Geometry of the species–area relationship in central European birds: testing the mechanism

Summary The species–area relationship (SAR) is one of the major patterns in community ecology, but the mechanisms that contribute to its exact shape have remained obscure. In continuous mainland areas, the SAR has been attributed to sampling effects (large areas contain species that are too rare to be present in small areas), habitat heterogeneity (large areas contain more types of habitat allowing more species to coexist), and population and metapopulation processes causing spatial aggregation. We tested the contribution of these effects to SARs using data on breeding bird distributions in the Czech Republic, their total population sizes and spatial distributions of their preferred habitats. 2. The relationship between number of species and sampled area is more or less linear on a log–log scale within the Czech Republic, although it reveals saturation when the area is expanded to the whole of central Europe. Neither sampling effect nor habitat heterogeneity alone explain the observed SAR shape: both models predict much higher species richness within any area and a SAR of much lower slope than observed. A combined model based on random sampling constrained by the amount of suitable habitat within an area gives quite realistic predictions of species numbers within different sample areas. Nevertheless, the observed pattern reveals much higher variance of species richness amongst areas, species often being significantly more spatially aggregated than predicted by habitat distribution. Moreover, the relationship between the amount of suitable habitat and the probability of quadrat occupancy is actually nonsignificant for about two‐thirds of species, indicating that assumptions of the combined model are unrealistic. Therefore, the shape and slope of SARs are actually affected both by habitat heterogeneity that represents the major driver of distribution of some species, and by spatial aggregation that is not attributable to habitat heterogeneity in other species.