Spatial variation in density and biomass of patellid limpets inside and outside a marine protected area

Although differences in the mean density, biomass and size of rocky intertidal biota inside and outside MPAs have been well documented, little is known about the variability in these attributes at different spatial scales. This topic merits investigation because it can influence the outcome of reproduction, competition and predation, and thereby affect the viability of populations. In this study, a nested sampling design was used to assess differences in density and biomass of seven species of patellid limpets at the scales of plots, sites and shores inside and outside a marine protected area (MPA) on the southeast coast of South Africa. At the scale of plots, significant variation was more common inside than outside the MPA. This probably reflects differences in the general pattern of space occupancy and quality of habitats available to limpets inside and outside the MPA. Significant variation at the scale of sites was rare. This suggests that either the processes contributing to variability at this scale counteract each other or that the sites were similar in terms of habitats and ecological processes. At the scale of shores, significant variation was more common in densities than in biomass, but both occurred with equal frequency inside and outside the MPA. Variation at this scale is probably driven by a combination of recruitment and/or mortality. Five species exhibited greater spatial variability inside than outside the MPA. The spatial patterns observed did not appear to be linked with differences in the mobility, habitat requirements or susceptibility of these species to exploitation. The lack of consistent patterns suggests that each species probably responds in a different way to the ecological processes operating at these spatial scales.