Evaluating the processes that generate spatial heterogeneity in species composition in ecological communities and along environmental gradients continues to be a major goal of research in community ecology. A classic view is that species turnover along environmental gradients reflects a deterministic sorting process whereby differentially-adapted species in the regional pool colonize and reach optimal abundances at different locations along a gradient in accordance with the environmental tolerance, competitive ability and niche requirements of species. However, it is widely recognized that stochastic factors, dispersal constraints and variable species pools can affect species distributions and community patterns independent of species environment-sorting. Indeed, a major current goal in ecology is to reconcile purely deterministic and purely neutral or stochastic perspectives of community assembly and asses the relative contribution of each class of processes to community organization. According to meta-community theory, limitations on dispersal and species availability can influence the importance of species sorting in meta-communities and influence the degree to which underlying gradients become expressed as pattern in the community. Sorting should be most strongly expressed in meta-communities that are embedded in landscapes with functionally rich species pools and where habitat connectivity and dispersal are sufficient to assure that suitably adapted species are available to colonize, compete and sort into their preferred sites. We present results from a long-term field experiment of grassland secondary succession and community assembly conducted in an abandoned agricultural field in northeastern Kansas. We assessed patterns of plant succession and community differentiation over an eight year period along a soil N fertilization gradient and in response to a manipulation of the available species pool, achieved through the use of a multi-species seed sowing treatment.
In general we found that the strength of species sorting and the degree of community differentiation along the fertilization gradient became stronger over successional time as the community transitioned from early dominance by annuals to later dominance by perennials. However, this occurred only in plots that had been exposed to an experimentally-enhanced species pool. These findings are in line with the meta-community perspective and the notion that species-environment sorting, and the degree to which underlying environmental gradients become expressed in spatial patterns of community composition across a meta-community, are contingent on levels of dispersal, species availability and habitat connectivity.