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Inter- and intraspecific consumer trait variation determine consumer diversity effects in multispecies predator–prey systems
Abstract
This study investigated how inter- and intraspecific trait variations determine consumer diversity loss effects in a short-term microcosm experiment, using consumer and prey biovolume production and composition as the main response variables. Three levels of ciliate diversity were created, all feeding on a 3-species microalgal prey mixture. Ciliates differed in consumer specialisation, feeding on 1 (specialist, S), 2 (intermediate, I) or all 3 (generalist, G) microalgal species. Intraspecific trait variation was incorporated by including 3 different clones of I and setting up ciliate combinations with either monoclonal or polyclonal populations of I. Both increasing inter- and intraspecific consumer diversity increased total ciliate biovolume. On the species level, total ciliate biovolume was high wherever G was included, indicating a positive selection effect for a competitively superior species. Polyclonal I monocultures exceeded the biovolume of all monoclonal ones (transgressive overyielding) based on complementary differences of clone-specific feeding niches. This effect was also observed in multispecies combinations. Both inter- and intraspecific consumer diversity decreased prey evenness. Despite being able to feed on all prey species, G displayed specific grazing preferences within its dietary niche. Furthermore, G exhibited an induced offence, forming giant cells that fed on other ciliates. S responded with an inducible defence, escaping predation by the intraguild predator. Overall, our study demonstrated highly complex trophic interactions driven by consumer selectivity, grazing rates, selective feeding and phenotypic plasticity, and indicated that both inter- and intraspecific consumer trait variations determine the consequences of consumer diversity loss on ecosystem functioning.
... In contrast to inducible defenses, inducible offenses have received substantially less attention (Banerji and Morin, 2014;Kopp and Tollrian, 2003;Mougi et al., 2011;Padilla, 2001), especially in the context of reciprocal phenotypic plasticity in predator-prey systems (Mougi and Kishida, 2009). Furthermore, such interactions have mostly been studied under constant environmental conditions, although environmental factors such as resource supply are often highly variable in nature, so that adaptive strategies including phenotypic plasticity to overcome periods of resource limitation can be beneficial in variable environments (Flöder et al., 2018). ...
... In the framework of this study, the term "superior" is based on the ability to produce a higher population biovolume by either escaping predation through a strong defense ensuring survival (H1) or also by other traits (H2). Despite high grazing and growth rates and its ability to expand its resource spectrum by preying on intraguild prey, the IG predator S. mytilus was previously shown to exhibit low resistance to food depletion (Flöder et al., 2018), and to form resting cysts triggered by resource limitation (Walker et al., 1975). In contrast, E. octocarinatus was shown to exhibit a high resistance to algal resource depletion before, i.e., a high starvation resistance (Flöder et al., 2018). ...
... Despite high grazing and growth rates and its ability to expand its resource spectrum by preying on intraguild prey, the IG predator S. mytilus was previously shown to exhibit low resistance to food depletion (Flöder et al., 2018), and to form resting cysts triggered by resource limitation (Walker et al., 1975). In contrast, E. octocarinatus was shown to exhibit a high resistance to algal resource depletion before, i.e., a high starvation resistance (Flöder et al., 2018). We thus expected pulsed prey supply entailing periods of resource depletion to promote E. octocarinatus, and a continuous resource supply to promote active S. mytilus (hypothesis H3). ...
... For instance, intraspecific trait variation has been shown to markedly affect predator-prey dynamics in an experimental rotifer-microalgal system (Yoshida et al., 2003). Flöder et al. (2018) demonstrated in a microcosm experiment using ciliate predators and microalgal prey that the effect of intraspecific trait variation concerning specialization, selectivity, and grazing rate was comparable to the effect of interspecific trait variation. Differences in the feeding niches of three different clones of the ciliate Coleps hirtus resulted in a transgressive overyielding, that is, in a higher biomass production of the polyclonal culture compared with each of the three monoclonal cultures, including the most productive one. ...
... In the present study, we specifically incorporated intraspecific predator trait variation to investigate how this trait variation affects the outcome of competition between two different herbivorous freshwater ciliate species, using three different clones of Coleps hirtus and a monoclonal culture of Euplotes octocarinatus (the same as used in Flöder et al., 2018). ...
... Highly selective regarding microalgal prey (Flöder et al., 2018;Wilks & Sleigh, 2004, Euplotes was able to feed and grow only on Cryptomonas. However, Euplotes could use the accompanying bacteria as an additional resource. ...
Trait variation among heterospecific and conspecific organisms may substantially affect community and food web dynamics. While the relevance of competition and feeding traits have been widely studied for different consumer species, studies on intraspecific differences are more scarce, partly owing to difficulties in distinguishing different clones of the same species. Here, we investigate how intraspecific trait variation affects the competition between the freshwater ciliates Euplotes octocarinatus and Coleps hirtus in a nitrogen-limited chemostat system. The ciliates competed for the microalgae Cryptomonas sp. (Cry) and Navicula pelliculosa (Nav), and the bacteria present in the cultures over a period of 33 days. We used monoclonal Euplotes and three different Coleps clones (Col 1, Col 2, and Col 3) in the experiment that could be distinguished by a newly developed rDNA-based molecular assay based on the internal transcribed spacer (ITS) regions. While Euplotes feeds on Cry and on bacteria, the Coleps clones cannot survive on bacteria alone but feed on both Cry and Nav with clone-specific rates. Experimental treatments comprised two-species mixtures of Euplotes and one or all of the three different Coleps clones, respectively. We found intraspecific variation in the traits “selectivity” and “maximum ingestion rate” for the different algae to significantly affect the competitive outcome between the two ciliate species. As Nav quickly escaped top-down control and likely reached a state of low food quality, ciliate competition was strongly determined by the preference of different Coleps clones for Cry as opposed to feeding on Nav. In addition, the ability of Euplotes to use bacteria as an alternative food source strengthened its persistence once Cry was depleted. Hence, trait variation at both trophic levels codetermined the population dynamics and the outcome of species competition.
... Many of these studies restricted their focus to primary producer diversity, and were able to show its correlation with relevant ecosystem functions (reviewed by Cardinale et al. 2011). However, during the last two decades, more sophisticated theoretical and experimental studies linking both plant and consumer diversity to these ecosystem functions were conducted (see Thébault and Loreau 2003, Borer et al. 2012, Seabloom et al. 2017, Flöder et al. 2018, and reviews by Duffy et al. 2007, Griffin et al. 2013, Barnes et al. 2018. In a recent experimental study, Wohlgemuth et al. (2017) demonstrated that producer diversity effects on the biomass distribution and production at higher trophic levels crucially depends on particular traits of the consumer level, such as specialization and selectivity. ...
... In addition, we reveal intricate and complicated interactions between the degree of diversity at different trophic levels and these ecosystem functions. These interactions complicate the comparison of studies on the links between diversity and functioning in a bitrophic context , Wohlgemuth et al. 2017, Flöder et al. 2018, Daam et al. 2019. ...
In this thesis, a collection of studies is presented that advance research on complex food webs in several directions. Food webs, as the networks of predator-prey interactions in ecosystems, are responsible for distributing the resources every organism needs to stay alive. They are thus central to our understanding of the mechanisms that support biodiversity, which in the face of increasing severity of anthropogenic global change and accelerated species loss is of highest importance, not least for our own well-being. The studies in the first part of the thesis are concerned with general mechanisms that determine the structure and stability of food webs. It is shown how the allometric scaling of metabolic rates with the species' body masses supports their persistence in size-structured food webs (where predators are larger than their prey), and how this interacts with the adaptive adjustment of foraging efforts by consumer species to create stable food webs with a large number of coexisting species. The importance of the master trait body mass for structuring communities is further exemplified by demonstrating that the specific way the body masses of species engaging in empirically documented predator-prey interactions affect the predator's feeding rate dampens population oscillations, thereby helping both species to survive. In the first part of the thesis it is also shown that in order to understand certain phenomena of population dynamics, it may be necessary to not only take the interactions of a focal species with other species into account, but to also consider the internal structure of the population. This can refer for example to different abundances of age cohorts or developmental stages, or the way individuals of different age or stage interact with other species. Building on these general insights, the second part of the thesis is devoted to exploring the consequences of anthropogenic global change on the persistence of species. It is first shown that warming decreases diversity in size-structured food webs. This is due to starvation of large predators on higher trophic levels, which suffer from a mismatch between their respiration and ingestion rates when temperature increases. In host-parasitoid networks, which are not size-structured, warming does not have these negative effects, but eutrophication destabilises the systems by inducing detrimental population oscillations. In further studies, the effect of habitat change is addressed. On the level of individual patches, increasing isolation of habitat patches has a similar effect as warming, as it leads to decreasing diversity due to the extinction of predators on higher trophic levels. In this case it is caused by dispersal mortality of smaller and therefore less mobile species on lower trophic levels, meaning that an increasing fraction of their biomass production is lost to the inhospitable matrix surrounding the habitat patches as they become more isolated. It is further shown that increasing habitat isolation desynchronises population oscillations between the patches, which in itself helps species to persist by dampening fluctuations on the landscape level. However, this is counteracted by an increasing strength of local population oscillations fuelled by an indirect effect of dispersal mortality on the feeding interactions. Last, a study is presented that introduces a novel mechanism for supporting diversity in metacommunities. It builds on the self-organised formation of spatial biomass patterns in the landscape, which leads to the emergence of spatio-temporally varying selection pressures that keep local communities permanently out of equilibrium and force them to continuously adapt. Because this mechanism relies on the spatial extension of the metacommunity, it is also sensitive to habitat change. In the third part of the thesis, the consequences of biodiversity for the functioning of ecosystems are explored. The studies focus on standing stock biomass, biomass production, and trophic transfer efficiency as ecosystem functions. It is first shown that increasing the diversity of animal communities increases the total rate of intra-guild predation. However, the total biomass stock of the animal communities increases nevertheless, which also increases their exploitative pressure on the underlying plant communities. Despite this, the plant communities can maintain their standing stock biomass due to a shift of the body size spectra of both animal and plant communities towards larger species with a lower specific respiration rate. In another study it is further demonstrated that the generally positive relationship between diversity and the above mentioned ecosystem functions becomes steeper when not only the feeding interactions but also the numerous non-trophic interactions (like predator interference or competition for space) between the species of an ecosystem are taken into account. Finally, two studies are presented that demonstrate the power of functional diversity as explanatory variable. It is interpreted as the range spanned by functional traits of the species that determine their interactions. This approach allows to mechanistically understand how the ecosystem functioning of food webs with multiple trophic levels is affected by all parts of the food web and why a high functional diversity is required for efficient transportation of energy from primary producers to the top predators. The general discussion draws some synthesising conclusions, e.g. on the predictive power of ecosystem functioning to explain diversity, and provides an outlook on future research directions.
... Many of these studies restricted their focus to primary producer diversity, and were able to show its correlation with relevant ecosystem functions (reviewed by Cardinale et al. 2011). However, during the last two decades, more sophisticated theoretical and experimental studies linking both plant and consumer diversity to these ecosystem functions were conducted (see Thébault and Loreau 2003, Tirok and Gaedke 2010, Borer et al. 2012, Filip et al. 2014, Klauschies et al. 2016, Schneider et al. 2016, Seabloom et al. 2017, Flöder et al. 2018, and reviews by Duffy et al. 2007, Griffin et al. 2013, Barnes et al. 2018). In a recent experimental study, Wohlgemuth et al. (2017) demonstrated that producer diversity effects on the biomass distribution and production at higher trophic levels crucially depends on particular traits of the consumer level, such as specialization and selectivity. ...
... In addition, we reveal intricate and complicated interactions between the degree of diversity at different trophic levels and these ecosystem functions. These interactions complicate the comparison of studies on the links between diversity and functioning in a bitrophic context (Filip et al. 2014, Wohlgemuth et al. 2017, Flöder et al. 2018, Daam et al. 2019. ...
It is well known that functional diversity strongly affects ecosystem functioning. However, even in rather simple model communities consisting of only two or, at best, three trophic levels, the relationship between multitrophic functional diversity and ecosystem functioning appears difficult to generalize, because of its high contextuality. In this study, we considered several differently structured tritrophic food webs, in which the amount of functional diversity was varied independently on each trophic level. To achieve generalizable results, largely independent of parametrization, we examined the outcomes of 128,000 parameter combinations sampled from ecologically plausible intervals, with each tested for 200 randomly sampled initial conditions. Analysis of our data was done by training a random forest model. This method enables the identification of complex patterns in the data through partial dependence graphs, and the comparison of the relative influence of model parameters, including the degree of diversity, on food‐web properties. We found that bottom‐up and top‐down effects cascade simultaneously throughout the food web, intimately linking the effects of functional diversity of any trophic level to the amount of diversity of other trophic levels, which may explain the difficulty in unifying results from previous studies. Strikingly, only with high diversity throughout the whole food web, different interactions synergize to ensure efficient exploitation of the available nutrients and efficient biomass transfer to higher trophic levels, ultimately leading to a high biomass and production on the top level. The temporal variation of biomass showed a more complex pattern with increasing multitrophic diversity: while the system initially became less variable, eventually the temporal variation rose again because of the increasingly complex dynamical patterns. Importantly, top predator diversity and food‐web parameters affecting the top trophic level were of highest importance to determine the biomass and temporal variability of any trophic level. Overall, our study reveals that the mechanisms by which diversity influences ecosystem functioning are affected by every part of the food web, hampering the extrapolation of insights from simple monotrophic or bitrophic systems to complex natural food webs.
... Many of these studies restricted their focus to primary producer diversity, and were able to show its correlation with relevant ecosystem functions (reviewed by Cardinale et al. (2011)). However, during the last two decades, more sophisticated theoretical and experimental studies linking both plant and consumer diversity to these ecosystem functions were conducted (see Tirok and Gaedke (2010); Borer et al. (2012); Filip et al. (2014); Klauschies et al. (2016); Schneider et al. (2016); Flöder et al. (2018), and reviews by Duffy et al. (2007) and Griffin et al. (2013)). In a recent experimental study, Wohlgemuth et al. (2017) demonstrated that producer diversity effects on the biomass distribution and production at higher trophic levels crucially depends on particular traits of the consumer level, such as specialisation and selectivity. ...
... In addition, we reveal intricate and complicated interactions between the degree of diversity at different trophic levels and these ecosystem functions. These interactions complicate comparison of the numerous studies on the links between diversity and functioning to each other (Filip et al., 2014;Wohlgemuth et al., 2017;Flöder et al., 2018;Daam et al., 2019). ...
It is well known that functional diversity strongly affects ecosystem functioning. However, even in rather simple model communities consisting of only two or, at best, three trophic levels, the relationship between multitrophic functional diversity and ecosystem functioning appears difficult to generalize, due to its high contextuality. In this study, we considered several differently structured tritrophic food webs, in which the amount of functional diversity was varied independently on each trophic level. To achieve generalizable results, largely independent of parametrization, we examined the outcomes of 128,000 parameter combinations sampled from ecologically plausible intervals, with each tested for 200 randomly sampled initial conditions. Analysis of our data was done by training a Random Forest model. This method enables the identification of complex patterns in the data through partial dependence graphs, and the comparison of the relative influence of model parameters, including the degree of diversity, on food web properties. We found that the effects of functional diversity on any trophic level are intimately linked to the amount of diversity on other trophic levels, which may explain the difficulty in unifying results from previous studies. Strikingly, with high diversity throughout the whole food web, different interactions synergize to ensure efficient exploitation of the available nutrients and efficient biomass transfer, ultimately leading to a high biomass and production on the top level. The temporal variation of biomass showed a more complex pattern with increasing multitrophic diversity: while the system initially became less variable, eventually the temporal variation rose again due to the increasingly complex dynamical patterns. Importantly, top diversity and food web parameters affecting the top level were of highest importance to determine the biomass and temporal variability of any trophic level. Therefore, given its high ecological importance and vulnerability to global change, it is essential to preserve diversity on the top trophic level.
... Taken together, the type and the quantity of resource supply seem to determine mixotroph performance and competitive ability. Most studies, however, have been performed under constant environmental conditions, although resource supply is often highly variable in nature, so that adaptive strategies to overcome periods of resource limitation can be beneficial in variable environments (Flöder et al., 2018). ...
The outcome of species competition strongly depends on the traits of the competitors and associated trade‐offs, as well as on environmental variability. Here, we investigate the relevance of consumer trait variation for species coexistence in a ciliate consumer–microalgal prey system under fluctuating regimes of resource supply. We focus on consumer competition and feeding traits, and specifically on the consumer's ability to overcome periods of resource limitation by mixotrophy, that is, the ability of photosynthetic carbon fixation via algal symbionts in addition to phagotrophy. In a 48‐day chemostat experiment, we investigated competitive interactions of different heterotrophic and mixotrophic ciliates of the genera Euplotes and Coleps under different resource regimes, providing prey either continuously or in pulses under constant or fluctuating light, entailing periods of resource depletion in fluctuating environments, but overall providing the same amount of prey and light. Although ultimate competition results remained unaffected, population dynamics of mixotrophic and heterotrophic ciliates were significantly altered by resource supply mode. However, the effects differed among species combinations and changed over time. Whether mixotrophs or heterotrophs dominated in competition strongly depended on the genera of the competing species and thus, species‐specific differences in the minimum resource requirements that are associated with feeding on shared prey, nutrient uptake, light harvesting, and access to additional resources such as bacteria. Potential differences in the curvature of the species' resource‐dependent growth functions may have further mediated the species‐specific responses to the different resource supply modes. Overall, our study demonstrates that genus‐ or species‐specific traits other than that related to nutritional mode may override the relevance of acquired phototrophy by heterotrophs in competitive interactions, and that the potential advantage of photosynthetic carbon fixation of symbiont‐bearing mixotrophs in competition with pure heterotrophs may differ greatly among different mixotrophs, playing out under different environmental conditions and depending on the specific requirements of the species. Complex trophic interactions determine the outcome of competition, which can only be understood by taking on a multidimensional trait perspective.
... The copyright holder for this preprint this version posted November 6, 2023. ; https://doi.org/10.1101/2023.11.06.564392 doi: bioRxiv preprint similar to those of the heterotrophic P. bursaria at high light (Berk et al. 1991 (Flöder et al. 2018). ...
The outcome of species competition strongly depends on the traits of the competitors and associated trade-offs, as well as on environmental variability. Here we investigate the relevance of consumer trait variation for species coexistence in a ciliate consumer - microalgal prey system under fluctuating regimes of resource supply. We focus on consumer competition and feeding traits, and specifically on the consumer's ability to overcome periods of resource limitation by mixotrophy, i.e. the ability of photosynthetic carbon fixation via algal symbionts in addition to phagotrophy. In a 48-day chemostat experiment, we investigated competitive interactions of different heterotrophic and mixotrophic ciliates of the genera Euplotes and Coleps under different resource regimes, providing prey either continuously or in pulses under constant or fluctuating light, entailing periods of resource depletion in fluctuating environments, but overall providing the same amount of prey and light. Although ultimate competition results remained unaffected, population dynamics of mixotrophic and heterotrophic ciliates were significantly altered by resource supply mode. However, the effects differed among species combinations and changed over time. Whether mixotrophs or heterotrophs dominated in competition strongly depended on the genera of the competing species and thus species-specific differences in the minimum resource requirements that are associated with feeding on shared prey, nutrient uptake, light harvesting and access to additional resources such as bacteria. Potential differences in the curvature of the species' resource-dependent growth functions may have further mediated the species-specific responses to the different resource supply modes. In addition, while the presence of a heterotrophic competitor may have a direct negative effect on the growth rate of a mixotrophic species through grazing on a shared prey species, its presence may also have an indirect positive effect on the growth rate of the mixotroph by reducing competition between the autotroph and mixotroph for shared nutrients and light. Our study thus demonstrates that complex trophic interactions determine the outcome of competition, which can only be understood by taking on a multidimensional trait perspective.
... Microcosm experiments demonstrated negative effects of copepods and cladocerans on ciliates due to competition and predation (Wickham, 1998;Burns and Schallenberg, 2001;Kunzmann et al., 2019) as they are a preferred food item of copepods (Paffenhöfer et al., 2005;Vargas et al., 2006;Kunzmann et al., 2019). However, trophic relationships expected from single feeding trials in the laboratory may not even hold in more species-rich microcosms (Flöder et al., 2018), which hampers extrapolation to field conditions. Thus, a number of studies used field data to infer feeding relationships of ciliates (Cleven, 2004;Posch et al., 2015) or the potential impact of ciliate predators or competitors (Wickham, 1998). ...
Ciliates represent a crucial link between phytoplankton and bacteria and mesozooplankton in pelagic food webs, but little is known about the processes influencing the dynamics of individual species. Using long-term, high-frequency observations, we compared the diversity and the temporal variability in biomass and species composition of the ciliate community in large, deep, mesotrophic Lake Constance to that of the phytoplankton and rotifer communities in the same lake. Furthermore, we used boosted regression trees to evaluate possible environmental predictors (temperature, three prey groups, four predator/competitor groups) influencing ciliate net growth. The biomass of all ciliate species showed a common, recurrent seasonal pattern, often with peaks in spring and summer. The ciliate community was more diverse than the rotifer community, exhibited highly synchronous dynamics and its species were regularly encountered during the season. The top-down control by copepods likely contributes to the ciliates’ synchronized decline prior to the clear-water phase when food concentration is still high. The high temporal autocorrelation of the ciliate biomasses together with the inter-annual recurrent seasonal patterns and the low explanatory power of the environmental predictors suggest that the dynamics of individual ciliate species are strictly controlled, yet it remains difficult to determine the responsible factors.
... Microcosm experiments demonstrated negative effects of copepods and cladocerans on ciliates due to competition and predation (Wickham, 1998;Burns and Schallenberg, 2001; as they are a preferred food item of copepods (Paffenhöfer et al., 2005;Vargas et al., 2006;. However, trophic relationships expected 77 from single feeding trials in the lab may not even hold in more species rich microcosms (Flöder et al., 2018), which hampers extrapolation to field conditions. Thus, a number of studies used field data to infer feeding relationships of ciliates (Cleven, 2004;Posch et al., 2015) or the potential impact of ciliate predators or competitors (Wickham, 1998). ...
Plankton food webs are the basis of marine and limnetic ecosystems. Especially aquatic ecosystems of high biodiversity provide important ecosystem services for humankind as providers of food, coastal protection, climate regulation, and tourism. Understanding the dynamics of biomass and coexistence in these food webs is a first step to understanding the ecosystems. It also lays the foundation for the development of management strategies for the maintenance of the marine and freshwater biodiversity despite anthropogenic influences. Natural food webs are highly complex, and thus often equally complex methods are needed to analyse and understand them well. Models can help to do so as they depict simplified parts of reality. In the attempt to get a broader understanding of the complex food webs, diverse methods are used to investigate different questions. In my first project, we compared the energetics of a food chain in two versions of an allometric trophic network model. In particular, we solved the problem of unrealistically high trophic transfer efficiencies (up to 70%) by accounting for both basal respiration and activity respiration, which decreased the trophic transfer efficiency to realistic values of ≤30%. Next in my second project I turned to plankton food webs and especially phytoplankton traits. Investigating a long-term data set from Lake Constance we found evidence for a trade-off between defence and growth rate in this natural phytoplankton community. I continued working with this data set in my third project focusing on ciliates, the main grazer of phytoplankton in spring. Boosted regression trees revealed that temperature and predators have the highest influence on net growth rates of ciliates. We finally investigated in my fourth project a food web model inspired by ciliates to explore the coexistence of plastic competitors and to study the new concept of maladaptive switching, which revealed some drawbacks of plasticity: faster adaptation led to higher maladaptive switching towards undefended phenotypes which reduced autotroph biomass and coexistence and increased consumer biomass. It became obvious that even well-established models should be critically questioned as it is important not to forget reality on the way to a simplistic model. The results showed furthermore that long-term data sets are necessary as they can help to disentangle complex natural processes. Last, one should keep in mind that the interplay between models and experiments/ field data can deliver fruitful insights about our complex world.
Consumer diversity effects on ecosystem functioning are highly context dependent and are determined by consumer specialization and other consumer and prey specific traits such as growth and grazing rates. Despite complex reciprocal interactions between consumers and their prey, few experimental studies have focused on prey diversity effects on consumer dynamics and trophic transfer. In microbial microcosms, we investigated effects of algal prey diversity (one, two and four species) on the production, evenness and grazing rates of 4 ciliate consumers, differing in grazing preferences and rates. Prey diversity increased prey biovolume in the absence of consumers and had opposing effects on different consumers, depending on their specialization and their preferred prey. Consumers profited from prey mixtures compared to monocultures of non-preferred prey, but responded negatively if preferred prey species were offered together with other species. Prey diversity increased consumer evenness by preventing dominance of specific consumers, demonstrating that the loss of prey species may have cascading effects resulting in reduced consumer diversity. Our study emphasizes that not only the degree of specialization but also the selectivity for certain prey species within the dietary niche may alter the consequences of changing prey diversity in a food web context.
The effects of prey heterogeneity and consumer identity on the strength of predator limitation of prey biomass were explored experimentally under controlled laboratory conditions. In this study, I utilized a model aquatic community composed of zooplankton as top predators, algae as prey, and nutrients as basal resources. To examine the effects of prey heterogeneity, I created a food chain initially composed of a single edible prey and a food web initially composed of a diverse assemblage of algae. These two prey treatments were then fully crossed with two predator treatments (a large-bodied zooplankter, Daphnia pulex, and a small-bodied species, Ceriodaphnia quadrangula), and two levels of productivity. Prey heterogeneity had clear effects on the ability of predators to limit overall prey biomass. In food chains, predators had strong negative effects on algae, and algal biomass exhibited a narrow response to enrichment. In contrast, predator limitation was weak in food webs with the consequence that predator and prey biomass both showed positive increases with productivity. The prey community in food webs also exhibited a striking increase in the relative abundance of large inedible algae with enrichment, in keeping with model predictions. These results indicate that prey heterogeneity can have substantial effects on predator-prey dynamics and trophic structure and can serve to shift systems from strong top-down control to ones in which prey are colimited by predators and resources. Comparisons between top predators showed that Daphnia, compared to Ceriodaphnia, more strongly limited the biomass of large algae in food webs at high productivity and total algal biomass in all nutrient-enriched treatments (both chains and webs). Thus, consumer identity and ecological context (productivity and heterogeneity of prey communities) may mediate the strength of zooplankton-algae interactions and the efficacy of trophic cascades.
Ecosystem functioning is affected by horizontal (within trophic groups) and vertical (across trophic levels) biodiversity. Theory predicts that the effects of vertical biodiversity depend on consumer specialization. In a microcosm experiment, we investigated ciliate consumer diversity and specialization effects on algal prey biovolume, evenness and composition, and on ciliate biovolume production. The experimental data was complemented by a process‐based model further analyzing the ecological mechanisms behind the observed diversity effects. Overall, increasing consumer diversity had no significant effect on prey biovolume or evenness. However, consumer specialization affected the prey community. Specialist consumers showed a stronger negative impact on prey biovolume and evenness than generalists. The model confirmed that this pattern was mainly driven by a single specialist with a high per capita grazing rate, consuming the two most productive prey species. When these were suppressed, the prey assemblage became dominated by a less productive species, consequently decreasing prey biovolume and evenness. Consumer diversity increased consumer biovolume, which was stronger for generalists than for specialists and highest in mixed combinations, indicating that consumer functional diversity, i.e. more diverse feeding strategies, increased resource use efficiency. Overall, our results indicate that consumer diversity effects on prey and consumers strongly depend on species‐specific growth and grazing rates, which may be at least equally important as consumer specialization in driving consumer diversity effects across trophic levels.
Synthesis
In a microcosm experiment, we investigated multitrophic consumer diversity and specialization effects using ciliate consumers and microalgal prey. Consumer diversity increased consumer biovolume, which was highest in combinations containing both generalists and specialists. Specialist consumers showed a stronger negative effect on prey biovolume and evenness than generalists. These experimental data were supported by a process‐based model, indicating that the large effect of the specialists was based on high per capita grazing rate on the two most productive prey species. Species‐specific traits such as growth and grazing rates were equally important for multitrophic diversity effects than consumer specialization.
Abstract The concept that diversity promotes reliability of ecosystem function depends on the pattern that community-level biomass shows lower temporal variability than species-level biomasses. However, this pattern is not universal, as it relies on compensatory or independent species dynamics. When in contrast within-trophic level synchronization occurs, variability of community biomass will approach population-level variability. Current knowledge fails to integrate how species richness, functional distance between species, and the relative importance of predation and competition combine to drive synchronization at different trophic levels. Here we clarify these mechanisms. Intense competition promotes compensatory dynamics in prey, but predators may at the same time increasingly synchronize, under increasing species richness and functional similarity. In contrast, predators and prey both show perfect synchronization under strong top-down control, which is promoted by a combination of low functional distance and high net growth potential of predators. Under such conditions, community-level biomass variability peaks, with major negative consequences for reliability of ecosystem function.
Apparent competition is of broad interest due to its effects on community structure and potential applications in agriculture, restoration, and medicine. It is well-established that apparent competition can result from changes in predator abundance or behavior caused by interactions with alternate prey, but no previous empirical study has demonstrated that apparent competition can also result from prey-induced changes in predator morphology. This trait-mediated alternative mechanism of apparent competition would expand the range of conditions under which apparent competition might occur in nature and identify new ways to generate or modify apparent competition in applied contexts. Here, as a proof of concept, we show that trait-mediated apparent competition can arise from inducible offenses and show how it operates using experiments involving three ciliates. When it feeds on Colpidium kleini, the intraguild predator Tetrahymena vorax increases in size to the extent that it can then consume Paramecium aurelia, an even larger prey. When feeding only on bacteria, however, Tetrahymena remains smaller and is unable to consume Paramecium. This trait-mediated indirect effect leads to the predatory exclusion of Paramecium, while Tetrahymena and Colpidium coexist. Developmental expansions such as those underlying the interactions observed in our study are not limited to ciliates, such as Tetrahymena, but occur among many diverse taxa and may have a surprising degree of influence over the structure and dynamics of food webs.
There is growing interest in the effects of changing marine biodiversity on a variety of community properties and ecosystem processes such as nutrient use and cycling, productivity, stability, and trophic transfer. We review published marine experiments that manipulated the number of species, genotypes, or functional groups. This research reveals several emerging generalities. In studies of primary producers and sessile animals, diversity often has a weak effect on production or biomass, especially relative to the strong effect exerted by individual species. However, sessile taxon richness did consistently decrease variability in community properties, and increased resistance to, or recovery from disturbance or invasion. Multitrophic-level studies indicate that, relative to depauperate assemblages of prey species, diverse ones (a) are more resistant to top-down control, (b) use their own resources more completely, and (c) increase consumer fitness. In contrast, predator diversity can either increase or de...
Global diversity loss has renewed the interest in the underlying mechanisms that explain species diversity and its maintenance in natural systems. This gave the impetus to significant new development in competition research. The aim of this chapter is to review important research results on competition among microalgae and on the effects of environmental fluctuation or disturbance on community structure.
It covers mechanistic competition theory as well as new approaches questioning the central theorem of the classical theory. Recent results of competition experiments and bio essays are included, that have shown how factors other than macro-nutrients may affect the structure of microalgal communities. To explain the influence of disturbance on community structure, we report different approaches to define disturbance and explain the meaning of disturbance within the framework of this review. Intermediate disturbance hypothesis and dynamic equilibrium theory build the theoretical background for discussing competition in variable environments. Experimental results, models and simulations show how environmental fluctuation influences community structure and how different mechanisms may interact.
It is well established that species richness of primary producers and primary consumers can enhance efficiency of resource uptake and biomass production of respective trophic levels. At the level of secondary consumers (predators), however, conclusions about the functional role of biodiversity have been mixed. We take advantage of a recent surge of published experiments (totaling 46 since 2005) to both evaluate general effects of predator richness on aggregate prey suppression (top-down control) and explore sources of variability among experiments. Our results show that, across experiments, predator richness enhances prey suppression relative to the average single predator species (mean richness effect), but not the best-performing species. Mean richness effects in predator experiments were stronger than those for primary producers and detritivores, suggesting that relationships between richness and function may increase with trophic height in food webs. The strength of mean predator richness effects increased with the spatial and temporal scale of experiments, and the taxonomic distinctness (TD, used as a proxy of phylogenetic diversity) of species present. This latter result suggests that TD captures important aspects of functional differentiation among predators and that measures of biodiversity that go beyond species richness may help to better predict the effects of predator species loss.
1. A host specialist parasitoid is thought to have greater efficiency in locating hosts or greater ability to overcome host defence than a generalist species. This leads to the prediction that a specialist should locate and parasitise more hosts than a generalist in a given arena. The work reported here tested these predictions by comparing the host‐searching behaviour of Diadegma semiclausum (a specialist) and Cotesia plutellae (an oligophagous species), two parasitoids of larval Plutella xylostella .
2. Both parasitoids employed antennal search and ovipositor search when seeking hosts but D. semiclausum also seemed to use visual perception in the immediate vicinity of hosts.
3. Larvae of P. xylostella avoided detection by parasitoids by moving away from damaged plant parts after short feeding bouts. When they encountered parasitoids, the larvae wriggled vigorously as they retreated and often hung from silk threads after dropping from a plant.
4. These two parasitoids differed in their responses to host defences. Diadegma semiclausum displayed a wide‐area search around feeding damage and waited near the silk thread for a suspended host to climb up to the leaf, then attacked it again. Cotesia plutellae displayed an area‐restricted search and usually pursued the host down the silk thread onto the ground.
5. Diadegma semiclausum showed a relatively fixed behavioural pattern leading to oviposition but C. plutellae exhibited a more plastic behavioural pattern.
6. The time spent by the two parasitoids on different plants increased with increasing host density, but the time spent either on all plants or a single plant by D. semiclausum was longer than that of C. plutellae . Diadegma semiclausum visited individual plants more frequently than C. plutellae before it left the patch, and stung hosts at more than twice the rate of C. plutellae .
7. The results indicated that the host‐location strategies employed by D. semiclausum were adapted better to the host's defensive behaviour, and thus it was more effective at detecting and parasitising the host than was C. plutellae .
Phagotrophic protists are diverse and abundant in aquatic and terrestrial environments,
making them fundamental to the transfer of matter/energy within their respective food webs. Recognising
their grazing impact is essential to evaluate the role of protists in ecosystems, and this includes
appreciating prey selectivity. Efforts have been made by groups and individuals to understand selective
grazing behaviour by protists: many approaches and perspectives have been pursued, not all of
which are compatible. This article, which is not a review, is the product of our discourse on this subject
at the SAME 10 meeting. It is the work of individuals, assembled for their breadth of backgrounds,
approaches, views, and expertise. Firstly, to communicate ideas and approaches, we
develop a framework for selective feeding processes and suggest 6 steps: searching, contact, capture,
processing, ingestion, digestion. We then separate study approaches into 2 categories: (1) those
examining whole organisms at the community, population, and individual levels, and (2) those examining
physiology and molecular attributes. Finally, we explore general problems associated with the
field of protistan selective feeding (e.g. linking food selection into food webs and modeling). We do
not present all views on any one topic, nor do we cover all topics; instead, we offer opinions and
suggest avenues for continued study. Overall, this paper should stimulate further discourse on the
subject and provide a roadmap for the future.
Reductions in genetic diversity can have widespread ecological consequences: populations with higher genetic diversity are more stable, productive and resistant to disturbance or disease than populations with lower genetic diversity. These ecological effects of genetic diversity differ from the more familiar evolutionary consequences of depleting genetic diversity, because ecological effects manifest within a single generation. If common, genetic diversity effects have the potential to change the way we view and manage populations, but our understanding of these effects is far from complete, and the role of genetic diversity in sexually reproducing animals remains unclear. Here, we examined the effects of genetic diversity in a sexually reproducing marine invertebrate in the field. We manipulated the genetic diversity of experimental populations and then measured individual survival, growth, and fecundity, as well as the size of offspring produced by individuals in high and low genetic diversity populations. Overall, we found greater genetic diversity increased performance across all metrics, and that complementarity effects drove the increased productivity of our high-diversity populations. Our results show that differences in genetic diversity among populations can have pervasive effects on population productivity within remarkably short periods of time.
The relationship between species diversity and the stability and production of trophic levels continues to receive intense scientific interest. Though facilitation is commonly cited as an essential underlying mechanism, few studies have provided evidence of the impact that indirect facilitation may have on diversity–ecosystem functioning relationships. In this laboratory study, we examined the effect of zooplankton species diversity on trophic structure (total algal and zooplankton biomass) and temporal stability of total zooplankton biomass. We utilized four species of pond zooplankton grown in either monoculture or in polyculture. When comparing responses in polycultures with responses averaged across monocultures, a positive effect of diversity on total zooplankton biomass was observed. This occurred as a result of positive facilitative effects among competing zooplankton. Daphnia pulex, a biomass dominant in monoculture, was negatively affected by the presence of interspecific competitors. In contrast, Diaphanosoma brachyurum, a species that performed poorly in monoculture, was strongly and positively affected by the presence of interspecific competitors, driving positive diversity effects on total zooplankton biomass. Positive temporal covariances among zooplankton were detected in several polyculture replicates, increasing temporal variability of total zooplankton biomass. However, this destabilizing effect was weak relative to effects of high biomass yields in polyculture which caused temporal biomass variability (as measured by the coefficient of variation) to be lower in polyculture relative to monocultures. Zooplankton diversity effects on total algal biomass were not detected. However, increased zooplankton diversity significantly altered the size structure of algae, increasing the relative abundance of large, grazer-resistant algae.
This paper concludes a collection of contributions presented at the 8th Workshop of the International Association of Phytoplankton Taxonomy and Ecology. It derives a consensus as to the virtues and strengths of J. H. Connell's Intermediate Disturbance Hypothesis (IDH), its applicability to phytoplankton ecology and its theoretical and practical weaknesses. The view is expressed that the IDH is too useful a concept to reject and that, as a word model, it provides a powerful link between diversity and disturbance. The more robust investigations that are necessary to consolidate the tenancy of IDH need to concentrate upon the separation and quantification of the stimulus- and response-components of disturbance.
In freshwater ecosystems production of both pelagic and benthic microalgae tends to be limited by phosphorus and light. However, the availability of these resources to pelagic and benthic communities differs due to differences in habitat structure. In a well mixed epilimnion individual phytoplankton cells should receive similar light intensities and nutrient concentrations per unit time. Benthic microalgae colonize substrates forming three-dimensional mats. Their access to nutrients and light is partially determined by the vertical architecture of the mat and often restricted to the canopy layer algae. In addition, algal mats will always be shaded by the phytoplankton inhabiting the water column above.
We investigated competition between pelagic and benthic freshwater microalgae in a three factorial experiment, in which we manipulated phosphorus concentration, light intensity and the source of the inoculum (pelagic, benthic and benthic-pelagic). We found significant and interacting effects of phosphorus, light and inoculum on algal biovolume, chlorophyll-a concentration and cellular phosphorus content. When cultured separately, both benthic and pelagic microalgae benefited from the enhanced supply of phosphorus and light (higher biovolume, Chl-a). In combined culture, pelagic microalgae benefited from the increased light supply and became dominant irrespective of whether high or low phosphorus concentrations were added. In contrast, benthic microalgae profited from low light intensity combined with high nutrient concentrations. Cellular phosphorus contents in benthic and in mixed cultures were highest under low light supply, whereas in pelagic cultures no signifi- cant differences in cellular phosphorus content were detected.
Ecology Letters (2012) 15: 492–501
Feedbacks between ecological and evolutionary change may play important roles in community and ecosystem functioning, but a complete eco-evolutionary feedback loop has not been demonstrated at the community level, and we know little about molecular mechanisms underlying this kind of eco-evolutionary dynamics. In predator–prey (rotifer-alga) microcosms, cyclical changes in predator abundance generated fluctuating selection for a heritable prey defence trait, cell clumping. Predator population growth was affected more by prey evolution than by changes in prey abundance, and changes in predator abundance drove further prey evolution, completing the feedback loop. Within a predator–prey cycle, genes up-regulated as clumping declined were down-regulated as clumping increased, and vice-versa. Genes changing most in expression tended to be associated with defence or its cost. Expression patterns of individual genes differed greatly between consecutive cycles (often reversing direction), suggesting that a particular phenotype may be produced by several (perhaps many) different gene transcription pathways.
Microalgal biovolume is commonly calculated to assess the relative abundance (as biomass or carbon) of co-occurring algae varying in shape and/or size. However, a standardized set of equations for biovolume calculations from microscopically measured linear dimensions that includes the entire range of microalgal shapes is not available yet. In comparison with automated methods, the use of microscopical measurements allows high taxonomic resolution, up to the species level, and has fewer sources of error. We present a set of geometric shapes and mathematical equations for calculating biovolumes of >850 pelagic and benthic marine and freshwater microalgal genera. The equations are designed to minimize the effort of microscopic measurement. The similarities and differences between our proposal for standardization and previously published proposals are discussed and recommendations for quality standards given.
Humans are altering the composition of biological communities through a variety of activities that increase rates of species invasions and species extinctions, at all scales, from local to global. These changes in components of the Earth's biodiversity cause concern for ethical and aesthetic reasons, but they also have a strong potential to alter ecosystem properties and the goods and services they provide to humanity. Ecological experiments, observations, and theoretical developments show that ecosystem properties depend greatly on biodiversity in terms of the functional characteristics of organisms present in the ecosystem and the distribution and abundance of those organisms over space and time. Species effects act in concert with the effects of climate, resource availability, and disturbance regimes in influencing ecosystem properties. Human activities can modify all of the above factors; here we focus on modification of these biotic controls. The scientific community has come to a broad consensus on many aspects of the relationship between biodiversity and ecosystem functioning, including many points relevant to management of ecosystems. Further progress will require integration of knowledge about biotic and abiotic controls on ecosystem properties, how ecological communities are structured, and the forces driving species extinctions and invasions. To strengthen links to policy and management, we also need to integrate our ecological knowledge with understanding of the social and economic constraints of potential management practices. Understanding this complexity, while taking strong steps to minimize current losses of species, is necessary for responsible management of Earth's ecosystems and the diverse biota they contain. Based on our review of the scientific literature, we are certain of the following conclusions: 1)Species' functional characteristics strongly influence ecosystem properties. Functional characteristics operate in a variety of contexts, including effects of dominant species, keystone species, ecological engineers, and interactions among species (e.g., competition, facilitation, mutualism, disease, and predation). Relative abundance alone is not always a good predictor of the ecosystem-level importance of a species, as even relatively rare species (e.g., a keystone predator) can strongly influence pathways of energy and material flows. 2)Alteration of biota in ecosystems via species invasions and extinctions caused by human activities has altered ecosystem goods and services in many well-documented cases. Many of these changes are difficult, expensive, or impossible to reverse or fix with technological solutions. 3)The effects of species loss or changes in composition, and the mechanisms by which the effects manifest themselves, can differ among ecosystem properties, ecosystem types, and pathways of potential community change. 4)Some ecosystem properties are initially insensitive to species loss because (a) ecosystems may have multiple species that carry out similar functional roles, (b) some species may contribute relatively little to ecosystem properties, or (c) properties may be primarily controlled by abiotic environmental conditions. 5)More species are needed to insure a stable supply of ecosystem goods and services as spatial and temporal variability increases, which typically occurs as longer time periods and larger areas are considered. We have high confidence in the following conclusions: 1)Certain combinations of species are complementary in their patterns of resource use and can increase average rates of productivity and nutrient retention. At the same time, environmental conditions can influence the importance of complementarity in structuring communities. Identification of which and how many species act in a complementary way in complex communities is just beginning. 2)Susceptibility to invasion by exotic species is strongly influenced by species composition and, under similar environmental conditions, generally decreases with increasing species richness. However, several other factors, such as propagule pressure, disturbance regime, and resource availability also strongly influence invasion success and often override effects of species richness in comparisons across different sites or ecosystems. 3)Having a range of species that respond differently to different environmental perturbations can stabilize ecosystem process rates in response to disturbances and variation in abiotic conditions. Using practices that maintain a diversity of organisms of different functional effect and functional response types will help preserve a range of management options. Uncertainties remain and further research is necessary in the following areas: 1)Further resolution of the relationships among taxonomic diversity, functional diversity, and community structure is important for identifying mechanisms of biodiversity effects. 2)Multiple trophic levels are common to ecosystems but have been understudied in biodiversity/ecosystem functioning research. The response of ecosystem properties to varying composition and diversity of consumer organisms is much more complex than responses seen in experiments that vary only the diversity of primary producers. 3)Theoretical work on stability has outpaced experimental work, especially field research. We need long-term experiments to be able to assess temporal stability, as well as experimental perturbations to assess response to and recovery from a variety of disturbances. Design and analysis of such experiments must account for several factors that covary with species diversity. 4)Because biodiversity both responds to and influences ecosystem properties, understanding the feedbacks involved is necessary to integrate results from experimental communities with patterns seen at broader scales. Likely patterns of extinction and invasion need to be linked to different drivers of global change, the forces that structure communities, and controls on ecosystem properties for the development of effective management and conservation strategies. 5)This paper focuses primarily on terrestrial systems, with some coverage of freshwater systems, because that is where most empirical and theoretical study has focused. While the fundamental principles described here should apply to marine systems, further study of that realm is necessary. Despite some uncertainties about the mechanisms and circumstances under which diversity influences ecosystem properties, incorporating diversity effects into policy and management is essential, especially in making decisions involving large temporal and spatial scales. Sacrificing those aspects of ecosystems that are difficult or impossible to reconstruct, such as diversity, simply because we are not yet certain about the extent and mechanisms by which they affect ecosystem properties, will restrict future management options even further. It is incumbent upon ecologists to communicate this need, and the values that can derive from such a perspective, to those charged with economic and policy decision-making.
Neglecting the naturally existing functional diversity of communities and the resulting potential to respond to altered conditions may strongly reduce the realism and predictive power of ecological models. We therefore propose and study a predator-prey model that describes mutual feedback via species shifts in both predator and prey, using a dynamic trait approach. Species compositions of the two trophic levels were described by mean functional traits--prey edibility and predator food-selectivity--and functional diversities by the variances. Altered edibility triggered shifts in food-selectivity so that consumers continuously respond to the present prey composition, and vice versa. This trait-mediated feedback mechanism resulted in a complex dynamic behavior with ongoing oscillations in the mean trait values, reflecting continuous reorganization of the trophic levels. The feedback was only possible if sufficient functional diversity was present in both trophic levels. Functional diversity was internally maintained on the prey level as no niche existed in our system, which was ideal under any composition of the predator level due to the trade-offs between edibility, growth and carrying capacity. The predators were only subject to one trade-off between food-selectivity and grazing ability and in the absence of immigration, one predator type became abundant, i.e., functional diversity declined to zero. In the lack of functional diversity the system showed the same dynamics as conventional models of predator-prey interactions ignoring the potential for shifts in species composition. This way, our study identified the crucial role of trade-offs and their shape in physiological and ecological traits for preserving diversity.
Over the past several decades, a rapidly expanding field of research known as biodiversity and ecosystem functioning has begun to quantify how the world's biological diversity can, as an independent variable, control ecological processes that are both essential for, and fundamental to, the functioning of ecosystems. Research in this area has often been justified on grounds that (1) loss of biological diversity ranks among the most pronounced changes to the global environment and that (2) reductions in diversity, and corresponding changes in species composition, could alter important services that ecosystems provide to humanity (e.g., food production, pest/disease control, water purification). Here we review over two decades of experiments that have examined how species richness of primary producers influences the suite of ecological processes that are controlled by plants and algae in terrestrial, marine, and freshwater ecosystems. Using formal meta-analyses, we assess the balance of evidence for eight fundamental questions and corresponding hypotheses about the functional role of producer diversity in ecosystems. These include questions about how primary producer diversity influences the efficiency of resource use and biomass production in ecosystems, how primary producer diversity influences the transfer and recycling of biomass to other trophic groups in a food web, and the number of species and spatial /temporal scales at which diversity effects are most apparent. After summarizing the balance of evidence and stating our own confidence in the conclusions, we outline several new questions that must now be addressed if this field is going to evolve into a predictive science that can help conserve and manage ecological processes in ecosystems.
Ecology Letters (2010) 13: 989–997
Abstract
Adaptive variation in the traits determining ecological interactions can lead to evolution so rapid that ecological dynamics change course while in progress (i.e., ‘eco‐evolutionary dynamics’). However, little is known about how the qualitative properties of eco‐evolutionary dynamics (e.g., cycling, equilibrium, etc.) are affected by the amount of heritable variation present. Here, we show that a change in the range of variation in a heritable prey defense trait determines what dynamics are observed in an experimental predator–prey system. We combine modelling and laboratory experiments to show that initial defense trait variation determines whether populations exhibit eco‐evolutionary cycles in which heritable variation is maintained, or converge to an equilibrium at which the prey population becomes monomorphic. Our results show how small changes in the amount of adaptive genetic variance initially present can radically alter eco‐evolutionary dynamics, and can ultimately determine whether heritable variation is maintained or lost.
Biodiversity in freshwater food webs can be considered in terms of the number of species occupying unique trophic positions or the number of species within a trophic position. Either can affect energy flow or biomass partitioning, and the two aspects of diversity may feed back on one another. This chapter analyses the importance of biodiversity in aquatic food webs on three different levels. First, it asks how varying diversity within trophic groups affects the outcome of trophic interactions. It then presents a conceptual framework and contrasts this model against experimental manipulations of consumer or prey diversity. Second, it asks how consumers and resources affect biodiversity within trophic levels. This question has a long history of experimental and modeling studies, and these are reviewed. The role of biodiversity at local and regional spatial scales for freshwater food webs is examined. Dispersal among local habitats can constrain local species diversity and influence the outcome of food web interactions. The interactive effects of the regional pool versus resident diversity on the outcome of trophic interactions are discussed.
A model is developed to predict evolution in a population which contains a variety of ecologically specialized phenotypes. Individuals of each phenotype are assumed to specialize on a specific region of a resource axis present in the environment. The model, incorporating density-dependent effects on the fitness of the various phenotypes, predicts the number of individuals of each phenotype through time for both asexually and sexually reproducing populations. The lizard species, Anolis roquet, illustrates the kind of population treated by the model. The resource axis is prey size; small lizards preferentially utilize small prey, while large lizards exploit large prey. Field data on this species are used to estimate some of the parameters of the model. The model shows that there is an optimum number of individuals of each phenotype for a given set of resources and a given regime of interphenotypic competition. If the population has this optimum distribution of individuals, then the fitnesses of all phenotyp...
Recent experimental evidence indicates the importance in some pelagic systems of mixotrophic protists that combine photosynthetic ability with the ability to ingest bacteria. If both bacteria and phytoplankton are mineral nutrient limited, this should provide the mixotrophs with the double benefit of combining removal of their competitor with ingestion of the limiting nutrient in pelleted form. It is the objective of this study to expand the classical theories of competition and predation to explore the effect on the microbial food web of one trophic group possessing both strategies. In a chemostat scenario, we analyzed the two-species situation of a mixotroph preying on mineral-nutrient-limited bacteria, and also the situations when the mixotroph in addition has to compete with specialized photoautotrophic and phagotrophic protists, each superior to the mixotroph in their specialized nutritional modes. In the mixotroph-bacteria relationship, somewhat paradoxically, high predatory abilities will reduce the quantitative importance of predation in the mixotroph's nutrition. The reason is a strong reduction in prey abundance, allowing the mixotroph to survive as a photoautotroph despite its low competitive ability. In the three-species case with mixotrophs, bacteria, and specialized phagotrophs, it is shown that the mixotroph can compensate for a "price" paid in reduced affinity for bacterial prey by a sufficiently high affinity for mineral nutrients. In the other three-species case where the mixotroph has to compete with a specialized photoautotroph, the situation is more complex; there is an optimum value for the mixotroph's predatory ability at which mixotroph biomass is maximized. In the general situation with all four species (bacteria, mixotrophs, and specialized auto- and phagotrophs) potentially present, different mixotrophic strategies will alter the equilibrium composition of the consortium, with the mixotroph being most successful with a high affinity for nutrients and an intermediate affinity for bacteria. In the simple form used here, the model predicts no equilibrium with all four species simultaneously present. The theory is in principle directly applicable to laboratory experimentation.
Mixotrophy is a widespread phenomenon and plays an important role in aquatic food
webs by increasing the complexity of trophic interactions and enhancing trophic transfer up the
food web. Nevertheless, the effects of mixotrophic consumers on lower trophic levels have not
been investigated in the framework of recent biodiversity and ecosystem functioning research,
although mixotrophic interactions can lead to enhanced variability of those relationships. The
effects of ciliate consumer diversity and nutritional mode on algal prey biovolume and evenness
as well as consumer biovolume were investigated in 2 freshwater microcosm experiments. Different
species compositions in the consumer and algal assemblages of the 2 experiments resulted in
partially different interactions. Increasing consumer richness decreased prey biovolume in both
experiments. However, heterotrophic consumers decreased prey biovolume and evenness more
than mixotrophs in one experiment, driven by a strong grazer that was not present in the other
experiment. Consumer presence and richness affected prey evenness differently, preventing competitive
dominance among algae in one experiment but leading to unbalanced grazing in an initially
more even prey assemblage in the other experiment. Secondary production increased with
consumer richness in both experiments but was differently affected by the consumers’ nutritional
mode due to altered competitive interactions and niche overlap in different species combinations.
The present study demonstrated that consumer-specific characteristics, such as nutritional mode
and feeding preferences, may alter consumer diversity effects on prey.
1. The ciliate Euplotes octocarinatus responds to the presence of Stylonychia mytilus with a morphological transformation rendering its ingestion more difficult. Predation on Euplotes is reduced when an alternative prey, the flagellate Chlorogonium elongatum, is available. Lower predation is accompanied by reduced induction in Euplotes.
2. We quantified the motility of both ciliates in the presence and absence of Chlorogonium to test the hypothesis that the differential response in Euplotes is a consequence of behavioural changes affecting the encounter rate of the prey and the predator.
3. The results indicate that S. mytilus uses different feeding strategies for each prey. It is a filter feeder when small food particles (Chlorogonium) are abundant, however, it becomes a raptorial feeder when large prey with an escape capability (Euplotes) is available and small alternative prey are absent.
4. As the mobility of filter feeding Stylonychia is strongly limited, the presence of the flagellate may indirectly affect the defence level in Euplotes by reducing the frequency of contacts with the predator.
5. An experiment with Chaetogaster diastrophus, a more specialised predator not affected by the presence of Chlorogonium, as well as direct manipulation of the encounter rate by changing the surface area available for the crawling ciliates, supported the encounter rate hypothesis.
Summary • Inducible trophic polymorphisms are greatly underappreciated forms of phenotypic plasticity that allow organisms to respond dynamically to the environmental variation by enabling them to change the trophic level upon which they feed. Although inducible trophic polymorphisms occur in a diverse array of organisms, their costs, benefits and their consequences for long-term population and community dynamics are poorly understood. • We studied the inducible trophic polymorphism of the freshwater hymenostome ciliate Tetrahymena vorax, whose isogenic populations can contain three distinct morphs: pyriform, bacterivorous microstomes; larger, carnivorous macrostomes; and elongate, ‘tailed’ microstomes. We tested whether (i) the tailed microstome constitutes an inducible defence against macrostomes and (ii) the transformation of microstomes into macrostomes is size-dependent. We also describe the dynamics of the three morphs in the presence and absence of an intraguild prey (Colpidium) across a gradient of growth medium concentrations to infer potential trade-offs in the success of different morphs at different productivity levels. • Macrostomes do not discriminate between pyriform microstomes and readily consumed heterospecific prey (Colpidium). Tailed microstomes display greatly reduced susceptibility to consumption by macrostomes as compared with undefended, pyriform microstomes. Morph dynamics are consistent with the hypothesis that tailed microstomes function as an inducible defence against cannibalism; tailed microstomes and macrostomes appear simultaneously, in both the presence and absence of Colpidium. At low productivity, T. vorax achieves higher rates of growth when feeding on Colpidium instead of on bacteria. At higher productivity, this pattern is reversed, with growth rates maximized in the absence of Colpidium. • The reduced consumption rate of tailed microstomes by cannibalistic macrostomes, together with the simultaneous induction of tailed microstomes and macrostomes, suggests that both morphs comprise a coordinated adaptive response to the presence of intraguild prey.
High plant species richness can enhance primary production, animal diversity, and invasion resistance. Yet theory predicts that plant and herbivore diversity, which often covary in nature, should have countervailing effects on ecosystem properties. Supporting this, we show in a seagrass system that increasing grazer diversity reduced both algal biomass and total community diversity, and facilitated dominance of a grazer-resistant invertebrate. In parallel with previous plant results, however, grazer diversity enhanced secondary production, a critical determinant of fish yield. Although sampling explained some diversity effects, only the most diverse grazer assemblage maximized multiple ecosystem properties simultaneously, producing a distinct ecosystem state. Importantly, ecosystem responses at high grazer diversity often differed in magnitude and sign from those predicted from summed impacts of individual species. Thus, complex interactions, often opposing plant diversity effects, arose as emergent consequences of changing consumer diversity, advising caution in extrapolating conclusions from plant diversity experiments to food webs.
The effect of species diversity on ecosystem productivity is controversial, in large part because field experiments investigating this relationship have been fraught with difficulties. Unfortunately, there are few guidelines to aid researchers who must overcome these difficulties and determine whether global species losses seriously threaten the ecological and economic bases of terrestrial ecosystems. In response, I offer a set of hypotheses that describe how diversity might influence productivity in plant communities based on three well-known mechanisms: complementarity, facilitation, and the sampling effect. Emphasis on these mechanisms reveals the sensitivity of any diversity-productivity relationship to ecological context (i.e., where this relationship should be found); ecological context includes characteristics of the surrounding environment, temporal and spatial scales of observation, and the intensity of human management. In particular, the legitimacy of the sampling effect as a mechanism of productivity enhancement is dependent upon the degree to which stochastic events influence immigration and extinction processes in a given ecosystem. A mechanistic approach also requires that the three mechanisms be separated and quantified in diversity experiments, and I examine the most appropriate analyses for doing so, focusing on the overyielding technique. Finally, I question why productivity per se is a relevant management concern in non-agricultural systems once relationships among diversity, productivity, and the qualities of the surrounding environment are considered.
Increasing species richness of primary producers or consumers is proposed to increase primary and secondary production; however, the consequences of biodiversity change across trophic levels has been poorly investigated. We used a controlled marine microbial system to investigate the effects of simultaneous changes in biodiversity of consumer and prey species. Consumer (ciliates) and prey (algae) richness and identity were manipulated independently in a complete factorial design. The results showed clear biodiversity effects of both consumers and prey, within and across trophic levels. We found reduced prey and increased consumer biomass with increased consumer richness, with the most diverse prey assemblage supporting the highest biomass of consumers at the highest richness of consumers. Increasing prey richness did not increase resistance to consumption when consumers were present. Instead, our results indicated enhanced energy transfer with simultaneous increasing richness of consumers and prey.
Starting with the publication of some influential studies in the early 1990's, the topic of biodiversity and ecosystem functioning has emerged as a major field within ecological research. Within this framework, the diversity of genotypes, species and functional groups are considered as explanatory variables of ecosystem functions rather than response variables of factors such as productivity and disturbance. Biodiversity‐ecosystem functioning research has received considerable attention, and new publications are emerging at a high pace. Both the validity of experimental approaches and the way the results may be extrapolated to natural systems have, however, been widely discussed. The width of the debate regarding whether or not biodiversity is important for ecosystem functioning have encouraged many scientists to refine both experiments and theory, as well as develop novel methods to analyse the relationship between diversity and functioning. Aquatic ecologists have contributed greatly to the evolution of ideas and concepts within the field. In this review, we discuss how the paradigm that biodiversity is an important factor for the functioning of aquatic ecosystems is currently maturing with more realistic studies embracing both new and innovative approaches. We also suggest fruitful areas for future research.
Each of us is trapped in a place, a time, and a circumstance, and our attempts to use our minds to transcend those boundaries are, more often than not, ineffective .
From the book “Stumbling on happiness” by Daniel Gilbert (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The loss of a predator from an ecological community can cause large changes in community structure and ecosystem
processes, or have very little consequence for the remaining species and ecosystem. Understanding when and why the loss of a predator causes large changes in community structure and ecosystem processes is critical for understanding the functional consequences of biodiversity loss. We used experimental microbial communities to investigate how the removal of a large generalist predator affected the extinction frequency, population abundance and total biomass of its prey. We removed this predator in the presence or absence of an alternative, more specialist, predator in order to determine whether the specialist predator affected the outcome of the initial species removal. Removal of the large generalist predator altered some species’ populations but many were unaffected and no secondary extinctions were observed. The specialist predator, though rare, altered the response of the prey community to the removal of the large generalist predator. In the absence of the specialist predator, the effects of the removal were only measurable at the level of individual species. However, when the specialist predator was present, the removal of the large generalist predator affected the total biomass of prey species. The results demonstrate that the effect of species loss from high trophic levels may be very context-dependent, as rare species can have disproportionately large effects in food webs.
Various methods for the estimation of populations of algae and other small freshwater organisms are described. A method of counting is described in detail. It is basically that of Utermhl and uses an inverted microscope.If the organisms are randomly distributed, a single count is sufficient to obtain an estimate of their abundance and confidence limits for this estimate, even if pipetting, dilution or concentration are involved.The errors in the actual counting and in converting colony counts to cell numbers are considered and found to be small relative to the random sampling error.Data are also given for a variant of Utermhl''s method using a normal microscope and for a method of using a haemocytometer for the larger plankton algae.
Diversity within distinct trophic groups is proposed to increase ecosystem functions such as the productivity of this group
and the efficiency of resource use. This proposition has mainly been tested with plant communities, consumer assemblages,
and multitrophic microbial assemblages. Very few studies tested how this diversity–productivity relationship varies under
different environmental regimes such as disturbances. Coastal benthic assemblages are strongly affected by temporal instability
of abiotic conditions. Therefore, we manipulated benthic ciliate species richness in three laboratory experiments with three
diversity levels each and analyzed biomass production over time in the presence or absence of a single application of a disturbance
(ultraviolet-B [UVB] radiation). In two out of three experiments, a clear positive relationship between diversity and productivity
was found, and also the remaining experiment showed a small but nonsignificant effect of diversity. Disturbance significantly
reduced the total ciliate biomass, but did not alter the relation between species richness and biomass production. Significant
overyielding (i.e., higher production at high diversity) was observed, and additive partitioning indicated that this was caused
by niche complementarity between ciliate species. Species-specific contribution to the total biomass varied idiosyncratically
with species richness, disturbance, and composition of the community. We thus present evidence for a significant effect of
consumer diversity on consumer biomass in a coastal ciliate assemblage, which remained consistent at different disturbance
regimes.
Theory predicts that productivity and the relative efficiency of resource utilization determine the outcome of interactions between intraguild predators and prey. Interactions between two freshwater protists, Colpidium striatum (the intraguild prey) and Blepharisma americanum (the intraguild predator), in laboratory microcosms support the predictions of intraguild predation theory. Colpidium competitively excluded Blepharisma when bacterial production was low, even though Blepharisma had a potential advantage conferred by its ability to consume Colpidium. In contrast to competitive exclusion at low productivity, Colpidium and Blepharisma coexisted for many generations at higher levels of bacterial production. Theory predicts a similar transition from competitive exclusion to coexistence as productivity increases, if intraguild predators use resources less efficiently than intraguild prey/competitors. Other experiments showed that Colpidium depresses bacterial densities more than Blepharisma, a result consistent with greater efficiency of resource utilization by Colpidium. These results indicate that simple mathematical models can provide important insights into the population dynamics of intraguild predators and prey under different levels of productivity.
In 1973 C.S.Holling introduced the word "resilience" into the ecological literature as a way of helping to understand the non-linear dynamics observed in ecosystems. Ecological resilience was defined as the amount of disturbance that an ecosystem could withstand without changing self-organised processes and structures(defined as alternative stable states). Other authors conside resilience as a return time to a stable state following a perturbation. A new term, adaptive capacity, is introduced to describe the process that modify ecological resilience. Two definitions recognise the presence of multiple stable states(or stability domains), and hence resilience is the property that meiates transitions between these states. Transitions among stable states have been described for many ecosystems, including semi-arid rangelands, lakes, coral reefs and forests. In these systems, ecological resilience is maintained by keystonestructuring processes across a number of scales, sources of renewal and reformation, and functional biodiversity. In practice, maintaining a capacity for renewal in a dynamic environment provides an ecological buffer that protects the system from failure of management actions that are taken based upon incomplete understanding, and it allows managers to affordably learn and change.
Environmental factors regulate biodiversity through species sorting processes. Species distributions in communities affect ecosystem processes and environmental factors. These dynamics are determined by the properties (traits) of species in the community. The optimal temperatures for growth, the minimal amount of resource that sustains positive mass balance, and the amount of energy allocated to predator defenses are examples of such traits. Over time, the trait distributions in communities may change in response to environmental changes, which, in turn, changes the processes and consequently the structure of the system. The result of such processes is the focus of complex adaptive systems (CAS) theory. This paper gives an overview of how CAS theory can contribute to understanding the role of biodiversity on the ability of functional groups that make up the ecosystem to change their species compositions in response to changes in the environment. Any trait that requires investment of energy, mass, or time is subjected to a tradeoff for alternative use of this resource. Such interspecies tradeoff relationships can be used to make predictions about past environmental conditions, as well as the response of the properties of a group of species, e.g., total productivity and species distributions, to future changes in the environment. The trait-based framework presented here makes explicit predictions regarding the relation between the environment, trait distributions, and ecosystem processes. Trait variance, a measure of the width of the distribution of traits in the community, is proportional to the rate at which species within functional groups can replace each other in response to environmental changes. This adaptive capacity is crucial for the ecosystem's ability to maintain certain processes under times of change. Examples of empirical tradeoffs are given as well as how to formalize them to use in the CAS framework.
Inducible defenses of prey and inducible offenses of predators are drastic phenotypic changes activated by the interaction between a prey and predator. Inducible defenses occur in many taxa and occur more frequently than inducible offenses. Recent empirical studies have reported reciprocal phenotypic changes in both predator and prey. Here, we model the coevolution of inducible plasticity in both prey and predator, and examine how the evolutionary dynamics of inducible plasticity affect the population dynamics of a predator-prey system. Under a broad range of parameter values, the proportion of predators with an offensive phenotype is smaller than the proportion of prey with a defensive phenotype, and the offense level is relatively lower than the defense level at evolutionary end points. Our model also predicts that inducible plasticity evolves in both species when predation success depends sensitively on the difference in the inducible trait value between the two species. Reciprocal phenotypic plasticity may be widespread in nature but may have been overlooked by field studies because offensive phenotypes are rare and inconspicuous.
Studies of whether plant community structure and ecosystem properties depend on the genetic diversity of component populations have been largely restricted to species monocultures and have involved levels of genetic differentiation that do not necessarily correspond to that exhibited by neighboring mature individuals in the field. We established experimental communities of varying intraspecific genetic diversity, using genotypes of eight species propagated from clonal material of individuals derived from a small (100-m2) limestone grassland community, and tested whether genetic diversity (one, four, and eight genotypes per species) influenced community composition and annual aboveground productivity across communities of one, four, and eight species. Eight-species communities were represented by common grass, sedge, and forb species, and four- and one-species communities were represented by four graminoids and the dominant grass Festuca ovina, respectively. After three years of community development, there was a marginal increase of species diversity with increased genetic diversity in four- and eight-species communities, and genetic diversity altered the performance of genotypes in monospecific communities of F. ovina. However, shifts in composition from genetic diversity were not sufficient to alter patterns of community productivity. Neighborhood models describing pairwise interactions between species indicated that genetic diversity decreased the intensity of competition between species in four-species mixtures, thereby promoting competitive equivalency and enhancing species equitability. In F. ovina monocultures, neighborhood models revealed both synergistic and antagonistic interactions between genotypes that were reduced in intensity on more stressful shallow soils. Although the dependence of F. ovina genotype performance on neighborhood genetic composition did not influence total productivity, such dependence was sufficient to uncouple genotype performance in genetic mixtures and monocultures. Our results point to an important connection between local genetic diversity and species diversity in this species-rich ecosystem but suggest that such community-level dependence on genetic diversity may not feedback to ecosystem productivity.
The individual functional traits of different species play a key role for ecosystem function in aquatic and terrestrial systems. We modeled a multispecies predator-prey system with functionally different predator and prey species based on observations of the community dynamics of ciliates and their algal prey in Lake Constance. The model accounted for differences in predator feeding preferences and prey susceptibility to predation, and for the respective trade-offs. A low food demand of the predator was connected to a high food selectivity, and a high growth rate of the prey was connected to a high vulnerability to grazing. The data and the model did not show standard uniform predator-prey cycles, but revealed both complex dynamics and a coexistence of predator and prey at high biomass levels. These dynamics resulted from internally driven alternations in species densities and involved compensatory dynamics between functionally different species. Functional diversity allowed for ongoing adaptation of the predator and prey communities to changing environmental conditions such as food composition and grazing pressure. The trade-offs determined whether compensatory or synchronous dynamics occurred which influence the variability at the community level. Compensatory dynamics were promoted by a joint carrying capacity linking the different prey species which is particularly relevant at high prey biomasses, i.e., when grazers are less efficient. In contrast, synchronization was enhanced by the coupling of the different predator and prey species via common feeding links, e.g., by a high grazing pressure of a nonselective predator. The communities had to be functionally diverse in terms of their trade-offs and their traits to yield compensatory dynamics. Rather similar predator species tended to cycle synchronously, whereas profoundly different species did not coexist. Compensatory dynamics at the community level thus required intermediately strong tradeoffs for functional traits in both predators and their prey.
We describe one of the first examples of reciprocal phenotypic plasticity in a predator-prey system: the interaction between an inducible defence and an inducible offence. When confronted with the predatory ciliate Lembadion bullinum, the hypotrichous ciliate Euplotes octocarinatus develops protective lateral wings, which inhibit ingestion by the predator. We show that L. bullinum reacts to this inducible defence by expressing an inducible offence - a plastic increase in cell size and gape size. This counteraction reduced the effect of the defence, but did not completely neutralize it. Therefore, the defence remained beneficial for E. octocarinatus. From L. bullinuds point of view, the increase in feeding rate because of the offence was not larger than the increase in mean cell volume and apparently, did not increase the predator's fitness. Therefore, the inducible offence of L. bullinum does not seem to be an effective counter-adaptation to the inducible defence of E. ortocarinatus.
Recent studies show that intraspecific genetic variation in asexual species may have large effects on community and ecosystem functions, increasing their stability, productivity, and species richness. However, major questions regarding its population-level impact remain empirically unanswered: (a) How does intraspecific genetic diversity affect the ecological characteristics of sexual species, in which recombination can alter the outcome of causal mechanisms such as selection and niche diversification? (b) Does genetic diversity increase population dynamic stability? (c) Is the impact of genetic diversity dependent on the selective environment? To answer these questions, I founded replicate flour beetle (Tribolium castaneum) populations with different degrees of ecologically relevant, heritable trait variation and monitored their dynamics for approximately eight generations. I show that population stability and persistence increased with greater genetic variation but that the stabilizing effect was independent of the selective habitat (different proportions of ancestral and novel resources). Alleles from a single founding strain underwent a selective sweep in the homogeneous ancestral habitat but not in a novel heterogeneous habitat. These results expand current understanding of the ecological impacts of genetic diversity by showing that genetically more diverse sexual populations persist longer and are more stable but that the selective environment determines the mechanistic basis of increased stability.