Search

Human-induced salinization caused by the use of road deicing salts, agricultural practices, mining operations, and climate change is a major threat to the biodiversity and functioning of freshwater ecosystems. Yet, it is unclear if freshwater ecosystems are protected from salinization by current water quality guidelines. Leveraging an experimental network of land-based and in-lake mesocosms across North America and Europe, we tested how salinization—indicated as elevated chloride (Cl−) concentration—will affect lake food webs and if two of the lowest Cl− thresholds found globally are sufficient to protect these food webs. Our results indicated that salinization will cause substantial zooplankton mortality at the lowest Cl− thresholds established in Canada (120 mg Cl−/L) and the United States (230 mg Cl−/L) and throughout Europe where Cl− thresholds are generally higher. For instance, at 73% of our study sites, Cl− concentrations that caused a ≥50% reduction in cladoceran abundance were at or below Cl− thresholds in Canada, in the United States, and throughout Europe. Similar trends occurred for copepod and rotifer zooplankton. The loss of zooplankton triggered a cascading effect causing an increase in phytoplankton biomass at 47% of study sites. Such changes in lake food webs could alter nutrient cycling and water clarity and trigger declines in fish production. Current Cl− thresholds across North America and Europe clearly do not adequately protect lake food webs. Water quality guidelines should be developed where they do not exist, and there is an urgent need to reassess existing guidelines to protect lake ecosystems from human-induced salinization. This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2115033119/-/DCSupplemental

There is increasing recognition of the importance of rapid adaptation in the dynamics of populations and communities. While the effects of rapid adaptation on the stability of populations h...

Recent studies suggest the necessity of understanding the interactive effects of predation and productivity on species coexistence and prey diversity. Models predict that coexistence of prey species with different competitive abilities can be achieved if inferior resource competitors are less susceptible to predation and if productivity and/or predation pressure are at intermediate levels. Hence, predator effects on prey diversity are predicted to be highly context dependent: enhancing diversity from low to intermediate levels of productivity or predation and reducing diversity of prey at high levels of productivity or predation. While several studies have examined the interactive effects of herbivory and productivity on primary producer diversity, experimental studies of such effects in predator-prey systems are rare. We tested these predictions using an aquatic field mesocosm experiment in which initial density of the zooplankton predator Notonecta undulata and productivity were manipulated to test their interactive effects on diversity of seven zooplankton, cladoceran species that were common in surrounding ponds. Two productivity levels were imposed via phosphorus enrichment at levels comparable to low and intermediate levels found within neighboring natural ponds. We used open systems to allow for natural dispersal and behaviorally-mediated numerical responses by the flight-capable predator. Effects of predators on zooplankton diversity depended on productivity level. At low and high productivity, prey species richness declined while at high productivity it showed a unimodal relationship with increasing the predator density. Effects of treatments were weaker when using Pielou's evenness index or the inverse Simpson index as measures of prey diversity. Our findings are generally consistent with model predictions in which predators can facilitate prey coexistence and diversity at intermediate levels of productivity and predation intensity. Our work also shows that the functional form of the relationship between prey diversity and predation intensity can be complex and highly dependent on environmental context.

There is increasing recognition of the importance of rapid adaptation in the dynamics of populations and communities. While the effects of rapid adaptation on the stability of populations have been shown in experimental systems, demonstration of their impacts in natural populations are rare. We examined the relationship between clonal dynamics and population stability of natural

Nutrient pollution constitutes a major threat to biodiversity, one whose magnitude is predicted to increase greatly in the near future. While the negative effects of excessive nitrogen and phosphorus loading on local species diversity are widely appreciated, a growing body of evidence indicates that increases in productivity (a correlate of nutrient supply) can also reduce predictability in community composition by driving community divergence and increases in beta diversity (or compositional dissimilarity among communities). Stochastic variation in dispersal history has frequently been cited as a cause of such patterns. However, underlying mechanisms have not received strong experimental scrutiny. I present results of a microcosm experiment testing the effects of enrichment and dispersal mode on zooplankton community structure. I show that beta diversity increases with enrichment but only in the presence of sequential dispersal and variation in dispersal history. Sequential dispersal and enrichment enhan...

A general prediction from simple metapopulation models is that spatially synchronized forcing can spatially synchronize population dynamics and destabilize metapopulations. In contrast, spatially asynchronous forcing is predicted to decrease population synchrony and promote temporal stability and population persistence, especially in the presence of dispersal. Only recently have studies begun to experimentally address these predictions. Moreover, few studies have experimentally examined how such processes operate in the context of competition communities. Stabilizing processes may continue to operate when placed within a metacommunity context with multiple competing consumers but only at low to intermediate levels of dispersal. High dispersal rates can reverse these predictions and lead to destabilization. We tested this under controlled conditions using an experimental aquatic system composed of three competing species of zooplankton. Metacommunities experienced different levels of dispersal and environmental forcing in the form of spatially synchronous or asynchronous pH perturbations. We found support that dispersal can have contrasting effects on population stability depending on the degree to which population dynamics were synchronized in space. Dispersal under synchronous forcing or no forcing had either neutral of positive effects on spatial population synchrony of all three zooplankton species. In these treatments, dispersal reduced population stability at the local and metapopulation levels for two of three species. In contrast, asynchronously varying environments reduced population synchrony relative to unforced systems, regardless of dispersal level. In these treatments, dispersal enhanced temporal stability and persistence of populations not by reducing population synchrony but by enhancing population minima and spatial averaging of abundances. High dispersal rates under asynchronous forcing reduced the abundance of one species, consistent with increasing regional competition and general metacommunity theory. However, no effects on its stability or persistence were observed. Our work highlights the context-dependent effects of dispersal on population dynamics in varying environments.

The presence of prey heterogeneity and weakly interacting prey species is frequently viewed as a stabilizer of predator-prey dynamics, countering the destabilizing effects of enrichment and reducing the amplitude of population cycles. However, prior model explorations have largely focused on long-term, dynamic attractors rather than transient dynamics. Recent theoretical work shows that the presence of prey that are defended from predation can have strongly divergent effects on dynamics depending on time scale: prey heterogeneity can counteract the destabilizing effects of enrichment on predator-prey dynamics at long time scales but strongly destabilize systems during transient phases by creating long periods of low predator/prey abundance and increasing extinction probability (an effect that is amplified with increasing enrichment). We tested these general predictions using a planktonic system composed of a zooplankton predator and multiple algal prey. We first parameterized a model of our system to generate predictions and tested these experimentally. Our results qualitatively supported several model predictions. During transient phases, presence of defended algal prey increased predator extinctions at low and high enrichment levels compared to systems with only a single edible prey. This destabilizing effect was moderated at higher dilution rates, as predicted by our model. When examining dynamics beyond initial oscillations, presence of the defended prey increased predator-prey temporal variability at high nutrient enrichment but had no effect at low nutrient levels. Our results highlight the importance of considering transient dynamics when assessing the role of stabilizing factors on the dynamics of food webs.

Understanding the factors that govern the stability of populations and communities has gained increasing importance as habitat fragmentation and environmental perturbations continue to escalate due to human activities. Dispersal is commonly viewed as essential to the maintenance of diversity in spatially subdivided com- munities, but few experiments have explored how dispersal interacts with the spatiotemporal components of environmental perturbations to determine community-level stability. We examined these processes using an experimental planktonic system composed of three com- peting species of zooplankton. We subjected zooplankton metacom- munities to varying levels of dispersal and pH perturbations that varied in their degree of spatial synchrony. We show that dispersal can reverse the destabilizing effects of environmental forcing when perturbations are spatially asynchronous. Asynchrony in pH pertur- bations generated spatially and temporally varying species refugia that promoted source-sink dynamics and allowed prolonged persis- tence of zooplankton species that were otherwise extirpated in syn- chronously varying metacommunities. This, in turn, increased local species diversity, promoted compensatory population dynamics, and enhanced local community-level stability. Our results indicate that patterns of spatial covariation in environmental variability are critical to predicting the effects of dispersal on the dynamics and persistence of communities.

Despite the recognition of the importance of seasonal forcing in nature, remarkably few studies have theoretically explored periodically forced community dynamics. Here we employ a novel approach called "successional state dynamics" (SSD) to model a seasonally forced predator-prey system. We first generated analytical predictions of the effects of altered seasonality on species persistence and the timing of community state transitions. We then parameterized the model using a zooplankton-phytoplankton system and tested quantitative predictions using controlled experiments. In the majority of cases, timing of zooplankton and algal population peaks matched model predictions. Decreases in growing-period length delayed algal blooms, consequently delaying peaks in zooplankton abundance. Predictions of increased probability of predator extinction at low growing-period lengths were also upheld experimentally. Our results highlight the utility of the SSD modeling approach as a framework for predicting the effects of altered seasonality on the structure and dynamics of multitrophic communities.

1. Allometric theory makes specific predictions about how density, and consequently biomass, scale with organism size within trophic levels, across trophic levels and across food webs. 2. Diversity-yield relationships suggest that more diverse food webs can sometimes support more biomass through mechanisms involving niche complementarity or selection effects that are sometimes attributed to organism size. 3. We combine the above two approaches and show that, generally, density and biomass scale with organism size within and between trophic levels as predicted by allometric theory. Further, food webs converged in total biomass despite persistent differences in the composition and size of the organisms among food webs; species richness explained deviations from the constant yield of biomass expected from size-abundance relationships. 4. Our results suggest that organism size plays only a transient role in controlling community biomass because population increases or decreases lead to rapid convergence in biomass. Species richness affects community biomass independently by effectively increasing the mass of organisms that can be supported in a given productivity regime.

Summary 1Previous evidence suggests that bacterially mediated decomposition of complex organic substrates increases with greater food web diversity. We attempted to identify changes in bacterial community composition and function associated with increased decomposition in more diverse food webs. 2We used aquatic microcosms where we manipulated productivity with different initial nutrient concentrations. We created a diversity gradient by establishing communities of eukaryotes with zero (bacteria alone), one, two or four microbe species (protists and rotifers) in each of four trophic levels: producers, herbivores, bacterivores and predators. The initial bacterial community was standardized across all treatments. To determine effects of productivity and diversity on the bacterial community, we measured: decomposition, abundance, diversity of colony morphotypes (a measure of composition) and community level physiological profiles (CLPP) (a functional profile based on carbon substrate utilization). 3Decomposition increased with greater eukaryotic species richness and was not influenced by productivity. Bacterial abundance remained constant with increasing eukaryotic species richness at low productivity, but significantly declined at high productivity. Eukaryotic species richness together with productivity influenced the composition of the bacterial community. However, the CLPP was strongly influenced by productivity and not species richness. 4Food web diversity and productivity interact to influence bacterial community composition and function. In more diverse food webs, bacterial activity (decomposition) increased despite lower population abundance.

Diversity-stability relationships have long been a topic of controversy in ecology, but one whose importance has been re-highlighted by increasing large-scale threats to global biodiversity. The ability of a community to recover from a perturbation (or resilience) is a common measure of stability that has received a large amount of theoretical attention. Yet, general expectations regarding diversity-resilience relations remain elusive. Moreover, the effects of productivity and its interaction with diversity on resilience are equally unclear. We examined the effects of species diversity, species composition, and productivity on population-and community-level resilience in experimental aquatic food webs composed of bacteria, algae, heterotrophic protozoa, and rotifers. Productivity manipulations were crossed with manipulations of the number of species and species compositions within trophic groups. Resilience was measured by perturbing communities with a nonselective, density-independent, mortality event and comparing responses over time between perturbed communities and controls. We found evidence that species diversity can enhance resilience at the community level (i.e., total community biomass), though this effect was more strongly expressed in low-productivity treatments. Diversity effects on resilience were driven by a sampling/selection effect, with resilient communities showing rapid response and dominance by a minority of species (primarily unicellular algae). In contrast, diversity had no effect on mean population-level resilience. Instead, the ability of a community's populations to recover from perturbations was dependent on species composition. We found no evidence of an effect of productivity, either positive or negative, on community- or population-level resilience. Our results indicate that the role of diversity as an insurer of stability may depend on the level of biological organization at which stability is measured, with effects emerging only when focusing on aggregate community properties.

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.

(2005). Cladoceran community assembly in a recently created lake. SIL Proceedings, 1922-2010: Vol. 29, No. 2, pp. 1020-1024.

Theory predicts that species diversity can enhance stability of community-level biomass while simultaneously decreasing population-level stability. Enrichment can theoretically destabilize communities but effects may become weaker with increasing diversity because of the inclusion of consumer-resistant prey. Few experiments using direct manipulations of species diversity have tested these predictions. We used laboratorybased aquatic food webs to examine the effects of species composition, diversity and enrichment on temporal variability of population- and community-level biomass. We found weak effects of enrichment on population- and community-level stability. However, diversity enhanced community-level stability while species composition had no influence. In contrast, composition effects outweighed diversity effects when stability was measured at the population level. We found no negative effects of diversity on population-level stability, in opposition to theory. Our results indicate that diversity can enhance stability in multitrophic systems, but effects vary with the scale of biological organization at which stability is measured.

Nonselective, density-independent mortality (DIM) is commonly thought to reduce the importance of competition, promoting coexistence of ecologically similar spe- cies. Yet, recent theory suggests that competition intensity and niche differences remain important under high DIM, even when competitive dominance is modified. Productivity is also thought to interact with DIM, potentially enhancing species coexistence. I explored these issues using a laboratory system composed of a diverse assemblage of bacteria as prey, and four species of competing protozoan bacterivores. Productivity manipulations were crossed with varying levels of DIM imposed on the bacterivores (in both polyculture and monoculture). DIM altered patterns of competitive dominance. At low DIM, Para- mecium aurelia and Paramecium caudatum dominated, while Colpoda inflata (a species absent at low DIM) was able to invade and dominate at high DIM. Productivity did not promote shifts in dominance and hindered Colpoda performance. Examination of compe- tition effect sizes provided clear evidence of competitive reversals and the importance of competition under low and high DIM. Variable competitive outcomes appeared to be me- diated by changes in the strength of apparent competition effects on bacterial diversity, in accordance with theory. Bacterial diversity was low at low levels of DIM, due to strong consumer effects. High DIM weakened consumer effects and increased bacterial diversity, likely facilitating invasion and coexistence.

SUMMARY 1. A large body of recent theory has recently developed focused on the relationship between the species diversity of competitor assemblages and the temporal stability of total competitor biomass. Many of these models predict that stability can increase with increasing diversity. 2. To explore natural relationships between zooplankton taxonomic diversity and temporal stability of total zooplankton biomass, 18 fishless, permanent ponds located in southern Michigan were surveyed over a 5 month period during a single growing season. 3. Results showed that temporal variability in total zooplankton biomass (measured as the coefficient of variation or CV) decreased with increasing mean zooplankton taxonomic richness. Thus, temporal stability increased with increasing taxonomic richness, consistent with theoretical predictions. 4. Decreases in the CV appeared to be because of portfolio effects (statistical averaging of species’ biomass fluctuations) rather than negative covariances among zooplankton taxa. 5. The CV of zooplankton biomass was also related to several environmental variables, suggesting that taxonomic richness may not be the only mediator of biomass stability. The CV decreased with increasing relative abundance of grazer-resistant algae (algae >35 lm in size) and the CV increased with increasing pond productivity.

Mortality (e.g. predation, disturbance) is often thought to lower the intensity of interspecific competition and thereby promote the coexistence of competing species. However, surprisingly few tests of this idea exist, especially for metazoans feeding on a self-renewing resource. Here we examined the effect of density-independent mortality on the coexistence of four species of pond zooplankton (microcrustacean grazers) in a series of laboratory microcosms. Across the experimental mortality gradient, consumer biomass decreased and resource abundance increased with an increase in mortality. Thus, the treatments resulted in an increase in resource availability per consumer (one measure of reduced competitive intensity). There was no significant effect of mortality treatment on species relative abundances or species evenness, and the zooplankter Diaphanosoma dominated community biomass at all mortality levels. Mortality rate did have a marginally significant effect on species richness (p < 0.07), but richness did not increase monotonically with mortality level. Instead, richness tended to be highest in the low- and no-mortality treatments.

Predation by fish has commonly been viewed as a primary driver of spatial and seasonal variation in Daphnia dominance and the size structure of zooplankton communities. Yet, previous research suggests thatlargeDaphniadonot always dominateinthe absenceof predation. Asalternativestotheplanktivory model, numerous mechanisms have been put forth, including the effect of resource competition and its interaction with resource quantity and quality and abiotic factors (e.g. temperature). Here results are presented of a field survey of 18 fishless, permanent ponds in southwest Michigan in which spatiotemporal variation in Daphnia pulex abundance and several potential determinants of this variation are explored. Results revealed a large amount of variation in D. pulex incidence and relative biomass, with some ponds exhibiting seasonal losses, some having few or no Daphnia, and some being dominated by D. pulex for the entire sample period. Redundancy analysis of zooplankton composition and pond environmental variables (biotic and abiotic) showed no relationship between D. pulex biomass and measures of Chaoborus abundance, algal resource production, or algal resource quality (including seston C:N:P). Instead, pH and temperature (both of which covaried) showed the strongest relationship with D. pulex biomass.

Empirical studies show that the relationship between species diversity and productivity can be strongly scale dependent. At the local scale of population dynamics the relationship is commonly unimodal or humped, but it is frequently monotonically increasing at larger spatial scales. Such contrasting patterns can occur if beta diversity (compositional dissimilarity among sites) also increases with productivity, but mechanistic reasons for this remain unresolved. Here we use theoretical models to explore the influence of ecological assembly on the diversity of multitrophic communities along gradients of productivity. We show that assembly dynamics can generate scale-dependent patterns con- sistent with those observed in nature; while unimodal productivity-diversity patterns are produced at the scale of local communities, positive monotonic patterns emerge when diversity is measured at larger spatial scales (among communities). This occurs due to increases in beta diversity with productivity. Our results suggest that increases in beta diversity, and resultant scale dependency, can depend on three vital and interactive elements: the presence of more than one consumer trophic level, a greater propensity for cyclic compositional change at high productivities, and the stochastic nature of species dispersal and invasion history.

If prey species exhibit trade-offs in their ability to utilize resources versus their ability to avoid predation, predators can facilitate prey turnover along gradients of productivity, shifting dominance from edible to inedible prey (the keystone predator effect). I tested this model under controlled, laboratory conditions, using a model aquatic system composed of zooplankton as the top consumer, a diverse community of algae as prey, and nutrients as basal resources. Nutrient manipulations (low and high) were crossed with presence–absence of zooplankton. Results supported theoretical predictions. Algal biomass increased in response to enrichment regardless of predator presence/absence. However, predators and nutrients had an interactive effect on algal biomass and size structure. At the low nutrient level, algal-prey were dominated by edible forms and attained similar biomass regardless of zooplankton presence/absence. At the high level of enrichment, presence of zooplankton favored higher levels of algal biomass and shifted dominance to large, inedible taxa. At the termination of the experiment, I performed a series of lab-based assays on the resultant algal community in order to quantify trade-offs among algal size classes in maximal population growth rates (as a measure of competitive ability for nutrients) and susceptibility to zooplankton grazing. Assays provided support for a size-based keystone trade-off. Small size classes of algae displayed higher maximal growth rates but were more susceptible to grazing effects. Large size classes were protected from grazing but showed low rates of population growth in response to enrichment.

Historically, fish predation has been viewed as a primary determinant of zooplankton community size structure and variable dominance by large-bodied Daphnia species. However, recent models have emphasized the relative carbon, nitrogen, and phos- phorus content of algae as a potential determinant of these patterns. In this research, I utilized five fishless ponds that varied inDaphnia pulex relative abundance and patterns of seasonal succession to explore the factors influencing Daphnia population performance in these systems. D. pulex population growth rates were measured in three treatments: (1) predators/competitors excluded, (2) competitors present and predators excluded, and (3) competitors and the predator Chaoborus present. The experiment was conducted twice in the growing season. In contrast to the top-down view, effects of competition outweighed effects of predation but were variable among ponds (being strongest in those ponds/times in which Daphnia relative abundance was low, and weakest when Daphnia were dominant). Competitor interaction strength was also temporally variable, increasing later in the season. Several potential determinants (biotic and abiotic) of variable competition intensity were explored using regressions. Only the C:P ratio of edible algae showed a significant rela- tionship; negative effects of competition increased as algae became more phosphorus lim- ited, in keeping with recent stoichiometric models. However, significant collinearities in several biotic/abiotic variables among ponds suggest that this interpretation may not be the only explanation for variable competition intensity.

Size-selective predation by fish is often considered to be a primary driver of seasonal declines in large-bodied Daphnia populations. However, large Daphnia commonly exhibit midsummer extinctions in ponds lacking planktivorous fish. A number of empirical and theoretical studies suggest that resource competition and its interaction with nutrient enrichment may determine variable dominance by large Daphnia. Low resource levels may favor competitive dominance by small-bodied taxa while large Daphnia may be favored under high resource conditions or following a nutrient/productivity pulse. Nutrient enrichment may also influence the strength of invertebrate predation on Daphnia by affecting how long vulnerable juveniles are exposed to predation. We investigated these hypotheses using an in situ mesocosm experiment in a permanent fishless pond that exhibited seasonal losses of Daphnia pulex. To explore the effects of nutrient enrichment, Daphnia plus a diverse assemblage of small-bodied zooplankton were exposed to three levels of enrichment (low, medium, and high). To explore the interaction between nutrient enrichment and invertebrate predation, we crossed the presence/absence of Notonecta undulata with low and high nutrient manipulations. We found no evidence of competitive reversals or shifts in dominance among nutrient levels, Daphnia performed poorly regardless of enrichment. This may have been due to shifts in algal composition to dominance by large filamentous green algae. Notonecta had significant negative effects on Daphnia alone, but no interaction with nutrient enrichment was detected. These results suggest that Daphnia are not invariably superior resource competitors compared to small taxa. Though predators can have negative effects, their presence is not necessary to explain poor Daphnia performance. Rather, abiotic conditions and/or resource-based effects are probably of greater importance.

Understanding the relationship between species richness and productivity is fundamental to the management and preservation of biodiversity. Yet despite years of study and intense theoretical interest, this relationship remains controversial. Here, we present the results of a literature survey in which we examined the relationship between species richness and productivity in 171 published studies. We extracted the raw data from published tables and graphs and subjected these data to a standardized analysis, using ordinary least-squares (OLS) regression and generalized linear-model (GLIM) regression to test for significant positive, negative, or curvilinear relationships between productivity and species diversity. If the relationship was curvilinear, we tested whether the maximum (or minimum) of the curve occurred within the range of productivity values observed (i.e., was there evidence of a hump?). A meta-analysis conducted on the distribution of standardized quadratic regression coefficients showed that ...

The effects of prey heterogeneity and consumer identity on the strength of predator limitation of prey biomass were explored experimentally under controlled labo- ratory conditions. In this study, I utilized a model aquatic community composed of zoo- plankton 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.

▪ Abstract Recent overviews have suggested that the relationship between species richness and productivity (rate of conversion of resources to biomass per unit area per unit time) is unimodal (hump-shaped). Most agree that productivity affects species richness at large scales, but unanimity is less regarding underlying mechanisms. Recent studies have examined the possibility that variation in species richness within communities may influence productivity, leading to an exploration of the relative effect of alterations in species number per se as contrasted to the addition of productive species. Reviews of the literature concerning deserts, boreal forests, tropical forests, lakes, and wetlands lead to the conclusion that extant data are insufficient to conclusively resolve the relationship between diversity and productivity, or that patterns are variable with mechanisms equally varied and complex. A more comprehensive survey of the ecological literature uncovered approximately 200 relationships, of which 30% were unimodal, 26% were positive linear, 12% were negative linear, and 32% were not significant. Categorization of studies with respect to geographic extent, ecological extent, taxonomic hierarchy, or energetic basis of productivity similarly yielded a heterogeneous distribution of relationships. Theoretical and empirical approaches increasingly suggest scale-dependence in the relationship between species richness and productivity; consequently, synthetic understanding may be contingent on explicit considerations of scale in analytical studies of productivity and diversity.

Heterogeneity among prey in their susceptibility to predation is a potentially important stabilizer of predator-prey interactions, reducing the magnitude of population oscillations and enhancing total prey population abundance. When microevolutionary responses of prey populations occur at time scales comparable to population dynamics, adaptive responses in prey defense can, in theory, stabilize predator-prey dynamics and reduce top-down effects on prey abundance. While experiments have tested these predictions, less explored are the consequences of the evolution of prey phenotypes that can persist in both vulnerable and invulnerable classes. We tested this experimentally using a laboratory aquatic system composed of the rotifer Brachionus calyciflorus as a predator and the prey Synura petersenii, a colony-forming alga that exhibits genetic variation in its propensity to form colonies and colony size (larger colonies are a defense against predators). Prey populations of either low initial genetic diversity and low adaptive capacity or high initial genetic diversity and high adaptive capacity were crossed with predator presence and absence. Dynamics measured over the last 127 days of the 167-day experiment revealed no effects of initial prey genetic diversity on the average abundance or temporal variability of predator populations. However, genetic diversity and predator presence/absence interactively affected prey population abundance and stability; diversity of prey had no effects in the absence of predators but stabilized dynamics and increased total prey abundance in the presence of predators. The size structure of the genetically diverse prey populations diverged from single strain populations in the presence of predators, showing increases in colony size and in the relative abundance of cells found in colonies. Our work sheds light on the adaptive value of colony formation and supports the general view that genetic diversity and intraspecific trait variation of prey can play a vital role in the short-term dynamics and stability of planktonic predator-prey systems.

Experiments show that consumer diversity can have important effects on the control of prey diversity and abundance. However, theory also indicates that the strength of consumer effects on such properties will vary depending on system productivity and disturbance regime. Using a laboratory-based system composed of ciliate consumers and bacterial prey, I explored the interactive effects of productivity, disturbance, and consumer diversity on prey diversity and trophic-level abundance. Consumer diversity had productivity-dependent effects on bacterial prey that were consistent with theoretical expectations. At low productivity, increasing consumer diversity reduced prey abundance while at high productivity no effects were detected due to compensatory responses among bacteria. In contrast, consumer diversity had weak effects on prey diversity at low productivity but significantly depressed prey diversity at high productivity. Disturbance on consumers enhanced prey diversity but did not alter consumer diversity effects on prey. These results indicate that consumer diversity may play an important role in the regulation of prey communities, but the strength of this effect varies with system productivity.

Prior ecological research has shown that spatial processes can enhance the temporal stability of populations in fluctuating environments. Less explored is the effect of dispersal on rapid adaptation and its concomitant impact on population dynamics. For asexually reproducing populations, theory predicts that dispersal in fluctuating environments can facilitate asynchrony among clones and enhance stability by reducing temporal variability of total population abundance. This effect is predicted when clones exhibit heritable variation in environmental optima and when fluctuations occur asynchronously among patches. We tested this in the field using artificial ponds and metapopulations composed of a diverse assemblage of Daphnia pulex clones. We directly manipulated dispersal presence/absence and environmental fluctuations in the form of nutrient pulses. Consistent with predictions, dispersal enhanced temporal asynchrony among clones in the presence of nutrient pulses; this in turn stabilized population dynamics. This effect only emerged when patches experienced spatially asynchronous nutrient pulses (dispersal had no effect when patches were synchronously pulsed). Clonal asynchrony was driven by strong positive selection for a single clone that exhibited a performance advantage under conditions of low resource availability. Our work highlights the importance of dispersal as a driver of eco-evolutionary dynamics and population stability in variable environments.

The ability of organisms to adapt and persist in the face of environmental change is accepted as a fundamental feature of natural systems. More contentious is whether the capacity of organisms to adapt (or “evolvability”) can itself evolve and the mechanisms underlying such responses. Using model gene networks, I provide evidence that evolvability emerges more readily when populations experience positively autocorrelated environmental noise (red noise) compared to populations in stable or randomly varying (white noise) environments. Evolvability was correlated with increasing genetic robustness to effects on network viability and decreasing robustness to effects on phenotypic expression; populations whose networks displayed greater viability robustness and lower phenotypic robustness produced more additive genetic variation and adapted more rapidly in novel environments. Patterns of selection for robustness varied antagonistically with epistatic effects of mutations on viability and phenotypic expression, suggesting that trade-offs between these properties may constrain their evolutionary responses. Evolution of evolvability and robustness was stronger in sexual populations compared to asexual populations indicating that enhanced genetic variation under fluctuating selection combined with recombination load is a primary driver of the emergence of evolvability. These results provide insight into the mechanisms potentially underlying rapid adaptation as well as the environmental conditions that drive the evolution of genetic interactions.

1. Allometric theory makes specific predictions about how density, and consequently biomass, scale with organism size within trophic levels, across trophic levels and across food webs. 2. Diversity-yield relationships suggest that more diverse food webs can sometimes support more biomass through mechanisms involving niche complementarity or selection effects that are sometimes attributed to organism size. 3. We combine the above two approaches and show that, generally, density and biomass scale with organism size within and between trophic levels as predicted by allometric theory. Further, food webs converged in total biomass despite persistent differences in the composition and size of the organisms among food webs; species richness explained deviations from the constant yield of biomass expected from size-abundance relationships. 4. Our results suggest that organism size plays only a transient role in controlling community biomass because population increases or decreases lead to rapid convergence in biomass. Species richness affects community biomass independently by effectively increasing the mass of organisms that can be supported in a given productivity regime.

A common prediction of evolutionary theory is that the strength of interspecific competition should decline over time among sympatric populations of competing species. Here we provide experimental evidence of historical declines in competition effects among competing zooplankton populations. Using diapausing eggs, we resurrected clones of three species of zooplankton obtained from different periods of community assembly in a single lake. We show that clones of Daphnia ambigua obtained from early in assembly when D. ambigua was dominant became extinct in competition with clones of Daphnia pulicaria and Daphnia dentifera (the current lake dominants). In contrast, D. ambigua clones obtained from later in the lake's history experienced weaker competition effects and persisted with D. dentifera. While we cannot rule out the role of intraspecific competition within D. ambigua, our results are in line with the view that natural selection favors reduced interaction strength among co-occurring species, facilitating coexistence and population persistence.

Experiments and theory in single trophic level systems dominate biodiversity and ecosystem functioning research and recent debates. All natural ecosystems contain communities with multiple trophic levels, however, and this can have important effects on ecosystem structure and functioning. Furthermore, many experiments compare assembled communities, rather than examining loss of species directly. We identify three questions around which to organise an investigation of how species loss affects the structure and functioning of multitrophic systems. 1) What is the distribution of species richness among trophic levels; 2) from which trophic levels are species most often lost; and 3) does loss of species from different trophic levels influence ecosystem functioning differently? Our analyses show that: 1) Relatively few high-quality data are available concerning the distribution of species richness among trophic levels. A new data-set provides evidence of a decrease in species richness as trophic height increases. 2) Multiple lines of evidence indicate that species are lost from higher trophic levels more frequently than lower trophic levels. 3) A theoretical model suggests that both the structure of food webs (occurrence of omnivory and the distribution of species richness among trophic levels) and the trophic level from which species are lost determines the impact of species loss on ecosystem functioning, which can even vary in the sign of the effect. These results indicate that, at least for aquatic systems, models of single trophic level ecosystems are insufficient for understanding the functional consequences of extinctions. Knowledge is required of food web structure, which species are likely to be lost, and also whether cascading extinctions will occur.

Large-bodied zooplankton of the genus Daphnia are thought to be keystone species in freshwater pelagic habitats, potentially able to exert strong grazing effects and enhance phosphorus limitation of algae. I examined the degree to which Daphnia pulex differ from small-bodied zooplankton in their effects on algal biomass, seston C:P and N:P, total nitrogen and total phosphorus. This was done with both survey data from natural ponds and an in situ experiment in which D. pulex was compared to a small zooplankton assemblage under low and high nutrient conditions and in two different ponds. D. pulex effects on algae were only evident under high nutrient conditions. In natural ponds, D. pulex dominance resulted in a significantly weaker chlorophyll–total phosphorus relationship, with the divergence between D. pulex and small zooplankton-dominated systems being greatest in highly enriched ponds. In the experiment, D. pulex exerted stronger top-down control in enriched treatments only and tended to graze algae to lower levels in the more productive pond. Dynamics of C:P over the course of the experiment did not reveal strong effects of zooplankton composition. However, data on the final date of the experiment provided some evidence that D. pulex can enhance phosphorus limitation of algae; total phosphorus was lower and C:P higher in D. pulex treatments. Survey results revealed no effects of D. pulex on seston C:P or N:P, suggesting that this species may not be an important factor governing phosphorus limitation of algae in natural ponds.