Nested versus independent sampling: Solving the mystery of contradictory species-area relationships

AO_SCPLOWBSTRACTC_SCPLOWSpecies-area relationships (SARs) describe how the number of species increases with the size of the area surveyed, and they usually take the shape of a power law on regional spatial scales. A meta-review of empirical data has shown that the exponent of the power law is on average larger when the areas are sampled in a nested manner, compared to sampling of independent areas such as islands of different sizes. As this is in contrast to ecological reasoning, we performed computer simulations of three qualitatively different models that generate species distributions in space and time by the mechanisms of speciation, dispersal, and extinction. We find that in all cases and over a wide parameter range the SARs obtained by nested sampling have a smaller slope in the regional scale than those obtained by independent sampling. We explain the discrepancy to the empirical data by the different spatial scales on which the two types of empirical investigations were performed.