Beta diversity in regenerating coastal dune forests in KwaZulu-Natal South Africa

Beta diversity, defined as the variation or turnover in species composition, is important to the understanding of how ecological communities assemble. Studies of beta diversity during secondary forest succession may thus afford the chance to understand community assembly from a known onset. This study examined the relationship between regeneration age and beta diversity within and between seral stages along a coastal dune forest successional sere for three taxa (trees, millipedes, and birds). These taxa represent different trophic levels and have different dispersal abilities. Niche-based processes (e.g. environmental filtering and niche diversification) and dispersal-based processes (e.g. dispersal limitation), or a mixture of the two, can influence beta diversity over the course of regeneration. However, stochastic community assembly processes (e.g. sampling and priority effects) can influence beta diversity in an unpredictable way. To determine whether these dune forest communities are developing deterministically (i.e. through environmental selection and/or dispersal limitation) or stochastically (i.e. via sampling and priority effects) with succession, a null model of beta diversity was also used. Beta diversity responses to regeneration age based on classical measures of compositional dissimilarity varied among taxa (e.g. tree beta diversity increased while millipede and bird beta diversity decreased). The choice of dissimilarity index (presence-absence vs. abundance) also had important consequences on beta diversity responses. The results of this study showed that deterministic processes such as niche diversification generally increased with increasing regeneration age, leading to greater compositional dissimilarity. However, this varied depending on whether presence-absence or abundance information was included. The null model for species turnover suggested that species-poor communities were not rarefied samples of species rich communities in older seral stages, but these communities experienced some degree of species turnover. Again, this differed among taxa. This leads to the conclusion that the post-mining development of coastal dune forest largely follows deterministic assembly rules and that stochastic sampling effects are of minor importance. However, there is apparent taxonomic and abundance dependency of beta diversity and inferred processes. Future studies that aim to clarify community assembly processes ought to adopt a null model approach and include species relative abundances. If not, inferences made about the processes driving beta diversity may be misleading.