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.

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.

Wood packaging material (WPM) is an important pathway for the spread of non‐native plant pests. To reduce the likelihood of plant pest movement with WPM, the International Standard for Phytosanitary Measures No. 15: “Regulation of Wood Packaging Material in International Trade” (ISPM 15) was developed in the framework of the International Plant Protection Convention. To be compliant with this Standard, WPM shipped internationally must be either heat‐treated or fumigated, regardless of any specific characteristics of the WPM. The objective of this survey was to determine if Bursaphelenchus xylophilus (Nematoda: Parasitaphelenchidae) or insects, especially Monochamus spp. (Coleoptera: Cerambycidae), were present in a specific subset of softwood boxes. While not compliant with ISPM 15, these boxes had been treated with wood preservatives, stored indoors for over five years, and, in addition to meeting various quality standards, almost all had a moisture content below 20 percent. United States Department of Agriculture staff inspected a sample of 630 softwood boxes, focusing on those showing signs of possible pest infestation. Based on a binomial distribution with an efficacy of detection equal to 0.95, a sample size of 630 provides a 95 percent confidence of detecting pests if >0.5 % of boxes are infested. No B. xylophilus or insect pests were found in the boxes, though other nematode genera ( Aphelenchoides, Aphelenchus , and Filenchus ), which feed on decomposing fungi in wood and do not damage trees, were found in 21 boxes. This study demonstrates that not all types of WPM present a high phytosanitary risk. It may be worthwhile to consider an amendment to ISPM 15 to differentiate between various risk categories of WPM in order to minimize costs and environmental impacts associated with treatments currently prescribed in ISPM 15.

A key goal of invasion biology is to identify the factors that favor species invasions. One potential indicator of invasiveness is the phylogenetic distance between a nonnative species and species in the recipient community. However, predicting invasiveness using phylogenetic information relies on an untested assumption: that both biotic resistance and facilitation weaken with increasing phylogenetic distance. We test the validity of this key assumption using a mathematical model in which a novel species is introduced into communities with varying ecological and phylogenetic relationships. Contrary to what is generally assumed, we find that biotic resistance and facilitation can either weaken or intensify with phylogenetic distance, depending on the mode of interspecific interactions (phenotype matching or phenotype differences) and the resulting evolutionary trajectory of the recipient community. Thus, we demonstrate that considering the mechanisms that drive phenotypic divergence between native and nonnative species can provide critical insight into the relationship between phylogenetic distance and invasibility.

Mutualistic microbial symbioses are one of the key innovations in the evolution of biological diversity, enabling the expansion of species' niches and the production of sophisticated structures such as the eukaryotic cell. For some of the best-studied cases, we are beginning to have network models of symbiotic metabolism, but this work is in its infancy and has not been developed with an evolutionary perspective. However, theoreticians have long been interested in how these symbioses arise and persist and have applied modelling approaches from economics, evolution, ecology, and sociobology to a number of fundamental questions. We provide an overview of these questions, followed by specific modelling examples. We cover economic game theory, including the Prisoner's Dilemma, the Snowdrift game, and biological markets. We also describe the eco-evolutionary framework of adaptive dynamics, inclusive fitness, and population genetic models. We aim to provide insight into the strengths and weaknesses of each approach and into how current evolutionary methods can benefit an understanding of the mechanistic basis of host-symbiont interactions elucidated by molecular network models.

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.

Social insect colonies are complex systems in which the interactions of many individuals lead to colony-level collective behaviors such as foraging. However, the emergent properties of collective behaviors may not necessarily be adaptive. Here, we examine symmetry breaking, an emergent pattern exhibited by some social insects that can lead colonies to focus their foraging effort on only one of several available food patches. Symmetry breaking has been reported to occur in several ant species. However, it is not clear whether it arises as an unavoidable epiphenomenon of pheromone recruitment, or whether it is an adaptive behavior that can be controlled through modification of the individual behavior of workers. In this paper, we used a simulation model to test how symmetry breaking is affected by the degree of non-linearity of recruitment, the specific mechanism used by individuals to choose between patches, patch size, and forager number. The model shows that foraging intensity on different trails becomes increasingly asymmetric as the recruitment response of individuals varies from linear to highly non-linear, supporting the predictions of previous work. Surprisingly, we also found that the direction of the relationship between forager number (i.e., colony size) and asymmetry varied depending on the specific details of the decision rule used by individuals. Limiting the size of the resource produced a damping effect on asymmetry, but only at high forager numbers. Variation in the rule used by individual ants to choose trails is a likely mechanism that could cause variation among the foraging behaviors of species, and is a behavior upon which selection could act.

Abstract 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.

The biotic environment can pose a challenge to introduced species; however, it is not known how rapid evolution in introduced and resident species influences the probability that the introduced species will become established. Here, we analyze the establishment phase of invasion with eco-evolutionary models of introduced species involved in predator-prey, mutualistic, or competitive interactions with a resident species. We find that, depending on the strength of the biotic interaction, establishment is impossible, guaranteed, or, in a narrow range, determined by genetic variation. Over this narrow range, rapid evolution of the introduced species always favors establishment, whereas resident evolution may either inhibit or facilitate establishment, depending on the interaction type. Coevolution can also either increase or decrease the chance of establishment, depending on the initial genotype frequencies as well as the interaction type. Our results suggest that the conditions under which genetic variation influences establishment success are limited, but they highlight the importance of considering the resident community's evolutionary response to introduced species as a component of its invasibility.

In the absence of predators, pollinators can often maximize their foraging success by visiting the most rewarding flowers. However, if predators use those highly rewarding flowers to locate their prey, pollinators may benefit from changing their foraging preferences to accept less rewarding flowers. Previous studies have shown that some predators, such as crab spiders, indeed hunt preferentially on the most pollinator-attractive flowers. In order to determine whether predation risk can alter pollinator preferences, we conducted laboratory experiments on the foraging behavior of bumble bees (Bombus impatiens) when predation risk was associated with a particular reward level (measured here as sugar concentration). Bees foraged in arenas containing a choice of a high-reward and a low-reward artificial flower. On a bee's first foraging trip, it was either lightly squeezed with forceps, to simulate a crab spider attack, or was allowed to forage safely. The foragers' subsequent visits were recorded for between 1 and 4 h without any further simulated attacks. Compared to bees that foraged safely, bees that experienced a simulated attack on a low-reward artificial flower had reduced foraging activity. However, bees attacked on a high-reward artificial flower were more likely to visit low-reward artificial flowers on subsequent foraging trips. Forager body size, which is thought to affect vulnerability to capture by predators, did not have an effect on response to an attack. Predation risk can thus alter pollinator foraging behavior in ways that influence the number and reward level of flowers that are visited.

Pollinators and their predators share innate and learned preferences for high quality flowers. Consequently, pollinators are more likely to encounter predators when visiting the most rewarding flowers. I present a model of how different pollinator species can maximize lifetime resource gains depending on the density and distribution of predators, as well as their vulnerability to capture by predators. For pollinator species that are difficult for predators to capture, the optimal strategy is to visit the most rewarding flowers as long as predator density is low. At higher predator densities and for pollinators that are more vulnerable to predator capture, the lifetime resource gain from the most rewarding flowers declines and the optimal strategy depends on the predator distribution. In some cases, a wide range of floral rewards provides near-maximum lifetime resource gains, which may favor generalization if searching for flowers is costly. In other cases, a low flower reward level provides the maximum lifetime resource gain and so pollinators should specialize on less rewarding flowers. Thus, the model suggests that predators can have qualitatively different top-down effects on plant reproductive success depending on the pollinator species, the density of predators, and the distribution of predators across flower reward levels.

With the growing recognition of exploiters as a prominent and enduring feature of many mutualisms, there is a need to understand the ecological and evolutionary dynamics of mutualisms in the context of exploitation. Here, we model coevolution between mutualist and exploiter birth rates, using an obligate pollinating seed parasite mutualism associated with a nonpollinating exploiter as a reference system. In this system, mutualist and exploiter larvae parasitize the host plant, competing for and consuming seeds. Evolution of the mutualist determines which exploiters can invade successfully. Subsequent coevolution with an exploiter has a strong, predictable influence on mutualist-exploiter coexistence, mutualist and exploiter phenotypes, and species abundances. Weak mutualist competition promotes "evolutionary purging" of the exploiter, while weak exploiter competition leads to "evolutionary suicide" of the system. When stable, long-term coexistence occurs, we identify two main "trait-abundance syndromes" that have three novel implications. (1) Persistent, highly parasitic exploiters can be favored by coevolution. (2) Even then, the density of coevolved mutualists can be high. (3) Low plant density results primarily from the evolution of mutualist, not exploiter, birth rate and density. To evaluate these predictions, studies are needed that identify and compare populations with and without exploiters and compare life-history traits of mutualists and exploiters.

Background: A major challenge for evolutionary biology is explaining altruism, particularly when it involves death of one party and occurs across species. Chimeric fruiting bodies of Dictyostelium discoideum and Dictyostelium purpureum develop from formerly independent amoebae, and some die to help others. Here we examine co-aggregation between D. discoideum and D. purpureum, determine its frequency and which party benefits, and the extent of fair play in contribution to the altruistic caste.Results: We mixed cells from both species in equal proportions, and then we analyzed 198 individual fruiting bodies, which always had either a D. discoideum or D. purpureum phenotype (D. discoideum-98, D. purpureum-100). Fifty percent of the fruiting bodies that looked like D. discoideum and 22% of the fruiting bodies that looked like D. purpureum were chimeric, though the majority of spores in any given fruiting body belonged to one species (D. discoideum fruiting bodies-0.85 +/- 0.03, D. purpureum fruiting bodies-0.94 +/- 0.02). Clearly, there is species level recognition occurring that keeps the cells mostly separate. The number of fruiting bodies produced with the D. discoideum phenotype increased from 225 +/- 32 fruiting bodies when D. discoideum was alone to 486 +/- 61 in the mix treatments. However, the number of D. discoideum spores decreased, although not significantly, from 2.75e(7) +/- 1.29e(7) spores in the controls to 2.06e(7) +/- 8.33e(6) spores in the mix treatments. D. purpureum fruiting body and spore production decreased from 719 +/- 111 fruiting bodies and 5.81e(7) +/- 1.26e(7) spores in the controls to 394 +/- 111 fruiting bodies and 9.75e(6) +/- 2.25e(6) spores in the mix treatments.Conclusion: Both species appear to favor clonality but can cooperate with each other to produce fruiting bodies. Cooperating amoebae are able to make larger fruiting bodies, which are advantageous for migration and dispersal, but both species here suffer a cost in producing fewer spores per fruiting body.