Regularity of species richness relationships to patch size and shape

Abstract

This study aims to assess the degree of regularity in the effect of patch size and patch shape on plant species richness across a macroscale region, and to evaluate the implications for nature conservation. Our study area covers south-eastern Norway and contains 16 agricultural landscapes with 2162 patches. To analyse regularity a local linear mixed model (LLMM) was applied. This procedure estimates the richness trends due to shared effects of size and shape, and simultaneously provides the landscape-specific random effect. The latter is a direct estimate of the degree of irregularity between the landscapes, conditioned on specific values of size and shape. The results show a positive interaction between the shape and size of patches, which is repeated for all landscapes. The shape of the patches produces more regular patterns in species richness than the size of patches. This we attribute to effects of dispersal and distance to neighbouring patches of different environmentally conditioned species pools. Large and complex patches have shorter average distance to neighbouring patches (of different types) than large simple-shaped (circular) patches have. We attribute the higher species richness of the former, given a similar area, to a higher number of species dispersed from the outside into the more complex plot. For small patches, however, the distance to the edge is short relative to normal dispersal distances, for patches of all shapes. This explains why the positive effect of shape complexity on species richness is stronger for large patches. This interpretation is supported by a strong spatial correlation conditioned on the most complex patches. Theories of dynamics in biodiversity in patchy landscapes must consider shape as a regulator at the same level as size, and both shape and size of patches should be simultaneously taken into account for management planning.