Identifying hotspots of molluscan species richness on rocky intertidal reefs

A significant question faced by environmental managers is how much survey effort is required in order to obtain an accurate representation of the species richness in an area. The appropriateness of rapid survey techniques for identifying biodiverse hotspots has not been previously tested for molluscs on intertidal rocky reefs. We used species inventories from standardized 4-h search surveys to rank 13 intertidal reefs in terms of their species richness and these were then compared to cumulative species records following repeated surveys from the same sites. A total of 172 surveys were conducted during low water spring tides over a 3 year period, with up to 20 surveys at a single site. Species richness in the inventories varied from 20 to 94 on the different reefs. There was a strong correlation between the number of species recorded in the standardized inventory and the total species richness from cumulative survey records (r = 0.969; P < 0.001). Importantly, the total species diversity recorded at each site was not related to the number of surveys that were conducted at that site (r = 0.110; P = 0.784). This confirms that a single standardized timed search produces a useful representation of molluscan species richness. The majority of molluscs recorded in this study were endemic to Australia (59%) and, significantly, the number of endemics was positively correlated to the total species richness found at each site (r = 0.992; P< 0.0001). Our data provide clear evidence for a local hotspot of molluscan species richness and endemism on the northern side of Bass Point, Shellharbour. We suggest that on a local scale biodiversity hotspots should only be identified as those sites that contain significantly more species than the local average. Two standard deviations above the mean appears to be an appropriate cut-off for identifying local biodiversity hotspots.