Supply-side biogeography: geographic patterns of settlement and early mortality for a barnacle approaching its range limit

Abstract

Species range limits are often associated with reduced adult densities, and this may reflect the failure of a particular life-history stage. For benthic marine invertebrates, settlement is a time of great mortality that strongly influences adult population structure, at least at local spatial scales. In south-eastern Australia we determined that adult abundance of the intertidal barnacle Tesseropora rosea declines over a 450 km region of rocky shore from the middle to the southern limit of its range, and we tested the hypothesis that this biogeographic pattern reflects variations in the production, settlement, or early post-settlement mortality of larvae or adult mortality. Sampling at 2 sites on 11 rocky shores in this region over 2 yr revealed that none of the life-history stages or demo- graphic processes displayed a latitudinal gradient or a clear decline towards the south, and settle- ment and adult mortality were highly variable among locations. Indeed local variation in early life- history processes and adult mortality appears to dictate regional variability and observed latitudinal patterns of adult abundance of T. rosea, but longer term studies spanning at least a decade may determine if storage from one strong year in recruitment can set patterns of adult abundance.