Scale-dependent frequency distributions of plant species in dune slacks: Dispersal and niche limitation

Question: How do distribution patterns change with increasing scale level, and can this shift be attributed to dispersal and/or niche limitation? Location: Dune slacks at the Belgian and North French coast. Method: Frequency distribution patterns of species were tested over different scale levels (ranging from 0.008 to 45 km(2)). Analyses were executed for the total species pool and for subsets of species with high and low dispersal rates and habitat generalist and specialist species. Results: Species distributions for the total species pool on scale levels larger than 1.5 km(2) were unimodal, with an overrepresentation of rare species. With decreasing scale level, the proportion of common species increased, leading to weak bimodality. Distributions of subsets of slowly dispersing and habitat specialist species are on all scale levels characterised by a strongly unimodal pattern. The subset of species with high dispersal rates and habitat generalist species has a higher proportion of common species, leading to a significant core peak. Conclusions: On all scale levels both dispersal and niche limitation are hampering the wider distribution of rare species. However, since isolation and habitat heterogeneity are limited on the smaller scale levels, a larger proportion of well dispersing and habitat generalist species is able to occupy a high number of patches, resulting in a significant core peak for the total species pool.