), is completely insufficient. A few studies from tropical Africa indicate that sunbird-flower networks are rather generalized. Unfortunately, these studies are limited to dry seasons and high elevations around the tree-line, the environments where also niche-based hypotheses often predict lower resource partitioning. In our study, we explore the specialization of sunbird-flower networks and their spatio-temporal variability on Mt. Cameroon (Cameroon). Using a combination of automatic video recording and personal observations, we constructed eight comprehensive sunbird-flower networks in four forest types occurring in different elevations and in both the dry and wet season. As reported by previous studies the montane forest plants, birds and whole networks were highly generalized. Nevertheless, we observed much higher specialization in forests in lower elevations. The wet season was also characterised by higher, but not significant, specialization. While less specialized flowering trees dominated in dry season networks, more specialized herbs and shrubs were visited during the wet season. Whereas our findings do not support the generally accepted assumption that Old World bird-flower networks are rather generalized, we need further studies to understand the differences in bird-flower specialization on individual continents.
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This study aims to clarify the community‐wide mating systems and pollination success of alpine plants. Selfing abilities and fruit‐set success of entomophilous alpine plants were compared using a dataset of 46 species that were obtained during the last 30 years in the Taisetsu Mountains, northern Japan. Almost all species showed self‐incompatibility or weak self‐compatibility, indicating that obligate outcrossing is a common mating system in alpine plants. This mating property leads to a clear seasonal pattern of fruit‐set success responding to the seasonal dynamics of pollinator activity. Most species are visited by bumble bees and/or flies, and the seasonal trend of pollination success was shown to vary among pollinator types. Pollen limitation in bee‐pollinated plants was intense early in the season and tended to decrease as the season progressed, reflecting the colony development cycle of bumble bees. Thus, the fruit‐set success of bee‐pollinated plants increased with the delay of flowering time. In contrast, seasonal trends in the pollen limitation and fruit‐set success of fly‐pollinated species were less clear. Mixed‐type plants visited by both bees and flies showed a pattern intermediate to that of bee‐type and fly‐type plants. The flowering phenology of alpine plant communities was greatly regulated by snowmelt patterns, and the pollination success of single species varied greatly among local populations distributed in different snowmelt habitats. These heterogeneous landscape features create diverse plant–pollinator interactions in alpine ecosystems. Based on these results, the ecological and evolutionary significance of the outcrossing syndrome in alpine plants is discussed. The mating system of alpine plants is strongly skewed toward the outcrossing mode in comparison with general pattern of mating systems in animal‐pollinated plants. The present study is the first report on the community‐wide predominance of obligate outcrossers, that is, outcrossing syndrome, in alpine plants.
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Bumble bees play an important role as pollinators of many crop plants and wild flowers. As in many wild bees, their abundance and diversity have declined in recent years, which may threaten the stability of pollination services. The observed decline is often linked with the loss or alteration of natural habitat, e.g., through urbanization, the conversion of natural habitat into largely sealed areas (concrete) inhabited by humans. The effects of urbanization on bumble bees remain as yet controversial with both positive and negative effects reported. We investigated how habitat isolation through increasing areas of concrete, as well as the diversity, abundance, and community composition of floral resources, determine bumble bee abundance and diversity in cities. We found plant species diversity and abundance to be more important than the amount of concrete in driving the abundance and species richness of common bumble bees in a German city. Moreover, plant species composition, i.e., the presence of specific plant species and families (e.g., Fabaceae), played a prominent role. In particular, flower-rich parks and gardens can offer a continuous food supply for bumble bees and attract bumble bee foragers even to isolated patches in the city center.
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The analysis of ecological networks has gained a very prominent foothold in ecology over the last years. While many publications try to elucidate patterns about the networks, others are primarily concerned with the role of specific species in the network. The core challenge here is to tell specialists from generalists. While field data and observations can be used to directly assess specialisation levels, the indirect way through networks is burdened with problems. Here, I review eight measures to quantify specialisation in pollination networks (degree, node specialisation, betweenness, closeness, strength, pollination support, Shannon's H and discrimination d'), the first four being based on binary, the others on weighted network data. All data and R-code are available as supplement and can be applied beyond pollination networks. The indices convey different concepts of specialisation and hence quantify different aspects. Still, there is some redundancy, with node specialisation and closeness quantifying the same properties, as do degree, betweenness and Shannon's H. Using artificial and real network data, I illustrate the interpretation of the different indices and the importance of using a null model to correct for expectations given the different observed frequencies of interactions. For a well-described network the distributions of specialisation values do not differ from null model expectations for most indices. Finally, I investigate the effect of cattle grazing on the specialisation of an important pollinator in eight replicated pollination networks as an illustration of how to employ the specialisation indices, null models and permutation-based statistics in the analysis of specialisation in pollination networks.
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Modified environmental conditions are driving phenological changes in ecosystems around the world. Many plants have already responded to warmer temperatures by flowering earlier and sustaining longer periods of growth. Changes in other environmental factors, like precipitation and atmospheric nitrogen (N) deposition, may also influence phenology but have been less studied. Alpine plants may be good predictors of phenological response patterns because environmental changes are amplified in mountain ecosystems and extreme conditions may make alpine plants particularly sensitive to changes in limiting factors like precipitation, temperature, and N. We tested the effects of increased snowpack, temperature, and N on alpine tundra plant phenology, using snow fence, open-top warming chamber, and N fertilization treatments at the Niwot Ridge Long Term Ecological Research (LTER) site. Flowering phenology of three abundant species was recorded during two growing seasons. Treatment responses varied among species and functional types. Forbs responded to warming by flowering earlier and responded to snowpack and N by flowering later; however, when both snow and N were increased simultaneously, phenology was unchanged. Graminoids flowered earlier in response to N addition. Our results demonstrate that changing environmental conditions influence plant phenology, and specifically highlight that N and multiple factor interactions can yield stronger responses than warming alone.
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Phenology-the timing of biological events-is highly sensitive to climate change. However, our general understanding of how phenology responds to climate change is based almost solely on incomplete assessments of phenology (such as first date of flowering) rather than on entire phenological distributions. Using a uniquely comprehensive 39-y flowering phenology dataset from the Colorado Rocky Mountains that contains more than 2 million flower counts, we reveal a diversity of species-level phenological shifts that bring into question the accuracy of previous estimates of long-term phenological change. For 60 species, we show that first, peak, and last flowering rarely shift uniformly and instead usually shift independently of one another, resulting in a diversity of phenological changes through time. Shifts in the timing of first flowering on average overestimate the magnitude of shifts in the timing of peak flowering, fail to predict shifts in the timing of last flowering, and underrepresent the number of species changing phenology in this plant community. Ultimately, this diversity of species-level phenological shifts contributes to altered coflowering patterns within the community, a redistribution of floral abundance across the season, and an expansion of the flowering season by more than I mo during the course of our study period. These results demonstrate the substantial reshaping of ecological communities that can be attributed to shifts in phenology.
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Climate change is predicted to alter global species diversity(1), the distribution of human pathogens' and ecosystem services(3). Forecasting these changes and designing adequate management of future ecosystem services will require predictive models encompassing the most fundamental biotic responses. However, most present models omit important processes such as evolution and competition(4,5). Here we develop a spatially explicit eco-evolutionary model of multi-species responses to climate change. We demonstrate that both dispersal and evolution differentially mediate extinction risks and biodiversity alterations through time and across climate gradients. Together, high genetic variance and low dispersal best minimized extinction risks. Surprisingly, high dispersal did not reduce extinctions, because the shifting ranges of some species hastened the decline of others. Evolutionary responses dominated during the later stages of climatic changes and in hot regions. No extinctions occurred without competition, which highlights the importance of including species interactions in global biodiversity models. Most notably, climate change created extinction and evolutionary debts, with changes in species richness and traits occuring long after climate stabilization. Therefore, even if we halt anthropogenic climate change today, transient eco-evolutionary dynamics would ensure centuries of additional alterations in global biodiversity.
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Growing concern about the influence of climate change on flowering plants, pollinators, and the mutualistic interactions between them has led to a recent surge in research. Much of this research has addressed the consequences of warming for phenological and distributional shifts. In contrast, relatively little is known about the physiological responses of plants and insect pollinators to climate warming and, in particular, how these responses might affect plant-pollinator interactions. Here, we summarize the direct physiological effects of temperature on flowering plants and pollinating insects to highlight ways in which plant and pollinator responses could affect floral resources for pollinators, and pollination success for plants, respectively. We also consider the overall effects of these responses on plant-pollinator interaction networks. Plant responses to warming, which include altered flower, nectar, and pollen production, could modify floral resource availability and reproductive output of pollinating insects. Similarly, pollinator responses, such as altered foraging activity, body size, and life span, could affect patterns of pollen flow and pollination success of flowering plants. As a result, network structure could be altered as interactions are gained and lost, weakened and strengthened, even without the gain or loss of species or temporal overlap. Future research that addresses not only how plant and pollinator physiology are affected by warming but also how responses scale up to affect interactions and networks should allow us to better understand and predict the effects of climate change on this important ecosystem service.
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Community studies have shown that plant species are often pollinated by multiple pollinators; however, networks of heterospecific pollen transfer (HPT) in natural communities remain largely unexplored. We analyzed pollen deposition on stigmas of 57 flowering species to build a picture of plant-plant interactions via HPT in a biodiverse alpine meadow in southwest China. Plant species were categorized as pollen donors or recipients by their link numbers and link qualities. We identified 3609 heterospecific pollen grains, representing 410 links among 69 pollen species. Each plant species received on average 7.2 pollen species and donated its pollen to 5.5 species; only a few species donated or received large amounts of pollen or pollen from a large number of species. Compared to specialized plants, generalized plants tended to receive more heterospecific pollen but exported no more pollen to other species. Plant position in the network was related to both floral traits (stigma position) and pollinator generalization level. When different species share the same pollinator, bidirectional HPT may occur, but this was rarely observed in the species-rich community, indicating that interspecific pollen interference was largely unidirectional. Our study highlights the importance of understanding how sympatric flowering plants reduce deleterious effects of HPT, for example via stigma position. This study is the first to present a pollen transfer network for an entire community and to unravel its properties using directed network analysis.
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Flowering phenology of alpine plant communities was observed at seven sites (two fellfields, two alpine meadows, and three snowbeds) in 1996 and 1997 in the Taisetsu Mts., northern Japan. These sites were selected along a gradient of snowmelt time. In total, flowering phenologies of 39 species were compared across the seven sites. The length of flowering season within commu- nities decreased with lateness of snowmelt from 88 days at the fellfield site to 32 days at the snowbed site. The onset of flowering varied among species at the early snowmelt sites, and it became concentrated within narrow periods among species at the later snowmelt sites. Interspecific overlap of anthesis was large in the late flowering season at the earliest snowmelt site, whereas it was large in the early flowering season at the snowbed sites. Such changes in flowering patterns at community level along the snowmelt gradient were considered to be induced by the difference in temperature sequence after snowmelt among the sites, i.e., the increasing pattern of the effective cumulative temperature. Thus, the time of snowmelt affects not only the flowering phenology of individual species but also the flowering pattern of a whole community. Some species growing in the fellfields changed the temperature-demand for onset of flowering which was considered as an adjustment to decrease the interspecific overlap of anthesis within the community.
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Unlabelled: • Premise of the study: While we have a good understanding of how co-flowering plants interact via pollinator foraging, we still know very little about how plants interact via heterospecific pollen (HP) receipt. To fill this gap, we sought to illuminate the extent of HP receipt and quantitatively evaluate the fitness consequences of HP receipt. We consider plant traits that could mediate the fitness costs of HP receipt in an effort to better understand the potential consequences of pollinator sharing in natural communities. • Methods: We survey the literature for occurrence of HP receipt and assess variation in the fitness effects of a standard HP treatment. We develop a conceptual framework for understanding variation in fitness consequences of HP receipt. • Key results: We find evidence for variation in HP receipt and its costs. Our framework predicts that certain traits (self-incompatibility, small, highly aperaturate or allelopathic pollen) will lead to detrimental HP donors, whereas others (self-compatibility, small or wet stigmas, short styles) will lead to vulnerable HP recipients. We also predict that detrimental effects of HP receipt will increase with decreasing phylogenetic distance between donor and recipient. • Conclusions: Our framework can guide much needed additional work so that we can evaluate whether and which plant traits contribute to the variation in the effects of HP receipt. This will be a step toward predicting the consequences of HP receipt in natural communities, and ultimately transform our understanding of the role of postpollination interactions in floral trait evolution and pollinator sharing.
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Using historic data sets, we quantified the degree to which global change over 120 years disrupted plant-pollinator interactions in a temperate forest understory community in Illinois, USA. We found degradation of interaction network structure and function and extirpation of 50% of bee species. Network changes can be attributed to shifts in forb and bee phenologies resulting in temporal mismatches, nonrandom species extinctions, and loss of spatial co-occurrences between extant species in modified landscapes. Quantity and quality of pollination services have declined through time. The historic network showed flexibility in response to disturbance; however, our data suggest that networks will be less resilient to future changes.
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Surveys in 1974 of bumble bee species distributions along elevational gradients (Pyke 1982) were revisited to reevaluate the original conclusion that coexistence of bumble bee species can be ascribed to niche differentiation, primarily on the basis of proboscis lengths and the associated corolla lengths of visited flowers. Each bee species largely visited a few plant species, which were preferred relative to other species. Bee proboscis length was correlated with average corolla length of visited flowers, but not when species with relatively long and short proboscises were considered separately. Bumble bee abundance was affected by presence or absence of major plant species and, contrary to the interpretation of Pyke (1982), elevation, with neither factor dominating. Multimodal distributions of proboscis lengths and altitudinal replacement of bee species of similar proboscis length were consistent with the original hypothesis that bumble bee species compete for floral resources, especially nectar, and cannot coexist if proboscis lengths are too similar, unless one species is a "nectar robber" and hence has exclusive use of some floral resources. However, observed overlap in elevational distributions of bumble bee species with similar proboscis length cannot be reconciled with this hypothesis unless other phenomena are invoked.
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We evaluate whether species interaction frequency can be used as a surrogate for the total effect of a species on another. Because interaction frequency is easier to estimate than per-interaction effect, using interaction frequency as a surrogate of total effect could facilitate the large-scale analysis of quantitative patterns of species-rich interaction networks. We show mathematically that the correlation between interaction frequency (I) and total effect (T) becomes more strongly positive the greater the variation of I relative to the variation of per-interaction effect (P) and the greater the correlation between I and P. A meta-analysis using data on I, P and T for animal pollinators and seed dispersers visiting plants shows a generally strong, positive relationship between T and I, in spite of no general relationship between P and I. Thus, frequent animal mutualists usually contribute the most to plant reproduction, regardless of their effectiveness on a per-interaction basis.
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This review examines the direct effects of climate change on insect herbivores. Temperature is identified as the dominant abiotic factor directly affecting herbivorous insects. There is little evidence of any direct effects of CO2 or UVB. Direct impacts of precipitation have been largely neglected in current research on climate change. Temperature directly affects development, survival, range and abundance. Species with a large geographical range will tend to be less affected. The main effect of temperature in temperate regions is to influence winter survival; at more northerly latitudes, higher temperatures extend the summer season, increasing the available thermal budget for growth and reproduction. Photoperiod is the dominant cue for the seasonal synchrony of temperate insects, but their thermal requirements may differ at different times of year. Interactions between photoperiod and temperature determine phenology; the two factors do not necessarily operate in tandem. Insect herbivores show a number of distinct life-history strategies to exploit plants with different growth forms and strategies, which will be differentially affected by climate warming. There are still many challenges facing biologists in predicting and monitoring the impacts of climate change. Future research needs to consider insect herbivore phenotypic and genotypic flexibility, their responses to global change parameters operating in concert, and awareness that some patterns may only become apparent in the longer term.
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Sympatric plant species with similar flowering phenologies and floral mor-phologies may compete for pollination, and as a consequence potentially influence each other's reproductive success and mating system. Two likely competitors are Mimulus ringens and Lobelia siphilitica, which co-occur in wet meadows of central and eastern North Amer-ica, produce blue zygomorphic flowers, and share several species of bumble bee pollinators. To test for effects of competition for pollination, we planted experimental arrays of Mimulus ringens, each consisting of genets with unique combinations of homozygous marker ge-notypes. In two arrays we planted mixtures of Mimulus and Lobelia, and in two additional arrays we planted Mimulus without a competitor for pollination. Bumble bee pollinators frequently moved between Mimulus and Lobelia flowers in the mixed-species arrays, with 42% of plant-to-plant movements being interspecific transitions. Pollinator movements between species were associated with a reduction in the amount of conspecific pollen arriving on Mimulus stigmas. The presence of Lobelia led to a significant 37% reduction in the mean number of Mimulus seeds per fruit. In addition, Mimulus had a significantly lower rate of outcrossing in the mixed-species arrays (0.43) than in the ''pure'' arrays (0.63). This is the first study to demonstrate that competition for pollination directly in-fluences outcrossing rates. Our work suggests that in self-compatible populations with genetic load, competition for pollination may not only reduce seed quantity, but may also lower seed quality.
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Recent reviews of plant–pollinator mutualistic networks showed that gen-eralization is a common pattern in this type of interaction. Here we examine the ecological correlates of generalization patterns in plant–pollinator networks, especially how interaction patterns covary with latitude, elevation, and insularity. We review the few published anal-yses of whole networks and include unpublished material, analyzing 29 complete plant– pollinator networks that encompass arctic, alpine, temperate, Mediterranean, and subtrop-ical–tropical areas. The number of interactions observed (I) was a linear function of network size (M) the maximum number of interactions: ln I 0.575 0.61 ln M; R 2 0.946. The connectance (C), the fraction of observed interactions relative to the total possible, decreased exponentially with species richness, the sum of animal and plant species in each community (A P): C 13.83 exp[0.003(A P)]. After controlling for species richness, the residual connectance was significantly lower in highland (1500 m elevation) than in lowland networks and differed marginally among biogeographic regions, with both alpine and trop-ical networks showing a trend for lower residual connectance. The two Mediterranean networks showed the highest residual connectance. After correcting for variation in network size, plant species were shown to be more generalized at higher latitude and lowland habitats, but showed increased specialization on islands. Oceanic island networks showed an im-poverishment of potential animal pollinators (lower ratio of animal to plant species, A : P, compared to mainland networks) associated with this trend of increased specialization. Plants, but not their flower-visiting animals, supported the often-repeated statements about higher specificity in the tropics than at higher latitudes. The pattern of interaction build-up as diversity increases in pollination networks does not differ appreciably from other mutualisms, such as plant–seed disperser networks or more complex food webs.
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When co-occurring plant species overlap in flowering phenology they may compete for the service of shared pollinators. Competition for pollination may lower plant reproductive success by reducing the number of pollinator probes or by decreasing the quality of pollen transport to or from a focal species. Pair-wise interactions between plants sharing pollinators have been well documented. However, relatively few studies have examined interactions for pollination among three or more plant species, and little is known about how the outcomes and mechanisms of competition for pollination may vary with competitor species composition. To better understand how the dynamics of competition for pollination may be influenced by changes in the number of competitors, we manipulated the presence of two competitors, Lythrum salicaria and Lobelia siphilitica, and quantified reproductive success for a third species, Mimulus ringens. Patterns of pollinator preference and interspecific transitions in mixed-species arrays were significantly influenced by the species composition of competitor plants present. Both pair-wise and three-species competition treatments led to a similar ∼ 40% reduction in Mimulus ringens seed set. However, the patterns of pollinator foraging we observed suggest that the relative importance of different mechanisms of competition for pollination may vary with the identity and number of competitors present. This variation in mechanisms of competition for pollination may be especially important in diverse plant communities where many species interact through shared pollinators.
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Recent ecological studies suggest that the landscape context of native habitat remnants may significantly influence plant and animal abundance and distribution within those remnants. Other research has revealed a weak link between landscape context and native community composition. To understand the relative importance of local and regional habitat characteristics for grassland butterflies, we assessed butterfly community diversity in four types of grassland habitats surrounded by varying amounts of urban development near Boulder, Colorado ( U.S.A.). We recorded butterfly species abundance and composition in 66 grassland study plots on five sampling dates in 1999 and 2000. Grasslands were of four types: native shortgrass, native mixed grass, native tallgrass, and planted hayfields. Grasslands also varied in quality, determined by the abundance of native versus exotic plant species. We observed highly significant effects of grassland type on butterfly species richness and composition. For example, tallgrass plots supported significantly higher butterfly species richness than shortgrass plots ( p < 0.01). Habitat quality also affected butterfly species richness and composition. Low-quality plots generally supported fewer species than moderate- or high-quality plots ( p < 0.05). Landscape context—the percentage of urbanization in the surrounding landscape—did not significantly predict butterfly species richness or composition. Our observations suggest that for the grassland butterfly communities in our study, (1) grassland type was the primary determinant of species richness and composition, (2) habitat quality secondarily affected butterfly community diversity, and (3) landscape context did not significantly predict butterfly species composition. Our findings emphasize the importance of maintaining high-quality grassland habitat to protect native butterfly diversity.
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This chapter first examines how water availability affects the diversity of flower functions integral to the process of sexual reproduction. It then reviews the evidence that water use by flowers is costly to vegetative growth. Third, it considers in detail how water acts to moderate other sources of environmental stress during anthesis. The second half of the chapter moves from a functional perspective to an evolutionary one. It addresses three major ways in which water relations might influence floral evolution. First, it examines how selection to reduce the water cost of flowers may lead to reductions in components of floral display. Next, it considers the adaptive significance of floral traits that influence the flower's water relations. The chapter proposes that floral traits may be targets of selection through parental environmental effects if they moderate the amount of water in the floral micro-environment or optimize the use of that water for temperature control. Last, it suggests that the plastic responses of floral traits to moisture regimes may alter the ecological interactions between plants and their animal pollinators. It shows that water stress can affect the mode of pollinator-mediated selection on floral traits indirectly by altering floral rewards and/or the flowering phenology.
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Elevation gradients provide ideal scenarios to study plant responses to environmental factors and to global warming. Physiological and morphological traits, growth, and reproduction in bilberry were investigated at 6 elevations along an elevation gradient from 350 to 2000 m asl. Chlorophyll content and growth increased with elevation, reaching a maximum at 950 m, and then decreased, with both variables being negatively influenced by high soil pH. By contrast, after removing the positive effect of tree canopy cover, the efficiency of photosynthesis did not show differences between elevations. The number of stomata per area increased with elevation, while leaf area reached maximum values at 950 m. Regarding reproductive traits, densities of flowers and fruits were not affected by elevation, and fruit set, seed set, and seed viability only varied between localities within elevation. Moreover, flower production was negatively correlated with soil pH. Reproductive success was not limited by pollen quantity at any of the elevations. However, elevation affected number of ovules, number of mature seeds per fruit, and fruit dry weight; these variables reached their highest values at around 1700 m. These results show that, while bilberry exhibited the most favourable vegetative performance at mid elevations, maximum reproductive output was observed at higher elevations.
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It was hypothesized that the local geographic distributions of bumblebees near Crested Butte, Colorado and the community patterns exhibited by these bumblebees are the products of competition for plants. To evaluate this hypothesis several transects were established and at regular intervals throughout a summer growing season, data were collected along each transect on how many bumblebees of each species and caste were visiting the various plant species. The distributions and abundances of the plant species involved were recorded qualitatively. Seven species of bumblebees accounted for 97% of all bumblebees observed and in this paper attention is restricted to these species. Five other species were observed in very low numbers. Each bumblebee species had a different distributional pattern. The seven bumblebee species form four groups in terms of both their proboscis lengths and the corolla lengths of the plants they preferentially visit. Long-, medium-, and short-tongued groups were most often observed foraging at flowers with long, medium, and short corollas, respectively. Furthermore proboscis lengths of bumblebees tended to be very similar within each group but quite dissimilar between groups. The fourth group consisted of a single short-tongued species which has well-developed mandibles which enable it to rob nectar from many plants with long corollas. It also feeds legitimately on short-corolla flowers. Some anomalies in the above groupings are mentioned and discussed. When the data on bumblebees and plant distributions are combined with data on flower preferences, a pattern consistent with the competition hypothesis emerges. Within each proboscis-length group, bumblebee species tend to replace one another altitudinally in a manner consistent with the hypothesis. The nectar-robbing species is most abundant in areas where a plant that is usually visited by hummingbirds is most common. Other species of bumblebees are unable to gain access to the nectar of this plant. In any uniform well-isolated area, similar floristically to the present study area, only three or four species of bumblebees appear to be able to coexist. Furthermore, a bumblebee community in such an area will apparently consist of a short-, a medium-, and a long-tongued species and in some cases a short-tongued nectar-robber. These patterns are also consistent with the competition hypothesis, as similarity in proboscis length reflects similarity in diet and the intensity of competition should, for these bumblebees, be closely related to diet similarity. It is possible that the observed distributional patterns could also be explained on the basis of different distributions of suitable nest sites for each bumblebee species or different responses to local variations in climatic conditions. Neither alternative seems able, however, to explain the observed patterns.
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We studied tradeoffs among current reproduction, vegetative growth, and resource storage, and the consequences of such tradeoffs for future growth and reproduction in an alpine tundra population of long-lived iteroparous herb, Oxytropis sericea from the Colorado Rocky Mountains. When plant reproductive effort was manipulated through the removal of inflorescence buds, branching and leaf production increased significantly in the year of treatment but the end-of-season nonstructural carbohydrate content in storage organs (taproots and branches) was not affected. In contrast, plants from which buds were removed, accumulated more nitrogen and phosphorus in their perennial branches (although not in taproots), compared to controls. Bud removal in 1996 and in 1997 led to greater vegetative growth in the following summer, with potential consequences for long term reproductive effort. Addition of mineral fertilizer to flowering individuals in 1996 stimulated some aspects of vegetative growth and reproduction in 1997 indicating that growth in these plants was nutrient-limited. Leaf photosynthetic rates were not affected by the removal of inflorescence buds, inflorescences or infructescences during 1996, but were significantly lower in plants prevented from fruiting in 1997, showing limited evidence for photosynthetic compensation of reproductive effort. We suggest that mineral nutrients, rather than photosynthate, constitute the currency underlying the future costs of reproduction in O. sericea.
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Interacting communities of species are organized into complex networks, and network analysis is reckoned to be a strong tool for describing their architecture. Many species assemblies show strong macroecological patterns, e.g. increasing species richness with decreasing latitude, but whether this latitudinal diversity gradient scales up to entities as complex as networks is unknown. We investigated this using a dataset of 54 community-wide pollination networks and hypothesized that pollination networks would display a latitudinal and altitudinal species richness gradient, increasing specialization towards the tropics, and that network topology would be affected by current climate.
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Mutualistic networks display distinct structural and organizational features such as nestedness, power-law degree distribution and asymmetric dependencies. Attention is now focused on how these structural properties influence network function. Most plant-pollinator networks are constructed using records of animals contacting flowers, which is based on the assumption that all visitors to flowers are pollinators; however, animals may visit flowers as nectar robbers, florivores, or to prey upon other visitors. To differentiate potential pollinator interactions from other interaction types, we examined individual bees that had visited flowers to detect if they carried pollen. Using these data, we constructed visitation and pollen-transport networks for a spinifex-dominated arid zone grassland. To determine how the structure of the visitation network reflects pollen transport, we compared the two networks using a null model approach to account for differences in network size. Differences in number of species, nestedness and connectance observed between the visitation and pollen-transport networks were within expected ranges generated under the null model. The pollen-transport network was more specialized, had lower interaction evenness, and fewer links compared to the visitation network. Almost half the number of species of the visitation network participated in the pollen-transport network, and one-third of unique visitation interactions resulted in pollen transport, highlighting that visitation does not always result in pollination. Floral visitor data indicate potential pollen transporters, but inferring pollination function from visitation networks needs to be performed cautiously as pollen transport resulted from both common and rare interactions, and depended on visitor identity. Although visitation and pollen-transport networks are structurally similar, the function of all species cannot be predicted from the visitation network alone. Considering pollen transport in visitation networks is a simple first step towards determining pollinators from non-pollinators. This is fundamental for understanding how network structure relates to network function.
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1. We revisited bumble bee survey data collected by Pyke in 1974 (Pyke, Ecology, 63, 555–573, 1982) to evaluate seasonal changes in abundances of bumble bees and their floral resources, diel patterns of bumble bee activity, and elevation effects on plant and bumble bee phenology. 2. Bumble bee abundance increased during summer as spring queens founded colonies that produced workers, followed by males and autumn queens. The number of plant species visited by bumble bees increased to a peak in midsummer, then declined. 3. The number of bumble bees recorded per person-hour peaked later than the number of flowering plant species used by the bees. Few autumn queens were observed. 4. Despite species differences in emergence times of spring queens, there were no apparent phenological differences among species in worker abundances. 5. Because flowering commences later at higher elevation, abundances of workers and males are also shifted later; therefore elevational comparisons must be seasonally adjusted. 6. These analyses provide basic information about important pollinating insects, and permit future investigations of elevational shifts over time to be properly adjusted for phenological and elevation effects in survey data.
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Seven of 12 species of bumblebees accounted for 97% of all bumblebees observed. These 7 species form 4 groups in terms of both their proboscis lengths and the corolla lengths of the plants they preferentially visit. Long-, medium-, and short-tongued groups were most often observed foraging at flowers with long, medium, and short corollas, respectively. Proboscis lengths tended to be very similar within each group but quite dissimilar between groups. The 4th group consisted of a single short-tongued species which has well- developed mandibles which enable it to rob nectar from many plants with long corollas. It also feeds legitimately on short-corolla flowers. When the data on bumblebees and plant distributions are combined with data on flower preferences, a pattern consistent with a competition hypothesis emerges. Within each proboscis-length group, bumblebee species tend to replace one another altitudinally in a manner consistent with the hypothesis.-from Author
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used a first-order, monthly snow model and observations to disentangle seasonal influences on 20th century,regional snowpack anomalies in the Rocky Mountains of western North America, where interannual variations in cool-season (November-March) temperatures are broadly synchronous, but precipitation is typically antiphased north to south and uncorrelated with temperature. Over the previous eight centuries, regional snowpack variability exhibits strong, decadally persistent north-south (N-S) antiphasing of snowpack anomalies. Contrary to the normal regional antiphasing, two intervals of spatially synchronized snow deficits were identified. Snow deficits shown during the 1930s were synchronized north-south by low cool-season precipitation, with spring warming (February-March) since the 1980s driving the majority of the recent synchronous snow declines, especially across the low to middle elevations. Spring warming strongly influenced low snowpacks in the north after 1958, but not in the south until after 1980. The post-1980, synchronous snow decline reduced snow cover at low to middle elevations by ~20% and partly explains earlier and reduced streamflow and both longer and more active fire seasons. Climatologies of Rocky Mountain snowpack are shown to be seasonally and regionally complex, with Pacific decadal variability positively reinforcing the anthropogenic warming trend.
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In many social insects, including bumblebees, the division of labor between workers relates to body size, but little is known about the factors influencing larval development and final size. We confirmed and extend the evidence that in the bumblebee Bombus terrestris the adult bee body size is positively correlated with colony age. We next performed cross-fostering experiments in which eggs were switched between incipient (before worker emergence) and later stage colonies with workers. The introduced eggs developed into adults similar in size to their unrelated nestmates and not to their same-age full sisters developing in their mother colony. Detailed observations revealed that brood tending by the queen decreases, but does not cease, in young colonies with workers. We next showed that both worker number and the queen presence influenced the final size of the developing brood, but only the queen influence was mediated by shortening developmental time. In colonies separated by a queen excluder, brood developmental time was shorter in the queenright compartment. These findings suggest that differences in body size are regulated by the brood interactions with the queen and workers, and not by factors inside the eggs that could vary along with colony development. Finally, we developed a model showing that the typical increase in worker number and the decrease in brood contact with the queen can account for the typical increase in body size. Similar self-organized social regulation of brood development may contribute to the optimization of growth and reproduction in additional social insects.
Article
The structure of pollination networks is described for two oceanic islands, the Azorean Flores and the Mauritian Ile aux Aigrettes. At each island site, all interactions between endemic, non-endemic native and introduced plants and pollinators were mapped. Linkage level, i.e. number of species interactions per species, was significantly higher for endemic species than for non-endemic native and introduced species. Linkage levels of the two latter categories were similar. Nine types of interaction may be recognized among endemic, non-endemic native and introduced plants and pollinators. Similar types had similar frequencies in the two networks. Specifically, we looked for the presence of 'invader complexes' of mutualists, defined as groups of introduced species interacting more with each other than expected by chance and thus facilitating each other's establishment. On both islands, observed frequencies of interactions between native (endemic and non-endemic) and introduced pollinators and plants differed from random. Introduced pollinators and plants interacted less than expected by chance. Thus, the data did not support the exist- ence of invader complexes. Instead, our study suggested that endemic super-generalist species, i.e. pollinators or plant species with a very wide pollination niche, include new invaders in their set of food plants or pollinators and thereby improve establishment success of the invaders. Reviewing other studies, super generalists seem to be a widespread island phenomenon, i.e. island pollination networks include one or a few species with a very high generalization level compared to co-occurring species. Low density of island species may lead to low interspecific competition, high abundance and ultimately wide niches and super generalization.
Article
Flower-visiting interactions, such as nectarivory and pollination, are considered to form networks with higher interaction specialization than other plant–animal facultative mutualisms. However, subsets within each network sometimes are different from the complete system. Social wasps are one subset within flower-visiting networks; they use nectar as a secondary food of adults. Some of the visited plants depend on wasps for pollination, and many are further benefited through predation of herbivores captured to feed wasp larvae. Therefore, mutual dependence is lower in the wasp subset compared to complete pollination networks, so we expected wasp–flower networks to exhibit more generalistic interactions. Quantitative datasets were built by recording wasp visits to flowers in six Brazilian localities in four ecoregions, and comparisons were made with complete pollination networks from the literature (with different taxa included). Nestedness (NODF = 0.39 ± 0.06) was similar in wasp–flower and complete pollination networks (NODF = 0.32 ± 0.18). Interaction specialization in wasp–flower networks was lower (median H2’ = 0.31 ± 0.09) than in complete pollination networks (median H2’ = 0.55 ± 0.17). Modularity in wasp–flower networks (M = 0.36 ± 0.05) was also lower than in complete pollination networks (M = 0.47 ± 0.09); there were on average 5 ± 1 modules on each network formed by species of different genera. In summary, our findings confirm that wasps that feed on nectar interact with similar subsets of plants; therefore, wasp–flower networks are more generalistic than other pollination networks. Our results corroborate the hypothesis that, despite some universal properties found in facultative mutualisms, parts of a system differ from the complete system. Furthermore, mutual dependence influences interaction specialization, and so it is an important structuring factor of mutualistic networks.
Article
Six field colonies and four laboratory colonies of bumblebees were used in a marking experiment during the summer of 1964 in Wisconsin to study the seasonal size increase of the workers. Mean sizes of weekly age groups differed significantly. The workers gradually increased in size in healthy colonies. In colonies disrupted by parasitism or transfer from the field to the laboratory, consecutive age groups of workers were- significantly smaller. After a period of adjustment, the colonies transferred from the field to the laboratory resumed the normal pattern of worker size increase.
Article
The mutually beneficial interactions between plants and their animal pollinators and seed dispersers form complex networks of species interdependence. Until very recently, the complexity of these networks precluded a community-wide approach to the investigation of mutualism. However, recent studies using tools and concepts from physics and sociology have allowed the exploration of this complexity within a rational framework. Regardless of differences across sites or species composition, networks of mutual benefit have a similar structure. Describing these network patterns is important for understanding both the generation of biodiversity and its responses to anthropogenic disturbances, such as habitat loss and species extinctions. This network approach is currently being applied to restoration ecology, biological invasions, and the conservation of endangered species.
Chapter
The competitive exclusion principle states that two species cannot coexist on the same limiting resource. It is possible to view bumblebees Bombus spp. (and other pollinators) as a limiting resource which is partitioned by the flowering plants in a community. I have chosen the alternative view which considers flowering plants as a limiting resource that is partitioned by bumblebees. The fact that bumblebees are annual species, while many of the plants they visit are long-lived perennials, lends support to this interpretation. Furthermore, many plant species may have other means of effecting pollination in addition to bumblebees. Because the relationship between bumblebees and flowers is generally mutualistic, consideration of both views is likely to be most fruitful.
Article
Foraging specialization plays an important role in the ability of social insects to efficiently allocate labor. However, relatively little is known about the degree to which individual bumble bees specialize on collecting nectar or pollen, when such preferences manifest, and if individuals can alter their foraging preferences in response to changes in the colony workforce. Using Bombus impatiens, we monitored all foraging visits made by every bee in multiple colonies and showed that individual foragers exhibit consistent lifetime foraging preferences. Based upon the distribution of foraging preferences, we defined three forager types (pollen specialists, nectar specialists, and generalists). In unmanipulated colonies, 16-36 % of individuals specialized (≥90 % of visits) on nectar or pollen only. On its first day of foraging, an individual's foraging choices (nectar only, pollen only, or nectar and pollen) significantly predicted its lifetime foraging preferences. Foragers that only collected pollen on their first day of foraging made 1.61- to 1.67-fold more lifetime pollen foraging visits (as a proportion of total trips) than foragers that only collected nectar on their first foraging day. Foragers were significantly larger than bees that stayed only in the nest. We also determined the effect of removing pollen specialists at early (brood present) or later (brood absent) stages in colony life. These results suggest that generalists can alter their foraging preferences in response to the loss of a small subset of foragers. Thus, bumble bees exhibit individual lifetime foraging preferences that are established early in life, but generalists may be able to adapt to colony needs.
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This is an R package (a piece of Software) to fit and do inference on mixed-effects models. The package is Free Software (hence open-source) and the package and much documentation about it is freely available from CRAN at https://cran.r-project.org/package=lme4
Article
Outcrossing in plants is influenced by the availability of pollinators and compatible mates, both of which may be modified by the population and community context in which plant–pollinator interactions occur. Although indirect interactions among plants through shared pollinators are often expected to be competitive, pollinator sharing may be beneficial when plant species jointly attract or maintain populations of pollinators. In this study, I tested the hypothesis that pollinator-sharing congeners facilitate reproduction in a focal taxon, Clarkia xantiana ssp. xantiana, and that positive interactions are most pro-nounced in small and sparse populations. Population surveys revealed that C. x. xantiana frequently coexists with pollinator-sharing congeners except at the periphery of its range. Populations varied extensively in size and density, with small populations more likely associated with pollinator-sharing congeners; conversely, populations occurring alone were more likely large. Flowering schedules in Clarkia communities ranged from segregated to aggregated. Although there was not strong evidence of character displacement, modes in flowering time were often staggered among Clarkia species resulting in a protracted flow-ering season within plant communities. Studies of bee pollinator availability in 17 popu-lations and pollen limitation to reproduction in 39 replicate populations revealed that pop-ulations occurring with multiple congeners had high pollinator availability and low pollen limitation of reproduction compared to populations occurring alone. Population size was inversely related to pollen limitation but did not affect pollinator availability, suggesting that Allee effects were caused by mate limitation. Intraspecific interactions were also pos-itive at a fine spatial scale where pollen deposition increased with the density of closely neighboring conspecifics across 11 populations. Overall, inter-and intraspecific interactions through shared pollinators were generally facilitative, suggesting that population viability and the coexistence of ecologically similar Clarkia species may be promoted by positive reproductive interactions.
Article
1 Taxocenes — monophyletic ecological assemblages — are a key focus of macroecology. Abundance (individuals per area) is a basic property of taxocenes but has received less attention than diversity, although the two are probably related. Abundance reflects a taxocene’s ability to harvest and sequester available energy and divide it among individuals. This paper explores how two properties of all taxocenes — trophic makeup and taxonomic level (e.g. genus, tribe, subfamily, family … ) — may contribute to patterns of local abundance at geographical scales.2 Forty-nine ground ant taxocenes, in habitats ranging from New World deserts to rain forests, were surveyed along a three-orders of magnitude productivity gradient using transects of 30 1-m2 quadrats at each site. Abundance — the number of nests per transect — varied over two orders of magnitude.3 Over 80% of the genera collected were omnivores. However, herbivore, omnivore, and predator taxa were added to ant taxocenes in roughly 1 order of magnitude steps up the productivity gradient. Specialist detritivores were added last.4 Net primary productivity and mean monthly temperature both consistently entered regression models predicting abundance. However, while productivity was the dominant predictor of abundance for higher taxa (families, subfamilies), temperature was the dominant predictor of abundance for lower taxa (tribes, genera). The answer to the question ‘What regulates the abundance of a taxocene?’ is thus sensitive to the taxonomic level of analysis.5 These data support the following scenario. Lower taxa are abiotic specialists given the insufficient number of genomes and generations required for the exploration of the entire abiotic envelope. Higher taxa, in contrast, consist of suites of abiotic specialists arrayed along the entire productivity gradient, with access to productivity everywhere the taxon occurs. If this scenario is true, individual species may respond to global changes in temperature; the higher taxa they belong to may most respond to global changes in productivity.
Article
Pollination networks are representations of all interactions between co-existing plants and their flower visiting animals at a given site. Although the study of networks has become a distinct sub-discipline in pollination biology, few studies have attempted to quantify spatio-temporal variation in species composition and structure of networks. We here investigate patterns of year-to-year change in pollination networks from six different sites spanning a large latitudinal gradient. We quantified level of species persistence and interactions among years, and examined year-to-year variation of network structural parameters in relation to latitude and sampling effort. In addition, we tested for correlations between annual variation in network parameters and short and long-term climate change variables. Numbers of plant and animal species and interactions were roughly constant from one year to another at all sites. However, composition of species and interactions changed from one year to another. Turnover was particularly high for flower visitors and interactions. On the other hand, network structural parameters (connectance, nestedness, modularity and centralization) remained remarkably constant between years, regardless of network size and latitude. Inter-annual variation of network parameters was not related to short or long term variation in climate variables (mean annual temperature and annual precipitation). We thus conclude that pollination networks are highly dynamic and variable in composition of species and interactions among years. However, general patterns of network structure remain constant, indicating that species may be replaced by topologically similar species. These results suggest that pollination networks are to some extent robust against factors affecting species occurrences.