Can marine reserves restore lost ecosystem functioning? A global synthesis

Marine protected areas (MPAs) have grown exponentially, emerging as a widespread tool to conserve biodiversity and enhance fisheries production. Although numerous empirical studies and global syntheses have evaluated the effects of MPAs on community structure (e.g., biodiversity), no broad assessment concerning their capacity to influence ecological processes (e.g., species interactions) exists. Here, we present meta-analyses that compare rates of predation and herbivory on a combined 32 species across 30 MPAs spanning 85° of latitude. Our analyses synthesize the fate of 15,225 field experiment assays, and demonstrate that MPAs greatly increased predation intensity on animals but not herbivory on macroalgae or seagrass. Predation risk, quantified as the odds of prey being eaten, was largely determined by predator abundance and biomass within reserves. At MPAs with the greatest predator accumulation, the odds of predation increased to nearly 49:1, as opposed to 1:1 at MPAs where predators actually declined. Surprisingly, we also found evidence that predation risk declined with increased sea-surface temperature. Greater predation risk within MPAs was consistent with predator and prey population abundance estimates, where predators increased 4.4-fold within MPAs, whereas prey decreased 2.2-fold. For herbivory, the lack of change may have been driven by functional redundancy and the inability of reserves to increase herbivore abundance relative to fished zones in our sample. Overall, this work highlights the capacity of MPAs to restore a critical ecosystem function such as predation, which mediates energy flows and community assembly within natural systems. However, our review of the literature also uncovers relatively few studies that have quantified the effects of MPAs on ecosystem function, highlighting a key gap in our understanding of how protected areas may alter ecological processes and deliver ecosystem services. From a historical perspective, these findings suggest that modern levels of predation in the coastal oceans may currently only be a fraction of the baseline prior to human exploitation.