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.     

Genetic variation tracks ecological segregation in Pacific island black flies

Geographic isolation is widely viewed as a key component of insular radiations on islands. However, strong ecological affinities may also reinforce isolation and promote genetic divergence. The black fly fauna in the Society Islands French Polynesia is notable for the number of closely related endemic species (31), and the morphological and habitat diversity of the larvae. Here, we measure ecological and morphological differences within and between two closely related species, Simulium oviceps and Simulium dussertorum and relate these differences to genetic distance. Phylogenetic analyses of a 920 bp fragment of the cytochrome oxidase I (COI) gene revealed a well-supported, ecologically divergent S. oviceps clade (larvae found in rivers instead of cascades) that shows little morphological differentiation. For both S. oviceps and S. dussertorum, genetic distance among populations is related to larval habitat, with cascade populations showing greater isolation from each other than river populations. Our data support the hypothesis that larval ecological shifts have played a role in the radiation of this black fly fauna.     

Shaping intraspecific variation: Development,ecology and the evolution of morphology and life history variation in tiger salamanders

The tiger salamander,Ambystoma tigrinum, is a geographically widespread, morphologically variable, polytipic species. It is among the most variable species of salamanders in morphology and life history with two larval morphs (typical and cannibal) and three adult morphs (metamorphosed, typical branchiate, cannibal branchiate) that vary in frequency between subspecies and between populations within subspecies. We report morphometric evidence suggesting that branchiate cannibals arose through intraspecific change in the onset or timing of development resulting in the wider head and hypertrophied tooth-bearing skull bones characteristic of this phenotype. We also quantified bilateral symmetry of gill raker counts and abnormalities, then evaluated fluctuating asymmetry as a measure of the developmental stability of each morph. There was a significant interaction between fluctuating asymmetry of developmental abnormalities in cannibals and typicals and the locality where they were collected, suggesting that relative stability of each phenotype could vary among populations. While altered timing of developmental events appears to have a role in the evolution and maintenance of morphs, novel phenotypes persist only under favorable ecological conditions. Predictability of the aquatic habitat, genetic variation, kinship, body size, intraspecific competition and predation all affect expression and survival of the morphs inA. tigrinum. This taxon provides an excellent model for understanding the diversity and complexity of developmental and ecological variables controlling the evolution and maintenance of novel phenotypes.     

Field and experimental evidence for competition's role in phenotypic divergence

Resource competition has long been viewed as a major cause of phenotypic divergence within and between species. Theory predicts that divergence arises because natural selection favors individuals that are phenotypically dissimilar from their competitors. Yet, there are few conclusive tests of this key prediction. Drawing on data from both natural populations and a controlled experiment, this paper presents such a test in tadpoles of two species of spadefoot toads (Spea bombifrons and S. multiplicata). These two species show exaggerated divergence in trophic morphology where they are found together (mixed-species ponds) but not where each is found alone (pure-species ponds), suggesting that they have undergone ecological character displacement. Moreover, in pure-species ponds, both species exhibit resource polymorphism. Using body size as a proxy for fitness, we found that in pure-species ponds disruptive selection favors extreme trophic phenotypes in both species, suggesting that intraspecific competition for food promotes resource polymorphism. In mixed-species ponds, by contrast, we found that trophic morphology was subject to stabilizing selection in S. multiplicata and directional selection in S. bombifrons. A controlled experiment revealed that the more similar an S. multiplicata was to its S. bombifrons tankmate in resource use, the worse was its performance. These results indicate that S. multiplicata individuals that differ from S. bombifrons would be selectively favored in competition. Our data therefore demonstrate how resource competition between phenotypically similar individuals can drive divergence between them. Moreover, our results indicate that how competition contributes to such divergence may be influenced not only by the degree to which competitors overlap in resource use, but also by the abundance and quality of resources. Finally, our finding that competitively mediated disruptive selection may promote resource polymorphism has potentially important implications for understanding how populations evolve in response to heterospecific competitors. In particular, once a population evolves resource polymorphism, it may be more prone to undergo ecological character displacement.     

Disruptive natural selection predicts divergence between the sexes during adaptive radiation

Evolution of sexual dimorphism in ecologically relevant traits, for example, via resource competition between the sexes, is traditionally envisioned to stall the progress of adaptive radiation. An alternative view is that evolution of ecological sexual dimorphism could in fact play an important positive role by facilitating sex‐specific adaptation. How competition‐driven disruptive selection, ecological sexual dimorphism, and speciation interact during real adaptive radiations is thus a critical and open empirical question. Here, we examine the relationships between these three processes in a clade of salamanders that has recently radiated into divergent niches associated with an aquatic life cycle. We find that morphological divergence between the sexes has occurred in a combination of head shape traits that are under disruptive natural selection within breeding ponds, while divergence among species means has occurred independently of this disruptive selection. Further, we find that adaptation to aquatic life is associated with increased sexual dimorphism across taxa, consistent with the hypothesis of clade‐wide character displacement between the sexes. Our results suggest the evolution of ecological sexual dimorphism may play a key role in niche divergence among nascent species and demonstrate that ecological sexual dimorphism and ecological speciation can and do evolve concurrently in the early stages of adaptive radiation.     

Sexual dimorphism and speciation on two ecological coins: patterns from nature and theoretical predictions

Adaptive divergence of phenotypes, such as sexual dimorphism or adaptive speciation, can result from disruptive selection via competition for limited resources. Theory indicates that speciation and sexual dimorphism can result from identical ecological conditions, but co-occurrence is unlikely because whichever evolves first should dissipate the disruptive selection necessary to drive evolution of the other. Here, we consider ecological conditions in which disruptive selection can act along multiple ecological axes. Speciation in lake populations of threespine sticklebacks (Gasterosteus aculeatus) has been attributed to disruptive selection due to competition for resources. Head shape in sticklebacks is thought to reflect adaptation to different resource acquisition strategies. We measure sexual dimorphism and species variation in head shape and body size in stickleback populations in two lakes in British Columbia, Canada. We find that sexual dimorphism in head shape is greater than interspecific differences. Using a numerical simulation model that contains two axes of ecological variation, we show that speciation and sexual dimorphism can readily co-occur when the effects of loci underlying sexually dimorphic traits are orthogonal to those underlying sexually selected traits.     

Character convergence under competition for nutritionally essential resources

Resource competition is thought to drive divergence in resource use traits (character displacement) by generating selection favoring individuals able to use resources unavailable to others. However, this picture assumes nutritionally substitutable resources (e.g., different prey species). When species compete for nutritionally essential resources (e.g., different nutrients), theory predicts that selection drives character convergence. We used models of two species competing for two essential resources to address several issues not considered by existing theory. The models incorporated either slow evolutionary change in resource use traits or fast physiological or behavioral change. We report four major results. First, competition always generates character convergence, but differences in resource requirements prevent competitors from evolving identical resource use traits. Second, character convergence promotes coexistence. Competing species always attain resource use traits that allow coexistence, and adaptive trait change stabilizes the ecological equilibrium. In contrast, adaptation in allopatry never preadapts species to coexist in sympatry. Third, feedbacks between ecological dynamics and trait dynamics lead to surprising dynamical trajectories such as transient divergence in resource use traits followed by subsequent convergence. Fourth, under sufficiently slow trait change, ecological dynamics often drive one of the competitors to near extinction, which would prevent realization of long-term character convergence in practice.     

Ecological character displacement caused by reproductive interference

We carried out a theoretical investigation of whether ecological character displacement can be caused by reproductive interference. Our model assumes that a quantitative character is associated with both resource use and species recognition, and that heterospecific mating incurs costs. The model shows that ecological character displacement can occur as a consequence of evolution of premating isolation; this conclusion is based on the premise that resource competition is less intense between species than within species and that the ecological character also contributes to premating isolation. When resource competition between species is intense, extinction of either species may occur by competitive exclusion before ecological character divergence. Some observational studies have shown that character displacement in body size is associated with not only resources use but also species recognition. We propose that body size displacement can occur as a consequence of evolution of premating isolation. Our results suggest that ecological character displacement results from reproductive character displacement.     

Larval ecomorphology of 13 Libellulidae (Anisoptera, Odonata) of the Middle Rio Doce Valley, Minas Gerais, Brazil.

In the lakes of the Middle Rio Doce, Minas Gerais (MG), two groups of larval Libellulidae are distinguished by preferences of habitat use: one uses mainly aquatic macrophytes and the other uses the bottom substrate. The goal of this work was to verify if there is a morphological distinction between the two groups of species. Thirteen body measures were taken from the larvae and analyzed. No difference was found between the two groups of species regarding the body size, but shape differences were observed for two morphological variables. The species that use mainly macrophytes tend to have larger relative measures of the labium and smaller measures of the abdomen width. Advantages in resource obtainment and in vulnerability to predation are probably the explanations for the morphological divergence among these larval groups.     

Adaptive radiation through phenological shift: the importance of the temporal niche in species diversification

Abstract 1. Phenological shift in oviposition in seed predators may be a key factor for adaptive radiation if temporal differences lead to less intense competition. 2. This hypothesis was tested at two sites in the French Alps in three sympatric species of larch cone flies grouped into two phenological groups (early and late) differing in adult emergence and oviposition timing by approximately 2 weeks. The present study assessed the intensity of competition within and between groups by measuring four larval traits. Cone traits were measured, and the impact of early species parasitism on cone development was assessed. 3. The occupation of the central axis of a developing cone by one early larva has a strong detrimental effect on cone growth and seed production. However, there was almost no correlation between the variables measured on the cones and on the larvae, suggesting that the resources available were not limiting. 4. Inter‐group competition had no significant effect on early larvae. In contrast, both inter‐ and intra‐group competition had a significant negative effect on late larvae length (–11% and –16% respectively), dry mass (–8% and –23%), and lipid mass (–15% and –26%). The intensity of competition was stronger among larvae in the same phenological group, which is consistent with the hypothesis that shifts in oviposition promote adaptive radiation in larch cone flies by reducing competition among larvae.     

Sexual selection's impacts on ecological specialization: an experimental test

In many species, individuals specialize on different resources, thereby reducing competition. Such ecological specialization can promote the evolution of alternative ecomorphs—distinct phenotypes adapted for particular resources. Elucidating whether and how this process is influenced by sexual selection is crucial for understanding how ecological specialization promotes the evolution of novel traits and, potentially, speciation between ecomorphs. We evaluated the population-level effects of sexual selection (as mediated by mate choice) on ecological specialization in spadefoot toad tadpoles that express alternative ecomorphs. We manipulated whether sexual selection was present or reversed by mating females to their preferred versus non-preferred males, respectively. We then exposed their tadpoles to resource competition in experimental mesocosms. The resulting distribution of ecomorphs was similar between treatments, but sexual selection generated poorer trait integration in, and lower fitness of, the more specialized carnivore morph. Moreover, disruptive and directional natural selection were weaker in the sexual selection present treatment. Nevertheless, this effect on disruptive selection was smaller than previously documented effects of ecological opportunity and competitor density. Thus, sexual selection can inhibit adaptation to resource competition and thereby hinder ecological specialization, particularly when females obtain fitness benefits from mate choice that offset the cost of producing competitively inferior offspring.     

Individual responses to population size structure: the role of size variation in controlling expression of a trophic polyphenism

We investigated how size structure affects development of alternative larval phenotypes in Arizona tiger salamanders, Ambystoma tigrinum nebulosum, by testing the hypothesis that population size structure per se is a significant component of an individual's environment. Larvae of this subspecies exhibit one of two feeding phenotypes; typical larvae eat zooplankton and macroinvertebrates and cannibalistic larvae feed primarily on conspecifics. Previous laboratory experiments showed that larval density positively affected expression of the cannibalistic phenotype. In this study we tested the hypothesis that size variation among larvae also serves as a cue triggering development of the cannibalistic phenotype. We report laboratory experiments and field observations showing that both an individual larva's position in a size distribution and the amount of size vaiation among larvae serve as cues stimulating development of cannibalistic larvae. Larval density and population size structure provide a larva with an indication of the abundance and vulnerability of potential conspecific prey. Size variation among larvae, in turn, appears to be influenced by larval density. Thus, a complex relationship exists between larval density, population size structure, and the frequency of cannibals within a habitat.     

ON THE COEXISTENCE AND COEVOLUTION OF ASEXUAL AND SEXUAL COMPETITORS

The coexistence and coevolution of sexual and asexual species under resource competition are explored with three models: a nongenetic ecological model, a model including single locus genetics, and a quantitative-genetic model. The basic assumption underlying all three models is that genetic differences are translated into ecological differences. Hence if sexual species are genetically more variable, they will be ecologically more variable. Under classical competition theory, this increased ecological variability can, in many cases, be an advantage to individual sexual genotypes and to the sexual species as a whole. The purpose of this paper is to determine the conditions when this advantage will outway three disadvantages of sexuality: the costs of males, of segregation, and of the additive component of recombination. All three models reach similar conclusions. Although asexuality confers an advantage, it is much less than a two-fold advantage because minor increases in the overall species niche width of the sexual species will offset the reproductive advantage of the asexual species. This occurs for two reasons. First, an increase in species niche width increases the resource base of the sexual species. Second, to the extent that the increase in niche width is due to increased differences between individuals, a reduction in intraspecific competition will result. This is not to imply that the sexual species will always win. The prime conditions that enable sexual species to stably coexist with or even supplant an asexual sister species are:

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Wall-making behavior as a proximate mechanism to generate variation in larval competition in Callosobruchus maculatus (Coleoptera: Bruchidae)

Variation from contest to scramble in larval competition types was observed among laboratory lines derived from a geographic strain of Callosobruchus maculatus. In contest competition, only one adult can emerge from a small bean because the successful larva monopolizes resources. In scramble competition, however, multiple adults can emerge from the bean because larvae share resources. To explain the variation in competition types, we used six lines of the geographic strain to test the hypothesis that the larval competition type is determined by the larval behavior of building walls, which prevent larvae from interfering with each other, allowing multiple adults to emerge from a single bean. We also investigated the proportions of wall-making in contest-scramble hybrid lines to test whether the formation of a wall structure was genetically determined. Results support our hypothesis that wall-making behavior determines the type of larval competition within a geographic strain, and that the behavior is genetically determined. Scramble-type lines exhibited higher frequencies of wall-making than contest-type lines when two larvae of the same line infested a bean. Larval competition type and the tendency towards wall formation in contest-scramble hybrid lines ranged intermediate of parental lines. We concluded that the variation in larval competition type is determined by the variation in larval wall-making behavior among laboratory lines derived from the geographic strain. We will discuss the evolution of scramble-type larvae in C. maculatus based on our results.     

A study of trophic niche partitioning between larval populations of reproductively isolated whitefish (Coregonus sp.) ecotypes

The abundance, spatio-temporal distribution, and feeding of larvae were compared from two reproductively isolated dwarf and normal ecotypes of whitefish ( Coregonus sp.) to test the hypothesis that larval ecotypes should reduce competition by using different resources. Contrary to a priori expectations, trophic niche partitioning between larval populations was much less pronounced than previously reported for the adult stages of fish ecotypes, presumably due to the lack of competition acting at this stage, and related to non-limiting food resources. Because this study was conducted in a single year and a single lake, we cannot however, strictly rule out the possibility that resource-based competition and resulting niche partitioning may occur at the larval stage in these species complexes. Nevertheless, this suggests for the first time, that resource-based selection at the larval stage may be relatively unimportant compared to that occurring at older life-history stages in driving the divergence and the development of reproductive isolation in sympatric fish ecotypes.     

Character displacement as the "best of a bad situation": fitness trade-offs resulting from selection to minimize resource and mate competition

Abstract Character displacement has long been considered a major cause of adaptive diversification. When species compete for resources or mates, character displacement minimizes competition by promoting divergence in phenotypes associated with resource use (ecological character displacement) or mate attraction (reproductive character displacement). In this study, we investigated whether character displacement can also have pleiotropic effects that lead to fitness trade-offs between the benefits of avoiding competition and costs accrued in other fitness components. We show that both reproductive and ecological character displacement have caused spadefoot toads to evolve smaller body size in the presence of a heterospecific competitor. Although this shift in size likely arose as a by-product of character displacement acting to promote divergence between species in mating behavior and larval development, it concomitantly reduces offspring survival, female fecundity, and sexual selection on males. Thus, character displacement may represent the "best of a bad situation" in that it lessens competition, but at a cost. Individuals in sympatry with the displaced phenotype will have higher fitness than those without the displaced trait because they experience reduced competition, but they may have reduced fitness relative to individuals in allopatry. Such a fitness trade-off can limit the conditions under which character displacement evolves and may even increase the risk of "Darwinian extinction" in sympatric populations. Consequently, character displacement may not always promote diversification in the manner that is often expected.     

Kin recognition and cannibalism in polyphenic salamanders

We investigated kin discrimination among larvae of Arizona tigersalamanders (Ambystoma tigrinum nebulosum) which occur as "typical"morphs that feed mostly on invertebrate prey and occasionallyon conspecifics, and as "cannibal" morphs that feed primarilyon conspecifics. When housed with smaller larvae that differedin relatedness, both cannibals and typicals preferentially consumedless-related individuals. Cannibals ate typicals much quickerwhen the choice was between nonkin and siblings than when thechoice was between nonkin and cousins, indicating that cannibalscould distinguish different categories of relatives. Cannibalswere less likely to eat a larval sibling that was a cannibalmorph than a sibling that was a typical morph. Occluding animals'nares temporarily eliminated kin discrimination, implying thatolfaction is important in recognition. Larvae from differentsibships varied considerably in their ability to discriminatekin, and the greater the probability that a larva from a givensibship would develop into a cannibal morph, the more likelythe members of that sibship were to discriminate kin. Our resultsenable us to infer the functional significance of kin recognitionin this species and to develop an evolutionary model of themechanisms underlying the joint control of kin recognition andcannibalistic polyphenism.     

Snake diets and the deep history hypothesis

The structure of animal communities has long been of interest to ecologists. Two different hypotheses have been proposed to explain origins of ecological differences among species within present‐day communities. The competition–predation hypothesis states that species interactions drive the evolution of divergence in resource use and niche characteristics. This hypothesis predicts that ecological traits of coexisting species are independent of phylogeny and result from relatively recent species interactions. The deep history hypothesis suggests that divergences deep in the evolutionary history of organisms resulted in niche preferences that are maintained, for the most part, in species represented in present‐day assemblages. Consequently, ecological traits of coexisting species can be predicted based on phylogeny regardless of the community in which individual species presently reside. In the present study, we test the deep history hypothesis along one niche axis, diet, using snakes as our model clade of organisms. Almost 70% of the variation in snake diets is associated with seven major divergences in snake evolutionary history. We discuss these results in the light of relevant morphological, behavioural, and ecological correlates of dietary shifts in snakes. We also discuss the implications of our results with respect to the deep history hypothesis. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 476–486.     

Timing of resource input and larval competition between invasive and native container-inhabiting mosquitoes (Diptera: Culicidae).

Container-inhabiting mosquito species are subject to both intraspecific and interspecific competition during larval development in resource-limited habitats. The arrival of an invasive species, Aedes albopictus, in the U.S. has altered competitive interactions among container-inhabiting mosquito species and, in some cases, has led to displacement of these species. Resource enrichment of container habitats has been shown to alleviate competitive interactions and to promote species co-existence; however, the importance of the timing of enrichment has yet to be explored. Larval competition between Ae. albopictus and a native species, Ochlerotatus triseriatus, was explored when resources were added either gradually or in a single pulse. Replacement series experiments revealed that Ae. albopictus was able to outcompete and displace Oc. triseriatus via resource monopolization when all resources were made available simultaneously; however, when the same resource amount was added over time, survival was high for both species, leading to co-existence. Timing of resource input also had an effect in monospecific treatments, indicating that intraspecific competition impacts survival as well. Duration of larval development was influenced by both species presence and by timing of resource input for Oc. triseriatus. These results indicate competitive outcome is condition-specific and that timing of resource input can determine whether a dominant invasive competitor displaces a native species, or if the two species are able to co-exist. Both intraspecific and interspecific competition occur at different temporal scales due to species-specific differences in larval developmental time. Timing of resource availability in container habitats can impact mosquito survival via competitive interactions, which can ultimately influence vector population size and behavior, possibly influencing vector-borne disease transmission.     

Resource Competition and Coevolution in Sticklebacks

Threespine stickleback in young postglacial lakes provide a compelling example of coevolution between species that compete for resources. Coexisting pairs of stickleback species are highly divergent in habitat, diet, and body size and shape, whereas stickleback occurring alone in lakes are intermediate. We used experiments in ponds to test mechanisms of divergence between coexisting species. The results support the hypothesis of coevolution by resource competition between stickleback, but we found evidence that interactions with natural enemies also contribute to divergence. Natural selection arising from these interactions selects against intermediate phenotypes, included hybrids, and thus has contributed to the origin and persistence of stickleback species.