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Distance From Nest and Climate Explain Geographical Trends of Harvester Ant's Food Resource Use: A Multi‐Species Approach
Abstract
Main
Animals not only forage for abundant and nearby resources, but their diets can also be influenced by abiotic and geographic factors. This often results in non‐random interactions among species. We investigate how seed density, distance from nest, abiotic (e.g., climate stability, temperature, precipitation) and geographic factors (e.g., latitude, elevation and continental hemisphere) influence the removal of food items (i.e., seeds and dead arthropods) by Pogonomyrmex species.
Location
South and North America, from Patagonia to the Rocky Mountains.
Taxon
Genus Pogonomyrmex (Formicidae: Hymenoptera).
Methods
Conducting standardised experiments, we performed a seed removal experiment and an assessment of the items retrieved by ant workers of 160 nests from eight Pogonomyrmex spp. at 16 sites extending the American continent.
Results
Pogonomyrmex ants native to North America removed more seeds than their South American counterpart. In general, results align with optimal foraging theory, indicating a higher probability of seed removal near ant nests. High climate stability correlated with lower seed predation rates, emphasising seed consumption's significance in historically arid environments. Increased precipitation and temperature led to reduced removal of food resources, suggesting reduced water availability and lower mean temperatures increases the consumption of seeds by harvester ants.
Conclusions
Overall, Pogonomyrmex ants' food resource use is influenced by a combination of factors such as region, distance from the nest and climate. This study underscores harvester ants' potential impact on plant distribution over large spatial scales consuming seeds from the immediate proximity of their nest and preventing establishment.
... mutualism, antagonism). Moreover, its implementation can reveal how time shape species interactions (Schleuning et al., 2014;Luna et al., 2022;Anjos et al., 2024). Therefore, due to the higher fern and insect diversity towards the warm, humid and stable tropics, we might expect that insect-fern interactions can be shaped by climate over large spatial scales (Schleuning et al., 2012;R. ...
Historically, ferns have been described as underutilized by insects. However, studies have shown a diversity of insects interacting with ferns, although the evolutionary and ecological drivers of these interactions are still to be untangled.
To fill these gaps, we compiled more than 100 yr of global data on insect–fern interactions from the literature comprising 374 fern and 649 insect species. With this database we assessed how fern trophic specialization, phylogenetic relationships and climate have shaped their interactions with insects.
Our findings showed that interactions between ferns and insects can be explained by the phylogenetic relations among them. We observed that insect orders part of the Endopterygota clade tend to interact with similar fern species, which might be a result of the inheritance of Endopterygota ancestors probably due to phylogenetic niche conservationism. Under an ecological context, fern specialization increased with temperature, precipitation, and climatic stability. Our results show that climate might be one of the main factors explaining the spatial variation of insect–fern interactions, postulate also supported by the observed phylogenetic clustering of the studied ferns species.
Our study highlights the intricate and multifaceted nature of insect–fern interactions, where evolutionary history and ecological factors converge to shape these relationships.
Ants (Hymenoptera: Formicidae) are one of the most dominant terrestrial organisms worldwide. They are hugely abundant, both in terms of sheer numbers and biomass, on every continent except Antarctica and are deeply embedded within a diversity of ecological networks and processes. Ants are also eusocial and colonial organisms-their lifecycle is built on the labor of sterile worker ants who support a small number of reproductive individuals. Given the climatic changes that our planet faces, we need to understand how various important taxonomic groups will respond; this includes the ants. In this review, we synthesize the available literature to tackle this question. The answer is complicated. The ant literature has focused on temperature, and we broadly understand the ways in which thermal changes may affect ant colonies, populations, and communities. In general, we expect that species living in the Tropics, and in thermally variable microhabitats, such as the canopy and leaf litter environments, will be negatively impacted by rising temperatures. Species living in the temperate zones and those able to thermally buffer their nests in the soil or behaviorally avoid higher temperatures, however, are likely to be unaffected or may even benefit from a changed climate. How ants will respond to changes to other abiotic drivers associated with climate change is largely unknown, as is the detail on how altered ant populations and communities will ramify through their wider ecological networks. We discuss how eusociality may allow ants to adapt to, or tolerate, climate change in ways that solitary organisms cannot and we identify key geographic and phylogenetic hotspots of climate vulnerability and resistance. We finish by emphasizing the key research questions that we need to address moving forward so that we may fully appreciate how this critical insect group will respond to the ongoing climate crisis.
Critical thermal limits (CTLs) constrain the performance of organisms, shaping their abundance, current distributions, and future distributions. Consequently, CTLs may also determine the quality of ecosystem services as well as organismal and ecosystem vulnerability to climate change. As some of the most ubiquitous animals in terrestrial ecosystems, ants are important members of ecological communities. In recent years, an increasing body of research has explored ant physiological thermal limits. However, these CTL data tend to centre on a few species and biogeographical regions. To encourage an expansion of perspectives, we herein review the factors that determine ant CTLs and examine their effects on present and future species distributions and ecosystem processes. Special emphasis is placed on the implications of CTLs for safeguarding ant diversity and ant‐mediated ecosystem services in the future. First, we compile, quantify, and categorise studies on ant CTLs based on study taxon, biogeographical region, methodology, and study question. Second, we use this comprehensive database to analyse the abiotic and biotic factors shaping ant CTLs. Our results highlight how CTLs may affect future distribution patterns and ecological performance in ants. Additionally, we identify the greatest remaining gaps in knowledge and create a research roadmap to promote rapid advances in this field of study.
The study of ecological networks has progressively evolved from a mostly descriptive science to one that attempts to elucidate the processes governing the emerging structure of multitrophic communities. To move forward, we propose a conceptual framework using trait‐based inference of ecological processes to improve our understanding of network assembly and our ability to predict network reassembly amid global change. The framework formalizes the view that network assembly is governed by processes shaping the composition of resource and consumer communities within trophic levels and those dictating species’ interactions between trophic levels. To illustrate the framework and show its applicability, we (1) use simulations to explore network structures emerging from the interactions of these assembly processes, (2) develop a null model approach to infer the processes underlying network assembly from observational data, and (3) use the null model approach to quantify the relative influence of bottom‐up (resource‐driven) and top‐down (consumer‐driven) assembly modes on plant–frugivore networks along an elevational gradient. Simulations suggest that assembly processes governing the formation of pairwise interactions have a greater influence on network structure than those governing the composition of communities within trophic levels. Our case study further shows that the mode of network assembly along the gradient is mainly bottom‐up controlled, suggesting that the filtering of plant traits has a larger effect on network structure relative to the filtering of frugivore traits. Combined with increasingly available trait and interaction data, the framework provides a timely toolbox to infer assembly processes operating within and between trophic levels and to test competing hypotheses about the assembly mode of resource–consumer networks along environmental gradients and among biogeographic regions. It is a step toward a more process‐based network ecology and complete integration of multitrophic interactions in the prediction of future biodiversity.
The study of ecological networks has progressively evolved from a mostly descriptive science to one that attempts to elucidate the processes governing the emerging structure of multitrophic communities. To move forward, we propose a conceptual framework using trait-based inference of ecological processes to improve our understanding of network assembly and our ability to predict network reassembly amid global change. The framework formalizes the view that network assembly is governed by processes shaping the composition of resource and consumer communities within trophic levels and those dictating species' interactions between trophic levels. To illustrate the framework and show its applicability, we (1) use simulations to explore network structures emerging from the interactions of these assembly processes, (2) develop a null model approach to infer the processes underlying network assembly from observational data, and (3) use the null model approach to quantify the relative influence of bottom-up (resource-driven) and top-down (consumer-driven) assembly modes on plant-frugivore networks along an elevational gradient. Simulations suggest that assembly processes governing the formation of pairwise interactions have a greater influence on network structure than those governing the composition of communities within trophic levels. Our case study further shows that the mode of network assembly along the gradient is mainly bottom-up controlled, suggesting that the filtering of plant traits has a larger effect on network structure relative to the filtering of frugivore traits. Combined with increasingly available trait and interaction data, the framework provides a timely toolbox to infer assembly processes operating within and between trophic levels and to test competing hypotheses about the assembly mode of resource-consumer networks along environmental gradients and among biogeographic regions. It is a step toward a more process-based network ecology and complete integration of multitrophic interactions in the prediction of future biodiversity.
Trophic specialisation is known to vary across space, but the environmental factors explaining such variation remain elusive. Here we used a global dataset of flower-visitor networks to evaluate how trophic specialisation varies between latitudinal zones (tropical and temperate) and across elevation gradients, while considering the environmental variation inherent in these spatial gradients. Specifically, we assessed the role of current (i.e., net primary productivity, temperature, and precipitation) and historical (i.e., temperature and precipitation stability) environmental factors in structuring the trophic specialisation of floral visitors. Spatial variations in trophic specialisation were not explained by latitudinal zones or elevation. Moreover, regardless of network location on the spatial gradient, there was a tendency for higher trophic specialisation in sites with high productivity and precipitation, whereas historical temperature stability was related to lower trophic specialisation. We highlight that both energetic constraints in animal foraging imposed by climate and resource availability may drive the global variation in trophic specialisation.
Long-term field studies help unveil mechanisms of grass soil seed bank (SSB) persistence and resilience. We wonder whether grazing is a disruptive force that changes grass SSB composition and size, while heavy rainfall is a positive bottom-up force that restores the grass SSB in grazed areas of the Monte desert, Argentina. We used core sampling to study the grass SSB in cattle-free (17 year) and grazed (10 year) grasslands. The SSB size in cattle-free sites usually responded in a continuous positive fashion to rainfall, and the proportion of palatable grasses in the SSB was similar among years. Most grass species formed a transient SSB, but the presence of a stand of perennial plants would prevent their local extinction. In contrast, the SSB size in the grazed habitats showed a threshold response to rainfall (i.e. it did not increase under low-moderate rainfall), the seeds of palatable grasses were always scarce, and the proportion of seeds of less palatable grasses increased under extreme rainy conditions. Seed production from grasses that grew tillers from axillary buds during a drought was zero, suggesting that the bank of plants could have a role in the replenishment of the grass SSB only in mesic years. Within the time span assessed here, continuous heavy grazing together with scarce as well as heavy rainfall were disruptive forces that reduced the number and proportion of seeds from palatable grasses in the SSB, which are also the species preferred by seed-eating animals.
Drylands cover 41% of the earth’s land surface and include 45% of the world’s agricultural land. These regions are among the most vulnerable ecosystems to anthropogenic climate and land use change and are under threat of desertification. Understanding the roles of anthropogenic climate change, which includes the CO2 fertilization effect, and land use in driving desertification is essential for effective policy responses but remains poorly quantified with methodological differences resulting in large variations in attribution. Here, we perform the first observation-based attribution study of desertification that accounts for climate change, climate variability, CO2 fertilization as well as both the gradual and rapid ecosystem changes caused by land use. We found that, between 1982 and 2015, 6% of the world’s drylands underwent desertification driven by unsustainable land use practices compounded by anthropogenic climate change. Despite an average global greening, anthropogenic climate change has degraded 12.6% (5.43 million km2) of drylands, contributing to desertification and affecting 213 million people, 93% of who live in developing economies. Drylands cover nearly half of Earth’s surface, yet how they will fare in light of anthropogenic climate change is debated. Here the authors find that over the past 40 years climate change has pushed ~13% of drylands towards desertification threatening hundreds of millions of people in developing nations.
Mountains contribute disproportionately to the terrestrial biodiversity of Earth, especially in the tropics, where they host hotspots of extraordinary and puzzling richness. With about 25% of all land area, mountain regions are home to more than 85% of the world’s species of amphibians, birds, and mammals, many entirely restricted to mountains. Biodiversity varies markedly among these regions. Together with the extreme species richness of some tropical mountains, this variation has proven challenging to explain under traditional climatic hypotheses. However, the complex climatic characteristics of rugged mountain regions differ fundamentally from those of lowland regions, likely playing a key role in generating and maintaining diversity. With ongoing global changes in climate and land use, the role of mountains as refugia for biodiversity may well come under threat.
As continental and global-scale paleoclimate model data become more readily available, biologists can now ask spatially explicit questions about the tempo and mode of past climate change and the impact of those changes on biodiversity patterns. In particular, researchers have focused on climate stability as a key variable that can drive expected patterns of richness, phylogenetic diversity and functional diversity. Yet, climate stability measures are not formalized in the literature and tools for generating stability metrics from existing data are nascent. Here we define “deviation” of a climate variable as the mean standard deviation between time slices over time elapsed; “stability” is defined as the inverse of this deviation. Finally, climate stability is the product of individual climate variable stability estimates. We also present an R package, climateStability, which contains tools for researchers to generate climate stability estimates from their own data.
1. Harvester ants perform important ecological functions, such as seed predation and redistribution of nutrients in the soil, through a complex of ecological interactions. Most studies are static descriptions of network structure, while their temporal organisations and the factors that modulate it have been neglected.
2. This study describes the temporal organisation and the influence of climatic variables (e.g. temperature and rainfall) on network structure between a harvester ant species and its food resources (seeds and invertebrates) in the Brazilian savanna over a 1‐year period. In addition, considering the ant–seed networks, the relationship between the size of seeds collected by ants and the distance travelled was examined, as well as how this relationship behaves over time.
3. For all network types (ant–seed, ant–invertebrate, and combined) there was great structural variation throughout the year. Among climatic variables, only rainfall influenced the properties of ant–invertebrate networks, presenting a positive influence on diversity of interactions and a negative influence on network specialisation. Finally, this study shows that harvester ants tend to collect larger seeds in the vicinity of the nest (< 5 m) and smaller seeds at greater distances, and this relationship appears to be constant over time.
4. This study highlights that ecological networks are useful tools with which to understand the temporal organisation of ecological functions. In addition, climatic changes, mainly in the rainfall regime, may interfere directly in the availability of resources and ultimately in the interactions between harvester ants and their food resources.
The harvester ant Pogonomyrmex mendozanus is endemic to the central Monte desert. In this region, extensive grazing reduces grass cover and seed abundance in the soil bank. In previous studies carried out in ungrazed areas, this ant species consumes and prefers seeds of perennial grasses over forbs and shrubs. However, in grazed habitats, P. mendozanus expands its diet by including seeds from other plant groups along with fruits, dead arthropods and other non-seed items. The broader diet could be due to a flexible foraging strategy or could reflect intrinsic, context-dependent food preferences of the populations inhabiting heavy grazed areas, different from those populations studied under ungrazed conditions. We carried out food preference experiments in heavy grazed areas in the central Monte desert in order to discriminate between both hypotheses. We performed choice experiments using a paired comparison design, offering seeds and non-seed food items present in the diet of P. mendozanus. Ants preferred large- and medium-sized perennial grass seeds and, among the non-seed food items, arthropods were preferred. Forb and shrub seeds along with fruits showed intermediate preference levels, whereas the least preferred items were small grass seeds and flowers. Food preferences were similar to those reported previously in ungrazed conditions, supporting the hypothesis that seed preferences are stereotyped in this species and strongly suggesting that it behaves as an expansive specialist by including less preferred items in its diet in heavy grazed habitats where the most preferred seeds are scarce.
Species interactions have long been predicted to increase in intensity toward the tropics and low elevations because of gradients in climate, productivity, or biodiversity. Despite their importance for understanding global ecological and evolutionary processes, plant-animal interaction gradients are particularly difficult to test systematically across large geographic gradients, and evidence from smaller, disparate studies is inconclusive. By systematically measuring postdispersal seed predation using 6995 standardized seed depots along 18 mountains in the Pacific cordillera, we found that seed predation increases by 17% from the Arctic to the Equator and by 17% from 4000 meters above sea level to sea level. Clines in total predation, likely driven by invertebrates, were consistent across treeline ecotones and within continuous forest and were better explained by climate seasonality than by productivity, biodiversity, or latitude. These results suggest that species interactions play predictably greater ecological and evolutionary roles in tropical, lowland, and other less seasonal ecosystems.
Colonial breeding is an evolutionary puzzle, as the benefits of breeding in high densities are still not fully explained. Although the dynamics of existing colonies are increasingly understood, few studies have addressed the initial formation of colonies, and empirical tests are rare. Using a high-resolution larval drift model, we here document that the distribution of seabird colonies along the Norwegian coast can be explained by variations in the availability and predictability of fish larvae. The modelled variability in concentration of fish larvae is, in turn, predicted by the topography of the continental shelf and coastline. The advection of fish larvae along the coast translates small-scale topographic characteristics into a macroecological pattern, viz. the spatial distribution of top-predator breeding sites. Our findings provide empirical corroboration of the hypothesis that seabird colonies are founded in locations that minimize travel distances between breeding and foraging locations, thereby enabling optimal foraging by central-place foragers.
Competition and resource partitioning have been studied extensively as explanations for community structure and species coexistence in North American seed-harvester ants, but the biogeographical and historical context within which these communities have formed are poorly known. This paper examines biogeography of North American seed-harvester ants by constructing functional groups that serve to identify ecologically equivalent ant species that share a common evolutionary ancestry. Species were thus separated into functional groups based on genus and foraging method (solitary and group/trunk trail foraging species). Ecology, behavior, and micro- and macro-distribution patterns are then discussed and synthesized for species in each functional group. Overall, this discussion includes 42 ant species in the myrmicine genera Aphaenogaster, Messor, Pheidole, Ephebomyrmex, and Pogonomyrmex, about 5% of the total ant species in the United States. The general pattern is that each functional group consists of a highly structured assemblage of ecologically equivalent ant species that are similar with respect to morphology, ecology, and behavior. The primary difference among species in each functional group is that each species occurs in specific micro- and macro-habitats, with the species replacing one another across contact zones that are delineated by abiotic habitat features. On a regional scale, species in these five ant genera are broadly sympatric, but only one species per functional group predominates at a locale. Re-examination of previously noted longitudinal clines of ant species richness and diversity suggest that rainfall is indeed correlated with these community attributes. However, patterns of species loss and species replacement across this rainfall gradient suggest that fewer ant species in drier locales is effected through limitations on mating flights and foundress survival rather than by seed abundance, as previously suggested.
South American species in the seed-harvester ant genus Pogonomyrmex (subfamily Myrmicinae) are interesting biologically because of their numerous queen phenotypes and life histories. This paper provides a taxonomic revision and reviews the natural history for 21 South American species of Pogonomyrmex so that we can better study their rich and interesting ecology, life history, and evolution. Species treated herein comprise all South American species-groups except for the brevibarbis and rastratus-groups. The following taxa are raised to species: pencosensis Forel 1914 and serpens Santschi 1922. The following new synonomies are proposed: bruchi Forel 1913 is synonomized under coarctatus Mayr 1868 and cunicularius carnivora Santschi 1925 under serpens Santschi 1922. The following new species is described: tinogasta. This paper redescribes workers of all species, and I describe queens and diagnose males for the following species: bispinosus (ergatoid queen, male), inermis (queen, male), laticeps (male), lobatus (queen, male), micans (queen), naegelii (ergatoid queen), pencosensis (ergatoid queen), serpens (ergatoid queen), tinogasta (brachypterous queen), and uruguayensis (queen, male). A neotype was designated for the untraceable or possibly lost type of P bispinosus, and a holotype or lectotype was designated from syntypes for all other previously described taxa in order to provide a single name-bearing specimen and to facilitate future taxonomic studies. Of the 21 species treated herein, five species have ergatoid (wingless) queens (bispinosus, cunicularius, pencosensis, serpens, mayri), two have brachypterous (short-winged) queens (mendozanus, tinogasta), and two have dimorphic queens (winged and ergatoid in naegelii, brachypterous and ergatoid in laticeps). I also provide keys for workers and queens (in English and Spanish), photographs of all castes, distribution maps, and a summary of known biology.
AimLarge-scale patterns linking energy availability, biological productivity and diversity form a central focus of ecology. Despite evidence that the activity and abundance of animals may be limited by climatic variables associated with regional biological productivity (e.g. mean annual precipitation and annual actual evapotranspiration), it is unclear whether plant–granivore interactions are themselves influenced by these climatic factors across broad spatial extents. We evaluated whether climatic conditions that are known to alter the abundance and activity of granivorous animals also affect rates of seed removal.LocationEleven sites across temperate North America.Methods
We used a common protocol to assess the removal of the same seed species (Avena sativa) over a 2-day period. Model selection via the Akaike information criterion was used to determine a set of candidate binomial generalized linear mixed models that evaluated the relationship between local climatic data and post-dispersal seed predation.ResultsAnnual actual evapotranspiration was the single best predictor of the proportion of seeds removed. Annual actual evapotranspiration and mean annual precipitation were both positively related to mean seed removal and were included in four and three of the top five models, respectively. Annual temperature range was also positively related to seed removal and was an explanatory variable in three of the top four models.Main conclusionsOur work provides the first evidence that energy and precipitation, which are known to affect consumer abundance and activity, also translate to strong, predictable patterns of seed predation across a continent. More generally, these findings suggest that future changes in temperature and precipitation could have widespread consequences for plant species composition in grasslands, through impacts on plant recruitment.
The Janzen–Connell hypothesis proposes that specialist natural enemies, such as herbivores and pathogens, maintain diversity in plant communities by reducing survival rates of conspecific seeds and seedlings located close to reproductive adults or in areas of high conspecific density. Variation in the strength of distance- and density-dependent effects is hypothesized to explain variation in plant species richness along climatic gradients, with effects predicted to be stronger in the tropics than the temperate zone and in wetter habitats compared to drier habitats.
We conducted a comprehensive literature search to identify peer-reviewed experimental studies published in the 40+ years since the hypothesis was first proposed. Using data from these studies, we conducted a meta-analysis to assess the current weight of evidence for the distance and density predictions of the Janzen–Connell hypothesis.
Overall, we found significant support for both the distance- and density-dependent predictions. For all studies combined, survival rates were significantly reduced near conspecifics compared to far from conspecifics, and in areas with high densities of conspecifics compared to areas with low conspecific densities. There was no indication that these results were due to publication bias.
The strength of distance and density effects varied widely among studies. Contrary to expectations, this variation was unrelated to latitude, and there was no significant effect of study region. However, we did find a trend for stronger distance and density dependence in wetter sites compared to sites with lower annual precipitation. In addition, effects were significantly stronger at the seedling stage compared to the seed stage.
Synthesis. Our study provides support for the idea that distance- and density-dependent mortality occurs in plant communities world-wide. Available evidence suggests that natural enemies are frequently the cause of such patterns, consistent with the Janzen–Connell hypothesis, but additional studies are needed to rule out other mechanisms (e.g. intraspecific competition). With the widespread existence of density and distance dependence clearly established, future research should focus on assessing the degree to which these effects permit species coexistence and contribute to the maintenance of diversity in plant communities.
Maximum likelihood or restricted maximum likelihood (REML) estimates of the
parameters in linear mixed-effects models can be determined using the lmer
function in the lme4 package for R. As for most model-fitting functions in R,
the model is described in an lmer call by a formula, in this case including
both fixed- and random-effects terms. The formula and data together determine a
numerical representation of the model from which the profiled deviance or the
profiled REML criterion can be evaluated as a function of some of the model
parameters. The appropriate criterion is optimized, using one of the
constrained optimization functions in R, to provide the parameter estimates. We
describe the structure of the model, the steps in evaluating the profiled
deviance or REML criterion, and the structure of classes or types that
represents such a model. Sufficient detail is included to allow specialization
of these structures by users who wish to write functions to fit specialized
linear mixed models, such as models incorporating pedigrees or smoothing
splines, that are not easily expressible in the formula language used by lmer.
In 1978, Mares and Rosenzweig assessed seed removal rates by granivores at a site in the northern Monte desert of Argentina, and concluded that granivory in South America is `much depressed'. In this study, spatial and temporal patterns of seed removal by small mammals, ants, and birds in the central Monte desert were analysed, and results compared with data available from other arid zones of the world. Ants were found to be the most important granivores in spring-summer, while birds were more important in autumn-winter. No differences were found in seed removal rates between microhabitats in winter, but in the summer both ants and birds removed more seeds from under the canopy of shrubs and trees than from exposed microhabitats. The impact of ants appeared to be lower in South America than in other continents, but removal by birds at the study site was only exceeded in North America, and removal by mammals only in North America and South Africa. Results indicate that granivory in South America is not abnormally depressed. Instead, current data suggest that seed removal in North America is exceptionally high, and that low levels are actually the norm for most arid zones.
It is commonly accepted that species interactions such as granivory are more intense in the tropics. However, this has rarely been tested. A global dataset of post-dispersal seed removal by invertebrates and vertebrates for 79 native plant species from semi-natural and natural terrestrial habitats ranging from 55° N to 45° S, was compiled from the global literature to test the hypothesis that post-dispersal seed removal by invertebrates and vertebrates is more intense at lower latitudes. We also quantified the relationship between post-dispersal seed removal by vertebrates and by invertebrates to global climatic features including temperature, actual evapotranspiration (AET) and rainfall seasonality. Linear mixed effect models were applied to describe the relationships between seed removal and latitude, hemisphere and climatic variables controlling for the effect of seed mass. Post-dispersal seed removal by invertebrates was negatively related to latitude. In contrast, post-dispersal seed removal by vertebrates was positively but weakly related to latitude. Mean annual temperature and actual evapotranspiration were positively related to post-dispersal seed removal by invertebrates, but not to post-dispersal seed removal by vertebrates, which was only marginally negatively related to rainfall seasonality. The inclusion of seed mass improved the fit of all models, but the term for seed mass was not significant in any model. Although a good climatic model for predicting post-dispersal seed predation by vertebrates at the global level was not found, our results suggest different and opposite latitudinal patterns of post-dispersal seed removal by invertebrates vs vertebrates. This is the first time that a negative relationship between post-dispersal seed removal by invertebrates and latitude, and a positive relationship with temperature and AET have been documented at a global-scale. These results have important implications for understanding global patterns in plant-animal interactions, and the factors that shape plant reproductive ecology, and also for predicting how this plant-animal interaction might respond to climate change.
Despite recognition of key biotic processes in shaping the structure of biological communities, few empirical studies have explored the influences of abiotic factors on the structural properties of mutualistic networks. We tested whether temperature and precipitation contribute to temporal variation in the nestedness of mutualistic ant–plant networks. While maintaining their nested structure, nestedness increased with mean monthly precipitation and, particularly, with monthly temperature. Moreover, some species changed their role in network structure, shifting from peripheral to core species within the nested network. We could summarize that abiotic factors affect plant species in the vegetation (e.g., phenology), meaning presence/absence of food sources, consequently an increase/decrease of associations with ants, and finally, these variations to fluctuations in nestedness. While biotic factors are certainly important, greater attention needs to be given to abiotic factors as underlying determinants of the structures of ecological networks.
The proportion of foragers in ant colonies is a fairly constant species-specific characteristic that could be determined by intrinsic or extrinsic factors. If intrinsic factors are relevant, species with similar life history characteristics (e.g., colony size and foraging strategies) would be expected to have a similar proportion of foragers in their colonies. Within the genus Pogonomyrmex, North American species can vary largely in their colony size, whereas only species with small colonies are known in South America. We studied the characteristics of the foraging subcaste in three sympatric South American species of Pogonomyrmex harvester ants, and compared it with the available information on other species of the same genus. We used two mark-recapture methods and colony excavations to estimate the number and proportion of foragers in the colonies of P. mendozanus, P. inermis, and P. rastratus, and to test the relationship between forager external activity levels and abundance per colony. Forager abundance in the three studied species was lower than in most North American species. The percentage of foragers in their colonies ranged 7–15 %, more similar to North American species with large colonies than to those with small colony size. Foraging activity was positively correlated with forager abundance in all three species, implying that colony allocation to number of foragers allows for higher food acquisition. Further comparative studies involving a wider range of traits in South and North American species would allow to unveil the role of environmental factors in shaping each species’ particular traits.
Some species are severely affected by global change, whereas others can survive or even thrive under the same circumstances. Species with more flexible behaviours can better cope with changes in the environment than those with more stereotyped behaviours; therefore, knowing the limits of behavioural flexibility is critical to predict their responses to global change.
Livestock grazing is the most widespread land use in deserts and a major driver of vegetation change. In turn, unpredictable and sporadic rainfall can increase the negative impact of grazing. The interaction of both factors is more relevant nowadays as a consequence of climate change.
In the central Monte desert, cattle grazing and inter‐annual variations in rainfall reduce the abundance of seeds preferentially consumed by granivorous animals. There we analysed the dietary flexibility of the specialist harvester ant Pheidole spininodis in response to inter‐annual variations in seed availability during four consecutive years at sites with contrasting grazing pressure.
Pheidole spininodis expanded its diet by including other items and decreasing seed consumption as seed availability decreased over the years, but this response did not differ between grazing conditions, probably because the drastic reduction in seed abundance at the regional scale blurred the contrast between grazed and ungrazed sites.
Our results highlight the potential dietary flexibility of a specialist species that, under adverse conditions, can modify its diet. Future studies should investigate whether these changes in the diet of P. spininodis in contexts of low resource availability may have negative effects on its survival, as previously reported for other granivorous species.
Current climate change is disrupting biotic interactions and eroding biodiversity worldwide. However, species sensitive to aridity, high temperatures and climate variability might find shelter in microclimatic refuges, such as leaf rolls built by arthropods. To explore how the importance of leaf shelters for terrestrial arthropods changes with latitude, elevation, and climate, we conducted a distributed experiment comparing arthropods in leaf rolls vs. control leaves across 52 sites along an 11,790 km latitudinal gradient. We then probed the impact of short‐ versus long‐term climatic impacts on roll use, by comparing the relative impact of conditions during the experiment versus average, baseline conditions at the site. Leaf shelters supported larger organisms and higher arthropod biomass and species diversity than non‐rolled control leaves. However, the magnitude of the leaf rolls’ effect differed between long‐ and short‐term climate conditions, metrics (species richness, biomass, and body size), and trophic groups (predators vs. herbivores). The effect of leaf rolls on predator richness was influenced only by baseline climate, increasing in magnitude in regions experiencing increased long‐term aridity, regardless of latitude, elevation, and weather during the experiment. This suggests that shelter use by predators may be innate, and thus, driven by natural selection. In contrast, the effect of leaf rolls on predator biomass and predator body size decreased with increasing temperature, and increased with increasing precipitation, respectively, during the experiment. The magnitude of shelter usage by herbivores increased with the abundance of predators and decreased with increasing temperature during the experiment. Taken together, these results highlight that leaf roll use may have both proximal and ultimate causes. Projected increases in climate variability and aridity are therefore likely to increase the importance of biotic refugia in mitigating the effects of climate change on species persistence.
How ecological interactions vary across spatial and environmental gradients has received increasing attention in recent years, contributing to the revelation of the drivers of biodiversity. However, it is still unclear how the structure of ecological interactions varies across large spatial scales and which climatic factors are associated with such variation. Here, specific predictions were derived and tested to evaluate how climatic factors and latitude are associated with the structure of bat-fruit interaction networks throughout the Neotropical region. For each study site (n = 44 sites, encompassing 48 degrees of latitude), four metrics were used to describe the network structure (i.e., network size, connectance, modularity, and nestedness). In general, an increase in modularity and a decrease in connectance and nestedness was observed towards lower latitudes and in sites with lower precipitation seasonality. Moreover, plant richness within networks increased towards lower latitudes and in sites with higher annual precipitation, whereas bat richness increased at lower latitudes and in sites with lower precipitation seasonality. These findings partially confirm both energy and seasonality hypotheses and suggest that fruit-bearing plant richness and fruit availability associated with annual precipitation and precipitation seasonality can be important correlates shaping the structure of ecological interactions throughout the Neotropical region.
This paper provides a taxonomic revision and reviews natural history for 35 South American species of the seed-harvesting ant genus Pogonomyrmex. Species treated herein mostly comprise the P. rastratus-group; four species are revived from synonomy, three taxa are elevated from subspecies to species, five taxa are synonymized, and 20 new species are described. The following taxa are revived from synonomy: P. intermedia Menozzi, P. semistriata Emery, P. spinolae Emery, and P. weiseri Santschi. The following taxa are raised from subspecies to species: P. leonis Kusnezov, P. pulchellus Santschi, and P. sanmartini Kusnezov. The following new synonymies are proposed, with the senior synonym listed first, and the junior synonym(s) in parentheses: P. carbonarius Mayr (= P. kusnezovi Cuezzo & Claver, = P. weiseri var. neuquensis Santschi, = P. variabilis Santschi); P. vermiculatus Emery (= P. vermiculatus var. chubutensis Forel, = P. vermiculatus var. jorgenseni Forel). The following new species are described: P. apterogenos, P. araucania, P. atacama, P. bolivianus, P. colca, P. cusquena, P. excelsior, P. forelii, P. granulatus, P. lagunabravensis, P. loaensis, P. mapuche, P. maulensis, P. pichachen, P. propinqua, P. santschii, P. strioligaster, P. tafi, P. varicolor, and P. wilsoni. One species treated herein has brachypterous queens (P. atacama), one species has dimorphic queens (winged and brachypterous in P. longibarbis), and two species have ergatoid (permanently wingless) queens and ergatoid males (P. apterogenos, P. laguanbravensis); the latter two are the only known ant species in which both sexual castes are only ergatoid. I also provide keys for workers and queens (in English and Spanish), diagnoses for males, photographs of known castes, distribution maps, and a summary of known biology.
The evolution of ecological networks
Plants and the animals that eat their fruits and disperse their seeds form complex networks of mutualistic interactions. The structures of many such networks and the ecological forces that shape them are well known, but their deeper evolutionary history has received little attention. Burin et al. address this knowledge gap in a study of frugivorious bird species in documented seed-dispersal networks around the world (see the Perspective by Bello and Barreto). Species occupying central positions in frugivory networks, which thus interact with many plant species, tend to belong to lineages that are more stable over macroevolutionary time scales. These patterns are more evident in regions with warmer and wetter climates and provide evidence that evolutionary processes can leave a signal on the structure of current ecological networks.
Science , this issue p. 733 see also p. 682
1. The factors that drive resource removal by insect predators hold the clue to understanding their role in structuring ecological communities and their evolution. Harvester ants are formidable seed predators and invertebrate carcass feeders. However, the extent to which neutral and niche‐based factors drive the selection and removal of preferred food resources by such ants has been neglected in the literature.
2. In this study authors evaluated how the richness of plants and their abundance, along with resource proximity, density, nutrient content, and seasonal changes affect the selective harvest of resources by the harvester ant Pogonomyrmex barbatus in a Neotropical semi‐arid region where resource availability changes across rainy and dry seasons.
3. In accordance with the neutral‐niche theory shaping biotic interactions, the authors observed that P. barbatus more frequently collects the seeds of the most abundant plant species located within their foraging areas, both in the dry and rainy seasons. The authors also observed that resources located in the proximity of nests, and those that are available at high density, are removed more often in the dry, whereas protein‐rich resources located at close range were removed most often in the rainy (i.e. niche‐based factors). Moreover, temporal fluctuations in the collection of seeds and invertebrates by P. barbatus suggest that the phenology of plants is a factor that can affect the availability and collection of resources.
4. The findings exemplify how the dynamics between an insect predator and its food resource can be simultaneously explained by both neutral and niche‐based factors.
1. Harvester ants are major seed predators in arid environments. However, given that many harvester ants are partly omnivorous and therefore potentially attracted to the elaiosomes of myrmecochorous seeds, it is unclear if these ants act as predators or dispersers when removing myrmecochorous seeds.
2. We describe the outcomes of interactions between the harvester ant, Pogonomyrmex naegelii, and myrmecochorous plant, Microstachys serrulata, in a Brazilian savanna. We: (i) evaluated the role of elaiosome in seed removal by P. naegelii; (ii) investigated the fate and viability of removed seeds; (iii) tested if soils associated with P. naegelii nests are nutrient‐enriched; (iv) compared seedling survival; and (v) the density of seedling and adult M. serrulata nearby to P. naegelii nests compared with those away from these nests (i.e. controls).
3. Rates of removal of M. serrulata seeds were two‐fold higher with elaiosomes than without. The ant attractant oleic acid was the dominant fatty acid in elaiosomes, but it was absent from seeds. Removed seeds are taken into nests, and Tetrazolium tests indicated that 95% of seeds remain viable. Soils associated with P. naegelii nests were not nutrient‐enriched, and seedling survival was similar nearby to P. naegelii nests compared with control areas. However, densities of both seedling and adult M. serrulata were higher nearby to P. naegelii nests than in control areas.
4. Our findings show that P. naegelii switches its role from seed predator for most plant species to be the dominant seed disperser for M. serrulata, playing a key role in the distribution of adult plants.
Aim
Similar to species richness, ecological interactions can vary across latitudinal and environmental gradients. Knowing the patterns and drivers of such variation could help us to better understand the role of species interactions in maintaining biodiversity. In this study, we analysed the macroecological patterns of the structure and interaction beta diversity of interaction networks involving trees and ants.
Location
Twenty‐nine sites encompassing 20 degrees of latitude throughout the Neotropical savanna.
Time period
2010–2015.
Major taxa studied
Trees and arboreal nesting ants.
Methods
For each site, we built an interaction network and calculated network size, interaction diversity (Shannon diversity of interactions), specialization, modularity, nestedness, and interaction dissimilarity (contribution of each network to the regional pool of possible interactions). We also determined how interaction beta diversity varied among all sampling sites. Net primary productivity (NPP), temperature and rainfall were evaluated as potential correlates of the observed changes in network descriptors and interaction beta diversity.
Results
We found no latitudinal gradient in network specialization, nestedness or modularity. However, sites at higher latitudes had larger networks, higher interaction diversity and higher interaction dissimilarity, and this was correlated mainly with the latitudinal variation in NPP. Interaction rewiring generated by the reassembly of the interactions between the same species in different sites was the main contributor to the total interaction beta diversity. However, the level of interaction rewiring was independent of the geographical and environmental distance between sampling sites.
Main conclusions
Ant–tree network structure remained relatively invariant across the latitudinal and environmental gradient possibly due to high interaction rewiring among the partners. Moreover, our findings show that more productive sites, located at higher latitudes, have high dissimilarity to the regional pool of possible interactions (i.e., strong interaction filtering), indicating that these sites significantly contribute to the maintenance of interaction biodiversity in Neotropical savannas.
Seed dispersal mediated by animals is a pivotal ecological interaction in the tropics. Despite a long tradition of tropical seed dispersal studies, only recently the drivers of the structure of seed dispersal networks are beginning to be uncovered at macroecological scales. The knowledge on tropical seed dispersal comes mainly from avian dispersal studies in the Neotropics while other frugivores and tropical regions are strongly understudied. The networks sampled with a combination of visitation census and seed recovery from feces seem more reliable of the number of detected links and web asymmetry than networks based on a single method. Our review reveals that keystone species in most networks share a set of functional traits likely influenced by species phylogeny. Woody plants bearing small berries (in the Melastomataceae, Myrtaceae, Moraceae, and Urticaceae families) were the most frequent keystone plants whereas two groups of keystone animals could be identified, namely: small obligate frugivores (Pipridae and Thraupidae) and large animals including a variety of taxonomic groups such as cracids, rodents, monkeys, and megafauna. Large keystone species tend to face a higher extinction risk leading to a concern on the sustainability of the dispersal services they provide, mainly to large-seeded plant species that are essential to ecosystem functioning.
Granivory is an important interaction in the arid and semi-arid zones of the world, since seeds form an abundant and nutritious resource in these areas. While species of the genus Pogonomyrmex have been studied in detail as seed predators, their impact on seed abundance in the soil has not yet been explored in sufficient depth. We studied the impact of the harvesting activities of the ant Pogonomyrmex barbatus on seed abundance in the soil of the Zapotitlán valley, Mexico. We found that P. barbatus activity significantly impacts the abundance of seeds in the soil, which is lower in the sites where P. barbatus forages than it is in sites with no recorded foraging. We also found that P. barbatus distributes intact seeds of three tree species, two of which are nurse plants, and could consequently be promoting the establishment of these species. Using tools derived from graph theory, we observed that the ant-seed interactions exhibit a nested pattern; where more depredated seed species seem to be the more spatially abundant in the environment. This study illustrates the complex foraging ecology of the harvester ant P. barbatus and elucidates its effect on the soil seed bank in a semi-arid environment.
Selective foraging by granivores can have important consequences for the structure and composition of plant communities, and potentially severe consequences for rare plant species. To understand how granivore foraging behavior affects common and rare plant species, diet selection should be viewed relative to the availability of alternative seed options, and with consideration of the individual attributes of those seeds (e.g., morphology, nutrient content). We examined the foraging decisions of Owyhee harvester ants, Pogonomyrmex salinus (Olsen), in semiarid grassland dominated by two species of grass, Poa secunda and Bromus tectorum, and two species of mustard, Sisymbrium altissimum and Lepidium papilliferum. The latter is a rare plant endemic to southwestern Idaho, and its seeds are readily consumed by P. salinus. We examined the diets of P. salinus colonies in June and July over three years and compared these values to the weekly availability of seeds on the ground in a 3–12 -m radius around individual ant colonies. Small-seeded species (P. secunda, S. altissimum, and L. papilliferum) were usually overrepresented in the diet of ants relative to their availability, whereas the large seeds of B. tectorum were largely avoided despite being abundant and nutritious. The reduced travel time associated with carrying small seeds may overshadow differences in nutritional content among seed types, except in times when small seeds are in short supply. Lepidium papilliferum appears particularly vulnerable to seed predation, likely in part because it grows in dense patches that are easily exploited by foragers.
Changes in the distribution of rainfall and the occurrence of extreme rain events will alter the size and persistence of aquatic ecosystems. Such alterations may affect the structure of local aquatic communities in terms of species composition, and by altering species interactions. In many aquatic ecosystems, leaf litter sustains detrital food webs and could regulate the responses of communities to changes in rainfall. Few empirical studies have focused on how rainfall changes will affect aquatic communities and none have evaluated if basal resource diversity can increase resistance to such rainfall effects. In this study, we used water-holding terrestrial bromeliads, a tropical aquatic ecosystem, to test how predicted rainfall changes and litter diversity may affect community composition and trophic interactions. We used structural equation modeling to investigate the combined effects of rainfall changes and litter diversity on trophic interactions. We demonstrated that changes in rainfall disrupted trophic relationships, even though there were only minor direct effects on species abundance, richness and community composition. Litter diversity was not able to reduce the impact of changes in rainfall on trophic interactions. We suggest that changes in rainfall can alter the way in which species interact with each other, decreasing the linkages among trophic groups. Such reductions in biotic interactions under climate change will have critical consequences for the functioning of tropical aquatic ecosystems. This article is protected by copyright. All rights reserved.
Intensification of the hydrological cycle is expected to accompany a warming climate. It has been suggested that changes in the spatial distribution of precipitation will amplify differences between dry and wet regions, but this has been disputed for changes over land. Furthermore, precipitation changes may differ not only between regions but also between different aspects of precipitation, such as totals and extremes. Here we investigate changes in these two aspects in the world's dry and wet regions using observations and global climate models. Despite uncertainties in total precipitation changes, extreme daily precipitation averaged over both dry and wet regimes shows robust increases in both observations and climate models over the past six decades. Climate projections for the rest of the century show continued intensification of daily precipitation extremes. Increases in total and extreme precipitation in dry regions are linearly related to the model-specific global temperature change, so that the spread in projected global warming partly explains the spread in precipitation intensification in these regions by the late twenty-first century. This intensification has implications for the risk of flooding as the climate warms, particularly for the world's dry regions.
This report describes the foraging pattern of the Florida harvester ant Pogonomyrmex badius in a high-density population of colonies. The foraging pattern has both promoted and been influenced by the colony distribution. Pogonomyrmex badius forages from short trails which extend into a surrounding foraging range. Direction of foraging trails is influenced by the location of a colony's near neighbors. Seasonal nest relocations always occur along a foraging trail, usually the main trail. Foraging ranges are not actively defended, but are used almost exclusively by foragers from a single colony. Foraging ranges will be extended into an area abandoned by neighboring foragers, indicating that forager presence may define each colony's range. Colony distribution has remained essentially the same for several years, despite seasonal nest relocations and addition of new colonies. Establishment of trials and exclusive foraging ranges by each colony minimizes encounters with neighboring foragers and guarantees access to available resources; this pattern also promotes maintenance of the existing colony distribution and partitioning of resources.
1. The abundance and composition of soil seed banks is a key determinant of plant community structure. Harvester ants can remove huge quantities of preferred seeds close to the nest affecting composition and spatial distribution of plants.
2. In the central Monte desert (Argentina) ants of the genus Pogonomyrmex have high seed removal rates, especially of the five main grasses. The aim of this study was to establish if their foraging activity affects spatial patterns of the soil seed bank around their nests. Our hypotheses were: (1) removal by ants decreases seed abundance of preferred species in the soil; and (2) the effect varies in time.
3. Soil seed abundance was assessed at different distances from Pogonomyrmex nests in the litter and in bare soil at the beginning, the middle, and the end of the season (late spring‐early autumn).
4. A lower seed abundance of preferred species was observed close to the nest in the litter at the end of the season. Non‐preferred species showed no distance gradient.
5. The lower foraging activity and seed consumption at the beginning of the season could explain the temporal variation of the spatial effect. This was only observed in the litter, probably because of the higher removal frequency in this substrate.
6. Colonies of Pogonomyrmex spp. could enhance the heterogeneity of soil seed banks in the central Monte desert from the summer to the beginning of the autumn. Implications for vegetation dynamics depend on the degree to which seed density limits perennial grasses recruitment after ant activity season.
This study investigates the evolutionary history of a hyperdiverse clade, the ant subfamily Myrmicinae (Hymenoptera: Formicidae), based on analyses of a data matrix comprising 251 species and 11 nuclear gene fragments. Under both maximum likelihood and Bayesian methods of inference, we recover a robust phylogeny that reveals six major clades of Myrmicinae, here treated as newly defined tribes and occurring as a pectinate series: Myrmicini, Pogonomyrmecini trib.n., Stenammini, Solenopsidini, Attini and Crematogastrini. Because we condense the former 25 myrmicine tribes into a new six-tribe scheme, membership in some tribes is now notably different, especially regarding Attini. We demonstrate that the monotypic genus Ankylomyrma is neither in the Myrmicinae nor even a member of the more inclusive formicoid clade—rather it is a poneroid ant, sister to the genus Tatuidris (Agroecomyrmecinae). Several species-rich myrmicine genera are shown to be nonmonophyletic, including Pogonomyrmex, Aphaenogaster, Messor, Monomorium, Pheidole, Temnothorax and Tetramorium. We propose a number of generic synonymies to partially alleviate these problems (senior synonym listed first): Pheidole = Anisopheidole syn.n. = Machomyrma syn.n.; Temnothorax = Chalepoxenus syn.n. = Myrmoxenus syn.n. = Protomognathus syn.n.; Tetramorium = Rhoptromyrmex syn.n. = Anergates syn.n. = Teleutomyrmex syn.n. The genus Veromessor stat.r. is resurrected for the New World species previously placed in Messor; Syllophopsis stat.r. is resurrected from synonymy under Monomorium to contain the species in the hildebrandti group; Trichomyrmex stat.r. is resurrected from synonymy under Monomorium to contain the species in the scabriceps- and destructor-groups; and the monotypic genus Epelysidris stat.r. is reinstated for Monomorium brocha. Bayesian divergence dating indicates that the crown group Myrmicinae originated about 98.6 Ma (95% highest probability density 87.9–109.6 Ma) but the six major clades are considerably younger, with age estimates ranging from 52.3 to 71.1 Ma. Although these and other suprageneric taxa arose mostly in the middle Eocene or earlier, a number of prominent, species-rich genera, such as Pheidole, Cephalotes, Strumigenys, Crematogaster and Tetramorium, have estimated crown group origins in the late Eocene or Oligocene. Most myrmicine species diversity resides in the two sister clades, Attini and Crematogastrini, which are estimated to have originated and diversified extensively in the Neotropics and Paleotropics, respectively. The newly circumscribed Myrmicini is Holarctic in distribution, and ancestral range estimation suggests a Nearctic origin. The Pogonomyrmecini and Solenopsidini are reconstructed as being Neotropical in origin, but they have subsequently colonized the Nearctic region (Pogonomyrmecini) and many parts of the Old World as well as the Nearctic region (Solenopsidini), respectively. The Stenammini have flourished primarily in the northern hemisphere, and are most likely of Nearctic origin, but selected lineages have dispersed to the northern Neotropics and the Paleotropics. Thus the evolutionary history of the Myrmicinae has played out on a global stage over the last 100 Ma, with no single region being the principal generator of species diversity.This published work has been registered in ZooBank, http://zoobank.org/urn:lsid:zoobank.org:pub: BB6829C4-DA79-45FE-979E-9749E237590E.
Pogonomyrmex cunicularius pencosensis Forel es la dispersora dominante de semillas con eleosoma en el Desierto del Monte septentrional del Noroeste semiárido de Argentina. En este trabajo, se estudian varios aspectos de la biología de esta especie de hormiga, relacionados con su capacidad para dispersar semillas mirmecócoras. Se presentan datos sobre la nidificación, el tamaño de la colonia, la dieta y la estrategia de forrajeo de P. cunicularius pencosensis. Finalmente, se discute cómo todos estos aspectos de la historia natural de P. cunicularius pencosensis podrían influenciar positivamente sobre la dispersión de semillas mirmecócoras de plantas nativas del área de estudio, así como sus implicancias ecológicas y evolutivas.
Fjeldså, J., Lambin, E, and Mertens, B. 1999, Correlation between endemism and local ecoclimatic stability documented by comparing Andean bird distributions and remotely sensed land surface data. - Ecography 22: 63-78.
Relationships between large-scale patterns of biodiversity and ecoclimatic variability were examined using distributions of 789 Andean birds, recorded in 15’x 15’grid cells, and interannual differences in Normalized Difference Vegetation Index and Brightness Surface Temperature, calculated month by month and resampled to 15’cells. Following the east Andean treeline from 1°N to 18°S, there is no evidence of a latitudinal gradient in species richness, but a considerable local variation reflecting the habitat complexity in individual cells. The mean endemism (inverse range-size for all species present in a given cell) shows well marked peaks. Pairwise comparisons of‘peaks’and adjacent‘lows’of endemism provide strong evidence for linking peaks of endemism with local ecoclimatic stability. The most important single factor responsible for this correlation could be orographic moderation of the impacts a south polar winds. Presently manifested as occasional winter freezes in the southern part of the tropical zone, these winds may have been a major determinant of vegetational changes during Pleistocene glacial periods. The correlations suggest that most endemics are relict populations which survived periods of global climatic change in places where these impacts were moderated. It is suggested that, by retaining relict populations, these places play a significant role in the recruitment to the regional species pool. The stable places were also centres of Andean cultures and have dense human populations. The current conservation strategy of reserving areas with few people therefore needs to be supplemented with actions to secure sustainable landuse in certain densely populated areas.
The hypothesis that seed or seedling predation by host-specific herbivores can explain the coexistence of the large number of tree species in tropical forests, and produce a low density and uniform dispersion of adults of these tree species, is examined. On theoretical grounds, it is shown that spacing of conspecific adults can account for only a very small fraction of the observed tree species richness in tropical forests unless interadult distances are quite large. Moreover, in growing or declining populations, seed predation will not result in large-scale uniformity in adult dispersion patterns throughout the population. It results in uniformity in equilibrium populations only if the same spacing rule applies to all adult trees. However, adult trees differ enormously in seed production from year to year, and from young (small) to old (large) adults. High variance in seed production from tree to tree and year to year leads to heterogeneity in the intensity of seed predation and in the spacing distance between adults. This explains why the adults of most tropical tree species are clumped or randomly dispersed, and not uniformly spaced, in spite of heavy seed predation in many species. Because tree species are not everywhere as dense as whould be predicted from observed nearest neighbor distances, it is concluded that other factors than seed predation limit the abundance of tropical tree species and prevent single-species dominance. /// Проверяли гипотезу о том, что потребление семян или проростков фитофагами, специализированными к определенному виду кормового растения, может объяснить сосуществование большого числа видов деревьев в тропических лесах, низкую плотность и равномерное распределение зрелых деревьев этих видов. На теоретической основе показано, что пространственное распределение взрослых деревьев одного вида определяется лишь небольной частью всего видового богатства деревьев в тропическом лесу, хотя расстояния между эрелыми деревьями относительно большие. Более того, в развивающихся или угасающих популяциях выедание семян не приводит к крупномасштабной равномерности в характере пространственного распределения взрослых деревьев в пределах распространения популяции. Это приводит равномерности в уровновешенных популяциях лишь в случае, если характер пространственного распределения одинаков у всех взрослыхдеревьев. Однако взрослых деревья сильно различаются по продукции семян в разные годы и в разном возрасте. Большие колебания в продукции семян в разные годы, а таюже индивидуальные колебания приводят к возникновению различий в интенсивности потребления семян и неравномерности распределения взрослых деревьев. Это объясняет, почему распределения взрослых деревьев большинства тропических видов агрегировано или случайно и неравномерно, несмотря на интенсивное отчуждение семян фитофагами у многих видов. Так как отдельные виды деревьев не везде встречаются в той плотности, которую можно предсказать по расстоянию между ближайшими соседними деревьями, сделано заключение, что обилие тропических деревьев лимитируется рядом друтих факторов, помимо выедания семян, которые препетствуют и доминированию отдельных видов.