Historical regulation of local species richness across a geographic region

Regional processes that regulate the dispersal of a pool of species can inf luence the number and types of species that are able to integrate into loca l assemblages. The transgression of glacial lakes to marine fjord environme nts of the South Island fjords of New Zealand was used to examine the influ ence of this regional process on local species richness of subtidal, suspen sion-feeding epifaunal invertebrates. Surveys of epifaunal assemblages over 200 km of coastline, including 23 sites in 13 separate fjords, were used t o quantify species density (per 0.25 m(2) quadrat), species turnover along a transect (Routledge's beta (I)), and local species richness (S) with Chao 2 estimations. An extension of the curve of sea level rise over the past 1 8 000 yr was used to estimate the time at which rising sea levels overtoppe d the entrance sills of individual fjords, indicating the date at which the se habitats were available for colonization. Species density was nonsignifi cantly related to fjord age, whereas Routledge's beta (I) and local species richness (S) with Chao 2 estimations were all significantly related to fjo rd age, with fjord age explaining 26%, 58%, and 62% of the variation across the region, respectively. These results indicate that regional constraints on the dispersal of a pool of species can have a strong influence on the l ocal species richness, whereas variations at the quadrat level are likely t o be explained by processes operating on smaller spatial scales. The regula tion of species integration into local environments by regional processes m ay also affect the outcome of local processes that depend upon the number a nd type of species coexisting in local assemblages, underscoring the need t o consider historical factors in understanding the structure and function o f ecological communities.