Marine reserves can enhance ecological resilience
Ecological resilience, the magnitude of a perturbation a community can withstand and remain in a given state, is a critical component of ecosystem-based management. However, resilience is a controversial concept because it is difficult to measure empirically. In theory, spatially-explicit management with designated protected and unprotected areas might enhance resilience relative to non-spatial management by buffering against catastrophic shocks via rescue effects or it may degrade resilience by concentrating impacts in unprotected areas. For example, no-take marine reserves might enhance resilience of marine communities through the compartmentalization of a preserved fraction of the community that can provide larvae to recolonize former habitat in the event of an environmental or anthropogenic catastrophe in an unprotected area. On the other hand, intensification of mortality in unprotected areas caused by displaced fishing effort may decrease resilience relative to the alternative of reducing the total fishing effort across the whole region. Here we test whether reserve (spatial) management can increase resilience compared to conventional (non-spatial) fishery management using a dynamic model of a fish community with alternative stable states. The alternate states are predator-dominated (desirable) or competitor-dominated (undesirable) equilibria, mediated by a cultivation effect wherein adult predators consume the competitors of their own juveniles.
Relative to conventional fishery management alone, reserves increased the range of initial predator and competitor densities that result in reaching the desired state at equilibrium, thus enhancing our deterministic metric of ecological resilience. This increased resilience holds even when fishing effort is displaced from protected to unprotected areas in proportion to the areal coverage of reserves, i.e., when total fishing effort is not reduced and is instead concentrated outside reserve boundaries. Similarly, reserves increased resilience in stochastic simulations with variable reproduction, where we measured resilience as the probability of being in the desired state at the end of the time horizon. Furthermore, our results indicate that for degraded systems (those trapped in the undesirable, competitor-dominated state), some combination of reserves and culling of competitors or stock enhancement of adult predators may be the most effective approach for restoration of the preferred state. Although our example focuses on temperate marine fish communities, spatial management may enhance ecological resilience in other systems with alternative stable states, such as coral reefs and woodlands.