Testing Hypotheses About Evolutionary Change on Single Branches of a Phylogeny Using Evolutionary Contrasts

Interspecific comparisons of phenotypes are used extensively to test hypotheses about the evolutionary forces shaping phenotypic variation, but comparative data analysis is complicated by correlations due to the common ancestry of species. The method of evolutionary contrasts removes such correlations by estimating the amount of character change between pairs of closely related species that has occurred since their most recent common ancestors. The original method allows character change to be estimated only along pairs of branches on a phylogeny, but many hypotheses address change along single branches. In this article the method of evolutionary contrasts is extended to allow character change along a set of single branches on a phylogeny to be estimated, expected variances are presented, and it is shown that these extensions also result in a set of contrasts that are not correlated because of common ancestry. These extensions will allow hypotheses to be tested concerning character change associated with host or habitat shifts, changes in breeding system (e.g., monogamy vs. polygyny, monoecy vs. dioecy), changes in life history (e.g., semelparity vs. iteroparity), and changes in quantitative characters in many other situations in which one is interested in character change along single branches

Linking Local Species Interactions to Rates of Speciation in Communities

Regional species diversity limits the diversity of local communities by defining the pool of species that are available to colonize sites. Biogeographical processes that influence speciation and extinction rates determine the size and composition of this regional species pool. Community ecologists are beginning to recognize the importance of these biogeographical processes in influencing diversity patterns among local communities, but the potential for local interactions to influence biogeographical processes, especially speciation, has been little considered. In this paper I discuss one mechanism by which variation in the strengths of local interactions can influence the potential for population differentiation and thus for influencing speciation rates in the allopatric model of speciation. When more than one selective agent acts on the phenotype, the shape of the overall fitness surface depends critically on the relative strengths of the various selective agents. If one selective agent, which imparts strong selection, acts in all populations of a species, population differentiation via adaptation to local ecological conditions or via differentiation of sexual systems is retarded because the overall fitness surface in all populations strongly resembles the shape of the strongest selective agent. Consequently, the potential for speciation is reduced. Alternatively, if selective agents in various populations impart relatively equitable strengths of selection, the potential for differences in the overall fitness surfaces among populations is enhanced, which will enhance the potential for population differentiation and thus speciation. Analogous results obtain when multiple selective agents impact genetically correlated characters. Because the strength of selection generated by a species interaction should increase with the strength of the interaction, and because fewer species can usually coexist when the strengths of interactions are greater, the number of coexisting species and the potential for speciation in component taxa may covary among communities. This analysis indicates that the relative strengths of interactions can be as important to diversification in communities as the number of niche dimensions along which differentiation can occur

The Consequences of Changing the Top Predator in a Food Web: A Comparative Experimental Approach

Changing the top predator in a food web often results in dramatic changes in species composition at lower trophic levels; many species are extirpated and replaced by new species in the presence of the new top predator. These shifts in species composition also often result in substantial alterations in the strengths of species interactions. However, some species appear to be little affected by these changes that cause species turnover at other positions in the food web. An example of such a difference in species responses is apparent in the distributions of coenagrionid damselflies (Odonata: Zygoptera) among permanent water bodies with and without fish as top predators. Enallagma species segregate between ponds and lakes that do and do not support fish populations, with each lake type having a characteristic Enallagma assemblage. In contrast, species of Ischnura, the sister genus to Enallagma, are common to both fish and fishless ponds and lakes. Previous research has shown that Enallagma species segregate because they are differentially vulnerable to the top predators in each lake type: dragonflies in fishless lakes and fish in fish lakes. This paper reports the results of a series of laboratory and field experiments quantifying the mortality and growth effects of interactions in the food webs surrounding Enallagma and Ischnura species in both lake types. These results are compared to determine how features of the food web change to force segregation of Enallagma species between the lake types but permit Ischnura species to inhabit both. The results of experiments conducted in a fishless lake show that damselflies are not food limited in this lake type, but that they do strongly compete via interference mechanisms. Interference effects between the genera are symmetrical. Ischnura species have substantially higher growth rates than Enallagma species under all conditions in fishless lakes. Although both Enallagma and Ischnura experience substantial mortality from predation by dragonflies (Anax and Aeshna species, the top predators in fishless lakes), these dragonflies display a significant bias towards feeding on Ischnura. Mortality rates due to dragonfly predation are not density dependent. The results of experiments done in a fish lake indicate that damselflies are food limited and thus compete for resources in fish lakes. Ischnura growth rates are also substantially higher than Enallagma species in the fish-lake system. Dragonfly species that coexist with fish (Basiaeschna and Epitheca species) do not impose significant mortality on coexisting damselflies, but they do compete for resources with the damselflies, and they may also generate feeding interference in the damselflies. Fish impose significantly higher mortality on Ischnura species than on coexisting Enallagma species, and this mortality is negatively density dependent. The coexistence of Enallagma and Ischnura species is fostered in both lake types by trade-offs in their abilities to avoid predators and to utilize resources. Native Enallagma species are better at avoiding coexisting predators in each lake type, but these abilities come at the expense of the ability to utilize resources effectively and to avoid the predator found in the other lake type. In contrast, Ischnura are better at utilizing resources in both lake types, but these abilities come at the expense of effectively avoiding both fish and dragonflies. Understanding the trade-offs faced by species at similar trophic positions within a food web is critical to predicting changes in food webs following major environmental perturbations such as changing the top predator

Trade-Offs, Food Web Structure, and the Coexistence of Habitat Specialists and Generalists

Species differ greatly in the breadth of their environmental distributions. Within the same collection of habitats, some species occur in many habitats, while others are only able to exist in one of a few. Trade-offs in the abilities of species to perform in various ecological interactions are important both to facilitating species coexistence within a habitat and to limiting the distributions of species among habitats. In this article I use a food web model to explore how in the same collection of habitats some species may be limited by trade-offs to occupying only one habitat, while other species may face no trade-off between habitats and therefore be able to generalize in their habitat use. Food web interactions define the available niches within a habitat. Changes in food web structure cause some niches to be lost and replaced by other niches as one moves between habitats. Species occupying these niches will have more limited habitat distributions. However, other niches may be available in the food web structures of multiple habitats, and species occupying these niches will have broad habitat distributions. Understanding the structure of trade-offs within and between communities is fundamental to predicting the types of species with broad or narrow habitat distributions

Intraspecific Density Dependence and a Guild of Consumers Coexisting on One Resource

The importance of negative intraspecific density dependence to promoting species coexistence in a community is well accepted. However, such mechanisms are typically omitted from more explicit models of community dynamics. Here I analyze a variation of the Rosenzweig-MacArthur consumer–resource model that includes negative intraspecific density dependence for consumers to explore its effect on the coexistence of multiple consumers feeding on a single resource. This analysis demonstrates that a guild of multiple consumers can easily coexist on a single resource if each limits its own abundance to some degree, and stronger intraspecific density dependence permits a wider variety of consumers to coexist. The mechanism permitting multiple consumers to coexist works in a fashion similar to apparent competition or to each consumer having its own specialized predator. These results argue for a more explicit emphasis on how negative intraspecific density dependence is generated and how these mechanisms combine with species interactions to shape overall community structure

Antipredator Behavior and Physiology Determine Lestes Species Turnover Along the Pond-Permanence Gradient

Identifying key traits that shape trade-offs that restrict species to only a subset of environmental gradients is crucial to understanding and predicting species turnover. Previous field experiments have shown that larvae of Lestes damselfly species segregate along the entire gradient of pond permanence and predator presence and that differential predation risk and life history constraints together shape their distribution. Here, we report laboratory experiments that identify key differences in behavior and physiology among species that structure their distributions along this gradient. The absence of adaptive antipredator behavioral responses against large dragonfly larvae and fish of Lestes dryas, the only species to inhabit predator-free vernal ponds that dry each year, is consistent with its high vulnerability to predation and probably the key trait that excludes it from parts of the gradient with predators. The reciprocal dominance of two other Lestes species in permanent waters dominated by either dragonflies or fish can be explained by the lack of effective antipredator behaviors against dragonflies and fish, respectively. Maximal growth rates did not differ among Lestes along the gradient. However, in the natural predator environment of vernal ponds (only conspecific cannibals), the vernal-pond Lestes had higher growth rates than the other Lestes suggesting that this excludes other Lestes from vernal ponds. Similarly, Lestes species that inhabit temporary ponds (i.e., ponds that dry intermittently every few years but not every year) had a higher growth rate than the fishless permanent-pond Lestes in the presence of the syntopic dragonfly predator. These growth differences among Lestes in predator treatments were not due to differences in food intake, but due to differences in physiology. The vernal-pond Lestes converted more assimilated food into body mass compared to the other Lestes in the presence of conspecific larvae, and the temporary-pond Lestes had a higher conversion efficiency than the fishless permanent-pond Lestes in the presence of the syntopic dragonfly predators. In contrast, reductions in growth rate within species in the presence of syntopic predators were both physiologically and behaviorally mediated. The interplay between behavior and physiology may be a common feature of the growth/predation-risk trade-off, and their joint study is therefore critical to mechanistically link phenotype, performance, and community assembly along the freshwater habitat gradient

Predators and Life Histories Shape Lestes Damselfly Assemblages Along a Freshwater Habitat Gradient

Survey data from New England showed that assemblages of Lestes damselflies are organized along the entire gradient of pond permanence and predator presence. One assemblage occupies vernal ponds lacking large dragonfly predators and fish; four are largely confined to temporary ponds that typically contain dragonfly predators; one dominates fishless permanent ponds and lakes where dragonflies are the top predators; and one dominates permanent ponds and lakes where fish are the top predators. We determined the role of life history and predation in maintaining this striking pattern by conducting a series of transplant experiments in the field and a laboratory experiment manipulating presence and absence of local predators. Life history (1) shaped the ability of species to cope with drying regime, thereby excluding temporary‐pond Lestes from vernal ponds and permanent‐water Lestes from temporary ponds, and (2) generated size differences among species due to differences in the timing of hatching. This mediated the exclusion of temporary‐pond Lestes from permanent water bodies through asymmetric intraguild predation by permanent‐water Lestes. Dragonfly predation on permanent‐water Lestes had an indirect positive effect on the survival of temporary‐pond Lestes; however, this effect apparently is too small to allow coexistence of both Lestes groups. Predation by large dragonfly larvae excluded the Lestes species of vernal ponds from temporary ponds, and differential vulnerability to large dragonfly larvae and fish shaped the reciprocal dominance of L. eurinus and L. vigilax in fishless and fish‐containing permanent water bodies, respectively. Taken together, these results show that life history constraints and predation both shape the distributions of Lestes species along the pond permanence gradient in New England. We discuss the importance of this freshwater habitat gradient in shaping local and regional species diversity

Predators and Life Histories Shape Lestes Damselfly Assemblages Along a Freshwater Habitat Gradient

Survey data from New England showed that assemblages of Lestes dam- selflies are organized along the entire gradient of pond permanence and predator presence. One assemblage occupies vernal ponds lacking large dragonfly predators and fish; four are largely confined to temporary ponds that typically contain dragonfly predators; one dom- inates fishless permanent ponds and lakes where dragonflies are the top predators; and one dominates permanent ponds and lakes where fish are the top predators. We determined the role of life history and predation in maintaining this striking pattern by conducting a series of transplant experiments in the field and a laboratory experiment manipulating presence and absence of local predators. Life history (1) shaped the ability of species to cope with drying regime, thereby excluding temporary-pond Lestes from vernal ponds and permanent- water Lestes from temporary ponds, and (2) generated size differences among species due to differences in the timing of hatching. This mediated the exclusion of temporary-pond Lestes from permanent water bodies through asymmetric intraguild predation by permanent- water Lestes. Dragonfly predation on permanent-water Lestes had an indirect positive effect on the survival of temporary-pond Lestes; however, this effect apparently is too small to allow coexistence of both Lestes groups. Predation by large dragonfly larvae excluded the Lestes species of vernal ponds from temporary ponds, and differential vulnerability to large dragonfly larvae and fish shaped the reciprocal dominance of L. eurinus and L. vigilax in fishless and fish-containing permanent water bodies, respectively. Taken together, these results show that life history constraints and predation both shape the distributions of Lestes species along the pond permanence gradient in New England. We discuss the importance of this freshwater habitat gradient in shaping local and regional species diversity

Coexistence of the Niche and Neutral Perspectives in Community Ecology

The neutral theory for community structure and biodiversity is dependent on the assumption that species are equivalent to each other in all important ecological respects. We explore what this concept of equivalence means in ecological communities, how such species may arise evolutionarily, and how the possibility of ecological equivalents relates to previous ideas about niche differentiation. We also show that the co-occurrence of ecologically similar or equivalent species is not incompatible with niche theory as has been supposed, because niche relations can sometimes favor coexistence of similar species. We argue that both evolutionary and ecological processes operate to promote the introduction and to sustain the persistence of ecologically similar and in many cases nearly equivalent species embedded in highly structured food webs. Future work should focus on synthesizing niche and neutral perspectives rather than dichotomously debating whether neutral or niche models provide better explanations for community structure and biodiversity

On the Evidence for Species Coexistence: A Critique of the Coexistence Program

A major challenge in ecology is to understand how the millions of species on Earth are organized into biological communities. Mechanisms promoting coexistence are one such class of organizing processes, which allow multiple species to persist in the same trophic level of a given web of species interactions. If some mechanism promotes the coexistence of two or more species, each species must be able to increase when it is rare and the others are at their typical abundances; this invasibility criterion is fundamental evidence for species coexistence regardless of the mechanism. In an attempt to evaluate the level of empirical support for coexistence mechanisms in nature, we surveyed the literature for empirical studies of coexistence at a local scale (i.e., species found living together in one place) to determine whether these studies satisfied the invasibility criterion. In our survey, only seven of 323 studies that drew conclusions about species coexistence evaluated invasibility in some way in either observational or experimental studies. In addition, only three other studies evaluated necessary but not sufficient conditions for invasibility (i.e., negative density dependence and a trade-off in performance that influences population regulation). These results indicate that, while species coexistence is a prevalent assumption for why species are able to live together in one place, critical empirical tests of this fundamental assumption of community structure are rarely performed. These tests are central to developing a more robust understanding of the relative contributions of both deterministic and stochastic processes structuring biological communities