Interactive effects of depth and marine protection on predation and herbivory patterns

摘要

The establishment of marine protected areas (MPAs) worldwide has shown that closure of areas to fishing results in major changes in the structure of marine ecosystems. The removal of high-order consumers by fishing has both direct and indirect effects that can lead to important changes in the strength of trophic interactions. Although the effects of marine protection on biological interactions are beginning to be understood, our knowledge is largely restricted to shallow-water assemblages. However, depth gradients are also characterised by significant differences in the intensity of trophic linkages, and these may be interacting with any effects derived from protection. In this study, the individual and combined effects of depth and marine reserve protection on predation and herbivory were determined across 3 regions in the NW Mediterranean (Catalunya, Mallorca and Menorca) using juvenile urchins and palatable algae as bioassays. Marine protection did not strongly influence fish herbivory, which generally decreased with depth. We found no evidence of depth-related changes in predation rates or the size of predatory fishes, but there was a strong effect of protection on predation rates that was only consistently observed across regions in shallow water (5 m depth), but not at greater depths (15 and 30 m). This increase in predation of sea urchins within MPAs in shallow waters can have important community-wide conseguences, as herbivorous sea urchins are commonly most abundant in these shallow habitats and predator-urchin interactions have important cascading effects on algal communities.