Predator effects on prey dynamics are conventionally studied by measuring changes in prey abundance attributed to consumption by predators. We revisit four classic examples of predator—prey systems ...often cited in textbooks and incorporate subsequent studies of nonconsumptive effects of predators (NCE), defined as changes in prey traits (e.g., behavior, growth, development) measured on an ecological time scale. Our review revealed that NCE were integral to explaining lynx—hare population dynamics in boreal forests, cascading effects of top predators in Wisconsin lakes, and cascading effects of killer whales and sea otters on kelp forests in nearshore marine habitats. The relatives roles of consumption and NCE of wolves on moose and consequent indirect effects on plant communities of Isle Royale depended on climate oscillations. Nonconsumptive effects have not been explicitly tested to explain the link between planktonic alewives and the size structure of the zooplankton, nor have they been invoked to attribute keystone predator status in intertidal communities or elsewhere. We argue that both consumption and intimidation contribute to the total effects of keystone predators, and that characteristics of keystone consumers may differ from those of predators having predominantly NCE. Nonconsumptive effects are often considered as an afterthought to explain observations inconsistent with consumption-based theory. Consequently, NCE with the same sign as consumptive effects may be overlooked, even though they can affect the magnitude, rate, or scale of a prey response to predation and can have important management or conservation implications. Nonconsumptive effects may underlie other classic paradigms in ecology, such as delayed density dependence and predator-mediated prey coexistence. Revisiting classic studies enriches our understanding of predator—prey dynamics and provides compelling rationale for ramping up efforts to consider how NCE affect traditional predator—prey models based on consumption, and to compare the relative magnitude of consumptive and NCE of predators.
Most empirical and theoretical studies of resource use and population dynamics treat conspecific individuals as ecologically equivalent. This simplification is only justified if interindividual niche ...variation is rare, weak, or has a trivial effect on ecological processes. This article reviews the incidence, degree, causes, and implications of individual‐level niche variation to challenge these simplifications. Evidence for individual specialization is available for 93 species distributed across a broad range of taxonomic groups. Although few studies have quantified the degree to which individuals are specialized relative to their population, between‐individual variation can sometimes comprise the majority of the population’s niche width. The degree of individual specialization varies widely among species and among populations, reflecting a diverse array of physiological, behavioral, and ecological mechanisms that can generate intrapopulation variation. Finally, individual specialization has potentially important ecological, evolutionary, and conservation implications. Theory suggests that niche variation facilitates frequency‐dependent interactions that can profoundly affect the population’s stability, the amount of intraspecific competition, fitness‐function shapes, and the population’s capacity to diversify and speciate rapidly. Our collection of case studies suggests that individual specialization is a widespread but underappreciated phenomenon that poses many important but unanswered questions.
Disruptive selection is often assumed to be relatively rare, because it is dynamically unstable and hence should be transient. However, frequency‐dependent interactions such as intraspecific ...competition may stabilize fitness minima and make disruptive selection more common. Such selection helps explain the maintenance of genetic variation and may even contribute to sympatric speciation. There is thus great interest in determining when and where disruptive selection is most likely. Here, we show that there is a general trend toward weak disruptive selection on trophic morphology in three‐spine stickleback (Gasterosteus aculeatus) in 14 lakes on Vancouver Island. Selection is inferred from the observation that, within a lake, fish with intermediate gill raker morphology exhibited slower growth than phenotypically extreme individuals. Such selection has previously been shown to arise from intraspecific competition for alternate resources. However, not all environments are equally conducive to disruptive selection, which was strongest in intermediate‐sized lakes where both littoral and pelagic prey are roughly balanced. Also, consistent with theory, we find that sexual dimorphism in trophic traits tends to mitigate disruptive selection. These results suggest that it may be possible to anticipate the kinds of environments and populations most likely to experience disruptive selection.
Divergence in habitat use among closely related species is a common characteristic of adaptive radiations. Large differences in the size structure of prey between habitats could strengthen disruptive ...selection on generalist predators and lead to a divergence in trophic position among species in an adaptive radiation. Using threespine stickleback (Gasterosteus aculeatus) in freshwater lakes as a model system, we examined whether divergence in habitat use coincides with shifts in trophic position. We examined the habitat use and trophic position of individual sticklebacks from divergent lake environments that have only one stickleback species (allopatric lakes) and from lakes that have a pair of benthic and limnetic stickleback species (sympatric lakes). In two sympatric lakes, the limnetic species had a higher trophic position than the benthic species, and in both allopatric and sympatric lakes, sticklebacks specializing on pelagic prey had a higher trophic position for a given size than sticklebacks specializing on benthic prey. Furthermore, the trophic position of pelagic specialists was correlated with individual variation in their gill raker length. Our results indicate that gill raker length is an important trait that underlies differentiation in both habitat use and trophic position among stickleback species, populations, and individuals.
Hybrid viability decreases with divergence time, a pattern consistent with a so‐called speciation clock. However, the actual rate at which this clock ticks is poorly known. Most speciation‐clock ...studies have used genetic divergence as a proxy for time, adopting a molecular clock and often far‐distant calibration points to convert genetic distances into age. Because molecular clock assumptions are violated for most genetic datasets and distant calibrations are of questionable utility, the actual rate at which reproductive isolation evolves may be substantially different than current estimates suggest. We provide a robust measure of the tempo at which hybrid viability declines with divergence time in a clade of freshwater fishes (Centrarchidae). This incompatibility clock is distinct from a speciation clock because speciation events in centrarchids appear to be driven largely by prezygotic isolation. Our analyses used divergence times estimated with penalized likelihood applied to a phylogeny derived from seven gene regions and calibrated with six centrarchid fossils. We found that hybrid embryo viability declined at mean rate of 3.13% per million years, slower than in most other taxa investigated to date. Despite measurement error in both molecular estimated ages and hatching success of hybrid crosses, divergence time explained between 73% and 90% of the variation in hybrid viability among nodes. This high correlation is consistent with the gradual accumulation of many genetic incompatibilities of small effect. Hybrid viability declined with the square of time, consistent with an increasing rate of accumulation of incompatibilities between divergent genomes (the snowball effect). However, the quadratic slope is due to a lag phase resulting from heterosis among young species pairs, a phenomenon rarely considered in predictions of hybrid fitness. Finally, we found that reciprocal crosses often show asymmetrical hybrid viabilities. We discuss several alternative explanations for this result including possible deleterious cytonuclear interactions. Speciation‐clock studies have been a small cottage industry recently, but there are still novel insights to be gained from analyses of more taxonomic groups. However, between‐group comparisons require more careful molecular‐clock calibration than has been the norm.
Question: Why would individuals that inhabit the same environment choose to feed on different subsets of the available resources? Mathematical method: We outline a flexible model that combines ...phenotypic variation with optimal diet theory and population dynamics. We then apply this model to investigate the role of different types of trade-offs, phenotype diversity and level of competition in determining the degree of individual specialization. Key assumptions: The foragers in the model are omniscient and maximize energy intake per time unit. Conclusion: Numerical simulations match empirical observations that changes in population density can alter the degree of individual specialization. Forager density and phenotypic variation affected prey densities, which in turn affected forager diet breadth and fitness (energy income). We propose that this feedback can explain the empirical relationship between forager density and the degree of individual specialization in the forager population.
Faced with sudden environmental changes, animals must either adapt to novel environments or go extinct. Thus, study of the mechanisms underlying rapid adaptation is crucial not only for the ...understanding of natural evolutionary processes but also for the understanding of human-induced evolutionary change, which is an increasingly important problem 1–8. In the present study, we demonstrate that the frequency of completely plated threespine stickleback fish (Gasterosteus aculeatus) has increased in an urban freshwater lake (Lake Washington, Seattle, Washington) within the last 40 years. This is a dramatic example of “reverse evolution,”9 because the general evolutionary trajectory is toward armor-plate reduction in freshwater sticklebacks 10. On the basis of our genetic studies and simulations, we propose that the most likely cause of reverse evolution is increased selection for the completely plated morph, which we suggest could result from higher levels of trout predation after a sudden increase in water transparency during the early 1970s. Rapid evolution was facilitated by the existence of standing allelic variation in Ectodysplasin (Eda), the gene that underlies the major plate-morph locus 11. The Lake Washington stickleback thus provides a novel example of reverse evolution, which is probably caused by a change in allele frequency at the major plate locus in response to a changing predation regime.
Models of adaptive speciation are typically concerned with demonstrating that it is possible for ecologically driven disruptive selection to lead to the evolution of assortative mating and hence ...speciation. However, disruptive selection could also lead to other forms of evolutionary diversification, including ecological sexual dimorphisms. Using a model of frequency-dependent intraspecific competition, we show analytically that adaptive speciation and dimorphism require identical ecological conditions. Numerical simulations of individual-based models show that a single ecological model can produce either evolutionary outcome, depending on the genetic independence of male and female traits and the potential strength of assortative mating. Speciation is inhibited when the genetic basis of male and female ecological traits allows the sexes to diverge substantially. This is because sexual dimorphism, which can evolve quickly, can eliminate the frequency-dependent disruptive selection that would have provided the impetus for speciation. Conversely, populations with strong assortative mating based on ecological traits are less likely to evolve a sexual dimorphism because females cannot simultaneously prefer males more similar to themselves while still allowing the males to diverge. This conflict between speciation and dimorphism can be circumvented in two ways. First, we find a novel form of speciation via negative assortative mating, leading to two dimorphic daughter species. Second, if assortative mating is based on a neutral marker trait, trophic dimorphism and speciation by positive assortative mating can occur simultaneously. We conclude that while adaptive speciation and ecological sexual dimorphism may occur simultaneously, allowing for sexual dimorphism restricts the likelihood of adaptive speciation. Thus, it is important to recognize that disruptive selection due to frequency-dependent interactions can lead to more than one form of adaptive splitting.
Many physiological traits consist of two hierarchically related levels: physical structures and the emergent functional properties of those structures. Because selection tends to act on the emergent ...functional traits, the evolution of structural phenotypes will depend on the nature of the form‐function relationship. Complex physiological or biomechanical traits are often characterized by many‐to‐one mapping: numerous structural phenotypes can yield equivalent functions. We suggest that this redundancy can promote the evolution of phenotypic diversity, and we illustrate this effect with a combination of empirical and analytical studies of a complex biomechanical trait, the four‐bar linkage found in the jaws of labrid fishes. We show that labrid jaws are subject to many‐to‐one mapping of form‐to‐jaw mechanical properties but that some mechanical types have higher levels of morphological redundancy than others. This variation in redundancy has affected the diversity and distribution of labrid jaw shapes: labrid species are disproportionately concentrated around functional traits with higher potential for redundancy. Many‐to‐one mapping can also mitigate evolutionary constraints imposed by mechanical trade‐offs by allowing a species to simultaneously optimize multiple functional properties. Many‐to‐one mapping may be an important factor in generating the uneven patterns of diversity in physiological traits.
Only a fraction of the individuals in a given prey population are likely to be killed and consumed by predators. In contrast, nearly all individuals experience the chronic effects of predation risk. ...When threatened by predators, prey adopt defensive tactics whose costs can lead to reduced growth, maturation rates, survivorship, fecundity, or population density. This nonconsumptive impact of predation risk on prey is known as a "trait-mediated interaction" (TMI) because it results from changes in prey traits such as behavior or physiology. Ecological theory suggests that the strength of TMI effects will reflect a balance between the conflicting demands of reproduction vs. predator avoidance. Competitor density and resource availability are expected to alter the balance between these conflicting forces. We conducted a meta-analysis of experimental studies that measured TMI effect size while varying competitor and/or resource density. The threat of predation had an overall negative effect on prey performance, but the strength of this effect varied with the level of competition. High competition exacerbated the negative effect of intimidation on prey density but moderated the negative effect of intimidation on prey life history and growth. We discuss these results in light of previously published theoretical expectations. Our results highlight the variable and context-dependent nature of interspecific interactions.
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