Mark A. McPeek

THE EVOLUTION OF DISPERSAL IN SPATIALLY AND TEMPORALLY VARYING ENVIRONMENTS

Using a simple two-patch model, we examine how patterns of spatial and temporal variation in carrying capacities affect natural selection on dispersal. The size of the population in each patch is regulated separately, according to a discrete-generation logistic equation, and individuals disperse from each patch at propensities determined by their genotype. We consider genotypes that express the same dispersal propensities in both patches and genotypes that express patch-specific disperal propensities. Contrary to previous analyses, our results show that some level of dispersal is favored by selection under almost all regimes of habitat variability, including a spatially varying and temporally constant environment. Furthermore, two very different polymorphisms are favored under different conditions. When carrying capacities vary spatially but not temporally, any number of genotypes with patch-specific dispersal propensities in ratios inversely proportional to the ratio of the carrying capacities can coexist. This result extends previous analyses to show that dispersal is favored in such an environment if individuals can alter dispersal propensities in response to environmental cues. In contrast, when carrying capacities vary both spatially and temporally but differ in mean or variance, a polymorphism of only two genotypes (a high-dispersal and a no-dispersal genotype) is favored when the only genotypes possible are ones expressing the same dispersal propensity in both patches. However, this dimorphism can be invaded and replaced by one genotype with a particular combination of patch-specific dispersal propensities in a ratio also inversely proportional to the ratio of the average population sizes. We discuss a number of testable predictions this model suggests about the amounts of phenotypic and genetic variation in dispersal characters that are expected both within and between populations, and the degree to which the expression of phenotypic characters affecting dispersal propensity should be sensitive to environmental conditions. The model also suggests novel mechanisms for coexistence between competing species in varying environments.

Early bursts of body size and shape evolution are rare in comparative data.

George Gaylord Simpson famously postulated that much of lifes diversity originated as adaptive radiations—more or less simultaneous divergences of numerous lines from a single ancestral adaptive type. However, identifying adaptive radiations has proven difficult due to a lack of broad‐scale comparative datasets. Here, we use phylogenetic comparative data on body size and shape in a diversity of animal clades to test a key model of adaptive radiation, in which initially rapid morphological evolution is followed by relative stasis. We compared the fit of this model to both single selective peak and random walk models. We found little support for the early‐burst model of adaptive radiation, whereas both other models, particularly that of selective peaks, were commonly supported. In addition, we found that the net rate of morphological evolution varied inversely with clade age. The youngest clades appear to evolve most rapidly because long‐term change typically does not attain the amount of divergence predicted from rates measured over short time scales. Across our entire analysis, the dominant pattern was one of constraints shaping evolution continually through time rather than rapid evolution followed by stasis. We suggest that the classical model of adaptive radiation, where morphological evolution is initially rapid and slows through time, may be rare in comparative data.

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.

DIRECT AND INDIRECT EFFECTS OF PREDATORS ON TWO ANURAN SPECIES ALONG AN ENVIRONMENTAL GRADIENT

This study examines direct and indirect interactions that influence the dis- tribution of larvae of the bullfrog (Rana catesbeiana) and green frog (R. clamitans) along the environmental gradient of permanent to temporary ponds. The bullfrog is found in permanent ponds that typically contain fish, whereas the green frog is widely distributed along the gradient and is most successful in ponds where the bullfrog is absent. In a set of experimental ponds, bullfrogs were most abundant in ponds containing fish (bluegill, Le- pomis macrochirus) and were rare in permanent ponds that lacked fish. In contrast, green frogs were most abundant in ponds that had been drained the previous fall, second most abundant in those that lacked fish, and sparse in ponds containing bluegill. When presence and absence of bluegill were experimentally manipulated in divided ponds, bullfrogs sur- vived well in the presence of bluegill, whereas no individuals survived in the absence of bluegill. Green frogs survived in low numbers under both treatments with a tendency to better survivorship in the absence of bluegill. A series of laboratory choice experiments was conducted with three major predator types: the dragonfly larva Anax junius, the salamander Ambystoma tigrinum, and L. macrochirus. Bullfrog larvae were more vulner- able to Anax and Ambystoma than were green frogs (because of higher activity levels), and green frogs were more vulnerable to bluegill than were bullfrogs (because tadpoles were less noxious). We argue that the high densities of bullfrogs found in the presence of bluegill reflect an indirect facilitation through two pathways: bluegill had strong negative effects on invertebrate and salamander predators of bullfrogs and on the green frog, which is a bullfrog competitor. The direct and indirect effects of these suites of predators appear to explain the differences in species abundances along the environmental gradient. Finally, we discuss the trade-offs at the individual level contributing to these differences in species performance along the gradient.

The dynamics of evolutionary stasis

Abstract The fossil record displays remarkable stasis in many species over long time periods, yet studies of extant populations often reveal rapid phenotypic evolution and genetic differentiation among populations. Recent advances in our understanding of the fossil record and in population genetics and evolutionary ecology point to the complex geographic structure of species being fundamental to resolution of how taxa can commonly exhibit both short-term evolutionary dynamics and long-term stasis.

Behavioral Differences between Enallagma Species (Odonata) Influencing Differential Vulnerability to Predators

In previous research I have shown that fish predation contributes to the exclusion of a group of Enallagma species (Odonata: Zygoptera) from lakes containing fish, whereas dragonfly predation contributes to the exclusion of another group of Enal- lagma species from fishless lakes. In the study reported in this paper I performed a series of laboratory experiments to identify behavioral differences between the two groups that could contribute to their differential vulnerabilities to the two predators. In one experiment I observed the behaviors of four Enallagma species, two from fishless lakes and two from lakes containing fish, in the presence of no predators, dragonfly larvae (Anaxjunius), and fish (Lepomis macrochirus) to identify behavioral differences that may cause the species to be differentially detected by the predators. In another experiment, I observed Aeshna dragonflies feeding on Enallagma larvae to evaluate whether species in the two groups differ in how they attempt to avoid attacking dragonflies. The Enallagma species from fishless lakes walked in rapid bursts of 2-3 s and frequently performed a conspicuous behavioral display, while the Enallagma species from lakes containing fish walked very slowly for durations of 20-30 s and almost never performed the conspicuous display. The species from lakes containing fish also swam for longer durations and remained motionless longer following swims than species from fishless lakes. The species from fishless lakes also greatly reduced their movement and responsiveness toward prey and increased the duration of motionless periods in the presence of dragonflies, but only reduced the number and duration of the conspicuous display in the presence of fish. In contrast, the species from lakes containing fish reduced their movement and re- sponsiveness toward prey and increased the duration of motionless periods in the presence of both fish and dragonflies. In videotaped feeding trials, larvae of an Enallagma species from fishless lakes usually swam away from attacking Aeshna dragonflies. Larvae of an Enallagma species from lakes containing fish usually did not respond to attacking dragon- flies, and were usually captured without attempting to evade the dragonfly. These behavioral differences are consistent with observed differences in their vulnerabilities to predators, and probably contribute to causing the pattern of habitat separation observed in the field.

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.

The Ecological Dynamics of Clade Diversification and Community Assembly

Clades diversify in an ecological context, but most macroevolutionary models do not directly encapsulate ecological mechanisms that influence speciation and extinction. A data set of 245 chordate, arthropod, mollusk, and magnoliophyte phylogenies had a majority of clades that showed rapid lineage accumulation early with a slowing more recently, whereas a small but significant minority showed accelerated lineage accumulation in their recent histories. Previous analyses have demonstrated that macroevolutionary birth‐death models can replicate the pattern of slowing lineage accumulation only by a strong decrease in speciation rate with increasing species richness and extinction rate held extremely low or absent. In contrast, the metacommunity model presented here could generate the full range of patterns seen in the real phylogenies by simply manipulating the degree of ecological differentiation of new species at the time of speciation. Specifically, the metacommunity model predicts that clades showing decelerating lineage accumulation rates are those that have diversified by ecological modes of speciation, whereas clades showing accelerating lineage accumulation rates are those that have diversified primarily by modes of speciation that generate little or no ecological diversification. A number of testable predictions that integrate data from molecular systematics, community ecology, and biogeography are also discussed.

LIFE HISTORIES AND THE STRENGTHS OF SPECIES INTERACTIONS: COMBINING MORTALITY, GROWTH, AND FECUNDITY EFFECTS

Interactive effects of one species on another may simultaneously influence mortality, growth, and fecundity. To quantify the strength of an interaction between two species, we must therefore use techniques that integrate these various responses into es- timates of overall effect. Demographic models of populations provide such a framework. Here we develop a demographic model describing the life history of a hemimetabolous insect to evaluate the relative importance of predator effects on mortality and growth of damselflies (Enallagma boreale) in fishless ponds and mayflies (Baetis bicaudatus) in trout streams. Previous experiments have shown that dragonfly predators in fishless ponds inflict direct mortality and cause reduced growth rates in Enallagma damselflies. Parameterization of the demographic model from these data show, however, that only the direct mortality effects of dragonflies should significantly influence damselfly population dynamics. This is because damselfly size at emergence does not influence adult female fecundity, so the effects of dragonflies on damselfly larval growth do not influence adult fecundity. Likewise, both trout and stonefly predators inflict mortality on larval Baetis mayflies and cause decreases in growth rates. However, our demographic analyses indicate that the growth effects of both predators should dominate the population-dynamic effects on Baetis. This is because size at emergence translates directly into adult fecundity in mayflies. We also present data suggesting that developmental responses to changes in environmental conditions (e.g., predator abundances, resource availabilities) differ between species depending on these same life history parameters. The biological significance of lethal vs. sublethal predator impacts must be evaluated in a demographic framework to identify whether alterations in growth rate, and the timing of and size at metamorphosis, significantly influence population dynamics. The demographic model used for any particular organism must be tailored to its life history, but the various impacts of interactions with other species can all be integrated into estimates of projected population growth that can then be readily compared among species with different life histories.

DEMOGRAPHY OF AN AGE-STRUCTURED ANNUAL: RESAMPLED PROJECTION MATRICES, ELASTICITY ANALYSES, AND SEED BANK EFFECTS'

The role of age—structured seed banks in influencing population dynamic parameters was investigated in a natural population of the winter annual Collinsia verna. Seed persistence was quantified by creating experimental seed banks in the field with seeds of known age. Survival and fecundities of adult plant stages were determined in quadrats of naturally occurring individuals in the field population. Bootstrapping was applied to the resulting data set to estimate means and 95% confidence intervals for population growth rate, stable age distributions, reproductive values, and elasticities. Analyses were performed to determine the extent of variation in these demographic parameters between two consecutive years and between three transects within each year. The results indicate that the presence of a seed bank, even of short duration, was critical to the demography of this population. The population was expanding rapidly in the 1st year of the study (growth rate 1.80), but was declining during the 2nd year (0.41). While the seed bank was demographically important in some transects during the good year (Year 1) the demographic effects of the seed bank were seen most significantly in the poor year (Year 2). The results of this study indicate the need for including age—structured seed banks in demographic analyses and the potential importance of seeds in age—structured seed banks in the shaping of plant life histories.