Interpopulation variation in a fish predator drives evolutionary divergence in prey in lakes

An experimental test of the growth rate hypothesis as a predictive framework for microevolutionary adaptation

The growth rate hypothesis (GRH) posits that the relative body phosphorus content of an organism is positively related to somatic growth rate, as protein synthesis, which is necessary for growth, requires P-rich rRNA. This hypothesis has strong support at the interspecific level. Here, we explore the use of the GRH to predict microevolutionary responses in consumer body stoichiometry. For this, we subjected populations of the rotifer Brachionus calyciflorus to selection for fast population growth rate (PGR) in P-rich (HPF) and P-poor (LPF) food environments. With common garden transplant experiments, we demonstrate that in HP populations evolution toward increased PGR was concomitant with an increase in relative phosphorus content. In contrast, LP populations evolved higher PGR without an increase in relative phosphorus content. We conclude that the GRH has the potential to predict microevolutionary change, but that its application is contingent on the environmental context. Our results highlight the potential of cryptic evolution in determining the performance response of populations to elemental limitation of their food resources.

Eco-evolutionary consequences of habitat warming and fragmentation in communities

Eco-evolutionary dynamics can mediate species and community responses to habitat warming and fragmentation, two of the largest threats to biodiversity and ecosystems. The eco-evolutionary consequences of warming and fragmentation are typically studied independently, hindering our understanding of their simultaneous impacts. Here, we provide a new perspective rooted in trade-offs among traits for understanding their eco-evolutionary consequences. On the one hand, temperature influences traits related to metabolism, such as resource acquisition and activity levels. Such traits are also likely to have trade-offs with other energetically costly traits, like antipredator defences or dispersal. On the other hand, fragmentation can influence a variety of traits (e.g. dispersal) through its effects on the spatial environment experienced by individuals, as well as properties of populations, such as genetic structure. The combined effects of warming and fragmentation on communities should thus reflect their collective impact on traits of individuals and populations, as well as trade-offs at multiple trophic levels, leading to unexpected dynamics when effects are not additive and when evolutionary responses modulate them. Here, we provide a road map to navigate this complexity. First, we review single-species responses to warming and fragmentation. Second, we focus on consumer-resource interactions, considering how eco-evolutionary dynamics can arise in response to warming, fragmentation, and their interaction. Third, we illustrate our perspective with several example scenarios in which trait trade-offs could result in significant eco-evolutionary dynamics. Specifically, we consider the possible eco-evolutionary consequences of (i) evolution in thermal performance of a species involved in a consumer-resource interaction, (ii) ecological or evolutionary changes to encounter and attack rates of consumers, and (iii) changes to top consumer body size in tri-trophic food chains. In these scenarios, we present a number of novel, sometimes counter-intuitive, potential outcomes. Some of these expectations contrast with those solely based on ecological dynamics, for example, evolutionary responses in unexpected directions for resource species or unanticipated population declines in top consumers. Finally, we identify several unanswered questions about the conditions most likely to yield strong eco-evolutionary dynamics, how better to incorporate the role of trade-offs among traits, and the role of eco-evolutionary dynamics in governing responses to warming in fragmented communities.

Legacy effects of fish but not elevation influence lake ecosystem response to environmental change

How communities reorganize during climate change depends on the distribution of diversity within ecosystems and across landscapes. Understanding how environmental and evolutionary history constrain community resilience is critical to predicting shifts in future ecosystem function. The goal of our study was to understand how communities with different histories respond to environmental change with regard to shifts in elevation (temperature, nutrients) and introduced predators. We hypothesized that community responses to the environment would differ in ways consistent with local adaptation and initial trait structure. We transplanted plankton communities from lakes at different elevations with and without fish in the Sierra Nevada Mountains in California to mesocosms at different elevations with and without fish. We examined the relative importance of the historical and experimental environment on functional (size structure, effects on lower trophic levels), community (zooplankton composition, abundance and biomass) and population (individual species abundance and biomass) responses. Communities originating from different elevations produced similar biomass at each elevation despite differences in species composition; that is, the experimental elevation, but not the elevation of origin, had a strong effect on biomass. Conversely, we detected a legacy effect of predators on plankton in the fishless environment. Daphnia pulicaria that historically coexisted with fish reached greater biomass under fishless conditions than those from fishless lakes, resulting in greater zooplankton community biomass and larger average size. Therefore, trait variation among lake populations determined the top-down effects of fish predators. In contrast, phenotypic plasticity and local diversity were sufficient to maintain food web structure in response to changing environmental conditions associated with elevation.

Cyclical environments drive variation in life-history strategies: a general theory of cyclical phenology

Cycles, such as seasons or tides, characterize many systems in nature. Overwhelming evidence shows that climate change-driven alterations to environmental cycles-such as longer seasons-are associated with phenological shifts around the world, suggesting a deep link between environmental cycles and life cycles. However, general mechanisms of life-history evolution in cyclical environments are still not well understood. Here, I build a demographic framework and ask how life-history strategies optimize fitness when the environment perturbs a structured population cyclically and how strategies should change as cyclicality changes. I show that cycle periodicity alters optimality predictions of classic life-history theory because repeated cycles have rippling selective consequences over time and generations. Notably, fitness landscapes that relate environmental cyclicality and life-history optimality vary dramatically depending on which trade-offs govern a given species. The model tuned with known life-history trade-offs in a marine intertidal copepod Tigriopus californicus successfully predicted the shape of life-history variation across natural populations spanning a gradient of tidal periodicities. This framework shows how environmental cycles can drive life-history variation-without complex assumptions of individual responses to cues such as temperature-thus expanding the range of life-history diversity explained by theory and providing a basis for adaptive phenology.

The evolution of eye size in response to increased fish predation in Daphnia

Variation in eye size is ubiquitous across taxa. Increased eye size is correlated with improved vision and increased fitness via shifts in behavior. Tests of the drivers of eye size evolution have focused on macroevolutionary studies evaluating the importance of light availability. Predator-induced mortality has recently been identified as a potential driver of eye size variation. Here, we tested the influence of increased predation by the fish predator, the alewife (Alosa pseudoharengus) on eye size evolution in waterfleas (Daphnia ambigua) from lakes in Connecticut. We quantified the relative eye size of Daphnia from lakes with and without alewife using wild-caught and third-generation laboratory reared specimens. This includes comparisons between lakes where alewife are present seasonally (anadromous) or permanently (landlocked). Wild-caught specimens did not differ in eye size across all lakes. However, third-generation lab reared Daphnia from lakes with alewife, irrespective of the form of alewife predation, exhibited significantly larger eyes than Daphnia from lakes without alewife. This genetically based increase in eye size may enhance the ability of Daphnia to detect predators. Alternatively, such shifts in eye size may be an indirect response to Daphnia aggregating at the bottom of lakes. To test these mechanisms, we collected Daphnia as a function of depth and found that eye size differed in Daphnia found at the surface versus the bottom of the water column between anadromous alewife and no alewife lakes. However, we found no evidence of Daphnia aggregating at the bottom of lakes. Such results indicate that the evolution of a larger eye may be explained by a connection between eyes and enhanced survival. We discuss the cause of the lack of concordance in eye size variation between our phenotypic and genetic specimens and the ultimate drivers of eye size.

Evolutionary history of Daphnia drives divergence in grazing selectivity and alters temporal community dynamics of producers

Consumers with different seasonal life histories encounter different communities of producers during specific seasonal phases. If consumers evolve to prefer the producers that they encounter, then consumers may reciprocally influence the temporal composition of producer communities. Here, we study the keystone consumer Daphnia ambigua, whose seasonal life history has diverged due to intraspecific predator divergence across lakes of New England. We ask whether grazing preferences of Daphnia have diverged also and test whether any grazing differences influence temporal composition patterns of producers. We reared clonal populations of Daphnia from natural populations representing the two diverged life history types for multiple generations. We conducted short‐term (24 hr) and long‐term (27 days) grazing experiments in equal polycultures consisting of three diatom and two green algae species, treated with no consumer, Daphnia from lakes with anadromous alewife, or from lakes with landlocked alewife. After 24 hr, life history and grazing preference divergence in Daphnia ambigua drove significant differences in producer composition. However, those differences disappeared at the end of the 27‐day experiment. Our results illustrate that, despite potentially more complex long‐term dynamics, a multitrophic cascade of evolutionary divergence from a predator can influence temporal community dynamics at the producer level.

Rapid evolution leads to differential population dynamics and top-down control in resurrected Daphnia populations

There is growing evidence of rapid genetic adaptation of natural populations to environmental change, opening the perspective that evolutionary trait change may subsequently impact ecological processes such as population dynamics, community composition, and ecosystem functioning. To study such eco‐evolutionary feedbacks in natural populations, however, requires samples across time. Here, we capitalize on a resurrection ecology study that documented rapid and adaptive evolution in a natural population of the water flea Daphnia magna in response to strong changes in predation pressure by fish, and carry out a follow‐up mesocosm experiment to test whether the observed genetic changes influence population dynamics and top‐down control of phytoplankton. We inoculated populations of the water flea D. magna derived from three time periods of the same natural population known to have genetically adapted to changes in predation pressure in replicate mesocosms and monitored both Daphnia population densities and phytoplankton biomass in the presence and absence of fish. Our results revealed differences in population dynamics and top‐down control of algae between mesocosms harboring populations from the time period before, during, and after a peak in fish predation pressure caused by human fish stocking. The differences, however, deviated from our a priori expectations. An S‐map approach on time series revealed that the interactions between adults and juveniles strongly impacted the dynamics of populations and their top‐down control on algae in the mesocosms, and that the strength of these interactions was modulated by rapid evolution as it occurred in nature. Our study provides an example of an evolutionary response that fundamentally alters the processes structuring population dynamics and impacts ecosystem features.

Contrasting gene expression programs correspond with predator-induced phenotypic plasticity within and across generations in Daphnia

Research has shown that a change in environmental conditions can alter the expression of traits during development (i.e., "within-generation phenotypic plasticity") as well as induce heritable phenotypic responses that persist for multiple generations (i.e., "transgenerational plasticity", TGP). It has long been assumed that shifts in gene expression are tightly linked to observed trait responses at the phenotypic level. Yet, the manner in which organisms couple within- and TGP at the molecular level is unclear. Here we tested the influence of fish predator chemical cues on patterns of gene expression within- and across generations using a clone of Daphnia ambigua that is known to exhibit strong TGP but weak within-generation plasticity. Daphnia were reared in the presence of predator cues in generation 1, and shifts in gene expression were tracked across two additional asexual experimental generations that lacked exposure to predator cues. Initial exposure to predator cues in generation 1 was linked to ~50 responsive genes, but such shifts were 3-4× larger in later generations. Differentially expressed genes included those involved in reproduction, exoskeleton structure and digestion; major shifts in expression of genes encoding ribosomal proteins were also identified. Furthermore, shifts within the first-generation and transgenerational shifts in gene expression were largely distinct in terms of the genes that were differentially expressed. Such results argue that the gene expression programmes involved in within- vs. transgeneration plasticity are fundamentally different. Our study provides new key insights into the plasticity of gene expression and how it relates to phenotypic plasticity in nature.

Maintaining mimicry diversity: optimal warning colour patterns differ among microhabitats in Amazonian clearwing butterflies

Mimicry is one of the best-studied examples of adaptation, and recent studies have provided new insights into the role of mimicry in speciation and diversification. Classical Müllerian mimicry theory predicts convergence in warning signal among protected species, yet tropical butterflies are exuberantly diverse in warning colour patterns, even within communities. We tested the hypothesis that microhabitat partitioning in aposematic butterflies and insectivorous birds can lead to selection for different colour patterns in different microhabitats and thus help maintain mimicry diversity. We measured distribution across flight height and topography for 64 species of clearwing butterflies (Ithomiini) and their co-mimics, and 127 species of insectivorous birds, in an Amazon rainforest community. For the majority of bird species, estimated encounter rates were non-random for the two most abundant mimicry rings. Furthermore, most butterfly species in these two mimicry rings displayed the warning colour pattern predicted to be optimal for anti-predator defence in their preferred microhabitats. These conclusions were supported by a field trial using butterfly specimens, which showed significantly different predation rates on colour patterns in two microhabitats. We therefore provide the first direct evidence to support the hypothesis that different mimicry patterns can represent stable, community-level adaptations to differing biotic environments.

Delayed effects of chlorpyrifos across metamorphosis on dispersal-related traits in a poleward moving damselfly

How exposure to contaminants may interfere with the widespread poleward range expansions under global warming is largely unknown. Pesticide exposure may negatively affect traits shaping the speed of range expansion, including traits related to population growth rate and dispersal-related traits. Moreover, rapid evolution of growth rates during poleward range expansions may come at a cost of a reduced investment in detoxification and repair thereby increasing the vulnerability to contaminants at expanding range fronts. We tested effects of a sublethal concentration of the widespread pesticide chlorpyrifos on traits related to range expansion in replicated edge and core populations of the poleward moving damselfly Coenagrion scitulum reared at low and high food levels in a common garden experiment. Food limitation in the larval stage had strong negative effects both in the larval stage and across metamorphosis in the adult stage. Exposure to chlorpyrifos during the larval stage did not affect larval traits but caused delayed effects across metamorphosis by increasing the incidence of wing malformations during metamorphosis and by reducing a key component of the adult immune response. There was some support for an evolutionary trade-off scenario as the faster growing edge larvae suffered a higher mortality during metamorphosis. Instead, there was no clear support for the faster growing edge larvae being more vulnerable to chlorpyrifos. Our data indicate that sublethal delayed effects of pesticide exposure, partly in association with the rapid evolution of faster growth rates, may slow down range expansions.

Local adaptation in transgenerational responses to predators

Environmental signals can induce phenotypic changes that span multiple generations. Along with phenotypic responses that occur during development (i.e. 'within-generation' plasticity), such 'transgenerational plasticity' (TGP) has been documented in a diverse array of taxa spanning many environmental perturbations. New theory predicts that temporal stability is a key driver of the evolution of TGP. We tested this prediction using natural populations of zooplankton from lakes in Connecticut that span a large gradient in the temporal dynamics of predator-induced mortality. We reared more than 120 clones of Daphnia ambigua from nine lakes for multiple generations in the presence/absence of predator cues. We found that temporal variation in mortality selects for within-generation plasticity while consistently strong (or weak) mortality selects for increased TGP. Such results provide us the first evidence for local adaptation in TGP and argue that divergent ecological conditions select for phenotypic responses within and across generations.

The alignment between phenotypic plasticity, the major axis of genetic variation and the response to selection

Phenotypic plasticity is the ability of a genotype to produce more than one phenotype in order to match the environment. Recent theory proposes that the major axis of genetic variation in a phenotypically plastic population can align with the direction of selection. Therefore, theory predicts that plasticity directly aids adaptation by increasing genetic variation in the direction favoured by selection and reflected in plasticity. We evaluated this theory in the freshwater crustacean Daphnia pulex, facing predation risk from two contrasting size-selective predators. We estimated plasticity in several life-history traits, the G matrix of these traits, the selection gradients on reproduction and survival, and the predicted responses to selection. Using these data, we tested whether the genetic lines of least resistance and the predicted response to selection aligned with plasticity. We found predator environment-specific G matrices, but shared genetic architecture across environments resulted in more constraint in the G matrix than in the plasticity of the traits, sometimes preventing alignment of the two. However, as the importance of survival selection increased, the difference between environments in their predicted response to selection increased and resulted in closer alignment between the plasticity and the predicted selection response. Therefore, plasticity may indeed aid adaptation to new environments.

Cryptic eco-evolutionary dynamics

Natural systems harbor complex interactions that are fundamental parts of ecology and evolution. These interactions challenge our inclinations and training to seek the simplest explanations of patterns in nature. Not least is the likelihood that some complex processes might be missed when their patterns look similar to predictions for simpler mechanisms. Along these lines, theory and empirical evidence increasingly suggest that environmental, ecological, phenotypic, and genetic processes can be tightly intertwined, resulting in complex and sometimes surprising eco-evolutionary dynamics. The goal of this review is to temper inclinations to unquestioningly seek the simplest explanations in ecology and evolution, by recognizing that some eco-evolutionary outcomes may appear very similar to purely ecological, purely evolutionary, or even null expectations, and thus be cryptic. We provide theoretical and empirical evidence for observational biases and mechanisms that might operate among the various links in eco-evolutionary feedbacks to produce cryptic patterns. Recognition that cryptic dynamics can be associated with outcomes like stability, resilience, recovery, or coexistence in a dynamically changing world provides added impetus for finding ways to study them.

Predator cannibalism can intensify negative impacts on heterospecific prey

Although natural populations consist of individuals with different traits, and the degree of phenotypic variation varies among populations, the impact of phenotypic variation on ecological interactions has received little attention, because traditional approaches to community ecology assume homogeneity of individuals within a population. Stage structure, which is a common way of generating size and developmental variation within predator populations, can drive cannibalistic interactions, which can affect the strength of predatory effects on the predator's heterospecific prey. Studies have shown that predator cannibalism weakens predatory effects on heterospecific prey by reducing the size of the predator population and by inducing less feeding activity of noncannibal predators. We predict, however, that predator cannibalism, by promoting rapid growth of the cannibals, can also intensify predation pressure on heterospecific prey, because large predators have large resource requirements and may utilize a wider variety of prey species. To test this hypothesis, we conducted an experiment in which we created carnivorous salamander (Hynobius retardatus) populations with different stage structures by manipulating the salamander's hatch timing (i.e., populations with large or small variation in the timing of hatching), and explored the resultant impacts on the abundance, behavior, morphology, and life history of the salamander's large heterospecific prey, Rana pirica frog tadpoles. Cannibalism was rare in salamander populations having small hatch-timing variation, but was frequent in those having large hatch-timing variation. Thus, giant salamander cannibals occurred only in the latter. We clearly showed that salamander giants exerted strong predation pressure on frog tadpoles, which induced large behavioral and morphological defenses in the tadpoles and caused them to metamorphose late at large size. Hence, predator cannibalism arising from large variation in the timing of hatching can strengthen predatory effects on heterospecific prey and can have impacts on various, traits of both predator and prey. Because animals commonly broaden their diet as they grow, such negative impacts of predator cannibalism on the heterospecific prey may be common in interactions between predators and prey species of similar size.

Predator-induced phenotypic plasticity within- and across-generations: a challenge for theory?

Much work has shown that the environment can induce non-genetic changes in phenotype that span multiple generations. Theory predicts that predictable environmental variation selects for both increased within- and across-generation responses. Yet, to the best of our knowledge, there are no empirical tests of this prediction. We explored the relationship between within- versus across-generation plasticity by evaluating the influence of predator cues on the life-history traits of Daphnia ambigua. We measured the duration of predator-induced transgenerational effects, determined when transgenerational responses are induced, and quantified the cues that activate transgenerational plasticity. We show that predator exposure during embryonic development causes earlier maturation and increased reproductive output. Such effects are detectable two generations removed from predator exposure and are similar in magnitude in response to exposure to cues emitted by injured conspecifics. Moreover, all experimental contexts and traits yielded a negative correlation between within- versus across-generation responses. That is, responses to predator cues within- and across-generations were opposite in sign and magnitude. Although many models address transgenerational plasticity, none of them explain this apparent negative relationship between within- and across-generation plasticities. Our results highlight the need to refine the theory of transgenerational plasticity.

Evolutionary divergence of adult body size and juvenile growth in sympatric subpopulations of a top predator in aquatic ecosystems

Evolutionary theory predicts that different selective regimes may contribute to divergent evolution of body size and growth rate among populations, but most studies have focused on allopatric populations. Here, we studied five sympatric subpopulations of anadromous northern pike (Esox lucius) in the Baltic Sea subjected to allopatric habitats for a short period of their life cycle due to homing behavior. We report differences in adult body size among subpopulations that were in part due to variation in growth rate. Body size of emigrating juveniles also differed among subpopulations, and differences remained when individuals were reared in a common environment, thus indicating evolutionary divergence among subpopulations. Furthermore, a QST-FST comparison indicated that differences had evolved due to divergent selection rather than genetic drift, possibly in response to differences in selective mortality among spawning habitats during the allopatric life stage. Adult and juvenile size were negatively correlated across subpopulations, and reconstruction of growth trajectories of adult fishes suggested that body size differences developed gradually and became accentuated throughout the first years of life. These results represent rare evidence that sympatric subpopulations can evolve differences in key life-history traits despite being subjected to allopatric habitats during only a very short fraction of their life.

Intraspecific phenotypic variation among alewife populations drives parallel phenotypic shifts in bluegill

Evolutionary diversification within consumer species may generate selection on local ecological communities, affecting prey community structure. However, the extent to which this niche construction can propagate across food webs and shape trait variation in competing species is unknown. Here, we tested whether niche construction by different life-history variants of the planktivorous fish alewife (Alosa pseudoharengus) can drive phenotypic divergence and resource use in the competing species bluegill (Lepomis macrochirus). Using a combination of common garden experiments and a comparative field study, we found that bluegill from landlocked alewife lakes grew relatively better when fed small than large zooplankton, had gill rakers better adapted for feeding on small-bodied prey and selected smaller zooplankton compared with bluegill from lakes with anadromous or no alewife. Observed shifts in bluegill foraging traits in lakes with landlocked alewife parallel those in alewife, suggesting interspecific competition leading to parallel phenotypic changes rather than to divergence (which is commonly predicted). Our findings suggest that species may be locally adapted to prey communities structured by different life-history variants of a competing dominant species.

Does variation in the intensity and duration of predation drive evolutionary changes in senescence?

The evolutionary theory of senescence predicts that increased rates of extrinsic mortality select for faster declines in fertility and survival with age. One predicted mechanism is that increased mortality favours alleles that enhance fitness early in life at the expense of survival or reproduction later in life (antagonistic pleiotropy). We tested these predictions in natural populations of Daphnia ambigua from lakes that vary in the severity and duration of fish predation. Daphnia are found in lakes with (i) anadromous alewife (Alosa pseudoharengus) that migrate between marine and freshwater, (ii) permanent landlocked alewife and (iii) no alewife. Daphnia are rare year-round in 'landlocked lakes' and are seasonally eliminated from the water column in 'anadromous lakes' due to the very strong predatory impact of anadromous alewife on populations of zooplankton. Previous work has also shown that intense seasonal bouts of predation by anadromous alewife has selected for increased allocation towards growth and reproduction in Daphnia found in lakes with anadromous alewife. Such variation in the risk of mortality and the expression of life-history traits early in life provides the raw materials to test the evolutionary theory of ageing. We reared replicate populations of Daphnia from all three lake types and quantified lifetime rates of offspring production and intrinsic survival. We found no differences in age-related declines in fertility or survival. Daphnia from anadromous lakes produced significantly more offspring throughout their lifetime despite no differences in life span or in the number of reproductive bouts compared with Daphnia from lakes with landlocked and no alewife. The lack of divergence in ageing contradicts the prediction that elevated mortality rates drive evolutionary shifts in ageing. We reconcile these results by considering the predictions of theoretical frameworks that incorporate feedbacks associated with increased mortality such as density- and condition-dependent processes. Our results, which are better explained by a consideration of these processes, thus call for a greater consideration of models that more explicitly consider the ecology of focal organisms.

The rate-size trade-off structures intraspecific variation in Daphnia ambigua life history parameters

The identification of trade-offs is necessary for understanding the evolution and maintenance of diversity. Here we employ the supply-demand (SD) body size optimization model to predict a trade-off between asymptotic body size and growth rate. We use the SD model to quantitatively predict the slope of the relationship between asymptotic body size and growth rate under high and low food regimes and then test the predictions against observations for Daphnia ambigua. Close quantitative agreement between observed and predicted slopes at both food levels lends support to the model and confirms that a ‘rate-size’ trade-off structures life history variation in this population. In contrast to classic life history expectations, growth and reproduction were positively correlated after controlling for the rate-size trade-off. We included 12 Daphnia clones in our study, but clone identity explained only some of the variation in life history traits. We also tested the hypothesis that growth rate would be positively related to intergenic spacer length (i.e. the growth rate hypothesis) across clones, but we found that clones with intermediate intergenic spacer lengths had larger asymptotic sizes and slower growth rates. Our results strongly support a resource-based optimization of body size following the SD model. Furthermore, because some resource allocation decisions necessarily precede others, understanding interdependent life history traits may require a more nested approach.

Intraspecific phenotypic variation in a fish predator affects multitrophic lake metacommunity structure

Contemporary insights from evolutionary ecology suggest that population divergence in ecologically important traits within predators can generate diversifying ecological selection on local community structure. Many studies acknowledging these effects of intraspecific variation assume that local populations are situated in communities that are unconnected to similar communities within a shared region. Recent work from metacommunity ecology suggests that species dispersal among communities can also influence species diversity and composition but can depend upon the relative importance of the local environment. Here, we study the relative effects of intraspecific phenotypic variation in a fish predator and spatial processes related to plankton species dispersal on multitrophic lake plankton metacommunity structure. Intraspecific diversification in foraging traits and residence time of the planktivorous fish alewife (Alosa pseudoharengus) among coastal lakes yields lake metacommunities supporting three lake types which differ in the phenotype and incidence of alewife: lakes with anadromous, landlocked, or no alewives. In coastal lakes, plankton community composition was attributed to dispersal versus local environmental predictors, including intraspecific variation in alewives. Local and beta diversity of zooplankton and phytoplankton was additionally measured in response to intraspecific variation in alewives. Zooplankton communities were structured by species sorting, with a strong influence of intraspecific variation in A. pseudoharengus. Intraspecific variation altered zooplankton species richness and beta diversity, where lake communities with landlocked alewives exhibited intermediate richness between lakes with anadromous alewives and without alewives, and greater community similarity. Phytoplankton diversity, in contrast, was highest in lakes with landlocked alewives. The results indicate that plankton dispersal in the region supplied a migrant pool that was strongly structured by intraspecific variation in alewives. This is one of the first studies to demonstrate that intraspecific phenotypic variation in a predator can maintain contrasting patterns of multitrophic diversity in metacommunities.

The cold-water connection: Bergmann's rule in North American freshwater fishes

Understanding general rules governing macroecological body size variations is one of the oldest pursuits in ecology. However, this science has been dominated by studies of terrestrial vertebrates, spurring debate over the validity of such rules in other taxonomic groups. Here, relationships between maximum body size and latitude, temperature, and elevation were evaluated for 29 North American freshwater fish species. Bergmann's rule (i.e., that body size correlates positively with latitude and negatively with temperature) was observed in 38% of species, converse Bergmann's rule (that body size correlates negatively with latitude and positively with temperature) was observed in 34% of species, and 28% of species showed no macroecological body size relationships. Most notably, every species that expressed Bergmann's rule was a cool- or cold-water species while every species that expressed converse Bergmann's rule was a warm-water species, highlighting how these patterns are likely connected to species thermal niches. This study contradicts previous research suggesting Bergmann's rule does not apply to freshwater fishes, and is congruent with an emerging paradigm of variable macroecological body size patterns in poikilotherms.

Evolution mediates the effects of apex predation on aquatic food webs

Ecological and evolutionary mechanisms are increasingly thought to shape local community dynamics. Here, I evaluate if the local adaptation of a meso-predator to an apex predator alters local food webs. The marbled salamander (Ambystoma opacum) is an apex predator that consumes both the spotted salamander (Ambystoma maculatum) and shared zooplankton prey. Common garden experiments reveal that spotted salamander populations which co-occur with marbled salamanders forage more intensely than those that face other predator species. These foraging differences, in turn, alter the diversity, abundance and composition of zooplankton communities in common garden experiments and natural ponds. Locally adapted spotted salamanders exacerbate prey biomass declines associated with apex predation, but dampen the top-down effects of apex predation on prey diversity. Countergradient selection on foraging explains why locally adapted spotted salamanders exacerbate prey biomass declines. The two salamander species prefer different prey species, which explains why adapted spotted salamanders buffer changes in prey composition owing to apex predation. Results suggest that local adaptation can strongly mediate effects from apex predation on local food webs. Community ecologists might often need to consider the evolutionary history of populations to understand local diversity patterns, food web dynamics, resource gradients and their responses to disturbance.

A cascade of evolutionary change alters consumer-resource dynamics and ecosystem function

It is becoming increasingly clear that intraspecific evolutionary divergence influences the properties of populations, communities and ecosystems. The different ecological impacts of phenotypes and genotypes may alter selection on many species and promote a cascade of ecological and evolutionary change throughout the food web. Theory predicts that evolutionary interactions across trophic levels may contribute to hypothesized feedbacks between ecology and evolution. However, the importance of 'cascading evolutionary change' in a natural setting is unknown. In lakes in Connecticut, USA, variation in migratory behaviour and feeding morphology of a fish predator, the alewife (Alosa pseudoharengus), drives life-history evolution in a species of zooplankton prey (Daphnia ambigua). Here we evaluated the reciprocal impacts of Daphnia evolution on ecological processes in laboratory mesocosms. We show that life-history evolution in Daphnia facilitates divergence in rates of population growth, which in turn significantly alters consumer-resource dynamics and ecosystem function. These experimental results parallel trends observed in lakes. Such results argue that a cascade of evolutionary change, which has occurred over contemporary timescales, alters community and ecosystem processes.

Linking genes to communities and ecosystems: Daphnia as an ecogenomic model

How do genetic variation and evolutionary change in critical species affect the composition and functioning of populations, communities and ecosystems? Illuminating the links in the causal chain from genes up to ecosystems is a particularly exciting prospect now that the feedbacks between ecological and evolutionary changes are known to be bidirectional. Yet to fully explore phenomena that span multiple levels of the biological hierarchy requires model organisms and systems that feature a comprehensive triad of strong ecological interactions in nature, experimental tractability in diverse contexts and accessibility to modern genomic tools. The water flea Daphnia satisfies these criteria, and genomic approaches capitalizing on the pivotal role Daphnia plays in the functioning of pelagic freshwater food webs will enable investigations of eco-evolutionary dynamics in unprecedented detail. Because its ecology is profoundly influenced by both genetic polymorphism and phenotypic plasticity, Daphnia represents a model system with tremendous potential for developing a mechanistic understanding of the relationship between traits at the genetic, organismal and population levels, and consequences for community and ecosystem dynamics. Here, we highlight the combination of traits and ecological interactions that make Daphnia a definitive model system, focusing on the additional power and capabilities enabled by recent molecular and genomic advances.

Fates beyond traits: ecological consequences of human-induced trait change

Human-induced trait change has been documented in freshwater, marine, and terrestrial ecosystems worldwide. These trait changes are driven by phenotypic plasticity and contemporary evolution. While efforts to manage human-induced trait change are beginning to receive some attention, managing its ecological consequences has received virtually none. Recent work suggests that contemporary trait change can have important effects on the dynamics of populations, communities, and ecosystems. Therefore, trait changes caused by human activity may be shaping ecological dynamics on a global scale. We present evidence for important ecological effects associated with human-induced trait change in a variety of study systems. These effects can occur over large spatial scales and impact system-wide processes such as trophic cascades. Importantly, the magnitude of these effects can be on par with those of traditional ecological drivers such as species presence. However, phenotypic change is not always an agent of ecological change; it can also buffer ecosystems against change. Determining the conditions under which phenotypic change may promote vs prevent ecological change should be a top research priority.

Fates beyond traits: ecological consequences of human-induced trait change

Human-induced trait change has been documented in freshwater, marine, and terrestrial ecosystems worldwide. These trait changes are driven by phenotypic plasticity and contemporary evolution. While efforts to manage human-induced trait change are beginning to receive some attention, managing its ecological consequences has received virtually none. Recent work suggests that contemporary trait change can have important effects on the dynamics of populations, communities, and ecosystems. Therefore, trait changes caused by human activity may be shaping ecological dynamics on a global scale. We present evidence for important ecological effects associated with human-induced trait change in a variety of study systems. These effects can occur over large spatial scales and impact system-wide processes such as trophic cascades. Importantly, the magnitude of these effects can be on par with those of traditional ecological drivers such as species presence. However, phenotypic change is not always an agent of ecological change; it can also buffer ecosystems against change. Determining the conditions under which phenotypic change may promote vs prevent ecological change should be a top research priority.