INTRAGUILD PREDATION DRIVES EVOLUTIONARY NICHE SHIFT IN THREESPINE STICKLEBACK

On the Origins of Phenotypic Parallelism in Benthic and Limnetic Stickleback

Rapid evolution of similar phenotypes in similar environments, giving rise to in situ parallel adaptation, is an important hallmark of ecological speciation. However, what appears to be in situ adaptation can also arise by dispersal of divergent lineages from elsewhere. We test whether two contrasting phenotypes repeatedly evolved in parallel, or have a single origin, in an archetypal example of ecological adaptive radiation: benthic-limnetic three-spined stickleback (Gasterosteus aculeatus) across species pair and solitary lakes in British Columbia. We identify two genomic clusters across freshwater populations, which differ in benthic-limnetic divergent phenotypic traits and separate benthic from limnetic individuals in species pair lakes. Phylogenetic reconstruction and niche evolution modeling both suggest a single evolutionary origin for each of these clusters. We detected strong phylogenetic signal in benthic-limnetic divergent traits, suggesting that they are ancestrally retained. Accounting for ancestral state retention, we identify local adaptation of body armor due to the presence of an intraguild predator, the sculpin (Cottus asper), and environmental effects of lake depth and pH on body size. Taken together, our results imply a predominant role for retention of ancestral characteristics in driving trait distribution, with further selection imposed on some traits by environmental factors.

Species divergence under competition and shared predation

Species competing for resources also commonly share predators. While competition often drives divergence between species, the effects of shared predation are less understood. Theoretically, competing prey species could either diverge or evolve in the same direction under shared predation depending on the strength and symmetry of their interactions. We took an empirical approach to this question, comparing antipredator and trophic phenotypes between sympatric and allopatric populations of threespine stickleback and prickly sculpin fish that all live in the presence of a trout predator. We found divergence in antipredator traits between the species: in sympatry, antipredator adaptations were relatively increased in stickleback but decreased in sculpin. Shifts in feeding morphology, diet and habitat use were also divergent but driven primarily by stickleback evolution. Our results suggest that asymmetric ecological character displacement indirectly made stickleback more and sculpin less vulnerable to shared predation, driving divergence of antipredator traits between sympatric species.

A test of frequency‐dependent selection in the evolution of a generalist phenotype

A solitary population of consumers frequently evolves to the middle of a resource gradient and an intermediate mean phenotype compared to a sympatric pair of competing species that diverge to either side via character displacement. The forces governing the distribution of phenotypes in these allopatric populations, however, are little investigated. Theory predicts that the intermediate mean phenotype of the generalist should be maintained by negative frequency‐dependent selection, whereby alternate extreme phenotypes are favored because they experience reduced competition for resources when rare. However, the theory makes assumptions that are not always met, and alternative explanations for an intermediate phenotype are possible. We provide a test of this prediction in a mesocosm experiment using threespine stickleback that are ecologically and phenotypically intermediate between the more specialized stickleback species that occur in pairs. We manipulated the frequency distribution of phenotypes in two treatments and then measured effects on a focal intermediate population. We found a slight frequency‐dependent effect on survival in the predicted direction but not on individual growth rates. This result suggests that frequency‐dependent selection might be a relatively weak force across the range of phenotypes within an intermediate population and we suggest several general reasons why this might be so. We propose that allopatric populations might often be maintained at an intermediate phenotype instead by stabilizing or fluctuating directional selection.

Adaptive divergence and the evolution of hybrid trait mismatch in threespine stickleback

Selection against mismatched traits in hybrids is the phenotypic analogue of intrinsic hybrid incompatibilities. Mismatch occurs when hybrids resemble one parent population for some phenotypic traits and the other parent population for other traits, and is caused by dominance in opposing directions or from segregation of alleles in recombinant hybrids. In this study, we used threespine stickleback fish (Gasterosteus aculeatus L.) to test the theoretical prediction that trait mismatch in hybrids should increase with the magnitude of phenotypic divergence between parent populations. We measured morphological traits in parents and hybrids in crosses between a marine population representing the ancestral form and twelve freshwater populations that have diverged from this ancestral state to varying degrees according to their environments. We found that trait mismatch was greater in more divergent crosses for both F₁ and F₂ hybrids. In the F₁, the divergence–mismatch relationship was caused by traits having dominance in different directions, whereas it was caused by increasing segregating phenotypic variation in the F₂. Our results imply that extrinsic hybrid incompatibilities accumulate as phenotypic divergence proceeds.

Fitness maps to a large-effect locus in introduced stickleback populations

Mutations of small effect underlie most adaptation to new environments, but beneficial variants with large fitness effects are expected to contribute under certain conditions. Genes and genomic regions having large effects on phenotypic differences between populations are known from numerous taxa, but fitness effect sizes have rarely been estimated. We mapped fitness over a generation in an F2 intercross between a marine and a lake stickleback population introduced to a freshwater pond. A quantitative trait locus map of the number of surviving offspring per F2 female detected a single, large-effect locus near Ectodysplasin (Eda), a gene having an ancient freshwater allele causing reduced bony armor and other changes. F2 females homozygous for the freshwater allele had twice the number of surviving offspring as homozygotes for the marine allele, producing a large selection coefficient, s = 0.50 ± 0.09 SE. Correspondingly, the frequency of the freshwater allele increased from 0.50 in F2 mothers to 0.58 in surviving offspring. We compare these results to allele frequency changes at the Eda gene in an Alaskan lake population colonized by marine stickleback in the 1980s. The frequency of the freshwater Eda allele rose steadily over multiple generations and reached 95% within 20 y, yielding a similar estimate of selection, s = 0.49 ± 0.05, but a different degree of dominance. These findings are consistent with other studies suggesting strong selection on this gene (and/or linked genes) in fresh water. Selection on ancient genetic variants carried by colonizing ancestors is likely to increase the prevalence of large-effect fitness variants in adaptive evolution.

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.

Evolutionary origins for ecological patterns in space

Historically, many biologists assumed that evolution and ecology acted independently because evolution occurred over distances too great to influence most ecological patterns. Today, evidence indicates that evolution can operate over a range of spatial scales, including fine spatial scales. Thus, evolutionary divergence across space might frequently interact with the mechanisms that also determine spatial ecological patterns. Here, we synthesize insights from 500 eco-evolutionary studies and develop a predictive framework that seeks to understand whether and when evolution amplifies, dampens, or creates ecological patterns. We demonstrate that local adaptation can alter everything from spatial variation in population abundances to ecosystem properties. We uncover 14 mechanisms that can mediate the outcome of evolution on spatial ecological patterns. Sometimes, evolution amplifies environmental variation, especially when selection enhances resource uptake or patch selection. The local evolution of foundation or keystone species can create ecological patterns where none existed originally. However, most often, we find that evolution dampens existing environmental gradients, because local adaptation evens out fitness across environments and thus counteracts the variation in associated ecological patterns. Consequently, evolution generally smooths out the underlying heterogeneity in nature, making the world appear less ragged than it would be in the absence of evolution. We end by highlighting the future research needed to inform a fully integrated and predictive biology that accounts for eco-evolutionary interactions in both space and time.

Trophic control changes with season and nutrient loading in lakes

Experiments have revealed much about top‐down and bottom‐up control in ecosystems, but manipulative experiments are limited in spatial and temporal scale. To obtain a more nuanced understanding of trophic control over large scales, we explored long‐term time‐series data from 13 globally distributed lakes and used empirical dynamic modelling to quantify interaction strengths between zooplankton and phytoplankton over time within and across lakes. Across all lakes, top‐down effects were associated with nutrients, switching from negative in mesotrophic lakes to positive in oligotrophic lakes. This result suggests that zooplankton nutrient recycling exceeds grazing pressure in nutrient‐limited systems. Within individual lakes, results were consistent with a ‘seasonal reset’ hypothesis in which top‐down and bottom‐up interactions varied seasonally and were both strongest at the beginning of the growing season. Thus, trophic control is not static, but varies with abiotic conditions – dynamics that only become evident when observing changes over large spatial and temporal scales.

The evolution of competitive ability for essential resources

Competition for limiting resources is among the most fundamental ecological interactions and has long been considered a key driver of species coexistence and biodiversity. Species' minimum resource requirements, their R*s, are key traits that link individual physiological demands to the outcome of competition. However, a major question remains unanswered-to what extent are species' competitive traits able to evolve in response to resource limitation? To address this knowledge gap, we performed an evolution experiment in which we exposed Chlamydomonas reinhardtii for approximately 285 generations to seven environments in chemostats that differed in resource supply ratios (including nitrogen, phosphorus and light limitation) and salt stress. We then grew the ancestors and descendants in a common garden and quantified their competitive abilities for essential resources. We investigated constraints on trait evolution by testing whether changes in resource requirements for different resources were correlated. Competitive abilities for phosphorus improved in all populations, while competitive abilities for nitrogen and light increased in some populations and decreased in others. In contrast to the common assumption that there are trade-offs between competitive abilities for different resources, we found that improvements in competitive ability for a resource came at no detectable cost. Instead, improvements in competitive ability for multiple resources were either positively correlated or not significantly correlated. Using resource competition theory, we then demonstrated that rapid adaptation in competitive traits altered the predicted outcomes of competition. These results highlight the need to incorporate contemporary evolutionary change into predictions of competitive community dynamics over environmental gradients. This article is part of the theme issue 'Conceptual challenges in microbial community ecology'.

Prey adaptation along a competition-defense tradeoff cryptically shifts trophic cascades from density- to trait-mediated

Trophic cascades have become a dominant paradigm in ecology, yet considerable debate remains about the relative strength of density- (consumptive) and trait-mediated (non-consumptive) effects in trophic cascades. This debate may, in part, be resolved by considering prey experience, which shapes prey traits (through genetic and plastic change) and influences prey survival (and therefore density). Here, we investigate the cascading role of prey experience through the addition of mosquitofish (Gambusia affinis) from predator-experienced or predator-naïve sources to mesocosms containing piscivorous largemouth bass (Micropterus salmoides), zooplankton, and phytoplankton. These two sources were positioned along a competition-defense tradeoff. Results show that predator-naïve mosquitofish suffered higher depredation rates, which drove a density-mediated cascade, whereas predator-experienced mosquitofish exhibited higher survival but fed less, which drove a trait-mediated cascade. Both cascades were similar in strength, leading to indistinguishable top-down effects on lower trophic levels. Therefore, the accumulation of prey experience with predators can cryptically shift cascade mechanisms from density- to trait-mediated.

Climate shapes population variation in dogwhelk predation on foundational mussels

Trait variation among populations is important for shaping ecological dynamics. In marine intertidal systems, seawater temperature, low tide emersion temperature, and pH can drive variation in traits and affect species interactions. In western North America, Nucella dogwhelks are intertidal drilling predators of the habitat-forming mussel Mytilus californianus. Nucella exhibit local adaptation, but it is not known to what extent environmental factors and genetic structure contribute to variation in prey selectivity among populations. We surveyed drilled mussels at sites across Oregon and California, USA, and used multiple regression and Mantel tests to test the effects of abiotic factors and Nucella neutral genetic relatedness on the size of mussels drilled across sites. Our results show that Nucella at sites characterized by higher and less variable temperature and pH drilled larger mussels. Warmer temperatures appear to induce faster handling time, and more stable pH conditions may prolong opportunities for active foraging by reducing exposure to repeated stressful conditions. In contrast, there was no significant effect of genetic relatedness on prey size selectivity. Our results emphasize the role of climate in shaping marine predator selectivity on a foundation species. As coastal climates change, predator traits will respond to localized environmental conditions, changing ecological interactions.

Eco-evolutionary feedbacks link prey adaptation to predator performance

Eco-evolutionary feedbacks may determine the outcome of predator-prey interactions in nature, but little work has been done to quantify the feedback effect of short-term prey adaptation on predator performance. We tested the effects of prey availability and recent (less than 100 years) prey adaptation on the feeding and growth rate of largemouth bass (Micropterus salmoides), foraging on western mosquitofish (Gambusia affinis). Field surveys showed higher densities and larger average body sizes of mosquitofish in recently introduced populations without bass. Over a six-week mesocosm experiment, bass were presented with either a high or low availability of mosquitofish prey from recently established populations either naive or experienced with bass. Naive mosquitofish were larger, less cryptic and more vulnerable to bass predation compared to their experienced counterparts. Bass consumed more naive prey, grew more quickly with naive prey, and grew more quickly per unit biomass of naive prey consumed. The effect of mosquitofish history with the bass on bass growth was similar in magnitude to the effect of mosquitofish availability. In showing that recently derived predation-related prey phenotypes strongly affect predator performance, this study supports the presence of reciprocal predator-prey trait feedbacks in nature.

A way forward with eco evo devo: an extended theory of resource polymorphism with postglacial fishes as model systems

A major goal of evolutionary science is to understand how biological diversity is generated and altered. Despite considerable advances, we still have limited insight into how phenotypic variation arises and is sorted by natural selection. Here we argue that an integrated view, which merges ecology, evolution and developmental biology (eco evo devo) on an equal footing, is needed to understand the multifaceted role of the environment in simultaneously determining the development of the phenotype and the nature of the selective environment, and how organisms in turn affect the environment through eco evo and eco devo feedbacks. To illustrate the usefulness of an integrated eco evo devo perspective, we connect it with the theory of resource polymorphism (i.e. the phenotypic and genetic diversification that occurs in response to variation in available resources). In so doing, we highlight fishes from recently glaciated freshwater systems as exceptionally well-suited model systems for testing predictions of an eco evo devo framework in studies of diversification. Studies on these fishes show that intraspecific diversity can evolve rapidly, and that this process is jointly facilitated by (i) the availability of diverse environments promoting divergent natural selection; (ii) dynamic developmental processes sensitive to environmental and genetic signals; and (iii) eco evo and eco devo feedbacks influencing the selective and developmental environments of the phenotype. We highlight empirical examples and present a conceptual model for the generation of resource polymorphism - emphasizing eco evo devo, and identify current gaps in knowledge.

Gene expression correlates of facultative predation in the blow fly Chrysomya rufifacies (Diptera: Calliphoridae)

Effects of intraguild predation (IGP) on omnivores and detritivores are relatively understudied when compared to work on predator guilds. Functional genetic work in IGP is even more limited, but its application can help answer a range of questions related to ultimate and proximate causes of this behavior. Here, we integrate behavioral assays and transcriptomic analysis of facultative predation in a blow fly (Diptera: Calliphoridae) to evaluate the prevalence, effect, and correlated gene expression of facultative predation by the invasive species Chrysomya rufifacies. Field work observing donated human cadavers indicated facultative predation by C. rufifacies on the native blow fly Cochliomyia macellaria was rare under undisturbed conditions, owing in part to spatial segregation between species. Laboratory assays under conditions of starvation showed predation had a direct fitness benefit (i.e., survival) to the predator. As a genome is not available for C. rufifacies, a de novo transcriptome was developed and annotated using sequence similarity to Drosophila melanogaster. Under a variety of assembly parameters, several genes were identified as being differentially expressed between predators and nonpredators of this species, including genes involved in cell-to-cell signaling, osmotic regulation, starvation responses, and dopamine regulation. Results of this work were integrated to develop a model of the processes and genetic regulation controlling facultative predation.

A Single Interacting Species Leads to Widespread Parallel Evolution of the Stickleback Genome

Biotic interactions are potent, widespread causes of natural selection and divergent phenotypic evolution and can lead to genetic differentiation with gene flow among wild populations ("isolation by ecology") [1-4]. Biotic selection has been predicted to act on more genes than abiotic selection thereby driving greater adaptation [5]. However, difficulties in isolating the genome-wide effect of single biotic agents of selection have limited our ability to identify and quantify the number and type of genetic regions responding to biotic selection [6-9]. We identified geographically interspersed lakes in which threespine stickleback fish (Gasterosteus aculeatus) have repeatedly adapted to the presence or absence of a single member of the ecological community, prickly sculpin (Cottus asper), a fish that is both a competitor and a predator of the stickleback [10]. Whole-genome sequencing revealed that sculpin presence or absence accounted for the majority of genetic divergence among stickleback populations, more so than geography. The major axis of genomic variation within and between the two lake types was correlated with multiple traits, indicating parallel natural selection across a gradient of biotic environments. A large proportion of the genome-about 1.8%, encompassing more than 600 genes-differentiated stickleback from the two biotic environments. Divergence occurred in 141 discrete genomic clumps located mainly in regions of low recombination, suggesting that genes brought to lakes by the colonizing ancestral population often evolved together in linked blocks. Strong selection and a wealth of standing genetic variation explain how a single member of the biotic community can have such a rapid and profound evolutionary impact.

The community and ecosystem consequences of intraspecific diversity: a meta-analysis

Understanding the relationships between biodiversity and ecosystem functioning has major implications. Biodiversity-ecosystem functioning relationships are generally investigated at the interspecific level, although intraspecific diversity (i.e. within-species diversity) is increasingly perceived as an important ecological facet of biodiversity. Here, we provide a quantitative and integrative synthesis testing, across diverse plant and animal species, whether intraspecific diversity is a major driver of community dynamics and ecosystem functioning. We specifically tested (i) whether the number of genotypes/phenotypes (i.e. intraspecific richness) or the specific identity of genotypes/phenotypes (i.e. intraspecific variation) in populations modulate the structure of communities and the functioning of ecosystems, (ii) whether the ecological effects of intraspecific richness and variation are strong in magnitude, and (iii) whether these effects vary among taxonomic groups and ecological responses. We found a non-linear relationship between intraspecific richness and community and ecosystem dynamics that follows a saturating curve shape, as observed for biodiversity-function relationships measured at the interspecific level. Importantly, intraspecific richness modulated ecological dynamics with a magnitude that was equal to that previously reported for interspecific richness. Our results further confirm, based on a database containing more than 50 species, that intraspecific variation also has substantial effects on ecological dynamics. We demonstrated that the effects of intraspecific variation are twice as high as expected by chance, and that they might have been underestimated previously. Finally, we found that the ecological effects of intraspecific variation are not homogeneous and are actually stronger when intraspecific variation is manipulated in primary producers than in consumer species, and when they are measured at the ecosystem rather than at the community level. Overall, we demonstrated that the two facets of intraspecific diversity (richness and variation) can both strongly affect community and ecosystem dynamics, which reveals the pivotal role of within-species biodiversity for understanding ecological dynamics.

The ecological importance of intraspecific variation

Human activity is causing wild populations to experience rapid trait change and local extirpation. The resulting effects on intraspecific variation could have substantial consequences for ecological processes and ecosystem services. Although researchers have long acknowledged that variation among species influences the surrounding environment, only recently has evidence accumulated for the ecological importance of variation within species. We conducted a meta-analysis comparing the ecological effects of variation within a species (intraspecific effects) with the effects of replacement or removal of that species (species effects). We evaluated direct and indirect ecological responses, including changes in abundance (or biomass), rates of ecological processes and changes in community composition. Our results show that intraspecific effects are often comparable to, and sometimes stronger than, species effects. Species effects tend to be larger for direct ecological responses (for example, through consumption), whereas intraspecific effects and species effects tend to be similar for indirect responses (for example, through trophic cascades). Intraspecific effects are especially strong when indirect interactions alter community composition. Our results summarize data from the first generation of studies examining the relative ecological effects of intraspecific variation. Our conclusions can help inform the design of future experiments and the formulation of strategies to quantify and conserve biodiversity.

Temporal Variation in Trophic Cascades

The trophic cascade has emerged as a key paradigm in ecology. Although ecologists have made progress in understanding spatial variation in the strength of trophic cascades, temporal variation remains relatively unexplored. Our review suggests that strong trophic cascades are often transient, appearing when ecological conditions support high consumer abundance and rapidly growing, highly edible prey. Persistent top-down control is expected to decay over time in the absence of external drivers, as strong top-down control favors the emergence of better-defended resources. Temporal shifts in cascade strength—including those driven by contemporary global change—can either stabilize or destabilize ecological communities. We suggest that a more temporally explicit approach can improve our ability to explain the drivers of trophic cascades and predict the impact of changing cascade strength on community dynamics.

Stabilizing mechanisms in a food web with an introduced omnivore

Intraguild predation (IGP) is an omnivorous food web configuration in which the top predator consumes both a competitor (consumer) and a second prey that it shares with the competitor. This omnivorous configuration occurs frequently in food webs, but theory suggests that it is unstable unless stabilizing mechanisms exist that can decrease the strength of the omnivore and consumer interaction. Although these mechanisms have been documented in native food webs, little is known about whether they operate in the context of an introduced species. Here, we study a marine mussel aquaculture system where the introduction of omnivorous mussels should generate an unstable food web that favors the extinction of the consumer, yet it persists. Using field and laboratory approaches, we searched for stabilizing mechanisms that could reduce interaction strengths in the food web. While field zooplankton counts suggested that mussels influence the composition and abundance of copepods, stable isotope results indicated that life‐history omnivory and cannibalism facilitated the availability of prey refugia, and reduced competition and the interaction strength between the mussel omnivore and zooplankton consumers. In laboratory experiments, however, we found no evidence of adaptive feeding which could weaken predator–consumer interactions. Our food web study suggests that the impact of an introduced omnivore may not only depend on its interaction with native species but also on the availability of stabilizing mechanisms that alter the strength of those interactions.

Recent evolution of extreme cestode growth suppression by a vertebrate host

Parasites can be a major cause of natural selection on hosts, which consequently evolve a variety of strategies to avoid, eliminate, or tolerate infection. When ecologically similar host populations present disparate infection loads, this natural variation can reveal immunological strategies underlying adaptation to infection and population divergence. For instance, the tapeworm Schistocephalus solidus persistently infects 0-80% of threespine stickleback (Gasterosteus aculeatus) in lakes on Vancouver Island. To test whether these heterogeneous infection rates result from evolved differences in immunity, we experimentally exposed laboratory-reared fish from ecologically similar high-infection and no-infection populations to controlled doses of Schistocephalus We observed heritable between-population differences in several immune traits: Fish from the naturally uninfected population initiated a stronger granulocyte response to Schistocephalus infection, and their granulocytes constitutively generate threefold more reactive oxygen species in cell culture. Despite these immunological differences, Schistocephalus was equally successful at establishing initial infections in both host populations. However, the no-infection fish dramatically suppressed tapeworm growth relative to high-infection fish, and parasite size was intermediate in F1 hybrid hosts. Our results show that stickleback recently evolved heritable variation in their capacity to suppress helminth growth by two orders of magnitude. Data from many natural populations indicate that growth suppression is widespread but not universal and, when present, is associated with reduced infection prevalence. Host suppression of helminth somatic growth may be an important immune strategy that aids in parasite clearance or in mitigating the fitness costs of persistent infection.

Different hunting strategies of generalist predators result in functional differences

The morphological, physiological, and behavioural traits of organisms are often used as surrogates for actual ecological functions. However, differences in these traits do not necessarily lead to functional differences and/or can be context-dependent. Therefore, it is necessary to explicitly test whether the surrogates have general ecological relevance. To investigate the relationship between the hunting strategies of predators (i.e., how, where, and when they hunt) and their function, we used euryphagous spiders as a model group. We used published data on the diet composition of 76 spider species based on natural prey and laboratory prey acceptance experiments. We computed differences in the position and width of trophic niches among pairs of sympatrically occurring species. Pairs were made at different classification levels, ranked according to the dissimilarity in their hunting strategies: congeners, confamiliars (as phylogenetic proxies for similarity in hunting strategy), species from the same main class of hunting strategy, from the same supra-class, and from different supra-classes. As for niche position computed from the natural prey analyses, species from the same class differed less than species from different classes. A similar pattern was obtained from the laboratory studies, but the congeners differed less than the species from the same classes. Niche widths were most similar among congeners and dissimilar among species from different supra-classes. Functional differences among euryphagous spiders increased continuously with increasing difference in their hunting strategy. The relative frequency of hunting strategies within spider assemblages can, therefore, influence the food webs through hunting strategy-specific predator-prey interactions.

Model-based analysis of causes for habitat segregation in Idotea species (Crustacea, Isopoda)

On the shore of the rocky island of Helgoland (North Sea) two closely related isopod species, Idotea balthicaPallas, 1772, and Idotea granulosaRathke, 1843, share a similar fundamental niche but inhabit well-separated habitats. Idotea balthica inhabits floating algae at the sea surface and accumulations of decaying algae on the seafloor, whereas I. granulosa primarily occurs in intertidal macroalgal belts. In laboratory experiments on individually reared isopods I. balthica outperformed I. granulosa with regard to growth, reproduction, and mortality in both a fully inundated habitat and in a tidal habitat with 5 h of daily emergence. We hypothesized that habitat segregation in the two isopod species is driven by one or multiple types of biotic interactions: (1) no interaction, (2) cannibalism, (3) intraguild predation, and (4) terrestrial predation. In order to evaluate how habitat segregation can be explained by each of these interaction types we employed a size-structured population model to account for the body-size-dependent predation. Net population growth rates were fitted to the simulations as a measure of population fitness. Experimental results served as database for parameter and process identification. As predation rates were unknown, we performed a sensitivity analysis for these. We found that below 5 h of daily tidal emergence either cannibalism or terrestrial predation sufficed to explain habitat segregation. Intraguild predation, in contrast, advantaged I. balthica in any case. From linear extrapolation of the effects occurring under conditions of 5 h of daily tidal emergence, we predict that contrasting physiological responses in I. balthica and I. granulosa would cause segregation even without any interaction if emergence lasted long enough.

Adaptive evolution of foraging-related trait in intraguild predation system

This paper considers a tri-trophic food chain in which the top predator (intraguild predator) also feeds on the basal resource. We refer to the model as intraguild predation. We analyze its dynamics from an evolutionary perspective. The attack rate or foraging effort of the middle species (intraguild prey) for the basal resources is assumed to be evolvable and is also assumed to be traded off with the vulnerability to being attacked by the top predator. We focus on the analysis of the evolutionary dynamics of the attack rate using the adaptive dynamics approximation of mutation limited evolution. In particular, the critical function analysis is applied. This study reveals that the evolutionary dynamics of the intraguild predation system is completely characterized by the concavity of the trade-off function and admits trichotomous dynamic scenarios: (1) when the trade-off function is more concave than the critical function, an evolutionary singular strategy exists and is a repeller; (2) when the trade-off function is less concave than the critical function, the evolutionary singular strategy is convergence stable and turns into an evolutionary branching point, in which case the monomorphic intraguild prey will split into two different types; (3) when the trade-off function is convex, the evolutionary singular strategy turns into a continuous stable strategy and is uninvadable. Our theoretical analysis suggests that the adaptive foraging behavior may strongly influence the community stability. Consequently, it may promote the diversity of intraguild prey and the persistence of the system on the evolutionary timescale, which highlight a more comprehensive mechanistic understanding of the intricate interplay between ecological and evolutionary force. This modeling approach provides a venue for research on indirect effects from an evolutionary perspective.

Speciation, Ecological Opportunity, and Latitude (American Society of Naturalists Address)

Evolutionary hypotheses to explain the greater numbers of species in the tropics than the temperate zone include greater age and area, higher temperature and metabolic rates, and greater ecological opportunity. These ideas make contrasting predictions about the relationship between speciation processes and latitude, which I elaborate and evaluate. Available data suggest that per capita speciation rates are currently highest in the temperate zone and that diversification rates (speciation minus extinction) are similar between latitudes. In contrast, clades whose oldest analyzed dates precede the Eocene thermal maximum, when the extent of the tropics was much greater than today, tend to show highest speciation and diversification rates in the tropics. These findings are consistent with age and area, which is alone among hypotheses in predicting a time trend. Higher recent speciation rates in the temperate zone than the tropics suggest an additional response to high ecological opportunity associated with low species diversity. These broad patterns are compelling but provide limited insights into underlying mechanisms, arguing that studies of speciation processes along the latitudinal gradient will be vital. Using threespine stickleback in depauperate northern lakes as an example, I show how high ecological opportunity can lead to rapid speciation. The results support a role for ecological opportunity in speciation, but its importance in the evolution of the latitudinal gradient remains uncertain. I conclude that per capita evolutionary rates are no longer higher in the tropics than the temperate zone. Nevertheless, the vast numbers of species that have already accumulated in the tropics ensure that total rate of species production remains highest there. Thus, tropical evolutionary momentum helps to perpetuate the steep latitudinal biodiversity gradient.

Intraguild predation leads to genetically based character shifts in the threespine stickleback

Intraguild predation is a common ecological interaction that occurs when a species preys upon another species with which it competes. The interaction is potentially a mechanism of divergence between intraguild prey (IG-prey) populations, but it is unknown if cases of character shifts in IG-prey are an environmental or evolutionary response. We investigated the genetic basis and inducibility of character shifts in threespine stickleback from lakes with and without prickly sculpin, a benthic intraguild predator (IG-predator). Wild populations of stickleback sympatric with sculpin repeatedly show greater defensive armor and water column height preference. We laboratory-raised stickleback from lakes with and without sculpin, as well as marine stickleback, and found that differences between populations in armor, body shape, and behavior persisted in a common garden. Within the common garden, we raised stickleback half-families from multiple populations in the presence and absence of sculpin. Although the presence of sculpin induced trait changes in the marine stickleback, we did not observe an induced response in the freshwater stickleback. Behavioral and morphological trait differences between freshwater populations thus have a genetic basis and suggest an evolutionary response to intraguild predation.

Extent of QTL Reuse During Repeated Phenotypic Divergence of Sympatric Threespine Stickleback

How predictable is the genetic basis of phenotypic adaptation? Answering this question begins by estimating the repeatability of adaptation at the genetic level. Here, we provide a comprehensive estimate of the repeatability of the genetic basis of adaptive phenotypic evolution in a natural system. We used quantitative trait locus (QTL) mapping to discover genomic regions controlling a large number of morphological traits that have diverged in parallel between pairs of threespine stickleback (Gasterosteus aculeatus species complex) in Paxton and Priest lakes, British Columbia. We found that nearly half of QTL affected the same traits in the same direction in both species pairs. Another 40% influenced a parallel phenotypic trait in one lake but not the other. The remaining 10% of QTL had phenotypic effects in opposite directions in the two species pairs. Similarity in the proportional contributions of all QTL to parallel trait differences was about 0.4. Surprisingly, QTL reuse was unrelated to phenotypic effect size. Our results indicate that repeated use of the same genomic regions is a pervasive feature of parallel phenotypic adaptation, at least in sticklebacks. Identifying the causes of this pattern would aid prediction of the genetic basis of phenotypic evolution.

Habitat-based polymorphism is common in stream fishes

Morphological differences (size and shape) across habitats are common in lake fish where differences relate to two dominant contrasting habitats: the pelagic and littoral habitat. Repeated occurrence of littoral and pelagic morphs across multiple populations of several lake fish species has been considered as important evidence that polymorphism is adaptive in these systems. It has been suggested that these habitat-based polymorphic differences are due to the temporal stability of the differences between littoral and pelagic habitats. Although streams are spatially heterogeneous, they are also more temporally dynamic than lakes and it is still an open question whether streams provide the environmental conditions that promote habitat-based polymorphism. We tested whether fish from riffle, run and pool habitats, respectively, differed consistently in their morphology. Our test compared patterns of morphological variation (size and shape) in 10 fish species from the three stream habitat types in 36 separate streams distributed across three watersheds. For most species, body size and shape (after controlling for body size) differed across riffle, run and pool habitats. Unlike many lake species, the nature of these differences was not consistent across species, possibly because these species use these habitat types in different ways. Our results suggest that habitat-based polymorphism is an important feature also in stream fishes despite the fact that streams are temporally variable in contrast to lake systems. Future research is required to assess whether the patterns of habitat-based polymorphism encountered in streams have a genetic basis or they are simply the result of within generation phenotypic plasticity.

Evolution of body shape in differently coloured sympatric congeners and allopatric populations of Lake Malawi's rock-dwelling cichlids

The cichlid fishes of Lake Malawi represent one of the most diverse adaptive radiations of vertebrates known. Among the rock-dwelling cichlids (mbuna), closely related sympatric congeners possess similar trophic morphologies (i.e. cranial and jaw structures), defend overlapping or adjacent territories, but can be easily distinguished based on male nuptial coloration. The apparent morphological similarity of congeners, however, leads to an ecological conundrum: theory predicts that ecological competition should lead to competitive exclusion. Hence, we hypothesized that slight, yet significant, ecological differences accompanied the divergence in sexual signals and that the divergence of ecological and sexual traits is correlated. To evaluate this hypothesis, we quantified body shape, a trait of known ecological importance, in populations of Maylandia zebra, a barred, widespread mbuna, and several sympatric nonbarred congeners. We found that the barred populations differ in body shape from their nonbarred sympatric congeners and that the direction of shape differences was consistent across all barred vs. nonbarred comparisons. Barred populations are generally deeper bodied which may be an adaptation to the structurally complex habitat they prefer, whereas the nonbarred species have a more fusiform body shape, which may be adaptive in their more open microhabitat. Furthermore, M. zebra populations sympatric with nonbarred congeners differ from populations where the nonbarred phenotype is absent and occupy less morphospace, indicating potential ecological character displacement. Mitochondrial DNA as well as published AFLP data indicated that the nonbarred populations are not monophyletic and therefore may have evolved multiple times independently. Overall our data suggest that the evolution of coloration and body shape may be coupled as a result of correlational selection. We hypothesize that correlated evolution of sexually selected and ecological traits may have contributed to rapid speciation as well as the maintenance of diversity in one of the most diverse adaptive radiations known.

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.

Genetic signature of adaptive peak shift in threespine stickleback

Transition of an evolving population to a new adaptive optimum is predicted to leave a signature in the distribution of effect sizes of fixed mutations. If they affect many traits (are pleiotropic), large effect mutations should contribute more when a population evolves to a farther adaptive peak than to a nearer peak. We tested this prediction in wild threespine stickleback fish (Gasterosteus aculeatus) by comparing the estimated frequency of large effect genetic changes underlying evolution as the same ancestor adapted to two lake types since the end of the ice age. A higher frequency of large effect genetic changes (quantitative trait loci) contributed to adaptive evolution in populations that adapted to lakes representing a more distant optimum than to lakes in which the optimum phenotype was nearer to the ancestral state. Our results also indicate that pleiotropy, not just optimum overshoot, contributes to this difference. These results suggest that a series of adaptive improvements to a new environment leaves a detectable mark in the genome of wild populations. Although not all assumptions of the theory are likely met in natural systems, the prediction may be robust enough to the complexities of natural environments to be useful when forecasting adaptive responses to large environmental changes.