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Floral nectar contains microbes that can influence nectar chemistry and pollinator visitation, and these microbial communities can be affected by pollinators in turn. Some flowers are also visited by nectar robbers, which feed on nectar through holes cut in floral tissue. If nectar robbers alter nectar microbial communities, they might have unexpected impacts on pollinator visitation. We investigated whether robbing could affect nectar microbial communities directly, by introducing microbes, or indirectly, by triggering a plant response to floral damage. We applied four treatments to flowers of Tecoma × "Orange Jubilee" (Bignoniaceae) in an arboretum setting: flowers were (1) covered to exclude all visitors; (2) available to both pollinators and nectar robbers and robbed naturally by carpenter bees; (3) available to pollinators only but cut at the base to simulate nectar robbing damage; or (4) available to pollinators only. We found that nectar in flowers accessible to any visitors was more likely to contain culturable microbes than flowers from which visitors were excluded. Microbial community composition and beta diversity were similar across treatments. Among flowers containing culturable microbes, flowers available to pollinators and nectar robbers had higher microbial abundance than flowers with simulated robbing, but there were no differences between flowers available to pollinators and robbers and unwounded flowers from which robbers were excluded. Overall, our results suggest that floral damage can affect some features of nectar microbial communities, but specific effects of nectar robbing are limited compared with the influence of visitation in general. [ABSTRACT FROM AUTHOR]

Predicting the impacts of global warming on mutualisms poses a significant challenge given the functional and life history differences that usually exist among interacting species. However, this is a critical endeavour since virtually all species on Earth depend on other species for survival and/or reproduction. The field of thermal ecology can provide physiological and mechanistic insights, as well as quantitative tools, for addressing this challenge. Here, we develop a conceptual and quantitative framework that connects thermal physiology to species' traits, species' traits to interacting mutualists' traits and interacting traits to the mutualism. We first identify the functioning of reciprocal mutualism‐relevant traits in diverse systems as the key temperature‐dependent mechanisms driving the interaction. We then develop metrics that measure the thermal performance of interacting mutualists' traits and that approximate the thermal performance of the mutualism itself. This integrated approach allows us to additionally examine how warming might interact with resource/nutrient availability and affect mutualistic species' associations across space and time. We offer this framework as a synthesis of convergent and critical issues in mutualism science in a changing world, and as a baseline to which other ecological complexities and scales might be added. [ABSTRACT FROM AUTHOR]

Understanding mechanisms that generate range limits is central to knowing why species are found where they are and how they will respond to environmental change. There is growing awareness that biotic interactions play an important role in generating range limits. However, current theory and data overwhelmingly focus on abiotic drivers and antagonistic interactions. Here we explore the effect that mutualists have on their partner's range limits: the geographic "footprint" of mutualism. This footprint arises from two general processes: modification of a partner's niche through environment‐dependent fitness effects and, for a subset of mutualisms, dispersal opportunities that lead suitable habitats to be filled. We developed a conceptual framework that organizes different footprints of mutualism and the underlying mechanisms that shape them, and evaluated supporting empirical evidence from the primary literature. In the available literature, we found that the fitness benefits and dispersal opportunities provided by mutualism can extend species' ranges; conversely, the absence of mutualism can constrain species from otherwise suitable regions of their range. Most studies found that the footprint of mutualism is driven by changes in the frequency of mutualist partners from range core to range edge, whereas fewer found changes in interaction outcomes, the diversity of partners, or varying sensitivities of fitness to the effects of mutualists. We discuss these findings with respect to specialization, dependence, and intimacy of mutualism. Much remains unknown about the geographic footprint of mutualisms, leaving fruitful areas for future work. A particularly important future direction is to explore the role of mutualism during range shifts under global change, including the promotion of shifts at leading edges and persistence at trailing edges. [ABSTRACT FROM AUTHOR]

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Human–wildlife cooperation occurs when humans and free‐living wild animals actively coordinate their behavior to achieve a mutually beneficial outcome. These interactions provide important benefits to both the human and wildlife communities involved, have wider impacts on the local ecosystem, and represent a unique intersection of human and animal cultures. The remaining active forms are human–honeyguide and human–dolphin cooperation, but these are at risk of joining several inactive forms (including human–wolf and human–orca cooperation). Human–wildlife cooperation faces a unique set of conservation challenges, as it requires multiple components—a motivated human and wildlife partner, a suitable environment, and compatible interspecies knowledge—which face threats from ecological and cultural changes. To safeguard human–wildlife cooperation, we recommend: (i) establishing ethically sound conservation strategies together with the participating human communities; (ii) conserving opportunities for human and wildlife participation; (iii) protecting suitable environments; (iv) facilitating cultural transmission of traditional knowledge; (v) accessibly archiving Indigenous and scientific knowledge; and (vi) conducting long‐term empirical studies to better understand these interactions and identify threats. Tailored safeguarding plans are therefore necessary to protect these diverse and irreplaceable interactions. Broadly, our review highlights that efforts to conserve biological and cultural diversity should carefully consider interactions between human and animal cultures. Please see AfricanHoneyguides.com/abstract‐translations for Kiswahili and Portuguese translations of the abstract. [ABSTRACT FROM AUTHOR]

1. Precise pollen placement on floral visitors can improve pollen transfer, but in many plant species, pollen is deposited onto the flexible proboscises of long‐tongued insects. These proboscises are curled and uncurled between floral visits, potentially causing pollen to be lost or displaced. Rates of pollen movement and loss resulting from proboscis curling, and hence the potential quality of long‐tongued insects as pollinators, are unknown. 2. Here, pollen loss and movement on the proboscises of Manduca sexta (Sphingidae) hawkmoths was experimentally measured. It was predicted that (i) proboscis curling causes pollen loss; (ii) pollen that is not lost is displaced from its deposition site; and (iii) repeated curls result in more displacement. Pollen from Datura wrightii, an important nectar plant for M. sexta, was placed distal to the knee bend on M. sexta proboscises, and the number and location of grains was recorded after proboscis curls. 3. Consistent with the hypotheses, proboscis curling caused significant pollen loss. (i) A single curl resulted in the loss of almost 75% of the pollen from the placement site; after repeated curling, 98% of grains were lost from this site. (ii) A single curl was also sufficient to displace pollen across all surfaces of the proboscis, but (iii) further curling did not affect its distribution across surfaces. 4. Together, these results suggest that precise pollen placement on the proboscises of hawkmoths would be unlikely to increase pollen transfer success. Strategies by which flowering plants might mitigate the effects of pollen loss from visitors with flexible pollen‐pickup structures are discussed. [ABSTRACT FROM AUTHOR]

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]

Within‐species variation in pollinator behavior is widely observed, but its causes have been minimally investigated. Pollinator sex is associated with large differences in behavior that may lead to predictable differences in flower foraging, but this expectation has not been explicitly tested. We investigate sex‐associated differences in nectar‐foraging behavior of the hawkmoth Hyles lineata, using pollen on the proboscis as a proxy for flower visitation. We tested two predictions emerging from the literature: (1) the sexes differ in the flower species they visit, (2) females are more specialized in flower choice. We also examined potential drivers underlying these predictions by performing field and laboratory experiments to test whether males (3) switch among flower species more frequently, or (4) fly farther and therefore encounter more species than females. Consistent with prediction (1), pollen load composition differed between the sexes, indicative of visitation differences. Contrary to prediction (2), females consistently carried more species‐rich pollen loads than males. (3) Both sexes switched between flower species at similar rates, suggesting that differences in floral fidelity are unlikely to explain why females are less specialized than males. (4) Males flew longer distances than females; coupled with larger between‐site differences in pollen composition for females, this result suggests that sex differences in mobility influence foraging, and that females may forage more frequently and in smaller areas than males. Together, our results demonstrate that sex‐associated foraging differences can be large and consistent over time, and highlight the importance of sex as a driver of variation in pollinator behavior. [ABSTRACT FROM AUTHOR]

PREMISE: In addition to its role as the male gamete, pollen is often used as a food reward for pollinators. Roughly 20,000 species of angiosperms are strictly pollen‐rewarding, providing no other rewards to their pollinators. However, the influence of this strategy on pollinator behavior and plant reproduction is poorly understood, especially relative to the nectar‐reward strategy. We performed a field experiment using the strictly pollen‐rewarding Lupinus argenteus to explore how the absence of nectar influences pollinator behavior and plant reproduction. METHODS: We added artificial nectar to Lupinus argenteus individuals to simulate a phenotype that would reward pollinators with both nectar and pollen. We compared bee pollinator behavior, via direct observation, and female reproduction between nectar‐added and nectarless control plants. RESULTS: Bees exhibited behavioral responses to the novel reward, collecting nectar as well as pollen and spending 27% longer per flower. Pollen transfer increased with flower visit duration. However, plants in the study population were not pollen‐limited; consequently, the observed changes in pollinator behavior did not result in changes in female components of plant reproduction. CONCLUSIONS: The addition of nectar to pollen‐rewarding plants resulted in modest increases in per‐flower pollinator visit duration and pollen transfer, but had no effect on reproduction because, at the place and time the experiment was conducted, plants were not pollen‐limited. These results suggest that a pollen‐only reward strategy may allow plants that are visited by pollen foragers to minimize some costs of reproduction by eliminating investment in other rewards, such as nectar, without compromising female plant fitness. [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]

Information on direct and indirect drivers of temporal variation in ant–plant interactions is scarce, compromising our ability to predict the functioning of these ecologically important interactions.We investigated the roles of precipitation, ant activity, abundance of young plant tissues bearing extrafloral nectaries (EFNs) and EFN phenotypes in the establishment of EFN‐mediated ant–plant interactions throughout the year in Amazonia, Brazil. We hypothesized that the frequency of ant–plant interactions follows a predictable seasonal pattern, being higher in wetter periods, during which plants invest more in the production of new plant tissues bearing EFNs, ultimately promoting ant attendance. We surveyed and tagged every understorey Bignonieae plant rooted inside 28,500‐m2 plots, and recorded ant–EFN interactions on each plant five times throughout the year. We also sampled ants with honey baits to estimate temporal variation in general ant activity.Contrary to our hypothesis, the proportion of plants tended by ants in each plot was higher in drier, not wetter, months. Ant attendance was indirectly and negatively related to precipitation, which was attributed to a decrease in the proportion of plants producing new EFN‐bearing plant tissues during the wetter period. Additionally, seasonal variation in an ant activity did not explain temporal patterns of plant attendance. At the plant level, ant attendance increased strongly with the number of recently formed shoot nodes, and ants almost never attended plants with limited or no young tissue. Among the 12 most abundant Bignonieae species, the amount of young tissue was the most important predictor of ant attendance, secondarily explained by the EFN secretory area.Synthesis. Our results suggest that seasonal variation in the production of new plant tissues bearing EFNs is the primary driver of the temporal patterns of EFN–plant attendance by ants in this system. Contrary to our expectations, production of new plant tissue is higher in the drier months of the year, which in turn boosts the frequency of interactions between ants and EFN‐bearing plants in the dry season. These results highlight the role of plant phenology in the remarkable variation encountered in ant visitation to EFN‐bearing plants in both space and time. [ABSTRACT FROM AUTHOR]

Indirect defence, the adaptive top‐down control of herbivores by plant traits that enhance predation, is a central component of plant–herbivore interactions. However, the scope of interactions that comprise indirect defence and associated ecological and evolutionary processes has not been clearly defined. We argue that the range of plant traits that mediate indirect defence is much greater than previously thought, and we further organise major concepts surrounding their ecological functioning. Despite the wide range of plant traits and interacting organisms involved, indirect defences show commonalities when grouped. These categories are based on whether indirect defences boost natural enemy abundance via food or shelter resources, or, alternatively, increase natural enemy foraging efficiency via information or alteration of habitat complexity. The benefits of indirect defences to natural enemies should be further explored to establish the conditions in which indirect defence generates a plant–natural enemy mutualism. By considering the broader scope of plant–herbivore–natural enemy interactions that comprise indirect defence, we can better understand plant‐based food webs, as well as the evolutionary processes that have shaped them. [ABSTRACT FROM AUTHOR]

Intraspecific variation in floral visitor behaviour and pollination efficiency has been much less studied than interspecific variation. Nevertheless, it is clear that large differences in these traits exist within species, and in particular between sexes within species. With the exception of a few well‐studied interactions, however, the consequences of these differences in the pollinators and visited plants remain to be investigated.In this review, we document large and consistent differences in the foraging patterns of male and female pollinators that have been demonstrated to directly affect plant reproduction or that have clear potential to do so.Males and females differ in visitation frequency, type of flowers visited, and per‐visit pollen transfer. Females gather more and different resources from flowers compared to males, and males generally tend to show more mobile foraging patterns than females.We argue that these sex‐associated patterns have broad generality across pollinators, and that sex‐associated differences can in some cases be larger than differences between species. We offer predictions about how these patterns will influence pollinator preference, specialization, and fidelity, as well as the cost, quality and quantity of pollination service to plants.In the face of increasing threats to plant–pollinator interactions, understanding their basic functioning and the variation inherent in their component parts is critical. We advocate for more attention to sex‐based differences among pollinators in particular, and the consequences of intraspecific variation more broadly. [ABSTRACT FROM AUTHOR]

Many plants that bear hidden or recessed floral nectar experience nectar robbing, the removal of nectar by a floral visitor through holes pierced in the corolla. Although robbing can reduce plant reproductive success, many studies fail to find such effects. We outline three mechanistic hypotheses that can explain when interactions between plants and nectar‐robbers should be commensal rather than antagonistic: the non‐discrimination (pollinators do not avoid robbed flowers), visitor prevalence (robber visitation is rare relative to pollinator visitation), and pollen saturation (stigmas receive sufficient pollen to fertilize all ovules with one or very few pollinator visits) hypotheses. We then explore these mechanisms in the North American subalpine, bumble bee‐pollinated and nectar‐robbed plant Corydalis caseana (Fumariaceae). We first confirmed that the effects of nectar robbing on female reproductive success were neutral in C. caseana. We then tested the three mechanisms underlying these neutral effects using a combination of observational studies and experiments. We found evidence for all three mechanisms. First, consistent with the non‐discrimination hypothesis, pollinators failed to discriminate against experimentally robbed flowers or inflorescences even though naturally robbed flowers offered significantly lower nectar rewards than unrobbed flowers. Second, C. caseana was more commonly visited by pollinators than by nectar robbers, in accordance with the visitor prevalence hypothesis. Third, stigmas of unvisited flowers as well as those visited once by pollinators were saturated with pollen, with all stigmas bearing pollen loads several orders of magnitude higher than the number of ovules per fruit, consistent with the pollen saturation hypothesis. Our investigation of the mechanisms driving the commensal outcome of nectar robbing in this system deepens our understanding of the ecology of nectar robbing and contributes to a more general understanding of the variation in the outcomes of interactions between species. [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]

Abstract: Many mutualisms are taken advantage of by organisms that take rewards from their partners but provide no benefit in return. In the absence of traits that limit exploitation, facultative exploiters (partners that can either exploit or cooperate) are widely predicted by mutualism theory to choose an exploitative strategy, potentially threatening mutualism stability. However, it is unknown whether facultative exploiters choose to exploit, and, if so, make this choice because it is the most beneficial strategy for them. We explored these questions in a subalpine plant‐insect community in which individuals of several bumble bee species visit flowers both “legitimately” (entering via the flower opening, picking up and depositing pollen, and hence behaving mutualistically) and via nectar robbing (creating holes through corollas or using an existing hole, bypassing stigmas and anthers). We applied foraging theory to (1) quantify handling costs, benefits and foraging efficiencies incurred by three bumble bee species as they visited flowers legitimately or robbed nectar in cage experiments, and (2) determine whether these efficiencies matched the food handling tactics these bee species employed in the field. Relative efficiencies of legitimate and robbing tactics depended on the combination of bee and plant species. In some cases (Bombus mixtus visiting Corydalis caseana or Mertensia ciliata), the robbing tactic permitted more efficient nectar removal. As both mutualism and foraging theory would predict, in the field, B. mixtus visiting C. caseana were observed more frequently robbing than foraging legitimately. However, for Bombus flavifrons visiting M. ciliata, the expectation from mutualism theory did not hold: legitimate visitation was the more efficient tactic. Legitimate visitation to M. ciliata was in fact more frequently observed in free‐flying B. flavifrons. Free‐flying B. mixtus also frequently visited M. ciliata flowers legitimately. This may reflect lower nectar volumes in robbed than unrobbed flowers in the field. These results suggest that a foraging ecology perspective is informative to the choice of tactics facultative exploiters make. In contrast, the simple expectation that exploiters should always have an advantage, and hence could threaten mutualism persistence unless they are deterred or punished, may not be broadly applicable. [ABSTRACT FROM AUTHOR]

Abstract: Aim: Leafcutter bees use plants as pollen and nectar sources, but also cut leaf discs and use them to line their nests. Which plant species they choose as nesting material and why they do so have remained obscure. We asked the following: (1) How are the plant species used by leafcutter bees distributed phylogenetically? (2) Does plant choice differ across geographical regions, and if so, in what ways? (3) Are the leaf plant species natives or exotics? (4) What plant and leaf traits predict selection of plant species by leafcutter bees? And (5) Does the abundance of individuals per species in the habitat influence leafcutter bees’ plant preferences? Location: Tropical South Asia, temperate eastern Canada and US Sonoran Desert Methods: We mapped taxa known to be used by leafcutter bees both from our own study and published literature onto the most recent angiosperm phylogeny. To determine what plant and leaf traits predict leaf selection, we monitored 6,120 individuals of 214 native and exotic plant species planted in a 3‐sq. km. arboretum in Arizona and recorded leaf damage inflicted by bees. Results: Megachile showed a strong preference for species in the rosid clade, particularly the phylogenetic cluster of Fabales, Fagales and Rosales. Thirty‐two to forty‐five percentage of the leaf plant species were exotic to a given region. Membership in the rosid clade and Fabaceae family predicted plant preference, whereas the plant species’ local abundance and geographical origin did not. Leaf water content, morphotype and the presence of latex were important factors influencing plant choice, whereas leaf shape and size did not. Main conclusion: These patterns point to plant groups whose availability should be monitored to assure persistence of leafcutter bees. Their preference for specific globally distributed plant clades, yet ability to readily adopt certain exotic plant species as nest resources, likely augments their ability to persist. [ABSTRACT FROM AUTHOR]

Premise of the Study: Organisms engage in multiple species interactions simultaneously. While pollination studies generally focus on plants and pollinators exclusively, secondary robbing, a behavior that requires other species (primary robbers) to first create access holes in corollas, is common. It has been shown that secondary robbing can reduce plants' female fitness; however, we lack knowledge about its impact on male plant fitness. Methods: We experimentally simulated primary and secondary robbing in the monocarpic perennial Ipomopsis aggregata (Polemoniaceae), quantifying indirect effects on pollinator‐mediated pollen (dye) donation. We also assessed whether continual nectar removal via the floral opening has similar effects on hummingbird‐pollinator behavior as continual secondary robbing through robber holes. Key Results: We found no significant indirect effects of secondary robbing on a component of Ipomopsis male fitness. Although robbing did reduce pollen (dye) donation due to avoidance of robbed plants by pollinating hummingbirds, pollen donation did not differ between the two robbing treatments. The effects of secondary robbing on hummingbird behavior resembled effects of chronic nectar removal by pollinators. Our results indicate that hummingbird pollinators may use a combination of cues, including cues given by the presence or absence of nectar, to make foraging decisions. Conclusions: Combined with prior research, this study suggests that secondary robbing is less costly to a component of male fitness than to female fitness in Ipomopsis, broadening our knowledge of the overall costs of mutualism exploitation to total plant fitness. [ABSTRACT FROM AUTHOR]

In mutualistic interactions, the decision whether to cooperate or cheat depends on the relative costs and benefits of each strategy. In pollination mutualisms, secondary nectar robbing is a facultative behavior employed by a diverse array of nectar-feeding organisms, and is thought to be a form of cheating. Primary robbers create holes in floral tissue through which they feed on nectar, whereas secondary robbers, which often lack chewing mouthparts, feed on nectar through existing holes. Because primary robbers make nectar more readily available to secondary robbers, primary robbers facilitate the behaviors of secondary robbers. However, the net effect of facilitation on secondary robber fitness has not been empirically tested: it is unknown whether the benefit secondary robbers receive is strong enough to overcome the cost of competing with primary robbers for a shared resource. We conducted foraging experiments using the bumble bee Bombus bifarius, which can alternatively forage 'legitimately' (from the floral opening) or secondary-rob. We measured the relative foraging efficiencies (handling time per flower, flowers visited per minute, proportion of foraging bout spent consuming nectar) of these alternative behaviors, and tested whether the frequency of primary robbing and nectar standing crop in primary-robbed flowers of Linaria vulgaris (Plantaginaceae) affected foraging efficiency. Surprisingly, there was no effect of primary robbing frequency on the foraging efficiency of secondary-robbing B. bifarius. Instead, foraging strategy was a major predictor of foraging efficiency, with legitimate foraging being significantly more efficient than secondary robbing. Legitimate foraging was the more common strategy used by B. bifarius in our study; however, it is rarely used by B. bifarius foraging on L. vulgaris in nature, despite indications that it is more efficient. Our results suggest the need for deeper investigations into why bees adopt secondary robbing as a foraging strategy, specifically, the environmental contexts that promote the behavior. [ABSTRACT FROM AUTHOR]

In plant-animal interactions, species are commonly labeled as either mutualists or antagonists, based on the most common, most studied, or most easily observed outcome. Nevertheless, evidence from simple systems comprising 2-4 species suggests that those labels are an oversimplification: individual species often function in both roles, either simultaneously or at different places or times. We include both mutualistic and antagonistic interactions between mammals and seeds in a multilayer network, to explore for the first time the community-level consequences of the dual roles played by some species. We tested whether negative and positive interactions within a plant-frugivore network are separated into different modules, or whether they overlap due to the presence of frugivores that both kill and disperse seeds. The frugivorous diets of nonvolant small mammals were studied at one dry tropical forest site in southeastern Brazil by analyzing fecal samples from individuals captured in live traps. Seed viability was assessed with a tetrazolium test to determine the outcome of those interactions, as estimated by whether or not seeds survived gut passage. Interactions were analyzed as a weighted multilayer network, subdivided into one potentially mutualistic (live seeds deposited) and one antagonistic (dead seeds deposited) layer. The two layers had similar structure with high overlap between them. Some mammal species exhibited highly central, dual roles, acting both as antagonists and mutualists, in many cases of the same plant species. Dispersal service by most of these small mammals is accompanied by seed destruction, suggesting that the selective pressures exerted by those animals on the plants is much more complex than often assumed. Our results demonstrate that the complexity of plant-frugivore networks can not be fully understood without proper incorporating measures of seed fate following gut passage. [ABSTRACT FROM AUTHOR]

Whether species interactions are static or change over time has wide-reaching ecological and evolutionary consequences. However, species interaction networks are typically constructed from temporally aggregated interaction data, thereby implicitly assuming that interactions are fixed. This approach has advanced our understanding of communities, but it obscures the timescale at which interactions form (or dissolve) and the drivers and consequences of such dynamics. We address this knowledge gap by quantifying the within-season turnover of plant-pollinator interactions from weekly censuses across 3 years in a subalpine ecosystem. Week-to-week turnover of interactions (1) was high, (2) followed a consistent seasonal progression in all years of study and (3) was dominated by interaction rewiring (the reassembly of interactions among species). Simulation models revealed that species' phenologies and relative abundances constrained both total interaction turnover and rewiring. Our findings reveal the diversity of species interactions that may be missed when the temporal dynamics of networks are ignored. [ABSTRACT FROM AUTHOR]

Mutualisms are only rarely one-to-one interactions: each species generally interacts with multiple mutualists. Exploitation is ubiquitous in mutualisms, and we would therefore expect that each mutualist interacts with multiple exploiters as well. Exploiter species may also interact with one another. For example, the action of one exploiter species might open the opportunity for exploitation by a second species., Exploitation is common in many plant-pollinator mutualisms: 'primary' nectar robbers feed through holes they make in flowers, which can be subsequently used by 'secondary' nectar robbers unable to create holes themselves. The overall effect of nectar robbing on plant fitness is often (although not always) negative. No study has separated the effects of interacting with primary vs. secondary robbers., Here, we examine the effects of primary vs. secondary nectar robbing on pollinator visitation rate and female fitness in Ipomopsis aggregata. Manipulating the type of nectar robbing that flowers experienced, we found that secondary nectar robbing inflicted fitness costs to plants beyond that inflicted by primary robbing alone. Secondary nectar robbing significantly reduced pollen receipt to flowers, as well as fruit and seed production. Although the causes are elusive, the effect may be attributed to changes in pollinator behaviour at these plants., Synthesis. Our findings provide evidence that interacting with multiple exploiters can lead to increased negative effects for mutualists, and highlight the importance of incorporating multiple exploiters into the conceptual framework of mutualism. [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]

Climate change-induced shifts in flowering phenology can expose plants to novel biotic and abiotic environments, potentially leading to decreased temporal overlap with pollinators and exposure to conditions that negatively affect fruit and seed set. We explored the relationship between flowering phenology and reproductive output in the common shrub pointleaf manzanita Arctostaphylos pungens in a lower montane habitat in southeastern Arizona, USA. Contrary to the pattern of progressively earlier flowering observed in many species, long-term records show that A. pungens flowering onset is shifting later and the flowering season is being compressed. This species can thus provide unusual insight into the effects of altered phenology. To determine the consequences of among- and within-plant variation in flowering time, we documented individual flowering schedules and followed the fates of flowers on over 50 plants throughout two seasons (2012 and 2013). We also measured visitation rates by potential pollinators in 2012, as well as both fruit mass and seeds per fruit of flowers produced at different times. Fruit set was positively related to visitation rate but declined with later dates of flower production in both years. Total fruit production per plant was positively influenced by flowering duration, which declined with later flowering onset, as did fruit mass. Individual flowering schedules were consistent between years, suggesting that plants that begin flowering late have lower reproductive output each year. These patterns suggest that if pointleaf manzanita flowering continues to shift later, its flowering season may continue to become shorter, compressing floral resource availability for pollinators and leading to reduced reproductive output. These results reveal the negative effects of delayed phenology on reproductive output in a long-lived plant. They highlight the value of using natural variation in flowering time, in combination with long-term data, to anticipate the consequences of phenological shifts. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Climate change is altering the timing of life history events in a wide array of species, many of which are involved in mutualistic interactions. Because many mutualisms can form only if partner species are able to locate each other in time, differential phenological shifts are likely to influence their strength, duration and outcome. At the extreme, climate change-driven shifts in phenology may result in phenological mismatch: the partial or complete loss of temporal overlap of mutualistic species. We have a growing understanding of how, when, and why phenological change can alter one type of mutualism - pollination. However, as we show here, there has been a surprising lack of attention to other types of mutualism. We generate a set of predictions about the characteristics that may predispose mutualisms in general to phenological mismatches. We focus not on the consequences of such mismatches but rather on the likelihood that mismatches will develop. We explore the influence of three key characteristics of mutualism: 1) intimacy, 2) seasonality and duration, and 3) obligacy and specificity. We predict that the following characteristics of mutualism may increase the likelihood of phenological mismatch: 1) a non-symbiotic life history in which co-dispersal is absent; 2) brief, seasonal interactions; and 3) facultative, generalized interactions. We then review the limited available data in light of our a priori predictions and point to mutualisms that are more and less likely to be at risk of becoming phenologically mismatched, emphasizing the need for research on mutualisms other than plant - pollinator interactions. Future studies should explicitly focus on mutualism characteristics to determine whether and how changing phenologies will affect mutualistic interactions. [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]

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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]

Recent years have seen an explosion in the diversity of partner control mechanisms hypothesised to stabilise cooperative behaviour among unrelated individuals. Game theory suggests numerous strategies, each with specific decision rules that allow cooperators to control a non-contributing partner. This diversity of hypothetical strategies seems likely to reflect diversity in the types of intraspecific cooperation and interspecific mutualism that exist in nature. It is therefore important to provide a framework that explains similarities and differences between the various hypothetical strategies and that predicts how key parameters that describe the natural history of natural systems favour different control mechanisms. We develop a novel unifying framework for pairwise interactions between unrelated individuals, in which we link specific control mechanisms to specific game structures. The latter are defined by unique combinations of the states of five parameters that describe investment, aspects of the payoff matrix, the number of interactions and partner choice. We find that specific control mechanisms potentially have utility in a limited number of game structures; conversely, each game structure may typically offer a few competing control mechanisms. Our framework offers theoreticians specific problems that await mathematical exploration, while at the same time offering empiricists guidelines for evaluating the game structure and corresponding control mechanisms in their systems. [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]

Animal foraging has been characterized as an attempt to maximize the intake of carbon and nitrogen at appropriate ratios. Plant species in over 90 families produce carbohydrate-rich extrafloral nectar (EFN), a resource attractive to ants and other omnivorous insects. This attraction can benefit the plant if those arthropods subsequently attack herbivores. This protective response has been attributed to the increased visitation and "ownership" of plants that provide a predictable source of fuel. Here, we propose and test an alternative (but non-mutually exclusive) hypothesis, that access to C-rich carbohydrates increases the ants' desire for N-rich protein and hence the likelihood that they will attack herbivorous insects on the host plant. This "deficit hypothesis" would be rejected if (1) EFN were itself a sufficiently balanced food source in terms of C and N, (2) ant dietary preferences. were similar in the presence vs. absence of EFN, (3) protein-hungry ants were not more predaceous, or (4) ants provided access to protein were more aggressive toward potential prey items than were ants provided access to carbohydrates. We test these predictions in a protective mutualism between a guild of desert ants and the barrel cactus Ferocactus wislizeni. C:N ratios of EFN exceeded that of ants or potential prey items by an order of magnitude (i.e., EFN is an N-poor food for ants). Baiting studies demonstrated that plant-tending ant species recruited more workers to N-rich protein baits than to C-rich sugar baits; this difference was more pronounced when the ants had access to F. wislizeni EFN. From these data, we infer that protein is a valuable resource and that its relative value increases when carbohydrates are readily available. Moreover, ant colonies provided access to supplemental carbohydrates responded more aggressively to surrogate herbivores than did control colonies (to which no additional resources were provided) or colonies provided protein. These results support the predictions of the "deficit" hypothesis, wherein plant protection is elicited by plant-mediated dietary imbalances. . [ABSTRACT FROM AUTHOR]

1. Hawkmoths (Sphingidae) are important plant associates at two life-history stages: larvae are herbivorous, whereas adults are nectar feeders and often pollinators. The diversity and identities of plants used for nectar is poorly known, however. 2. This study takes a community-level approach to hawkmoth nectar plant usage in a semi-arid grassland habitat in southern Arizona, U.S.A. 3. Pollen carried on the proboscis was identified from over 700 individuals of 14 hawkmoth species attracted to lights over a 2-year period. 4. Two plant species dominated pollen loads, suggesting that hawkmoths use these species extensively as nectar sources: Datura wrightii (Solanaceae), a classic hawkmoth-pollinated plant, and Agave palmeri (Agavaceae), which is known to be used extensively by bats. Field surveys indicate that both species are relatively rare in the flowering community. Little or no pollen was present on the moths from the most common plant species in flower during the study. 5. The dominance of Agave in pollen loads suggests that this typically bat-pollinated species may be subsidising pollinator populations of the hawkmoth-pollinated flora. 6. Three groups of hawkmoths within this community are identified based on larval diets (reported in the literature) and adult diets (documented here): those that, at a given site, heavily exploit the same plant species at both life-history stages ( Manduca sexta and M. quinquemaculata); those that have broad local associations at both life-history stages ( Hyles lineata); and those that exhibit narrow but non-overlapping local associations at the two life-history stages (all other hawkmoths at this site). [ABSTRACT FROM AUTHOR]

Mutualisms are ubiquitous in nature, as is their exploitation by both conspecific and heterospecific cheaters. Yet, evolutionary theory predicts that cheating should be favoured by natural selection. Here, we show theoretically that asymmetrical competition for partners generally determines the evolutionary fate of obligate mutualisms facing exploitation by third-species invaders. When asymmetry in partner competition is relatively weak, mutualists may either exclude exploiters or coexist with them, in which case their co-evolutionary response to exploitation is usually benign. When asymmetry is strong, the mutualists evolve towards evolutionary attractors where they become extremely vulnerable to exploiter invasion. However, exploiter invasion at an early stage of the mutualism's history can deflect mutualists’ co-evolutionary trajectories towards slightly different attractors that confer long-term stability against further exploitation. Thus, coexistence of mutualists and exploiters may often involve an historical effect whereby exploiters are co-opted early in mutualism history and provide lasting ‘evolutionary immunization’ against further invasion. [ABSTRACT FROM AUTHOR]

Contents Summary 412 I. Introduction 413 II A historical perspective on mutualism 413 III. Insect pollination 414 IV. Protection of plants by ants 417 V. Ant-mediated seed dispersal 419 VI. Discussion 420 Acknowledgements 423 References 424 Mutualisms (cooperative interactions between species) have had a central role in the generation and maintenance of life on earth. Insects and plants are involved in diverse forms of mutualism. Here we review evolutionary features of three prominent insect–plant mutualisms: pollination, protection and seed dispersal. We focus on addressing five central phenomena: evolutionary origins and maintenance of mutualism; the evolution of mutualistic traits; the evolution of specialization and generalization; coevolutionary processes; and the existence of cheating. Several features uniting very diverse insect–plant mutualisms are identified and their evolutionary implications are discussed: the involvement of one mobile and one sedentary partner; natural selection on plant rewards; the existence of a continuum from specialization to generalization; and the ubiquity of cheating, particularly on the part of insects. Plant–insect mutualisms have apparently both arisen and been lost repeatedly. Many adaptive hypotheses have been proposed to explain these transitions, and it is unlikely that any one of them dominates across interactions differing so widely in natural history. Evolutionary theory has a potentially important, but as yet largely unfilled, role to play in explaining the origins, maintenance, breakdown and evolution of insect–plant mutualisms. [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]

Pollinator attraction, pollen limitation, resource limitation, pollen donation and selective fruit abortion have all been proposed as processes explaining why hermaphroditic plants commonly produce many more flowers than mature fruit. We conducted a series of experiments in Arizona to investigate low fruit-to-flower ratios in senita cacti, which rely exclusively on pollinating seed-consumers. Selective abortion of fruit based on seed predators is of particular interest in this case because plants relying on pollinating seed-consumers are predicted to have such a mechanism to minimize seed loss. Pollinator attraction and pollen dispersal increased with flower number, but fruit set did not, refuting the hypothesis that excess flowers increase fruit set by attracting more pollinators. Fruit set of natural- and hand-pollinated flowers were not different, supporting the resource, rather than pollen, limitation hypothesis. Senita did abort fruit, but not selectively based on pollen quantity, pollen donors, or seed predators. Collectively, these results are consistent with sex allocation theory in that resource allocation to excess flower production can increase pollen dispersal and the male fitness function of flowers, but consequently results in reduced resources available for fruit set. Inconsistent with sex allocation theory, however, fruit production and the female fitness function of flowers may actually increase with flower production. This is because excess flower production lowers pollinator-to-flower ratios and results in fruit abortion, both of which limit the abundance and hence oviposition rates, of pre-dispersal seed predators. [ABSTRACT FROM AUTHOR]

Traits that are attractive to mutualists may also attract antagonists, resulting in conflicting selection pressures. Here we develop the idea that increased floral nectar production can, in some cases, increase herbivory. In these situations, selection for increased nectar production to attract pollinators may be constrained by a linked cost of herbivore attraction. In support of this hypothesis, we report that experimentally supplementing nectar rewards in Datura stramonium led to increased oviposition by Manduca sexta, a sphingid moth that pollinates flowers, but whose larvae feed on leaf tissue. We speculate that nectar composition may provide information about plant nutritional status or defense that floral visitors could use to make oviposition decisions. Thus, selection by floral visitors and leaf herbivores may be inextricably intertwined, and herbivores may represent a relatively unexplored agent Of selection on nectar traits. [ABSTRACT FROM AUTHOR]

Mutualisms (interspecific cooperative interactions) are ubiquitously exploited by organisms that obtain the benefits mutualists offer, while delivering no benefits in return. The natural history of these exploiters is well-described, but relatively little effort has yet been devoted to analysing their ecological or evolutionary significance for mutualism. Exploitation is not a unitary phenomenon, but a set of loosely related phenomena: exploiters may follow mixed strategies or pure strategies at either the species or individual level, may or may not be derived from mutualists, and may or may not inflict significant costs on mutualisms. The evolutionary implications of these different forms of exploitation, especially the threats they pose to the stability of mutualism, have as yet been minimally explored. Studies of this issue are usually framed in terms of a “temptation to defect” that generates a destabilizing conflict of interest between partners. I argue that this idea is in fact rather inappropriate for interpreting most observed forms of exploitation in mutualisms. I suggest several alternative and testable ideas for how mutualism can persist in the face of exploitation. [ABSTRACT FROM AUTHOR]

Focuses on a study which discussed self-pollination and its costs in a monoecious fig in a highly seasonal subtropical environment. Methods; Results; Discussion.

See also the Commentary by Selosse [ABSTRACT FROM AUTHOR]

Mutualisms, cooperative interactions between species, generally involve an economic exchange: species exchange commodities that are cheap for them to provide, for ones that cannot be obtained affordably or at all. But these associations can only succeed if effective partners can be enticed to interact. In some mutualisms, partners can actively seek one another out. However, plants, which use mutualists for a wide array of essential life history functions, do not have this option. Instead, natural selection has repeatedly favoured the evolution of rewards - nutritional substances (such as sugar-rich nectar and fleshy fruit) with which plants attract certain organisms whose feeding activities can then be co-opted for their own benefit. The trouble with rewards, however, is that they are usually also attractive to organisms that confer no benefits at all. Losing rewards to 'exploiters' makes a plant immediately less attractive to the mutualists it requires; if the reward cannot be renewed quickly (or at all), then mutualistic service is precluded entirely. Thus, it is in plants' interests to either restrict rewards to only the most beneficial partners or somehow punish or deter exploiters. Yet, at least in cases where the rewards are highly nutritious, we can expect counter-selection for exploiter traits that permit them to skirt such control. How, then, can mutualisms persist? In this issue, Orona-Tamayo et al. () describe a remarkable adaptation that safeguards one particularly costly reward from nonmutualists. Their study helps to explain the evolutionary success of an iconic interaction and illuminates one way in which mutualism as a whole can persist in the face of exploitation. [ABSTRACT FROM AUTHOR]

This article is a commentary on Chomicki et al., 211: 1358–1370. [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]