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Many different analyses have shown how antagonistic interactions (e.g. predation, disease, interference competition) can foster the coexistence of two species that compete for a single resource. In contrast, whether interactions with mutualist partners can similarly foster coexistence between resource competitors has been little considered. Here, we derive a mechanistic model of two plant species that compete for a single abiotic resource and that each produce nectar that supports a single shared pollinator species. In our model, plant coexistence requires three relationships: 1) one plant species must be better at utilizing the abiotic resource to produce more ovules; 2) this better resource competitor must also be more pollen‐limited in the absence of the pollinator; and 3) this species must produce much less nectar, such that pollinator abundance depends primarily on nectar produced by the less pollen‐limited species. Pollinators can also shape the competitive hierarchy among plant species without promoting coexistence, an influence determined by the amount of nectar produced by the entire plant assemblage that supports pollinator abundance. Our results show that patterns of pollen limitation and nectar production across competing plant species will be essential data to evaluating whether pollinators may foster the plants' coexistence. [ABSTRACT FROM AUTHOR]

1. Competition alters animal foraging, including promoting the use of alternative resources. It may also impact how animals feed when they are able to handle the same food with more than one tactic. Competition likely impacts both consumers and their resources through its effects on food handling, but this topic has received little attention. 2. Bees often use two tactics for extracting nectar from flowers: they can visit at the flower opening, or rob nectar from holes at the base of flowers. Exploitative competition for nectar is thought to promote nectar robbing. If so, higher competition among floral visitors should reduce constancy to a single foraging tactic as foragers will seek food using all possible tactics. To test this prediction, field observations and two experiments involving bumble bees visiting three montane Colorado plant species (Mertensia ciliata, Linaria vulgaris, Corydalis caseana) were used under various levels of inter‐ and intra‐specific competition for nectar. 3. In general, individual bumble bees remained constant to a single foraging tactic, independent of competition levels. However, bees that visited M. ciliata in field observations decreased their constancy and increased nectar robbing rates as visitation rates by co‐visitors increased. 4. While tactic constancy was high overall regardless of competition intensity, this study highlights some intriguing instances in which competition and tactic constancy may be linked. Further studies investigating the cognitive underpinnings of tactic constancy should provide insight on the ways in which animals use alternative foraging tactics to exploit resources. [ABSTRACT FROM AUTHOR]

Abstract: Aggregated distributions of parasite individuals across host individuals are nearly ubiquitous among parasitic taxa. The size and sex ratio of the population of one parasite species infecting a single host (hereafter “infrapopulation”) can influence parasite fitness through intraspecific competition, mate availability and the ability to attract vectors for transmission of parasite propagules. Competition for resources and for pollen and seed vector services may limit reproductive success (pollen receipt, fruit production and seed dispersal) in large infrapopulations of parasitic plants, while mate limitation or reduced ability to attract vectors may limit this success in small infrapopulations. Using a dioecious parasitic plant, desert mistletoe Phoradendron californicum, we experimentally removed reproductive tissue from male parasites in whole infrapopulations to test for independent effects of infrapopulation size and within‐host mate availability on female fitness. As desert mistletoe requires both pollen and seed vectors for successful reproduction, the species provides the opportunity to test how infrapopulation characteristics affect multiple components of parasite fitness. We found that insect‐mediated pollen receipt decreased for parasites on treated hosts, consistent with within‐host mate limitation. Additionally, the relationship between mate availability and fruit production per flower ranged from neutral to positive depending on year of the experiment. As expected if competition for host resources limits reproductive success more than mate availability in larger infrapopulations, the greater pollen receipt to females in large infrapopulations did not generally translate into increased mistletoe fruit production. Relationships between mistletoe fruit production per flower and infrapopulation size ranged from negative to neutral. Both pollen receipt and pollinator visitation increased with infrapopulation size, indicating that larger populations can be more attractive to pollen vectors independent of mate availability. However, we found no relationship between infrapopulation size and fruit removal by dispersers and, thus, no evidence that attraction of seed dispersal vectors increases with infrapopulation size. Synthesis. These results highlight the interactive roles of within‐host processes (resource competition, mate availability and vector attraction) in determining the fitness of biotically transmitted parasite individuals. [ABSTRACT FROM AUTHOR]

Quantifying the relationship between form and function can inform use of morphology as a surrogate for ecology. How the strength of this relationship varies continentally can inform understanding of evolutionary radiations; for example, does the relationship break down when certain lineages invade and diversify in novel habitats? The 75 species of Australian honeyeaters (Meliphagidae) are morphologically and ecologically diverse, with species feeding on nectar, insects, fruit, and other resources. We investigated Meliphagidae ecomorphology and community structure by (1) quantifying the concordance between morphology and ecology (foraging behavior), (2) estimating rates of trait evolution in relation to the packing of ecological space, and (3) comparing phylogenetic and trait community structure across the broad environmental gradients of the continent. We found that morphology explained 37% of the variance in ecology (and 62% vice versa), and we uncovered well-known bivariate relationships among the multivariate ecomorphological data. Ecological trait diversity declined less rapidly than phylogenetic diversity along a gradient of decreasing precipitation. We employ a new method (trait fields) and extend another (phylogenetic fields) to show that while species in phylogenetically clustered, arid-environment assemblages are similar morphologically, they are as varied in foraging behavior as those from more diverse assemblages. Thus, although closely related and similar morphologically, these arid-adapted species have diverged in ecological space to a similar degree as their mesic counterparts. [ABSTRACT FROM AUTHOR]

The incorporation of ecological processes into models of trait evolution is important for understanding past drivers of evolutionary change. Species interactions have long been thought to be key drivers of trait evolution. However, models for comparative data that account for interactions between species are lacking. One of the challenges is that such models are intractable and difficult to express analytically. Here we present phylogenetic models of trait evolution that include interspecific competition among chosen species. Competition is modeled as a tendency of sympatric species to evolve toward difference from one another, producing trait overdispersion and high phylogenetic signal. Themodel predicts elevated trait variance across species and a slowdown in evolutionary rate both across the clade and within each branch. Themodel also predicts a reduction in correlation between otherwise correlated traits. We use an approximate Bayesian computation approach to estimate model parameters. We find reasonable power to detect competition in sufficiently large (201species) trees compared with Brownian trait evolution and with Ornstein- Uhlenbeck and early burst models. We apply the model to examine the evolution of bill morphology of Darwin's finches and find evidence that competition affects the evolution of bill length. [ABSTRACT FROM AUTHOR]

Winning or losing a prior contest can influence the outcome of future contests, but it might also alter subsequent reproductive decisions. For example, losers may increase their investment in the current breeding attempt if losing a contest indicates limited prospects for future breeding. Using the burying beetle Nicrophorus vespilloides, we tested whether females adjust their prehatching and posthatching reproductive effort after winning or losing a contest with a same-sex conspecific. Burying beetles breed on carcasses of small vertebrates for which there is fierce intrasexual competition. We found no evidence that winning or losing a contest influenced reproductive investment decisions in this species. Instead, we show that a female's prior contest experience (regardless of its outcome) influenced the amount of posthatching care provided, with downstream consequences for the female's reproductive output; both winners and losers spentmore time provisioning food to their offspring and produced larger broods than females with no contest experience. We discuss the wider implications of our findings and present a conceptual model linking contestmediated adjustments in parental investment to population-level processes. We propose that the frequency of intraspecific contests could both influence and be influenced by population dynamics in species where contest experience influences the size and/or number of offspring produced. [ABSTRACT FROM AUTHOR]

Interspecific interactions are important structuring forces in ecological communities. Interactions can be disturbed when species are lost from a community. When interactions result in fitness gains for at least one participating organism, that organism may experience reduced fitness as a result of interaction disturbance. However, many species exhibit traits that enable individuals to persist and reproduce in spite of such disruptions, resulting in resilience to interaction disturbance. Such traits can result in interaction generalization, phenotypic and behavioral plasticity, and adaptive capacity. We discuss examples of these traits and use case studies to illustrate how restoration practitioners can use a trait-based approach to examine species of concern, identify traits that are associated with interspecific interactions and are relevant to resilience, and target such traits in restoration. Restoration activities that bolster interaction resilience could include, for example, reintroducing or supporting specific functional groups or managing abiotic conditions to reduce interaction dependence by at-risk species (e.g. providing structural complexity offering shelter and cover). Resilience may also be an important consideration in species selection for restoration. Establishment of resilient species, able to persist after interaction disturbance, may be essential to restoring to a functioning ecological community. Once such species are present, they could help support more specialized species that lack resilience traits, such as many species of concern. Understanding the conditions under which processes linked to resilience may enable species to persist and communities to reform following interaction disturbance is a key application of community ecology to ecological restoration. [ABSTRACT FROM AUTHOR]

Community interactions (e.g., predation, competition) can be characterized by two factors: their strengths and how they are structured between and within species. Both factors play a role in determining community dynamics. In addition to trophic interactions, dispersal acts as an interaction between separate populations. As with other interactions, the structure of dispersal can affect the stability of a system. However, the primary structure that has been studied in consumer-resource models has been hierarchical dispersal, where between-patch dispersal rates increase with trophic level. Here we use analytical, numerical, and simulation approaches on a two-patch, three-species metacommunity model to investigate the relationship between structure and community stability and resilience. We show that metacommunity stability is greater in systems with both weak and strong dispersal rates. Our system is stabilized by the formation of patterns when predators disperse frequently and herbivores disperse rarely, and via asynchrony when both predators and herbivores disperse infrequently. Our results show how interaction strengths within both trophic and spatial networks shape metacommunity stability. [ABSTRACT FROM AUTHOR]

Exploitation in cooperative interactions both within and between species is widespread. Although it is assumed to be costly to be exploited, mechanisms to control exploitation are surprisingly rare, making the persistence of cooperation a fundamental paradox in evolutionary biology and ecology. Focusing on between-species cooperation (mutualism), we hypothesize that the temporal sequence in which exploitation occurs relative to cooperation affects its net costs and argue that this can help explain when and where control mechanisms are observed in nature. Our principal prediction is that when exploitation occurs late relative to cooperation, there should be little selection to limit its effects (analogous to “tolerated theft” in human cooperative groups). Although we focus on cases in which mutualists and exploiters are different individuals (of the same or different species), our inferences can readily be extended to cases in which individuals exhibit mixed cooperative-exploitative strategies. We demonstrate that temporal structure should be considered alongside spatial structure as an important process affecting the evolution of cooperation. We also provide testable predictions to guide future empirical research on interspecific as well as intraspecific cooperation. [ABSTRACT FROM AUTHOR]

Multiparasitism (females of multiple species parasitizing the same host) is a ubiquitous phenomenon in parasitoids, yet the role of within-host competition has been mostly ignored in multiparasitoid-host models. Here we study the effect of varying the degree of competition at different life stages: competition over oviposition sites (between-adult competition) and larval competition over resources within the host (within-host competition). We adapt a Nicholson-Bailey model to allow for varying levels of between-adult competition (varying the overlap in species distributions) and withinhost competition (varying the number of offspring that can successfully emerge from a host). Surprisingly, while stronger between-adult competition reduces coexistence, stronger within-host competition promotes it. Asymmetric between-adult competition (a fecundity difference between the two species) reduces coexistence when compared to symmetric competition; this can be counteracted by asymmetric within-host competition (within-host competitive advantage of the lower-fecundity species), but only when within-host competition is strong and the correlation between the parasitoids' distributions is intermediate. We discuss our results in the context of the interaction between two parasitoid species, Nasonia vitripennis and Nasonia giraulti, which have strongly correlated distributions and high levels of multiparasitism in the field. We conclude that either low or asymmetric within-host competition is unlikely to explain their coexistence. [ABSTRACT FROM AUTHOR]

Pathogens live in diverse, competitive communities, yet the processes that maintain pathogen diversity remain elusive. Here, we use a species-rich, well-studied plant virus system, the barley yellow dwarf viruses, to examine the mechanisms that regulate pathogen diversity. We empirically parameterized models of three viruses, their two aphid vectors, and one perennial grass host. We found that high densities of both aphids maximized virus diversity and that competition limited the coexistence of two closely related viruses. Even limited ability to simultaneously infect (coinfect) host individuals strongly promoted virus coexistence; preventing coinfection led to priority effects. Coinfection generated stabilizing niche differences by allowing viruses to share hosts. However, coexistence also required trade-offs between vector generalist and specialist life-history strategies. Our predicted outcomes broadly concur with previous field observations. These results show how competition within individual hosts and vectorsmay lead to unexpected population-level outcomes between pathogens, including coexistence, competitive exclusion, and priority effects, and how contemporary coexistence theory can help to predict these outcomes. [ABSTRACT FROM AUTHOR]

Recent studies investigating feedbacks between evolution and ecology suggest that microevolution may affect community structure.Motivated by this, we use a quantitative genetics and Lotka-Volterra framework to understand the impact of eco-evolutionary feedbacks on an intraguild predation community in which the intraguild predator evolves between a phenotype specialized for attacking its competitor (the intraguild prey) and a phenotype specialized for attacking a common resource. We show that evolution can drive both sudden and gradual shifts in community structure. Evolutionary rescue of the community, in which evolution prevents the loss of species, occurs in two ways: (i) selection to a particular phenotype that supports coexistence and (ii) continuous evolution between prey- and resource-specialist phenotypes that support communitylevel Red Queen dynamics in which the community fluctuates between prey- and predator-dominated states. Paradoxically, the predator can evolve to extirpate itself from the community provided that the ecological dynamics support bistability. Strong trade-offs between specialist phenotypes can lead to trait-based alternative states of the community resembling either a food chain or exploitative competition. Finally, we show that rapid evolution can stabilize equilibria that are unstable for the ecological dynamics, whereas slow evolution can stabilize equilibria that are unstable for the evolutionary dynamics. Our results demonstrate that eco-evolutionary feedbacks can drive shifts in community structure and that the overall dynamics depend on the trade-off strength and evolutionary rate. [ABSTRACT FROM AUTHOR]

A fundamental assumption of coexistence theory is that competition inevitably decreases species diversity. Consequently, in the quest to understand the ecological regulators of diversity, there has been a great deal of focus on processes with the potential to reduce competitive exclusion. However, the notion that competition must decrease diversity is largely based on the outcome of two-species interaction experiments and models, despite the fact that species rarely interact only in pairs in natural systems. In a field experiment, we found that competition among native perennial plants in multispecies assemblages was far weaker than competition between those same species in pairwise arrangements and that indirect interactions appeared to weaken direct competitive effects. These results suggest that community assembly theory based on pairwise approaches may overestimate the strength of competition and likelihood of competitive exclusion in species-rich communities. We also found that Centaurea stoebe, a North American invader, retained strong competitive effects when competing against North American natives in both pairwise and multi-species assemblages. Our experimental results support an emerging body of theory suggesting that complex networks of competing species may generate strong indirect interactions that can maintain diversity and that ecological differentiation may not be necessary to attenuate competition. [ABSTRACT FROM AUTHOR]

The net effects of interspecific species interactions on individuals and populations vary in both sign (−, 0, +) and magnitude (strong to weak). Interaction outcomes are context-dependent when the sign and/or magnitude change as a function of the biotic or abiotic context. While context dependency appears to be common, its distribution in nature is poorly described. Here, we used meta-analysis to quantify variation in species interaction outcomes (competition, mutualism, or predation) for 247 published articles. Contrary to our expectations, variation in the magnitude of effect sizes did not differ among species interactions, and while mutualism was most likely to change sign across contexts (and predation least likely), mutualism did not strongly differ from competition. Both the magnitude and sign of species interactions varied the most along spatial and abiotic gradients, and least as a function of the presence/absence of a third species. However, the degree of context dependency across these context types was not consistent among mutualism, competition and predation studies. Surprisingly, study location and ecosystem type varied in the degree of context dependency, with laboratory studies showing the highest variation in outcomes. We urge that studying context dependency per se, rather than focusing only on mean outcomes, can provide a general method for describing patterns of variation in nature. [ABSTRACT FROM AUTHOR]

Mate choice frequently operates differently for males and females as a consequence of male competition for mates. Competitive interactions can alter the fitness payoffs of choice and the realization of preferences under natural conditions, yet the majority of male choice studies still use binary trials that ignore social factors. Here we test the importance of contest dynamics in male choice using a framework in which females are considered analogous to foraging patches that are subject to competition. We track the mate choices and interactions of 640 spiders (Nephila plumipes) before and after manipulation of competition within enclosures, modeling the expected fitness payoffs of each male's actual choices and comparing these with all alternative choices. Many males choose new mates once social conditions change and achieve higher fitness than predicted under random movement. Males do not simply move to larger females but choose favorable competitive environments that balance competition and female fecundity, thereby increasing their fitness payoffs. Further, we show for the first time that prior-residence effects, which are known to influence male contests, also have a strong influence in male reproductive contests and can shape male mate choice. These results highlight the importance of situating male choice studies in the relevant social context, as intrasexual interactions can have profound effects on the realization and payoffs of male mate-choice strategies. Keywords: male, mate choice, foraging, resident effect, prior residence. [ABSTRACT FROM AUTHOR]

Disentangling the mechanisms mediating the coexistence of habitat specialists and generalists has been a long-standing subject of investigation. However, the roles of species traits and environmental and spatial factors have not been assessed in a unifying theoretical framework. Theory suggests that specialist species are more competitive in natural communities. However, empirical work has shown that specialist species are declining worldwide due to habitat loss and fragmentation.We addressed the question of the coexistence of specialist and generalist species with a spatially explicit metacommunity model in continuous and heterogeneous environments. We characterized how species' dispersal abilities, the number of interacting species, environmental spatial autocorrelation, and disturbance impact community composition. Our results demonstrated that species' dispersal ability and the number of interacting species had a drastic influence on the composition of metacommunities. More specialized species coexisted when species had large dispersal abilities and when the number of interacting species was high. Disturbance selected against highly specialized species, whereas environmental spatial autocorrelation had a marginal impact. Interestingly, species richness and niche breadth were mainly positively correlated at the community scale but were negatively correlated at the metacommunity scale. Numerous diversely specialized species can thus coexist, but both species' intrinsic traits and environmental factors interact to shape the specialization signatures of communities at both the local and global scales. [ABSTRACT FROM AUTHOR]

The concept that diversity promotes reliability of ecosystem function depends on the pattern that community-level biomass shows lower temporal variability than species-level biomasses. However, this pattern is not universal, as it relies on compensatory or independent species dynamics. When in contrast within–trophic level synchronization occurs, variability of community biomass will approach population-level variability. Current knowledge fails to integrate how species richness, functional distance between species, and the relative importance of predation and competition combine to drive synchronization at different trophic levels. Here we clarify these mechanisms. Intense competition promotes compensatory dynamics in prey, but predators may at the same time increasingly synchronize, under increasing species richness and functional similarity. In contrast, predators and prey both show perfect synchronization under strong top-down control, which is promoted by a combination of low functional distance and high net growth potential of predators. Under such conditions, community-level biomass variability peaks, with major negative consequences for reliability of ecosystem function. [ABSTRACT FROM AUTHOR]

Adaptive speciation can occur when a population undergoes assortative mating and disruptive selection caused by frequency- dependent intraspecific competition. However, other interactions, such as mutualisms based on trait matching, may generate conflicting selective pressures that constrain species diversification. We used individual-based simulations to explore how different types of mutualism affect adaptive diversification. A magic trait was assumed to simultaneously mediate mate choice, intraspecific competition, and mutualisms. In scenarios of intimate, specialized mutualisms, individuals interact with one or few individual mutualistic partners, and diversification is constrained only if the mutualism is obligate. In other scenarios, increasing numbers of different partners per individual limit diversification by generating stabilizing selection. Stabilizing selection emerges from the greater likelihood of trait mismatches for rare, extreme phenotypes than for common intermediate phenotypes. Constraints on diversification imposed by increased numbers of partners decrease if the trait matching degree has smaller positive effects on fitness. These results hold after the relaxation of various assumptions. When trait matching matters, mutualism-generated stabilizing selection would thus often constrain diversification in obligate mutualisms, such as ant-myrmecophyte associations, and in low-intimacy mutualisms, including plant-seed disperser systems. Hence, different processes, such as trait convergence favoring the incorporation of nonrelated species, are needed to explain the higher richness of low-intimacy assemblages—shown here to be up to 1 order of magnitude richer than high-intimacy systems. [ABSTRACT FROM AUTHOR]

Reconciling how niche and neutral processes may be important in species coexistence has revealed two important weaknesses in our collective understanding of species diversity: few empirical studies have determined whether species are truly coexisting, and fewer still have properly evaluated whether coexistence is achieved through niche differentiation or ecological equivalence. Here, we ask whether two common barnacles, Jehlius cirratus and Notochthamalus scabrosus, coexist locally and whether the slight but persistent differences in their distribution provide sufficient fitness trade-offs to overcome differences in competition. Both species recovered after experimental reduction; that is, they coexist, with no indication of hierarchical exclusion. No fitness inequalities affected species performance or interference effects on vital rates at any shore level, indicating no trade-offs in intra-interspecific effects across the ecological gradient. Additionally, no relationship was found between per capita population growth rates of either species with its own relative abundance; that is, neither species has a demographic advantage when rare. Instead, a lottery for space during settlement largely determines species' distributions, evidenced by the positive correlation across sites and tidal elevations between the relative abundances of adults and the recruits of the prior season. We conclude that Jehlius and Notochthamalus coexist neutrally, or nearly so, but discuss whether small, nonsignificant, and probably ephemeral fitness differences, which are inconsistent across the tidal gradient, could provide enough niche differentiation to promote coexistence. [ABSTRACT FROM AUTHOR]

Social information transmission is important because it enables horizontal spread of behaviors, not only between conspecifics but also between individuals of different species. Because interspecific social information use is expected to take place among species with similar resource needs, it may have major consequences for the emergence of local adaptations, resource sharing, and community organization. Social information use is expected to be selective, but the conditions promoting it in an interspecific context are not well known. Here, we experimentally test whether pied flycatchers (Ficedula hypoleuca) use the clutch size of great tits (Parus major) in determining the quality of the observed individual and use it as a basis of decision making. We show that pied flycatchers copied or rejected a novel nest site feature preference of great tits experimentally manipulated to exhibit high or low fitness (clutch size), respectively. Our results demonstrate that the social transmission of behaviors across species can be highly selective in response to observed fitness, plausibly making the phenomenon adaptive. In contrast with the current theory of species coexistence, overlap between realized niches of species could dynamically increase or decrease depending on the observed success of surrounding individuals. [ABSTRACT FROM AUTHOR]

Sexual trait divergence has been shown to play a role in the evolution of reproductive isolation. While variation in multiple sexual signals is common among closely related species, little is known about the role of these different axes of phenotype variation with respect to the evolution of behavioral reproductive isolation. Here we study a unique population of barn swallows (Hirundo rustica transitiva) that can be distinguished phenotypically from its neighboring populations only on the basis of two features of male plumage: exaggerated expression of both long tail streamers and dark ventral coloration. Using phenotype manipulation experiments, we conducted a paternity study to examine whether both traits are sexually selected. Our results show that an exaggerated form of the local male phenotype (with both tail elongation and color darkening) is favored by local females, whereas males whose phenotypes were manipulated to look like males of neighboring subspecies suffered paternity losses from their social mates. These results confirm the multiple signaling role of the unique tail and color combination in our diverging population and suggest a novel possibility according to which multiple sexual signals may also be used to discriminate among males from nearby populations when prezygotic reproductive isolation is adaptive. [ABSTRACT FROM AUTHOR]

Interspecific competition mediates biodiversity maintenance and is an important selective pressure for evolution. Competition is often conceptualized as being exploitative (indirect) or involving direct interference. However, most empirical studies are phenomenological, focusing on quantifying effects of density manipulations, and most competition theory has characterized exploitation competition systems. The effects on resource use of traits associated with direct, interference competition has received far less attention. Here we examine the relationships of dietary ecology and phylogeny to heterospecific aggression in a guild of corallivorous reef fishes. We find that, among chaetodontids (butterflyfishes), heterospecific aggression depends on a synergistic interaction of dietary overlap and specialization: aggression increases with dietary overlap for interactions between specialists but not for interactions involving generalists. Moreover, behavioral dominance is a monotonically increasing function of dietary specialization. The strong, positive relationship of dominance to specialization suggests that heterospecific aggression may contribute to the maintenance of biodiversity where it promotes resource partitioning. Additionally, we find strong phylogenetic signals in dietary overlap and specialization but not behavioral dominance. Our results support the use of phylogeny as a proxy for ecological similarity among butterflyfishes, but we find that direct measures of dietary overlap and specialization predict heterospecific agression much better than phylogeny. [ABSTRACT FROM AUTHOR]

In multispecies assemblages, phylogenetic relatedness often predicts total community biomass. In assemblages dominated by a single species, increasing the number of genotypes increases total production, but the role of genetic relatedness is unknown. We used data from three published experiments and a field survey of eelgrass (Zostera marina), a habitat-forming marine angiosperm, to examine the strength and direction of the relationship between genetic relatedness and plant biomass. The genetic relatedness of an assemblage strongly predicted its biomass, more so than the number of genotypes. However, contrary to the pattern observed in multispecies assemblages, maximum biomass occurred in assemblages of more closely related individuals. The mechanisms underlying this pattern remain unclear; however, our data support a role for both trait differentiation and cooperation among kin. Many habitat-forming species interact intensely with conspecifics of varying relatedness; thus, genetic relatedness could influence the functioning of ecosystems dominated by such species. [ABSTRACT FROM AUTHOR]

In resource-limited populations, an increase in average body size can occur only with a decline in abundance. This is known as self-thinning, and the decline in abundance in food-limited populations is considered proportional to the scaling of metabolism with body mass. This popular hypothesis may be inaccurate, because self-thinning populations can also experience density-dependent competition, which could alter their energy use beyond the predictions of metabolic scaling. This study tested whether density-dependent competition has an energetic role in self-thinning, by manipulating the abundance of the fish Macquaria novemaculeata and tank size to partition the effects of competition from metabolic scaling.We found that self-thinning can be density dependent and that changes in intraspecific competition may be more influential than metabolic scaling on self-thinning relationships. The energetic mechanism we propose is that density-dependent competition causes variation in the allocation of energy to growth, which alters the energetic efficiency of self-thinning cohorts. The implication is that food-limited cohorts and populations with competitive strategies that encourage fast-growing individuals will have less body mass at equilibrium and higher mortality rates. This finding sheds light on the processes structuring populations and can be used to explain inconsistencies in the mass-abundance scaling of assemblages and communities (the energetic-equivalence rule). [ABSTRACT FROM AUTHOR]

Mutualisms are interspecific interactions that yield reciprocal benefits. Here, by adopting a consumer-resource perspective, we show how considering competition is necessary in order to understand the evolutionary and ecological dynamics of mutualism. We first review the ways in which competition shapes the ecology of mutualisms, using a graphical framework based on resource flows rather than net effects to highlight the opportunities for competition. We then describe the known mechanisms of competition and show how it is a critical driver of the evolutionary dynamics, persistence, and diversification of mutualism. We argue that empirical and theoretical research on the ecology and evolution of mutualisms will jointly progress by addressing four key points: (i) the existence and shape of physiological trade-offs among cooperation, competition, and other life-history and functional traits; (ii) the capacity for individuals to express conditional responses to variation in their mutualistic and competitive environment; (iii) the existence of heritable variation for mutualistic and competitive traits and their potentially conditional expression; and (iv) the structure of the network of consumer-resource interactions in which individuals are embedded. [ABSTRACT FROM AUTHOR]

nnual migrations of birds profoundly influence terrestrial communities. However, few empirical studies examine why birds migrate, in part due to the difficulty of testing causal hypotheses in long-distance migration systems. Short-distance altitudinal migrations provide relatively tractable systems in which to test explanations for migration. Many past studies explain tropical altitudinal migration as a response to spatial and temporal variation in fruit availability. Yet this hypothesis fails to explain why some coexisting, closely-related frugivorous birds remain resident year-round. We take a mechanistic approach by proposing and evaluating two hypotheses (one based on competitive exclusion and the other based on differences in dietary specialization) to explain why some, but not all, tropical frugivores migrate. We tested predictions of these hypotheses by comparing diets, fruit preferences, and the relationships between diet and preference in closely-related pairs of migrant and resident species. Fecal samples and experimental choice trials revealed that sympatric migrants and residents differed in both their diets and fruit preferences. Migrants consumed a greater diversity of fruits and fewer arthropods than did their resident counterparts. Migrants also tended to have slightly stronger fruit preferences than residents. Most critically, diets of migrants more closely matched their preferences than did the diets of residents. These results suggest that migrants may be competitively superior foragers for fruit compared to residents (rather than vice versa), implying that current competitive interactions are unlikely to explain variation in migratory behavior among coexisting frugivores. We found some support for the dietary specialization hypothesis, propose refinements to the mechanism underlying this hypothesis, and discuss how dietary specialization might ultimately reflect past interspecific competition. We recommend that future studies quantify variation in nutritional content of tropical fruits, and determine whether frugivory is a consequence or a cause of migratory behaviour. [ABSTRACT FROM AUTHOR]

1. While the relationship between facilitation and competition has been explored extensively in recent years, there is also a natural link between facilitation and mutualism, as both are interspecific interactions that confer benefits. Yet, the relationship between these two interactions has been minimally explored. 2. Here, I explore parallels and differences between mutualism and facilitation. Five focal areas organize current research on mutualism evolution: trait evolution; the continuum from specialization to generalization; the evolutionary origins and maintenance of the interaction; co-evolution of partners; and the prevalence and implications of cheating. These foci are also helpful for investigating how facilitation evolves, a much less explored issue. 3. Testable hypotheses regarding the evolution of facilitation include the following: selection should be stronger on traits of facilitated species than on traits of facilitators; facilitative interactions with mutualistic (++) and commensal (+0) outcomes should exhibit greater evolutionary stability than those with antagonistic (+−) outcomes; co-evolution should be possible in mutualistic and antagonistic facilitation only; when co-evolution occurs, it should produce a geographic mosaic of interaction outcomes; and antagonistic facilitation could lead to selection on facilitators to either escape or to tolerate the neighbours that benefit from them. 4. Synthesis. Three gaps in our knowledge currently impede progress on evolutionary questions surrounding facilitation. First, reciprocal effects are rarely investigated; facilitation might evolve like mutualism, commensalism or antagonism, depending on effects on the facilitator species. Secondly, the genetics of relevant traits are not yet well explored; the traits themselves are better known for facilitator species than for the facilitated, which are more likely to evolve in the context of the interaction. Finally, the fitness costs and benefits associated with facilitation have rarely been measured. Filling these gaps should permit rapid progress in understanding how facilitation arises, persists and evolves. [ABSTRACT FROM AUTHOR]

In generalized, multispecies mutualisms, competition among members of one guild can influence the net benefits that each species in the other guild receives. Hence seasonal factors that affect the dynamics of competition can also affect net benefits, especially if the benefit or cost of mutualism also varies seasonally. In the Sonoran Desert, two common species of generalist ants compete for access to extrafloral nectaries on the fishhook barrel cactus Ferocactus wislizeni, but their relative abundances vary seasonally; one ant dominates more cacti in winter/spring, and the other in summer/autumn. Recently, a third ant species, which also varies in abundance seasonally, has appeared at our study sites and is competing with resident ants for access to cactus nectaries. This empirical system motivated us to examine a metapopulation model of competition for patches in an open system with periodic forcing. We use the model to: (1) illustrate three ways in which competing species may differ in their sensitivities to environmental conditions that are consistent with seasonally displaced patterns of abundance; (2) ask under what conditions the invasion of a third competitor into a two-species system could alter the sensitivity of the system to environmental forcing at low vs. high frequency; and (3) show how differences among competitors in the pattern of seasonal forcing alone can dramatically alter the outcome of a competitor invasion. We discuss implications of these results for the ways in which seasonal variation among competing mutualists, and the benefits and costs of mutualism, may affect the functioning of generalized, facultative mutualisms. [ABSTRACT FROM AUTHOR]

The competitive exclusion principle is fundamental to understanding coexistence. Well‐established theories predict the conditions for coexistence in consumer–resource interactions. Given that species often compete for commodities offered by mutualists, competitive exclusion theory should also be critical to understanding how mutualisms function. We explicitly apply the competitive exclusion principle to mutualism and derive a rule analogous to Tilman's R* rule for exploitative competition. Coexistence is impossible when competitors compete solely for a shared partner‐provided commodity because superior competitors deplete that commodity sufficiently to exclude inferior competitors. We then investigate how competition between two guild members for a partner‐provided commodity and a resource external to the mutualism affects competitor coexistence. There are three key results. First, coexistence is possible via partitioning of a partner‐provided commodity and another resource. Second, unlike in classic R* Theory, competitive outcomes are influenced both by species' abilities to obtain commodities and their mutualisms with the shared commodity‐providing partner, which can indirectly alleviate competitors' commodity limitation. Third, the outcome of competition has important consequences for the commodity‐providing partner, which depend on the type of mutualism and the competitive abilities of competing mutualists. This theory provides a novel framework for investigating how competitors for mutualistic commodities coexist in nature. [ABSTRACT FROM AUTHOR]