The fundamental role of competition in the ecology and evolution of mutualisms

Extreme specificity in obligate mutualism-A role for competition?

Obligate mutualisms, reciprocally obligate beneficial interactions, are some of the most important mutualisms on the planet, providing the basis for the evolution of the eukaryotic cell, the formation and persistence of terrestrial ecosystems and the establishment and expansion of coral reefs. In addition, these mutualisms can also lead to the diversification of interacting partner species. Accompanying this diversification is a general pattern of a high degree of specificity among interacting partner species. A survey of obligate mutualisms demonstrates that greater than half of these systems have only one or two mutualist species on each side of the interaction. This is in stark contrast to facultative mutualisms that can have dozens of interacting mutualist species. We posit that the high degree of specificity in obligate mutualisms is driven by competition within obligate mutualist guilds that limits species richness. Competition may be particularly potent in these mutualisms because mutualistic partners are totally dependent on each other's fitness gains, which may fuel interspecific competition. Theory and the limited number of empirical studies testing for the role of competition in determining specificity suggest that competition may be an important force that fuels the high degree of specificity. Further empirical research is needed to dissect the relative roles of trait complementarity, mutualism regulation, and competition among mutualist guild members in determining mutualism specificity at local scales.

The complex circuitry of interactions determining coexistence among plants and mycorrhizal fungi

We present a mechanistic model of coexistence among a mycorrhizal fungus and one or two plant species that compete for a single nutrient. Plant-fungal coexistence is more likely if the fungus is better at extracting the environmental nutrient than the plant and the fungus acquires carbon from the plant above a minimum rate. When they coexist, their interaction can shift from mutualistic to parasitic at high nutrient availability. The fungus is a second nutrient source for plants and can promote the coexistence of two plant competitors if one is better at environmental nutrient extraction and the other is better at acquiring the nutrient from the fungus. Because it extracts carbon from both plants, the fungus also serves as a conduit of apparent competition between the plants. Consequently, the plant with the lower environmental nutrient extraction rate can drive the plant with the higher environmental nutrient extraction rate extinct at high carbon supply rates. This model illustrates mechanisms to explain several observed patterns, including shifts in plant-mycorrhizal growth responses and coexistence along nutrient gradients, equivocal results among experiments testing the effect of mycorrhizal fungi on plant diversity, and differences in plant diversity among ecosystems dominated by different mycorrhizal groups.

Mutualism at the leading edge: insights into the eco-evolutionary dynamics of host-symbiont communities during range expansion

The evolution of mutualism between host and symbiont communities plays an essential role in maintaining ecosystem function and should therefore have a profound effect on their range expansion dynamics. In particular, the presence of mutualistic symbionts at the leading edge of a host-symbiont community should enhance its propagation in space. We develop a theoretical framework that captures the eco-evolutionary dynamics of host-symbiont communities, to investigate how the evolution of resource exchange may shape community structure during range expansion. We consider a community with symbionts that are mutualistic or parasitic to various degrees, where parasitic symbionts receive the same amount of resource from the host as mutualistic symbionts, but at a lower cost. The selective advantage of parasitic symbionts over mutualistic ones is increased with resource availability (i.e. with host density), promoting mutualism at the range edges, where host density is low, and parasitism at the population core, where host density is higher. This spatial selection also influences the speed of spread. We find that the host growth rate (which depends on the average benefit provided by the symbionts) is maximal at the range edges, where symbionts are more mutualistic, and that host-symbiont communities with high symbiont density at their core (e.g. resulting from more mutualistic hosts) spread faster into new territories. These results indicate that the expansion of host-symbiont communities is pulled by the hosts but pushed by the symbionts, in a unique push-pull dynamic where both the host and symbionts are active and tightly-linked players.

Evolution of cooperation in a two-species system with a common resource pool

Understanding the evolution of cooperation is a major question in Evolutionary Biology. Here, we extend a previously proposed mathematical model in Evolutionary Game Theory that investigated how resource use by a single species composed of cooperators and defectors may lead to its maintenance or extinction. We include another species in the model, so as to investigate how different intra and interspecific interactions of cooperative or competitive nature among individuals that share the same essential resource may drive the survival and evolution of the species. Several outcomes emerge from the model, depending on the configuration of the payoff matrix, the individual contribution to the resource pool, the competition intensity between species, and the initial conditions of the system dynamics. Observed results include scenarios in which species thrive due to the action of cooperators, but also scenarios in which both species collapse due to lack of cooperation and, consequently, of resources. In particular, a high initial availability of resources may be the determinant factor to the survival of both species. Interestingly, cooperation may be more favored when individuals have less incentive to cooperate with others, and the survival of their populations may depend crucially on their competitive capacities.

Mutualism-enhancing mutations dominate early adaptation in a two-species microbial community

Species interactions drive evolution while evolution shapes these interactions. The resulting eco-evolutionary dynamics and their repeatability depend on how adaptive mutations available to community members affect fitness and ecologically relevant traits. However, the diversity of adaptive mutations is not well characterized, and we do not know how this diversity is affected by the ecological milieu. Here we use barcode lineage tracking to address this question in a community of yeast Saccharomyces cerevisiae and alga Chlamydomonas reinhardtii that have a net commensal relationship that results from a balance between competitive and mutualistic interactions. We find that yeast has access to many adaptive mutations with diverse ecological consequences, in particular those that increase and reduce the yields of both species. The presence of the alga does not change which mutations are adaptive in yeast (that is, there is no fitness trade-off for yeast between growing alone or with alga), but rather shifts selection to favour yeast mutants that increase the yields of both species and make the mutualism stronger. Thus, in the presence of the alga, adaptative mutations contending for fixation in yeast are more likely to enhance the mutualism, even though cooperativity is not directly favoured by natural selection in our system. Our results demonstrate that ecological interactions not only alter the trajectory of evolution but also dictate its repeatability; in particular, weak mutualisms can repeatably evolve to become stronger.

The ephemeral resource patch concept

Ephemeral resource patches (ERPs) - short lived resources including dung, carrion, temporary pools, rotting vegetation, decaying wood, and fungi - are found throughout every ecosystem. Their short-lived dynamics greatly enhance ecosystem heterogeneity and have shaped the evolutionary trajectories of a wide range of organisms - from bacteria to insects and amphibians. Despite this, there has been no attempt to distinguish ERPs clearly from other resource types, to identify their shared spatiotemporal characteristics, or to articulate their broad ecological and evolutionary influences on biotic communities. Here, we define ERPs as any distinct consumable resources which (i) are homogeneous (genetically, chemically, or structurally) relative to the surrounding matrix, (ii) host a discrete multitrophic community consisting of species that cannot replicate solely in any of the surrounding matrix, and (iii) cannot maintain a balance between depletion and renewal, which in turn, prevents multiple generations of consumers/users or reaching a community equilibrium. We outline the wide range of ERPs that fit these criteria, propose 12 spatiotemporal characteristics along which ERPs can vary, and synthesise a large body of literature that relates ERP dynamics to ecological and evolutionary theory. We draw this knowledge together and present a new unifying conceptual framework that incorporates how ERPs have shaped the adaptive trajectories of organisms, the structure of ecosystems, and how they can be integrated into biodiversity management and conservation. Future research should focus on how inter- and intra-resource variation occurs in nature - with a particular focus on resource × environment × genotype interactions. This will likely reveal novel adaptive strategies, aid the development of new eco-evolutionary theory, and greatly improve our understanding of the form and function of organisms and ecosystems.

When wax wanes: competitors for beeswax stabilize rather than jeopardize the honeyguide–human mutualism

Many mutualisms are exploited by third-party species, which benefit without providing anything in return. Exploitation can either destabilize or promote mutualisms, via mechanisms that are highly dependent on the ecological context. Here we study a remarkable bird–human mutualism, in which wax-eating greater honeyguides ( Indicator indicator ) guide humans ( Homo sapiens ) to wild bees' nests, in an exchange of knowledge about the location of nests for access to the wax combs inside. We test whether the depletion of wax by mammalian and avian exploiter species either threatens or stabilizes the mutualism. Using camera traps, we monitored feeding visits to wax comb made available following honey harvests. We found that greater honeyguides face competition for wax from conspecifics and nine exploiter species, five of which were not previously known to consume wax. Our results support the hypothesis that heterospecific exploiters stabilize the mutualism, because wax depletion by these competitors probably limits feeding opportunities for conspecific exploiters, favouring the early-arriving individual that guided humans to the bees’ nest. These findings highlight the importance of the ecological context of species interactions and provide further evidence for how mutualisms can persist because of, and not in spite of, exploitation by third-party species.

Competition for pollinators destabilizes plant coexistence

Mounting concern over the global decline of pollinators has fuelled calls for investigating their role in maintaining plant diversity^1,2. Theory predicts that competition for pollinators can stabilize interactions between plant species by providing opportunities for niche differentiation³, while at the same time can drive competitive imbalances that favour exclusion⁴. Here we empirically tested these contrasting effects by manipulating competition for pollinators in a way that predicts its long-term implications for plant coexistence. We subjected annual plant individuals situated across experimentally imposed gradients in neighbour density to either ambient insect pollination or a pollen supplementation treatment alleviating competition for pollinators. The vital rates of these individuals informed plant population dynamic models predicting the key theoretical metrics of species coexistence. Competition for pollinators generally destabilized the interactions between plant species, reducing the proportion of pairs expected to coexist. Interactions with pollinators also influenced the competitive imbalances between plant species, effects that are expected to strengthen with pollinator decline, potentially disrupting plant coexistence. Indeed, results from an experiment simulating pollinator decline showed that plant species experiencing greater reductions in floral visitation also suffered greater declines in population growth rate. Our results reveal that competition for pollinators may weaken plant coexistence by destabilizing interactions and contributing to competitive imbalances, information critical for interpreting the impacts of pollinator decline.

When does mutualism offer a competitive advantage? A game-theoretic analysis of host-host competition in mutualism

Due to their non-motile nature, plants rely heavily on mutualistic interactions to obtain resources and carry out services. One key mutualism is the plant-microbial mutualism in which a plant trades away carbon to a microbial partner for nutrients like nitrogen and phosphorous. Plants show much variation in the use of this partnership from the individual level to entire lineages depending upon ecological, evolutionary and environmental context. We sought to determine how this context dependency could result in the promotion, exclusion or coexistence of the microbial mutualism by asking if and when the partnership provided a competitive advantage to the plant. To that end, we created a 2 × 2 evolutionary game in which plants could either be a mutualist and pair with a microbe or be a non-mutualist and forgo the partnership. Our model includes both frequency dependence and density dependence, which gives us the eco-evolutionary dynamics of mutualism evolution. As in all models, mutualism only evolved if it could offer a competitive advantage and its net benefit was positive. However, surprisingly the model reveals the possibility of coexistence between mutualist and non-mutualist genotypes due to competition between mutualists over the microbially obtained nutrient. Specifically, frequency dependence of host strategies can make the microbial symbiont less beneficial if the microbially derived resources are shared, a phenomenon that increasingly reduces the frequency of mutualism as the density of competitors increases. In essence, ecological competition can act as a hindrance to mutualism evolution. We go on to discuss basic experiments that can be done to test and falsify our hypotheses.

Extrafloral nectar production induced by simulated herbivory does not improve ant bodyguard attendance and ultimately plant defence

Highly competitive and aggressive ant species are efficient bodyguards that monopolize the more attractive plants bearing extrafloral nectaries. Given that herbivory often increases the quality of extrafloral nectar, we hypothesized that plants damaged by herbivory would be more prone to interact with high-quality ant bodyguards and be better defended against herbivores. We performed an experiment with Chamaecrista nictitans plants. We induced anti-herbivore responses by applying jasmonic acid to a group of plants while keeping another group unmanaged. We measured extrafloral nectar production, censused ants visiting extrafloral nectaries and, subsequently, added herbivore mimics to measure the efficiency of ant anti-herbivore defence in both conditions. Induction increased the volume of extrafloral nectar and the mass of sugar per nectary without affecting the sugar concentration or the patterns of plant attendance and defence by ants. Thus, we found no evidence that defence-induced C. nictitans plants are more prone to interact with high-quality bodyguards or to receive better anti-herbivore defence. These findings highlight that increases in extrafloral nectar production are not always rewarded with increases in the biotic defences; instead, these rewards might be dependent on the traits of the nectar induced by herbivory events and/or on the ecological context in which the interaction is embedded. Consequently, herbivory might increase the costs of this induced biotic defence to plants bearing extrafloral nectaries when the induced defence does not increase the attractiveness of the plants to ants.

The Evolution of Resource Provisioning in Pollination Mutualisms

AbstractResource dynamics influence the contemporary ecology of consumer-resource mutualisms. Suites of resource traits, such as floral nectar components, also evolve in response to different selective pressures, changing the ecological dynamics of the interacting species at the evolutionary equilibrium. Here we explore the evolution of resource-provisioning traits in a biotically pollinated plant that produces nectar as a resource for beneficial consumers. We develop a mathematical model describing natural selection on two quantitative nectar traits: maximum nectar production rate and maximum nectar reservoir volume. We use this model to examine how nectar production dynamics evolve under different ecological conditions that impose varying cost-benefit regimes on resource provisioning. The model results predict that natural selection favors higher nectar production when ecological factors limit the plant or pollinator's abundance (e.g., a lower productivity environment or a higher pollinator conversion efficiency). We also find that nectar traits evolve as a suite in which higher costs of producing one trait select for a compensatory increase in investment in the other trait. This empirically explicit approach to studying the evolution of consumer-resource mutualisms illustrates how natural selection acting via direct and indirect pathways of species interactions generates patterns of resource provisioning seen in natural systems.

Phenological traits foster persistence of mutualistic networks by promoting facilitation

Morphological and phenological traits are key determinants of the structure of mutualistic networks. Both traits create forbidden links, but phenological traits can also decouple interaction in time. While such difference likely affects the indirect effects among species and consequently network persistence, it remains overlooked. Here, using a dynamic model, we show that networks structured by phenology favour facilitation over competition within guilds of pollinators and plants, thereby increasing network persistence, while the contrary holds for networks structured by morphology. We further show that such buffering of competition by phenological traits mostly beneficiate to specialists, the most vulnerable species otherwise, which propagate the most positive effects within guilds and promote nestedness. Our results indicate that beyond trophic mismatch, phenological shifts such as those induced by climate change are likely to affect indirect effects within mutualistic assemblages, with consequences for biodiversity.

How mutualisms influence the coexistence of competing species

Mutualisms are ubiquitous in nature and are thought to play important roles in the maintenance of biodiversity. For biodiversity to be maintained, however, species must coexist in the face of competitive exclusion. Chesson's coexistence theory provides a mechanistic framework for evaluating coexistence, yet mutualisms are conspicuously absent from coexistence theory and there are no comparable frameworks for evaluating how mutualisms affect the coexistence of competiting species. To address this conceptual gap, I develop theory predicting how multitrophic mutualisms mediate the coexistence of species competing for mutualistic commodities and other limiting resources using the niche and fitness difference concepts of coexistence theory. I demonstrate that failing to account for mutualisms can lead to erroneous conclusions. For example, species might appear to coexist on resources alone, when the simultaneous incorporation of mutualisms actually drives competitive exclusion, or competitive exclusion might occur under resource competition, when in fact, the incorporation of mutualisms generates coexistence. Existing coexistence theory cannot therefore be applied to mutualisms without explicitly considering the underlying biology of the interactions. By discussing how the metrics derived from coexistence theory can be quantified empirically, I show how this theory can be operationalized to evaluate the coexistence consequences of mutualism in natural communities.

Mycorrhizal fungi control phosphorus value in trade symbiosis with host roots when exposed to abrupt 'crashes' and 'booms' of resource availability

Biological market theory provides a conceptual framework to analyse trade strategies in symbiotic partnerships. A key prediction of biological market theory is that individuals can influence resource value - meaning the amount a partner is willing to pay for it - by mediating where and when it is traded. The arbuscular mycorrhizal symbiosis, characterised by roots and fungi trading phosphorus and carbon, shows many features of a biological market. However, it is unknown if or how fungi can control phosphorus value when exposed to abrupt changes in their trade environment. We mimicked an economic 'crash', manually severing part of the fungal network (Rhizophagus irregularis) to restrict resource access, and an economic 'boom' through phosphorus additions. We quantified trading strategies over a 3-wk period using a recently developed technique that allowed us to tag rock phosphate with fluorescing quantum dots of three different colours. We found that the fungus: compensated for resource loss in the 'crash' treatment by transferring phosphorus from alternative pools closer to the host root (Daucus carota); and stored the surplus nutrients in the 'boom' treatment until root demand increased. By mediating from where, when and how much phosphorus was transferred to the host, the fungus successfully controlled resource value.

Limitation by a shared mutualist promotes coexistence of multiple competing partners

Although mutualisms are often studied as simple pairwise interactions, they typically involve complex networks of interacting species. How multiple mutualistic partners that provide the same service and compete for resources are maintained in mutualistic networks is an open question. We use a model bacterial community in which multiple 'partner strains' of Escherichia coli compete for a carbon source and exchange resources with a 'shared mutualist' strain of Salmonella enterica. In laboratory experiments, competing E. coli strains readily coexist in the presence of S. enterica, despite differences in their competitive abilities. We use ecological modeling to demonstrate that a shared mutualist can create temporary resource niche partitioning by limiting growth rates, even if yield is set by a resource external to a mutualism. This mechanism can extend to maintain multiple competing partner species. Our results improve our understanding of complex mutualistic communities and aid efforts to design stable microbial communities.

Species richness and redundancy promote persistence of exploited mutualisms in yeast

Mutualisms, or reciprocally beneficial interspecific interactions, constitute the foundation of many ecological communities and agricultural systems. Mutualisms come in different forms, from pairwise interactions to extremely diverse communities, and they are continually challenged with exploitation by nonmutualistic community members (exploiters). Thus, understanding how mutualisms persist remains an essential question in ecology. Theory suggests that high species richness and functional redundancy could promote mutualism persistence in complex mutualistic communities. Using a yeast system (Saccharomyces cerevisiae), we experimentally show that communities with the greatest mutualist richness and functional redundancy are nearly two times more likely to survive exploitation than are simple communities. Persistence increased because diverse communities were better able to mitigate the negative effects of competition with exploiters. Thus, large mutualistic networks may be inherently buffered from exploitation.

Interactions among interactions: The dynamical consequences of antagonism between mutualists

Species often interact with multiple mutualistic partners that provide functionally different benefits and/or that interact with different life-history stages. These functionally different partners, however, may also interact directly with one another in other ways, indirectly altering net outcomes and persistence of the mutualistic system as a whole. We present a population dynamical model of a three-species system involving antagonism between species sharing a mutualist partner species with two explicit life stages. We find that, regardless of whether the antagonism is predatory or non-consumptive, persistence of the shared mutualist is possible only under a restrictive set of conditions. As the rate of antagonism between the species sharing the mutualist increases, indirect rather than direct interactions increasingly determine species' densities and sometimes result in complex, oscillatory dynamics for all species. Surprisingly, persistence of the mutualistic system is particularly dependent upon the degree to which each of the two mutualistic interactions is specialized. Our work investigates a novel mechanism by which changing ecological conditions can lead to extinction of mutualist partners and provides testable predictions regarding the interactive roles of mutualism and antagonism in net outcomes for species' densities.

Endosymbiosis before eukaryotes: mitochondrial establishment in protoeukaryotes

Endosymbiosis and organellogenesis are virtually unknown among prokaryotes. The single presumed example is the endosymbiogenetic origin of mitochondria, which is hidden behind the event horizon of the last eukaryotic common ancestor. While eukaryotes are monophyletic, it is unlikely that during billions of years, there were no other prokaryote-prokaryote endosymbioses as symbiosis is extremely common among prokaryotes, e.g., in biofilms. Therefore, it is even more precarious to draw conclusions about potentially existing (or once existing) prokaryotic endosymbioses based on a single example. It is yet unknown if the bacterial endosymbiont was captured by a prokaryote or by a (proto-)eukaryote, and if the process of internalization was parasitic infection, slow engulfment, or phagocytosis. In this review, we accordingly explore multiple mechanisms and processes that could drive the evolution of unicellular microbial symbioses with a special attention to prokaryote-prokaryote interactions and to the mitochondrion, possibly the single prokaryotic endosymbiosis that turned out to be a major evolutionary transition. We investigate the ecology and evolutionary stability of inter-species microbial interactions based on dependence, physical proximity, cost-benefit budget, and the types of benefits, investments, and controls. We identify challenges that had to be conquered for the mitochondrial host to establish a stable eukaryotic lineage. Any assumption about the initial interaction of the mitochondrial ancestor and its contemporary host based solely on their modern relationship is rather perilous. As a result, we warn against assuming an initial mutually beneficial interaction based on modern mitochondria-host cooperation. This assumption is twice fallacious: (i) endosymbioses are known to evolve from exploitative interactions and (ii) cooperativity does not necessarily lead to stable mutualism. We point out that the lack of evidence so far on the evolution of endosymbiosis from mutual syntrophy supports the idea that mitochondria emerged from an exploitative (parasitic or phagotrophic) interaction rather than from syntrophy.

Foundress numbers and the timing of selective events during interactions between figs and fig wasps

In intimate mutualisms between hosts and symbionts, selection can act repeatedly over the development times of the interacting individuals. Although much is now known about the overall ecological conditions that favor the evolution of mutualism, a current challenge is to understand how natural selection acts on the number and kinds of partners to shape the evolution and stability of these interactions. Using the obligate fig-fig wasp mutualism, our experiments showed that the proportion of figs developed to maturity increased quickly to 1.0 as the number of foundresses increased, regardless of whether the foundresses carried pollen. Selection against pollen-free wasps did not occur at this early stage in fig development. Within figs that developed, the proportion of galls producing adult wasps remained high as the number of pollen-carrying foundresses increases. In contrast, the proportion of galls producing adult wasps decreased as the number of pollen-free foundresses increased. Viable seed production increased as the number or proportion of pollen-carrying foundresses increased, but the average number of wasp offspring per pollen-carrying foundress was highest when she was the sole foundress. These results show that figs and their pollinator wasps differ in how fitness effects are distributed throughout the development of the interaction and depend on the number and proportion of pollen-carrying foundresses contributing to the interaction. These results suggest that temporal fluctuations in the local number and proportion of pollen-carrying wasps available to enter figs are likely to have strong but different effects on the figs and the wasps.

The Termite Fecal Nest: A Framework for the Opportunistic Acquisition of Beneficial Soil Streptomyces (Actinomycetales: Streptomycetaceae)

Mutualistic associations between insects and microorganisms must imply gains for both partners, and the emphasis has mostly focused on coevolved host-symbiont systems. However, some insect hosts may have evolved traits that allow for various means of association with opportunistic microbial communities, especially when the microbes are omnipresent in their environment. It was previously shown that colonies of the subterranean termite Coptotermes formosanus Shiraki (Blattodea: Rhinotermitidae) build nests out of fecal material that host a community of Streptomyces Waksman and Henrici (Actinomycetales: Streptomycetaceae). These Actinobacteria produce an array of bioactive metabolites that provides a level of protection for termites against certain entomopathogenic fungi. How C. formosanus acquires and maintains this association remains unknown. This study shows that the majority of Streptomyces isolates found in field termite fecal nest materials are identical to Streptomyces isolates from soils surrounding the nests and are not vertically inherited. A survey of Streptomyces communities from C. formosanus fecal nest materials sampled at 20 locations around the world revealed that all nests are reliably associated with a diverse Streptomyces community. The C. formosanus fecal nest material therefore provides a nutritional framework that can recruit beneficial Streptomyces from the soil environment, in the absence of long-term coevolutionary processes. A diverse Streptomyces community is reliably present in soils, and subterranean termite colonies can acquire such facultative symbionts each social cycle into their fecal nest. This association probably emerged as an exaptation from the existing termite nest structure and benefits both the termite and the opportunistic colonizing bacteria.

Aggressive bodyguards are not always the best: Preferential interaction with more aggressive ant species reduces reproductive success of plant bearing extrafloral nectaries

Variation in partner species and frequency of interaction between species pairs are potential drivers of the net outcome of generalized mutualisms. In ant-plant mutualisms, the quality of defence provided by ants is related to ant aggressiveness. Hence, we hypothesize that the performance of plants bearing extrafloral nectaries will be higher when they interact more frequently with more aggressive ant species. We estimated ant aggressiveness in the field by observing their behaviour towards soil baits. Afterwards, we observed the frequency with which individuals from these ant species visited plants through an entire reproductive cycle. We measured the production and persistence of plants reproductive structures through this period and the total seed production. Increasing in the interaction frequency with highly aggressive ants reduced the number of floral buds and seeds produced. Increased visitation frequency by less aggressive ants increased the number of floral buds and seeds per branch. The inverse relationship between ant aggressiveness and seed production may be influenced by the costs imposed by different mutualistic partners. Thus, frequent interaction with highly aggressive ants may lead to a higher accumulation of costs through time, resulting in a negative net outcome for the plants. Our results bring new evidence highlighting the importance to incorporate temporal aspects in the study of mutualistic interactions. We suggests that the quality of mutualistic partners must be understood as a function of its per-interaction benefit and their cumulative costs to their partner over time, what puts in check our current classification regarding partner quality in mutualistic systems.

Carbon allocation and competition maintain variation in plant root mutualisms

Plants engage in multiple root symbioses that offer varying degrees of benefit. We asked how variation in partner quality persists using a resource-ratio model of population growth. We considered the plant's ability to preferentially allocate carbon to mutualists and competition for plant carbon between mutualist and nonmutualist symbionts. We treated carbon as two nutritionally interchangeable, but temporally separated, resources-carbon allocated indiscriminately for the construction of the symbiosis, and carbon preferentially allocated to the mutualist after symbiosis establishment and assessment. This approach demonstrated that coexistence of mutualists and nonmutualists is possible when fidelity of the plant to the mutualist and the cost of mutualism mediate resource competition. Furthermore, it allowed us to trace symbiont population dynamics given varying degrees of carbon allocation. Specifically, coexistence occurs at intermediate levels of preferential allocation. Our findings are consistent with previous empirical studies as well the application of biological market theory to plantroot symbioses.

Trade-offs in an ant-plant-fungus mutualism

Species engaged in multiple, simultaneous mutualisms are subject to trade-offs in their mutualistic investment if the traits involved in each interaction are overlapping, which can lead to conflicts and affect the longevity of these associations. We investigate this issue via a tripartite mutualism involving an ant plant, two competing ant species and a fungus the ants cultivate to build galleries under the stems of their host plant to capture insect prey. The use of the galleries represents an innovative prey capture strategy compared with the more typical strategy of foraging on leaves. However, because of a limited worker force in their colonies, the prey capture behaviour of the ants results in a trade-off between plant protection (i.e. the ants patrol the foliage and attack intruders including herbivores) and ambushing prey in the galleries, which has a cascading effect on the fitness of all of the partners. The quantification of partners' traits and effects showed that the two ant species differed in their mutualistic investment. Less investment in the galleries (i.e. in fungal cultivation) translated into more benefits for the plant in terms of less herbivory and higher growth rates and vice versa. However, the greater vegetative growth of the plants did not produce a positive fitness effect for the better mutualistic ant species in terms of colony size and production of sexuals nor was the mutualist compensated by the wider dispersal of its queens. As a consequence, although the better ant mutualist is the one that provides more benefits to its host plant, its lower host-plant exploitation does not give this ant species a competitive advantage. The local coexistence of the ant species is thus fleeting and should eventually lead to the exclusion of the less competitive species.

Evolution on the bright side of life: microorganisms and the evolution of mutualism

Mutualistic interactions, where two interacting species have a net beneficial effect on each other's fitness, play a crucial role in the survival and evolution of many species. Despite substantial empirical and theoretical work in past decades, the impact of these interactions on natural selection is not fully understood. In addition, mutualisms between microorganisms have been largely ignored, even though they are ecologically important and can be used as tools to bridge the gap between theory and empirical work. Here, I describe two problems with our current understanding of natural selection in mutualism and highlight the properties of microbial mutualisms that could help solve them. One problem is that bias and methodological problems have limited our understanding of the variety of mechanisms by which species may adapt to mutualism. Another problem is that it is rare for experiments testing coevolution in mutualism to address whether each species has adapted to evolutionary changes in its partner. These problems can be addressed with genome resequencing and time-shift experiments, techniques that are easier to perform in microorganisms. In addition, microbial mutualisms may inspire novel insights and hypotheses about natural selection in mutualism.

The Ecological Dynamics of Natural Selection: Traits and the Coevolution of Community Structure

Natural selection has both genetic and ecological dynamics. The fitnesses of individuals change with their ecological context, and so the form and strength of selective agents change with abiotic factors and the phenotypes and abundances of interacting species. I use standard models of consumer-resource interactions to explore the ecological dynamics of natural selection and how various trait types influence these dynamics and the resulting structure of a community of coevolving species. Evolutionary optima favored by natural selection depend critically on the abundances of interacting species, and the traits of species can undergo dynamic cycling in limited areas of parameter space. The ecological dynamics of natural selection can also drive shifts from one adaptive peak to another, and these ecologically driven adaptive peak shifts are fundamental to the dynamics of niche differentiation. Moreover, this ecological differentiation is fostered in more productive and more benign environments where species interactions are stronger and where the selection gradients generated by species interactions are stronger. Finally, community structure resulting from coevolution depends fundamentally on the types of traits that underlie species interactions. The ecological dynamics of the process cannot be simplified, neglected, or ignored if we are to build a predictive theory of natural selection.

Synthesizing perspectives on the evolution of cooperation within and between species

Cooperation is widespread both within and between species, but are intraspecific and interspecific cooperation fundamentally similar or qualitatively different phenomena? This review evaluates this question, necessary for a general understanding of the evolution of cooperation. First, we outline three advantages of cooperation relative to noncooperation (acquisition of otherwise inaccessible goods and services, more efficient acquisition of resources, and buffering against variability), and predict when individuals should cooperate with a conspecific versus a heterospecific partner to obtain these advantages. Second, we highlight five axes along which heterospecific and conspecific partners may differ: relatedness and fitness feedbacks, competition and resource use, resource-generation abilities, relative evolutionary rates, and asymmetric strategy sets and outside options. Along all of these axes, certain asymmetries between partners are more common in, but not exclusive to, cooperation between species, especially complementary resource use and production. We conclude that cooperation within and between species share many fundamental qualities, and that differences between the two systems are explained by the various asymmetries between partners. Consideration of the parallels between intra- and interspecific cooperation facilitates application of well-studied topics in one system to the other, such as direct benefits within species and kin-selected cooperation between species, generating promising directions for future research.

Asymmetric competition for nectar between a large nectar thief and a small pollinator: an energetic point of view

There are two alternative hypotheses related to body size and competition for restricted food sources. The first one supposes that larger animals are superior competitors because of their increased feeding abilities, whereas the second one assumes superiority of smaller animals because of their lower food requirements. We examined the relationship between two unrelated species of different size, drinking technique, energy requirements and roles in plant pollination system, to reveal the features of their competitive interaction and mechanisms enabling their co-existence while utilising the same nectar source. We observed diurnal feeding behaviour of the main pollinator, the carpenter bee Xylocopa caffra and a nectar thief, the northern double-collared sunbird Cinnyris reichenowi on 19 clumps of Hypoestes aristata (Acanthaceae) in Bamenda Highlands, Cameroon. For comparative purpose, we established a simplistic model of daily energy expenditure and daily energy intake by both visitor species assuming that they spend all available daytime feeding on H. aristata. We revealed the energetic gain-expenditure balance of the studied visitor species in relation to diurnal changes in nectar quality and quantity. In general, smaller energy requirements and related ability to utilise smaller resources made the main pollinator X. caffra competitively superior to the larger nectar thief C. reichenowi. Nevertheless, sunbirds are endowed with several mechanisms to reduce asymmetry in exploitative competition, such as the use of nectar resources in times of the day when rivals are inactive, aggressive attacks on carpenter bees while defending the nectar plants, and higher speed of nectar consumption.

Population variation affects interactions between two California salt marsh plant species more than precipitation

Species that occur along broad environmental gradients often vary in phenotypic traits that make them better adapted to local conditions. Variation in species interactions across gradients could therefore be due to either phenotypic differences among populations or environmental conditions that shift the balance between competition and facilitation. To understand how the environment (precipitation) and variation among populations affect species interactions, we conducted a common garden experiment using two common salt marsh plant species, Salicornia pacifica and Jaumea carnosa, from six salt marshes along the California coast encompassing a large precipitation gradient. Plants were grown alone or with an individual of the opposite species from the same site and exposed to one of three precipitation regimes. J. carnosa was negatively affected in the presence of S. pacifica, while S. pacifica was facilitated by J. carnosa. The strength of these interactions varied by site of origin but not by precipitation treatment. These results suggest that phenotypic variation among populations can affect interaction strength more than environment, despite a threefold difference in precipitation. Geographic intraspecific variation may therefore play an important role in determining the strength of interactions in communities.

Adding biotic complexity alters the metabolic benefits of mutualism

Mutualism is ubiquitous in nature and plays an integral role in most communities. To predict the eco-evolutionary dynamics of mutualism it is critical to extend classic pair-wise analysis to include additional species. We investigated the effect of adding a third species to a pair-wise mutualism in a spatially structured environment. We tested the hypotheses that selection for costly excretions in a focal population (i) decreases when an exploiter is added (ii) increases when a third mutualist is added relative to the pair-wise scenario. We assayed the selection acting on Salmonella enterica when it exchanges methionine for carbon in an obligate mutualism with an auxotrophic Escherichia coli. A third bacterium, Methylobacterium extorquens, was then added and acted either as an exploiter of the carbon or third obligate mutualist depending on the nitrogen source. In the tripartite mutualism M. extorquens provided nitrogen to the other species. Contrary to our expectations, adding an exploiter increased selection for methionine excretion in S. enterica. Conversely, selection for cooperation was lower in the tripartite mutualism relative to the pair-wise system. Genome-scale metabolic models helped identify the mechanisms underlying these changes in selection. Our results highlight the utility of connecting metabolic mechanisms and eco-evolutionary dynamics.

Pollinator Competition as a Driver of Floral Divergence: An Experimental Test

Optimal foraging models of floral divergence predict that competition between two different types of pollinators will result in partitioning, increased assortative mating, and divergence of two floral phenotypes. We tested these predictions in a tropical plant-pollinator system using sexes of purple-throated carib hummingbirds (Anthracothorax jugularis) as the pollinators, red and yellow inflorescence morphs of Heliconia caribaea as the plants, and fluorescent dyes as pollen analogs in an enclosed outdoor garden. When foraging alone, males exhibited a significant preference for the yellow morph of H. caribaea, whereas females exhibited no preference. In competition, males maintained their preference for the yellow morph and through aggression caused females to over-visit the red morph, resulting in resource partitioning. Competition significantly increased within-morph dye transfer (assortative mating) relative to non-competitive environments. Competition and partitioning of color morphs by sexes of purple-throated caribs also resulted in selection for floral divergence as measured by dye deposition on stigmas. Red and yellow morphs did not differ significantly in dye deposition in the competition trials, but differences in dye deposition and preferences for morphs when sexes of purple-throated caribs foraged alone implied fixation of one or the other color morph in the absence of competition. Competition also resulted in selection for divergence in corolla length, with the red morph experiencing directional selection for longer corollas and the yellow morph experiencing stabilizing selection on corolla length. Our results thus support predictions of foraging models of floral divergence and indicate that pollinator competition is a viable mechanism for divergence in floral traits of plants.

Thiomonas sp. CB2 is able to degrade urea and promote toxic metal precipitation in acid mine drainage waters supplemented with urea

The acid mine drainage (AMD) in Carnoulès (France) is characterized by the presence of toxic metals such as arsenic. Several bacterial strains belonging to the Thiomonas genus, which were isolated from this AMD, are able to withstand these conditions. Their genomes carry several genomic islands (GEIs), which are known to be potentially advantageous in some particular ecological niches. This study focused on the role of the “urea island” present in the Thiomonas CB2 strain, which carry the genes involved in urea degradation processes. First, genomic comparisons showed that the genome of Thiomonas sp. CB2, which is able to degrade urea, contains a urea genomic island which is incomplete in the genome of other strains showing no urease activity. The urease activity of Thiomonas sp. CB2 enabled this bacterium to maintain a neutral pH in cell cultures in vitro and prevented the occurrence of cell death during the growth of the bacterium in a chemically defined medium. In AMD water supplemented with urea, the degradation of urea promotes iron, aluminum and arsenic precipitation. Our data show that ureC was expressed in situ, which suggests that the ability to degrade urea may be expressed in some Thiomonas strains in AMD, and that this urease activity may contribute to their survival in contaminated environments.

Cheaters must prosper: reconciling theoretical and empirical perspectives on cheating in mutualism

Cheating is a focal concept in the study of mutualism, with the majority of researchers considering cheating to be both prevalent and highly damaging. However, current definitions of cheating do not reliably capture the evolutionary threat that has been a central motivation for the study of cheating. We describe the development of the cheating concept and distill a relative-fitness-based definition of cheating that encapsulates the evolutionary threat posed by cheating, i.e. that cheaters will spread and erode the benefits of mutualism. We then describe experiments required to conclude that cheating is occurring and to quantify fitness conflict more generally. Next, we discuss how our definition and methods can generate comparability and integration of theory and experiments, which are currently divided by their respective prioritisations of fitness consequences and traits. To evaluate the current empirical evidence for cheating, we review the literature on several of the best-studied mutualisms. We find that although there are numerous observations of low-quality partners, there is currently very little support from fitness data that any of these meet our criteria to be considered cheaters. Finally, we highlight future directions for research on conflict in mutualisms, including novel research avenues opened by a relative-fitness-based definition of cheating.

Inherent demographic stability in mutualist-resource-exploiter interactions

Core principles of ecological theory predict that, in the absence of other factors, mutualisms should experience destabilizing positive feedback and should be vulnerable to extinction through competitive exclusion by exploiter species. Many effective stabilizing mechanisms address one issue or the other, and many turn upon additional features. Using an explicitly demographic approach, I show that indirect, demography-mediated interactions between mutualists and exploiters can enable mutualist-exploiter coexistence, which in turn can stabilize the abundances of mutualists, exploiters, and their shared resources. This occurs because of the distinct resource demographic responses that are inherent to interaction with mutualistic and exploitative partners and can occur in long-lasting, exclusive interactions, such as protection mutualisms, as well as in apparently very different, short-lived mutualistic interactions, such as pollination. The key necessary factor-demographic response to interspecific interaction-is common in nature. Some demographic structure is also necessary and is generated through interspecific interaction in long-lasting associations; it is also very common in natural populations. Thus, the explicitly demographic and multispecies approach taken here constitutes a potentially promising single explanation for the apparent stability of mutualism in a wide range of natural systems.

Dynamical transitions in a pollination-herbivory interaction: a conflict between mutualism and antagonism

Plant-pollinator associations are often seen as purely mutualistic, while in reality they can be more complex. Indeed they may also display a diverse array of antagonistic interactions, such as competition and victim–exploiter interactions. In some cases mutualistic and antagonistic interactions are carried-out by the same species but at different life-stages. As a consequence, population structure affects the balance of inter-specific associations, a topic that is receiving increased attention. In this paper, we developed a model that captures the basic features of the interaction between a flowering plant and an insect with a larval stage that feeds on the plant’s vegetative tissues (e.g. leaves) and an adult pollinator stage. Our model is able to display a rich set of dynamics, the most remarkable of which involves victim–exploiter oscillations that allow plants to attain abundances above their carrying capacities and the periodic alternation between states dominated by mutualism or antagonism. Our study indicates that changes in the insect’s life cycle can modify the balance between mutualism and antagonism, causing important qualitative changes in the interaction dynamics. These changes in the life cycle could be caused by a variety of external drivers, such as temperature, plant nutrients, pesticides and changes in the diet of adult pollinators.

A mutualistic endophyte alters the niche dimensions of its host plant

Mutualisms can play important roles in influencing species coexistence and determining community composition. However, few studies have tested whether such interactions can affect species distributions by altering the niches of partner species. In subalpine meadows of the Rocky Mountains, USA, we explored whether the presence of a fungal endophyte (genus Epichloë) may shift the niche of its partner plant, marsh bluegrass (Poa leptocoma) relative to a closely related but endophyte-free grass species, nodding bluegrass (Poa reflexa). Using observations and a 3-year field experiment, we tested two questions: (i) Do P. leptocoma and P. reflexa occupy different ecological niches? and (ii) Does endophyte presence affect the relative fitness of P. leptocoma versus P. reflexa in the putative niches of these grass species? The two species were less likely to co-occur than expected by chance. Specifically, P. leptocoma grew closer to water sources and in wetter soils than P. reflexa, and also had higher root colonization by mycorrhizal fungi. Endophyte-symbiotic P. leptocoma seeds germinated with greater frequency in P. leptocoma niches relative to P. reflexa niches, whereas neither endophyte-free (experimentally removed) P. leptocoma seeds nor P. reflexa seeds showed differential germination between the two niche types. Thus, endophyte presence constrained the germination and early survival of host plants to microsites occupied by P. leptocoma. However, endophyte-symbiotic P. leptocoma ultimately showed greater growth than endophyte-free plants across all microsites, indicating a net benefit of the symbiosis at this life history stage. Differential effects of endophyte symbiosis on different host life history stages may thus contribute to niche partitioning between the two congeneric plant species. Our study therefore identifies a symbiotic relationship as a potential mechanism facilitating the coexistence of two species, suggesting that symbiont effects on host niche may have community-level consequences.