Multiple, Alternative Stable States in Marine Metacommunities: risk of unintended consequences in MPA management scenarios

Marine area protection has had some spectacular failures, despite a century of effort to establish conservation measures in marine environments. We hypothesize that these failures may be explained by the existence of alternative stable equilibrium states in ecosystems. In other words, if multiple alternate basins of attraction exist, then the implementation of protection together with other disturbances may trigger a shift from the present regime to an unintended one. This can be induced by a shift to a different basin of attraction (leading to a negative effect), or based on the fact that the dynamic regime is trapped in a basin of attraction and cannot return to a desirable past condition (leading to a null effect).The common idea in mathematical ecology is that the canonical equation to formalize alternative stable states (ASS) is the Hill-equation. This is nonetheless a mathematical statement, which does not find any equivalent in term of ecological process. Besides, it rests on the assumption that the conditions for ASS to exist are local but cannot come from the dynamics of other remote connected systems. We demonstrated, from simple competition models in a metacommunity framework, that multiple stable states (which exist when inter-specific competitions are strong) allows the dynamics to alternate between ASS without to include a hill-type function. In the case of post-recruitment dispersal, this possibility only exists if local populations are rescued by the supplementation from a small flux of immigrants. The analysis of the relative importance between regional dispersal and local demographic processes provides an explicit and quantitative criterion to distinguish between the rescue effect and the mass effect. Finally, the possibility to shift between Alternate Stable States in the community (which implies several sites simultaneously) exist sustainably only, in the competition perspective, if at least, in one of the interconnected sites, the competition between the two targeted populations is weak, hence ensuring the conditions for their coexistence. This suggests that the pressure and intensity of the competition should be represented explicitly, through the quantification of the dynamics of available resources in the ecosystem. This paradigm is part of the new context of MPA development based on ecological principles and management of networked MPAs. The application of this so-called ecosystem-based approach should prevent negative consequences of management plan, and should optimize effort for MPA managers.

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