Grasslands are some of the most species-rich communities in the world at small-spatial scales, generating lots of interest in what enables large numbers of species to co-exist at scales where species interactions are expected to occur. Local species richness is influenced by the size of the regional species pool (i.e. the number of species that can potentially inhabit a site) and local abiotic and biotic factors. It is expected that the species pool is of greater importance in species-rich grasslands as these are relatively open to colonization, and high richness suggests local limitations are weak. We examined the relative importance of the species pool and local factors governing small-scale species richness and assembly patterns in temperate grasslands around the world. We sampled vegetation in 50 transects; transects were 10 m long and comprised of 100 contiguous 10 x 10 cm quadrats (total of 5000 quadrats). We calculated quadrat and transect-level richness and compared this among the study regions. We examined the relationship between transect richness (surrogate for species pool) and quadrat (small-scale) richness to determine if there is a positive linear relationship supporting the species pool hypothesis or if local communities were saturated suggesting local limitations. We further explored evidence for local limitations using a null model approach to compare several species richness measures to random expectations. We compared these tests among the regions to detect if community assembly is similar among temperate grasslands.
Results/Conclusions
Quadrat (10 x 10 cm) richness was high with a mean of 7.4 across all regions, and the highest richness (23 species) was found in the Spain-Morocco region. We found a positive curvilinear relationship between small-scale richness and transect richness indicating that communities are partially saturated at high richness. We found some evidence for local factors influencing assembly in most of the study regions but only at small-spatial scales. Local assembly patterns were similar, with few differences between the regions. We found evidence for both the species pool and local factors governing species richness patterns in ecologically similar but evolutionary distinct temperate grasslands. Most of the variation in small-scale richness is due to varying species pool size but there are still consistent, albeit, weak local limitation on community assembly.