Untangling natural variability of macrofaunal populations from protection effects

Marine protected areas are essential for conservation purposes. Remarkably, few studies have assessed the responses of small macrofaunal species to different protection levels in the Mediterranean Sea. Using a hierarchical sampling design spanning four orders of magnitude (1, 10, 100 and 1000 m), we investigated whether a marine protected area had an effect on crustacean (amphipod) and mollusc (mainly gastropod) assemblages associated with Posidonia oceanica seagrass meadows. Based on this investigation, we report spatial and temporal variability patterns of these assemblages in four different protection levels. We also discuss potential confounding effects on these areas, such as different habitat features. The structure of amphipod and gastropod assemblages based on density data was patchy at small and large spatial scales, and differed markedly among protection levels. Multiscale analyses showed that lower densities and/or biomasses of several taxa occurred within fully protected areas when compared to partially protected areas. Moreover, the seagrass meadows accounted for a low proportion of the total variability of the studied macrofaunal assemblages. We suggest therefore that the observed patchiness is likely to occur for multiple and interrelated reasons, ranging from the ecological and behavioural traits (e.g., dispersion and mobility) of macrofaunal species to protection-dependent factors such as fish predation. Multiscale spatial and temporal monitoring of macrofaunal assemblages in a long-term perspective, as well as experimental manipulations, are needed to better understand the protection effects.