Adaptive partner recruitment can help maintain an intra-guild diversity in mutualistic systems.

Mutualisms between assemblages of multiple species or strains (guilds) are considered unstable because of positive feedback between the guilds. Previous studies suggest that negative inter-guild feedback due to asymmetry in the exchange of benefits between the guilds can stabilize them, but preferential association for more beneficial partners may reduce such asymmetry and strengthen the positive inter-guild feedback. Here I develop a replicator dynamics model for mutualistic systems between two host and two symbiont strains to investigate conditions that stabilize mutualisms when feedback between host-symbiont guilds is positive. I assume that one symbiont strain is mutualistic for one host strain but parasitic for the other, whereas the other symbiont strain is the opposite. Hosts recruit their symbionts from the environment and discriminately offer them resources (partner preference), and only mutualistic symbionts return benefits to their hosts. I show that the two host and symbiont strains can coexist under strong partner preference by hosts if they adaptively adjust the number of associating symbionts, even when the intra-host strain competition is not so strong. Interestingly, there can be a stable coexistence equilibrium also under weak partner preference, but it disappears under intermediate levels of partner preference.

Effects of generalized and specialized adaptive defense by shared prey on intra-guild predation.

Intra-guild predation (IGP), predation on consumers which share common prey with the predators, is an important community module to understand a mechanism for persistence of complex food webs. However, classical theory suggests that persistence of an IGP system is unlikely particularly at high productivity, while empirical data do not support the prediction. Recently, adaptive defense by shared prey has been recognized to enhance coexistence of species and stability of the system. Some organisms having multiple predators in IGP systems employ two types of defenses; generalized defense that is effective against multiple predators and specialized one that is effective against only a specific predator species. We consider an IGP model including shared prey that can use the two types of defenses in combination against the consumer or omnivore. Assuming that the shared prey can change the allocation of defensive effort to increase its fitness, we show that the joint use of two types of adaptive defenses promotes three species coexistence and enhances stability of the IGP system when the specialized defense is more effective than the generalized one. When the system is unstable, a variety of oscillations appear and both the population densities and defensive efforts or only the population densities oscillate. Joint use of defenses against the consumer tends to increase the equilibrium population density of the shared prey with the defense efficiencies. In contrast, efficient generalized and specialized defenses against the omnivore often decrease the prey population. Consequently, adaptive defense by shared prey may not necessarily heighten the population size of the defender but sometimes increases densities of both the attackers and defender in IGP systems.

Coexistence of mutualists and non-mutualists in a dual-lattice model.

Evolution and maintenance of mutualism have been one of the major questions in evolutionary ecology, because it is often susceptible of invasion of non-mutualistic strategy. Some previous studies using dual-lattice model suggest that spatial structures of habitat can prevent non-mutualism from prevailing over mutualism, while the detail of the dynamics is not fully revealed. Here we explore population dynamics of the two strategies (mutualism and non-mutualism) in two species engaged in Prisoner's Dilemma game on a dual-lattice space, especially focusing on whether mutualists and non-mutualists can coexist in long-term dynamics. The habitat consists of two layers, each of which a population of species inhabits, and interspecific interaction is restricted between two corresponding sites of the layers. Each individual of the both species is either a mutualist or a non-mutualist and only the former pay cost c for benefit of the partner b. The payoff of the game affects the individuals' fecundity, while the mortality is constant. Reproduction is restricted to neighboring vacant sites of the focal individuals. Our computer simulations of the model show that even if b/c ratio remains constant, mutualists become dominant in both species over wider ranges of basic reproduction rate (reproduction rate without interspecific interaction) as b and c increase. If basic reproduction rates are asymmetric between the species or basic reproduction rates were sufficiently large, mutualists and non-mutualists can coexist in one or both species, while their population sizes often fluctuate. Transition of the final state between mutualism and non-mutualism happens rather discontinuously, then total population sizes change drastically at the transition. Moreover, we also find paradoxical cases of unilateral exploitation, i.e. one species consists of mutualists and other species non-mutualists. Additional simulations reveal that accidental extinction of the non-mutualists of one species can result in extinction of mutualist of the other species.

Evolutionary stability of one-to-many mutualisms.

One-to-many mutualisms - interspecific cooperations in which each host individual can potentially interact with multiple symbiont individuals while each symbiont individual can only one host individual - are widely found in nature, while their evolutionary stability has not been explored. It has been often thought that partner choice can stabilize multi-player mutualisms. However, in one-to-many mutualisms partner choice is inevitably asymmetric between hosts and symbionts, which might destabilize the system. Here I develop a simple mathematical model for an obligate one-to-many mutualism, with explicitly considering imperfect ability of symbiont choice by hosts. I fix the trait of hosts and concentrate on the evolutionary dynamics of cooperativeness in symbiont population. Each host chooses a constant number of symbionts from a potential symbiont population, a fraction of which are chosen through preferential choice depending on cooperativeness of the symbionts, while the rest are through random choice. After the association between the host and the symbionts is established, the host offers a constant amount of resource to each associating symbiont. It spends a part of the resource to increase the fitness of the host in proportion to its cooperativeness, and the rest for its own reproduction. I show that pure mutualist population is evolutionarily stable when the fraction of preferential choice c is large and the strength of preferential choice k is small, otherwise mutualists and cheaters coexist. In addition, in the coexistence state the frequency of mutualists increases with c. In contrast, it decreases with k, while the cooperativeness of mutualists increases. The two factors offset against each other, so that the fitness gain of host remains constant.

Dual lattice model of the evolution of facultative symbiosis with continuous Prisoner's Dilemma game.

Mutualism is ubiquitous in nature and is thought to have played a key role in the history of life. However, how mutualism could evolve despite being prone to unilateral exploitation is a puzzling question in evolutionary ecology. Some theoretical studies have shown that spatial structure of habitat can facilitate the emergence and maintenance of mutualism. However, they are based on the simple assumption that the trait in question is discrete: each individual is either a mutualist or a non-mutualist. In this article I develop a simple simulation model of coevolution of facultative symbiosis using a one-shot continuous Prisoner's Dilemma game to investigate the evolutionary dynamics of mutualism between two species. In this model I assume continuous traits for both species from -1 (fully deceptive) to 1 (fully cooperative). The habitat has a dual-lattice structure, each layer is inhabited by one species. Interspecific interaction is restricted between two corresponding sites of the two layers. Without limitation on the magnitude of a single mutation, I find that mutualism can arise and persist when the intrinsic reproduction rate is low (but is above a threshold) and the benefit/cost ratio of the cooperative strategy is large, which is consistent with Yamamura et al. [2004. Evolution of mutualism through spatial effects. J. Theor. Biol. 226, 421-428]. In these cases, extreme antagonism often evolves starting from a neutral population that seems nearly stable, but once mutualism arises, the cooperative individuals quickly increase and both the populations eventually become mutualistic on average, although they are polymorphic. However, when the effect of a single mutation was limited to be small, extreme antagonism is much likely to dominate unless the intrinsic reproduction rate is low. When only one species is allowed to evolve, mutualism arises when the initial strategy of the other species is cooperative. Otherwise, excessive deception evolves in the former, and the latter often becomes driven to extinction.

Effect of stochasticity in the availability of pollinators on the resource allocation within a flower.

In this article, we develop a simple model to study the effect of stochasticity in pollination on evolutionarily stable (ES) resource allocation within a hermaphrodite flower of animal-pollinating plants. For simplicity, we consider trade-off in resource allocation between attractive structure (petals etc.) and female function (seeds and fruits) with neglecting the amount of resource allocated to male function (pollens and stamens). We show that ES resource allocation does not much depend on the detail of the probability distribution of the number of pollinator visit on a flower, but on the probability that a flower fails to be visited. We also find that: (1) When the flowers are self-incompatible, the ES allocation to the attractive structure monotonically increases as the availability of pollinators in the environment decreases. (2) When there is strong positive correlation among flowers in the number of pollinator visit, the ES allocation is larger than the case without the correlation. (3) When the flowers are self-compatible and engage prior selfing, the ES allocation monotonically increases as the availability of pollinators in the environment decreases to a threshold, under which it suddenly decreases to zero.