How Does the Evolution of Universal Ecological Traits Affect Population Size? Lessons from Simple Models.

This article argues that adaptive evolutionary change in a consumer species should frequently decrease (and maladaptive change should increase) population size, producing adaptive decline. This conclusion is based on analysis of multiple consumer-resource models that examine evolutionary change in consumer traits affecting the universal ecological parameters of attack rate, conversion efficiency, and mortality. Two scenarios are investigated. In one, evolutionary equilibrium is initially maintained by opposing effects on the attack rate and other growth rate parameters; the environment or trait is perturbed, and the trait then evolves to a new (or back to a previous) equilibrium. Here evolution exhibits adaptive decline in up to one-half of all cases. The other scenario assumes a genetic perturbation having purely fitness-increasing effects. Here adaptive decline in the consumer requires that the resource be self-reproducing and overexploited and requires a sufficient increase in the attack rate. However, if the resource exhibits adaptive defense via behavior or evolution, adaptive decline may characterize consumer traits affecting all parameters. Favorable environmental change producing parameter shifts similar to those produced by adaptive evolution has similar counterintuitive effects on consumer population size. Many different food web models have already been shown to exhibit such counterintuitive changes in some species.

Hydra effects in stable communities and their implications for system dynamics.

A hydra effect occurs when the mean density of a species increases in response to greater mortality. We show that, in a stable multispecies system, a species exhibits a hydra effect only if maintaining that species at its equilibrium density destabilizes the system. The stability of the original system is due to the responses of the hydra-effect species to changes in the other species' densities. If that dynamical feedback is removed by fixing the density of the hydra-effect species, large changes in the community make-up (including the possibility of species extinction) can occur. This general result has several implications: (1) Hydra effects occur in a much wider variety of species and interaction webs than has previously been described, and may occur for multiple species, even in small webs; (2) conditions for hydra effects caused by predators (or diseases) often differ from those caused by other mortality factors; (3) introducing a specialist or a switching predator of a hydra-effect species often causes large changes in the community, which frequently involve extinction of other species; (4) harvest policies that attempt to maintain a constant density of a hydra-effect species may be difficult to implement, and, if successful, are likely to cause large changes in the densities of other species; and (5) trophic cascades and other indirect effects caused by predators of hydra-effect species can exhibit amplification of effects or unexpected directions of change. Although we concentrate on systems that are originally stable and models with no stage-structure or trait variation, the generality of our result suggests that similar responses to mortality will occur in many systems without these simplifying assumptions. In addition, while hydra effects are defined as responses to altered mortality, they also imply counterintuitive responses to changes in immigration and other parameters affecting population growth.

Is competition needed for ecological character displacement? Does displacement decrease competition?

Interspecific competition for resources is generally considered to be the selective force driving ecological character displacement, and displacement is assumed to reduce competition. Skeptics of the prevalence of character displacement often cite lack of evidence of competition. The present article uses a simple model to examine whether competition is needed for character displacement and whether displacement reduces competition. It treats systems with competing resources, and considers cases when only one consumer evolves. It quantifies competition using several different measures. The analysis shows that selection for divergence of consumers occurs regardless of the level of between-resource competition or whether the indirect interaction between the consumers is competition (-,-), mutualism (+,+), or contramensalism (+,-). Also, divergent evolution always decreases the equilibrium population size of the evolving consumer. Whether divergence of one consumer reduces or increases the impact of a subsequent perturbation of the other consumer depends on the parameters and the method chosen for measuring competition. Divergence in mutualistic interactions may reduce beneficial effects of subsequent increases in the other consumer's population. The evolutionary response is driven by an increase in the relative abundance of the resource the consumer catches more rapidly. Such an increase can occur under several types of interaction.

Why ratio dependence is (still) a bad model of predation.

The history of the idea that predation rates are functions of the ratio of prey density to predator density, known as ratio dependence, is reviewed and updated. When the term was introduced in 1989, it was already known that higher predator abundance often reduced an individual predator's average intake rate of prey. However, the idea that this effect was a universally applicable inverse proportionality was new. That idea was widely criticized in many articles in the early 1990s, and many of these criticisms have never been addressed. Nevertheless, ratio dependence seems to be gaining in popularity and is the subject of a recent monograph by the originators. This article revisits the most important objections to this theory, and assesses to what extent they have been answered by the theory's proponents. In this process, several new objections are raised. The counterarguments begin with the lack of a plausible, generally applicable mechanism that could produce ratio dependence. They include the fact that ratio dependence is a special case of predator-density effects, which, in turn, are only one of many non-prey species effects that influence the consumption rate of a particular prey. The proclaimed simplicity advantage of ratio dependence is at best small and is outweighed by its disadvantages; it predicts biologically implausible phenomena, and cannot easily be extended to describe multi-species systems, trait-mediated interactions, coevolution, and a number of other important ecological phenomena. Any potential small simplicity advantage disappears with corrections to remove unrealistic low-density dynamics caused by ratio dependence. The frequent occurrence of strong predator dependence does not make ratio dependence a better 'default' model of predation than prey dependence, and empirical studies of the full range of non-prey species effects on the consumption rates of predators are needed.

The evolutionary and behavioral modification of consumer responses to environmental change.

How will evolution or other forms of adaptive change alter the response of a consumer species' population density to environmentally driven changes in population growth parameters? This question is addressed by analyzing some simple consumer-resource models to separate the ecological and evolutionary components of the population's response. Ecological responses are always decreased population size, but evolution of traits that have effects on both resource uptake rate and another fitness-related parameter may magnify, offset, or reverse this population decrease. Evolution can change ecologically driven decreases in population size to increases; this is likely when: (1) resources are initially below the density that maximizes resource growth, and (2) the evolutionary response decreases the consumer's resource uptake rate. Evolutionary magnification of the ecological decreases in population size can occur when the environmental change is higher trait-independent mortality. Such evolution-driven decreases are most likely when uptake-rate traits increase and the resource is initially below its maximum growth density. It is common for the difference between the new eco-evolutionary equilibrium and the new ecological equilibrium to be larger than that between the original and new ecological equilibrium densities. The relative magnitudes of ecological and evolutionary effects often depend sensitively on the magnitude of the environmental change and the nature of resource growth.

Is feedback control effective for ecosystem-based fisheries management?

We investigate the effects of species interactions on the robustness of feedback control of the harvesting of prey species. We consider the consequences of feedback control of fishing effort. If a prey species is exploited, increasing fishing effort decreases predator abundance more than it does the prey abundance. Feedback control of fishing effort may cause the extinction of the predator, even if the prey population is well controlled. Even when fishing effort is controlled by predator density, it is difficult for the fishery and the predator to coexist, and, if they do so, the system exhibits complex dynamic behaviors. If the predator and fishery coexist, feedback control of fishing effort converges to a stable equilibrium, a synchronous cycle, or an asynchronous cycle. In the last case, the system undergoes more complex cycling with a longer period than that when the fishing effort is kept constant. These analyses suggest that there is no effective strategy that is robust against measurement errors, process errors and complex interactions in ecosystem dynamics.

Does consumption rate scale superlinearly?

Arising from S. Pawar, A. I. Dell & V. M. Savage 486, 485-489 10.1038/nature11131(2012)A recent paper by Pawar and colleagues has provided important insights into the consequences of foraging behaviour for food-web dynamics. One notable pattern predicted by their analysis is that consumption rate (c) scales superlinearly (cm(1.16)) with consumer body mass (m) in three-dimensional (3D), but not two-dimensional (2D), foraging spaces. Although we feel that the authors should be applauded for this interesting contribution, we argue that their result is not consistent with established life-history theory. To resolve this contradiction, progress in both fields is probably required, including new empirical studies in which consumption rate, metabolism and dimensionality are examined directly under natural conditions.

Evolution of the storage effect.

The storage effect, a mechanism that promotes species coexistence in temporally variable environments, poses a dilemma to evolutionary ecologists. Ecological studies have demonstrated its importance in natural communities, but evolutionary models have predicted that selection either impedes coexistence or diminishes the storage effect if there is coexistence. Here, we develop a lottery model of competition in which two species experience a trade-off in competitive ability between two types of years. We use an adaptive evolution framework to determine conditions favoring the evolution of the storage effect. Storage evolves via divergence of relative performance in the two environments under a wide range of biologically realistic conditions. It evolves between two initially identical species (or lineages) when the trade-off in performance is strong enough. It evolves for species having different initial trade-offs for both weak and strong trade-offs. Our simple 2-species-2-environment scenario can be extended to multiple species and environmental conditions. Results indicate that the storage effect should evolve in a broad range of situations that involve a trade-off in competitive ability among years, and are consistent with empirical observations. The findings show that storage can evolve in a manner and under conditions similar to other types of resource partitioning.

The eco-evolutionary responses of a generalist consumer to resource competition.

This article explores the combined evolutionary and ecological responses of resource uptake abilities in a generalist consumer to exploitative competition for one resource using a simple 2-resource model. It compares the sizes of ecologically and evolutionarily caused changes in population densities in cases where the original consumer has a strong or a weak trade-off in its abilities to consume the two resources. The analysis also compares the responses of the original species to competition when the competitor's population size is or is not limited by the shared resource. Although divergence in resource use traits in the resident generalist consumer is expected under all scenarios when resources are substitutable, the changes in population densities of the resources and resident consumer frequently differ between scenarios. The population of the original consumer often decreases as a result of its own adaptive divergence, and this decrease is often much greater than the initial ecological decrease. If the evolving consumer has a strong trade-off, the overlapped resource increases in equilibrium population density in response to being consumed by a generalist competitor. Some of these predictions differ qualitatively in alternative scenarios involving sustained variation in population densities or nutritionally essential resources.

Harvesting creates ecological traps: consequences of invisible mortality risks in predator-prey metacommunities.

Models of two-patch predator-prey metacommunities are used to explore how the global predator population changes in response to additional mortality in one of the patches. This could describe the dynamics of a predator in an environment that includes a refuge area where that predator is protected and a spatially distinct ("risky") area where it is harvested. The predator's movement is based on its perceived fitness in the two patches, but the risk from the additional mortality is potentially undetectable; this often occurs when the mortality is from human harvesting or from a novel type of top predator. Increases in undetected mortality in the risky area can produce an abrupt collapse of either the refuge population or of the entire predator population when the mortality rate exceeds a threshold level. This is due to the attraction of the risky patch, which has abundant prey due to its high predator mortality. Extinction of the refuge predator population does not occur when the refuge patch has a higher maximum per capita predator growth rate than the exploited patch because the refuge is then more attractive when the predator is rare. The possibility of abrupt extinction of one or both patches from high densities in response to a small increase in harvest is often associated with alternative states. In such cases, large reductions in mortality may be needed to avoid extinction in a collapsing predator population, or to reestablish an extinct population. Our analysis provides a potential explanation for sudden collapses of harvested populations, and it argues for more consideration of adaptive movement in designing protected areas.

The roles of spatial heterogeneity and adaptive movement in stabilizing (or destabilizing) simple metacommunities.

Adaptive consumer movement and between-patch heterogeneity have both been suggested to reduce population fluctuations in spatially subdivided systems. These conjectures are explored using models of two-patch consumer-resource systems with fitness dependent consumer movement and cyclic dynamics in at least one of the patches; neither conjecture applies generally to such systems. Under relatively low heterogeneity, highly accurate and rapid adaptive movement most often increases both the between-patch correlation of density and the variation in the total density of both species compared to a similar system having a low rate of random movement. However, such adaptive movement can decrease between-patch correlation and global population variability when (1) the consumer's movement is moderately sensitive to fitness differences and heterogeneity is relatively low, or (2) one of the patches would be stable in isolation, and the stable patch supports a sufficiently large consumer population. In both cases, the dynamics are typically either a stable equilibrium or a simple anti-phase cycle with low variation in total population size. Under adaptive movement, population variability is often lowest for intermediate levels of heterogeneity, but monotonic increases or decreases with increasing spatial heterogeneity are possible, depending on the fitness sensitivity of movement and how the characteristic that differs between patches affects within-patch stability and population size. High rates of random movement can lead to greater stability than adaptive movement when consumers are very efficient.

Simple life-history omnivory: responses to enrichment and harvesting in systems with intraguild predation.

This article analyzes the nature of top-down and bottom-up effects and alternative states in systems characterized by life-history omnivory. The analysis is based on a three-species food web with intraguild predation (IGP). The top predator population has juvenile and adult stages, which consume the basal resource and the intermediate prey, respectively; the prey consumes only the resource. The per capita reproduction of the adult predators depends on their consumption rate of prey, while the maturation rate of the juvenile predators depends on their resource consumption rate. Enriching the resource can increase or decrease the abundances of one or both of the two consumer species; an increased density is more likely in the intermediate species than in the systems where IGP is not based on stage differences. Alternative states that have or lack the predator occur frequently, particularly when the prey population is capable of reducing the resource to very low densities. These results differ from those of several other recent models of life-history omnivory. They suggest that life-history omnivory may be one of the primary reasons why exploited populations undergo sudden collapses and why collapsed populations fail to recover in spite of large reductions in the exploitation rate.

Modifying modifiers: what happens when interspecific interactions interact?

1. The strength of the trophic link between any given pair of species in a food web is likely to depend on the presence and/or densities of other species in the community. How these trophic interaction modifications (TIMs) interact with one another to produce a net modifying effect is an important but under-explored issue. 2. We review several specific types of TIMs that are well understood to address whether the magnitude of the net modification changes with the number of modifiers, and whether modifiers usually increase or decrease each other's effects. 3. Modifications of interactions are generally not independent. It is likely that TIMs interact antagonistically in the majority of cases; the magnitudes of TIMs decrease as more modifiers are added, or new TIMs reduce the magnitudes of modifications that are already present. 4. Individual modifications are likely to have a smaller effect in many-species systems than expected from independent combination of modifications measured in systems with relatively few species. Thus, models that lack explicit TIMs may in some cases yield adequate predictions for species-level perturbations, provided that the net effects of TIMs are implicitly included in measured interaction strengths. 5. Many types of TIMs share structural similarities. Nevertheless, a complete understanding of their effects may require theory that distinguishes different 'functional groups' of modifiers and addresses how these are structured according to trophic relationships.

How does adaptive consumer movement affect population dynamics in consumer-resource metacommunities with homogeneous patches?

This article uses simple models to explore the impact of adaptive movement by consumers on the population dynamics of a consumer-resource metacommunity consisting of two identical patches. Consumer-resource interactions within a patch are described by the Rosenzweig-MacArthur predator-prey model, and these dynamics are assumed to be cyclic in the absence of movement. The per capita movement rate from one patch to the other is an increasing function of the difference between the per capita birth minus death rate in the destination patch and that in the currently occupied patch. Several variations on this model are considered. Results show that adaptive movement frequently creates anti-phase cycles in the two patches; these suppress the predator-prey cycle and lead to low temporal variation of the total population sizes of both species. Paradoxically, even when movement is very sensitive to the fitness difference between patches, perfect synchrony of patches is often much less likely than in comparable systems with random movement. Under these circumstances adaptive movement of consumers often generates differences in the average properties of the two patches. In addition, mean global densities and responses to global perturbations often differ greatly from similar systems with no movement or random movement.

A multi-scale comparison of trait linkages to environmental and spatial variables in fish communities across a large freshwater lake.

Species present in communities are affected by the prevailing environmental conditions, and the traits that these species display may be sensitive indicators of community responses to environmental change. However, interpretation of community responses may be confounded by environmental variation at different spatial scales. Using a hierarchical approach, we assessed the spatial and temporal variation of traits in coastal fish communities in Lake Huron over a 5-year time period (2001-2005) in response to biotic and abiotic environmental factors. The association of environmental and spatial variables with trophic, life-history, and thermal traits at two spatial scales (regional basin-scale, local site-scale) was quantified using multivariate statistics and variation partitioning. We defined these two scales (regional, local) on which to measure variation and then applied this measurement framework identically in all 5 study years. With this framework, we found that there was no change in the spatial scales of fish community traits over the course of the study, although there were small inter-annual shifts in the importance of regional basin- and local site-scale variables in determining community trait composition (e.g., life-history, trophic, and thermal). The overriding effects of regional-scale variables may be related to inter-annual variation in average summer temperature. Additionally, drivers of fish community traits were highly variable among study years, with some years dominated by environmental variation and others dominated by spatially structured variation. The influence of spatial factors on trait composition was dynamic, which suggests that spatial patterns in fish communities over large landscapes are transient. Air temperature and vegetation were significant variables in most years, underscoring the importance of future climate change and shoreline development as drivers of fish community structure. Overall, a trait-based hierarchical framework may be a useful conservation tool, as it highlights the multi-scaled interactive effect of variables over a large landscape.

Prey persistence and abundance in systems with intraguild predation and type-2 functional responses.

The apparent prevalence of intraguild predation in productive environments has been regarded as puzzling because some simple models suggest that the intraguild prey species is often either reduced in abundance or driven extinct at high resource productivity. While various theoretical mechanisms that avoid this prediction have been uncovered, they have often been viewed as being narrowly applicable. This article examines the fate of the intraguild prey in models in which consumer species may have type-2 functional responses; these are usually characterized by sustained fluctuations in population density at high enough resource productivities. The models also include adaptive, but imperfect diet choice by the top predator. We concentrate on two situations: (1) the prey exhibits less saturation in its functional response to the resource than does the predator and (2) the predator is unable to persist on the basal resource alone. The reasons given by previous studies for discounting these cases are re-examined. The present analysis shows that prey abundance often increases with increasing productivity in both cases, as does the range of prey parameters that allows prey persistence. It is also possible for the prey to coexist with the predator in spite of having a larger equilibrium requirement for the resource. Different assumptions about the dynamics of diet choice can have a large impact on population responses to enrichment. We argue that the persistence and/or increase in abundance of intraguild prey at higher productivity should not be regarded as puzzling because observations are consistent with a range of theoretical models that reflect commonly observed mechanisms.

Adaptive changes in prey vulnerability shape the response of predator populations to mortality.

Simple models are used to explore how adaptive changes in prey vulnerability alter the population response of their predator to increased mortality. If the mortality is an imposed harvest, the change in prey vulnerability also influences the relationship between harvest effort and yield of the predator. The models assume that different prey phenotypes share a single resource, but have different vulnerabilities to the predator. Decreased vulnerability is assumed to decrease resource consumption rate. Adaptive change may occur by phenotypic changes in the traits of a single species or by shifts in the abundances of a pair of coexisting species or morphs. The response of the predator population is influenced by the shape of the predator's functional response, the shape of resource density dependence, and the shape of the tradeoff between vulnerability and food intake in the prey. Given a linear predator functional response, adaptive prey defense tends to produce a decelerating decline in predator population size with increased mortality. Prey defense may also greatly increase the range of mortality rates that allow predator persistence. If the predator has a type-2 response with a significant handling time, adaptive prey defense may have a greater variety of effects on the predator's response to mortality, sometimes producing alternative attractors, population cycles, or increased mean predator density. Situations in which there is disruptive selection on prey defense often imply a bimodal change in yield as a function of harvesting effort, with a minimum at intermediate effort. These results argue against using single-species models of density dependent growth to manage predatory species, and illustrate the importance of incorporating anti-predator behavior into models in applied population ecology.