Historical regulation of local species richness across a geographic region

Regional processes that regulate the dispersal of a pool of species can influence the number and types of species that are able to integrate into local assemblages. The transgression of glacial lakes to marine fjord environments of the South Island fjords of New Zealand was used to examine the influence of this regional process on local species richness of subtidal, suspension‐feeding epifaunal invertebrates. Surveys of epifaunal assemblages over 200 km of coastline, including 23 sites in 13 separate fjords, were used to quantify species density (per 0.25 m² quadrat), species turnover along a transect (Routledge's βI), and local species richness (S) with Chao 2 estimations. An extension of the curve of sea level rise over the past 18 000 yr was used to estimate the time at which rising sea levels overtopped the entrance sills of individual fjords, indicating the date at which these habitats were available for colonization. Species density was nonsignificantly related to fjord age, whereas Routledge's βI and local species richness (S) with Chao 2 estimations were all significantly related to fjord 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 local species richness, whereas variations at the quadrat level are likely to be explained by processes operating on smaller spatial scales. The regulation of species integration into local environments by regional processes may also affect the outcome of local processes that depend upon the number and type of species coexisting in local assemblages, underscoring the need to consider historical factors in understanding the structure and function of ecological communities.