Akihiko Mougi

Diversity of Interaction Types and Ecological Community Stability

Diversity of Interactions The diversity in interactions that occur between different species—such as predation, competition, and mutualism—is a fundamental feature of natural ecological systems. Using a theoretical model, Mougi and Kondoh (p. 349; see the Perspective by Boyd) show that the diversity of ecological relationship helps a biological community to be stable and thus may be key to the maintenance of biodiversity itself. A theoretical model incorporating multiple interactions shows how ecological complexity leads to ecosystem stability. Ecological theory predicts that a complex community formed by a number of species is inherently unstable, guiding ecologists to identify what maintains species diversity in nature. Earlier studies often assumed a community with only one interaction type, either an antagonistic, competitive, or mutualistic interaction, leaving open the question of what the diversity of interaction types contributes to the community maintenance. We show theoretically that the multiple interaction types might hold the key to understanding community dynamics. A moderate mixture of antagonistic and mutualistic interactions can stabilize population dynamics. Furthermore, increasing complexity leads to increased stability in a “hybrid” community. We hypothesize that the diversity of species and interaction types may be the essential element of biodiversity that maintains ecological communities.

Evolution towards oscillation or stability in a predator–prey system

We studied a prey–predator system in which both species evolve. We discuss here the conditions that result in coevolution towards a stable equilibrium or towards oscillations. First, we show that a stable equilibrium or population oscillations with small amplitude is likely to occur if the preys (hosts) defence is effective when compared with the predators (parasites) attacking ability at equilibrium, whereas large-amplitude oscillations are likely if the predators (parasites) attacking ability exceeds the preys (hosts) defensive ability. Second, a stable equilibrium is more likely if the preys defensive trait evolves faster than the predators attack trait, whereas population oscillations are likely if the predators trait evolves faster than that of the prey. Third, when the adaptation rates of both species are similar, the amplitude of the fluctuations in their abundances is small when the adaptation rate is either very slow or very fast, but at an intermediate rate of adaptation the fluctuations have a large amplitude. We also show the case in which the preys abundance and trait fluctuate greatly, while those of the predator remain almost unchanged. Our results predict that populations and traits in host–parasite systems are more likely than those in prey–predator systems to show large-amplitude oscillations.

Stability of competition-antagonism-mutualism hybrid community and the role of community network structure

Theory predicts that ecological communities of many interacting species are unstable, despite the fact that complex ecosystems persist in nature. A recent theoretical study hypothesised that coexistence of antagonism and mutualism can stabilise a community and even give rise to a positive complexity-stability relationship. Here, using a theoretical model, we extended the earlier hypothesis to include competition as a third major interaction type, and showed that interaction-type diversity generally enhances stability of complex communities. Furthermore, we report a new finding that the hierarchically structured antagonistic interaction network is important for the stabilizing effect of interaction type diversity to emerge in complex communities. The present study indicated that the complexities characterised by species number, connectance, species variation, and interaction type diversity synergistically contributed to maintaining communities, and posed an interesting question of how present complex communities emerged, and developed from simpler ecosystems.

Paradox of marine protected areas: suppression of fishing may cause species loss

A number of fish and invertebrate stocks have been depleted by overexploitation in recent years. To address this, marine protected areas (MPAs) are often established to protect biodiversity and recover stocks. We analyzed the potential impact of establishing MPAs on marine ecosystems using mathematical models. We demonstrate that establishment of an MPA can sometimes result in a considerable decline, or even extinction, of a species. We focus on a prey–predator system in two patches, one exposed to fishing activity and the other protected (MPA). Our analyses reveal that the establishment of the MPA can cause a reduction in prey abundance, and even extinction of the prey. Such unintended consequences are more likely to occur if the predator species is a generalist and if the MPA is intended to protect only the predatory species. Further, a mobile predator that migrates adaptively rather than randomly is associated with a greater reduction in prey abundance.

Predator-prey coevolution driven by size selective predation can cause anti-synchronized and cryptic population dynamics.

Population dynamics and evolutionary dynamics can occur on similar time scales, and a coupling of these two processes can lead to novel population dynamics. Recent theoretical studies of coevolving predator-prey systems have concentrated more on the stability of such systems than on the characteristics of cycles when they are unstable. Here I explore the characteristics of the cycles that arise due to coevolution in a system in which prey can increase their ability to escape from predators by becoming either significantly larger or significantly smaller in trait value (i.e., a bidirectional trait axis). This is a reasonable model of body size evolution in some systems. The results show that antiphase population cycles and cryptic cycles (large population fluctuation in one species but almost no change in another species) can occur in the coevolutionary system but not systems where only a single species evolves. Previously, those dynamical patterns have only been theoretically shown to occur in single species evolutionary models and the coevolutionary model which do not involve a bi-directional axis of adaptation. These unusual dynamics may be observed in predator-prey interactions when the density dependence in the prey species is strong.

Food-web complexity, meta-community complexity and community stability

What allows interacting, diverse species to coexist in nature has been a central question in ecology, ever since the theoretical prediction that a complex community should be inherently unstable. Although the role of spatiality in species coexistence has been recognized, its application to more complex systems has been less explored. Here, using a meta-community model of food web, we show that meta-community complexity, measured by the number of local food webs and their connectedness, elicits a self-regulating, negative-feedback mechanism and thus stabilizes food-web dynamics. Moreover, the presence of meta-community complexity can give rise to a positive food-web complexity-stability effect. Spatiality may play a more important role in stabilizing dynamics of complex, real food webs than expected from ecological theory based on the models of simpler food webs.

Interaction-type diversity hypothesis and interaction strength: the condition for the positive complexity-stability effect to arise

Recently, a theoretical hypothesis was proposed that the coexistence of antagonism and mutualism may stabilize ecological community and even give rise to a positive complexity-stability relationship (interaction-type diversity hypothesis). This hypothesis was derived from an analysis of community model, which was developed based on two specific assumptions about the interaction strengths: those are, (i) different interaction types, antagonism and mutualism, have quantitatively comparable magnitude of effects to population growth; and (ii) interaction strength decreases with increasing interaction links of the same interaction type. However, those assumptions do not necessarily hold in real ecosystems, leaving unclear how robust this hypothesis is. Here, using a model with those two assumptions relaxed, we show (i) that the balance of interaction strength is necessary for the positive complexity effect to arise and (ii) that interaction-type diversity hypothesis may still hold when interaction strength decreases with increasing links of all interaction type for some species.

Effects of marine protected areas on overfished fishing stocks with multiple stable states.

Marine protected areas (MPAs) have attracted much attention as a tool for sustainable fisheries management, restoring depleted fisheries stocks and maintaining ecosystems. However, even with total exclusion of fishing effort, depleted stocks sometimes show little or no recovery over a long time period. Here, using a mathematical model, we show that multiple stable states may hold the key to understanding the tendency for fisheries stocks to recover because of MPAs. We find that MPAs can have either a positive effect or almost no effect on the recovery of depleted fishing stocks, depending on the fish migration patterns and the fishing policies. MPAs also reinforce ecological resilience, particularly for migratory species. In contrast to previous reports, our results show that MPAs have small or sometimes negative effects on the recovery of sedentary species. Unsuitable MPA planning might result in low effectiveness or even deterioration of the existing condition.

Unusual predator-prey dynamics under reciprocal phenotypic plasticity.

Recent theories and experiments have shown that plasticity, such as an inducible defense or an inducible offense in predator-prey interactions, strongly influences the stability of the population dynamics. However, such plastic adaptation has not been expected to cause unusual dynamics such as antiphase cycles, which occur in experimental predator-prey systems with evolutionary adaptation in the defensive trait of prey. Here I show that antiphase cycles and cryptic cycles (a large population fluctuation in one species with almost no change in the population of the other species) can occur in a predator-prey system when both member species can change their phenotypes through adaptive plasticity (inducible defenses and offenses). I consider a familiar type of predator-prey system in which both species can change their morphology or behavior through phenotypic plasticity. The plasticity, that is, the ability to change between distinct phenotypes, is assumed to occur so as to maximize their fitness. I examined how the reciprocal adaptive plasticity influences the population dynamics. The results show that unusual dynamics such as antiphase population cycles and cryptic cycles can occur when both species show inducible plasticity. The unusual dynamics are particularly likely to occur when the carrying capacity of the prey is small (the density dependence of the preys growth is strong). The unusual predator-prey dynamics may be induced by phenotypic plasticity as long as the phenotypic change occurs to maximize fitness.

Allelopathic adaptation can cause competitive coexistence

The maintenance of plant diversity is often explained by the ecological and evolutionary consequences of resource competition. Recently, the importance of allelopathy for competitive interactions has been recognized. In spite of such interest in allelopathy, we have few theories for understanding how the allelopathy influences the ecological and evolutionary dynamics of competing species. Here, I study the coevolutionary dynamics of two competing species with allelopathy in an interspecific competition system, and show that adaptive trait dynamics can cause cyclic coexistence. In addition, very fast adaptation such as phenotypic plasticity is likely to stabilize the population cycles. The results suggest that adaptive changes in allelopathy can lead to cyclic coexistence of plant species even when their ecological characters are very similar and interspecific competition is stronger than intraspecific competition, which should destroy competitive coexistence in the absence of adaptation.