Comparing species-area relationships of native and exotic species

The species-area relationship (SAR) is one of the most general patterns in ecology. Recently, SARs have been employed as tools for comparing the ecology and biogeography of native and exotic species across spatial scales and exploring the influence of invasive species on native biodiversity. Here, we assess published studies to determine if SARs differ between native and exotic assemblages. We conducted a literature search to find studies that estimated the exponent (z) of the power-law SAR for native and exotic species across the same set of locales. We also compiled intercepts (c) of SARs where available. We used linear mixed models to test if z and c differed between native and exotic SARs and if this relationship differed across taxa. Our literature search produced 36 native-exotic pairs from 23 studies with which to compare the exponent of the power-law SAR. Further, SAR intercepts were available for 21 native-exotic pairs. Overall, exotic SAR exponents (z) did not differ from those of natives. However, this pattern did not hold across all taxonomic groups. Plant assemblages, which are best represented in our data (61% of total), mirrored the overall pattern showing no differences in exponents between native and exotic SARs. On the other hand, SAR exponents were greater for both native bird and animal assemblages. The intercepts (c) of native SARs were significantly greater than those of exotics for all taxa combined and for each individual taxonomic grouping. Our results suggest processes driving the increase in species richness with area are similar for native and exotic plant species, but not for animals. Expanding studies that compare SARs of native and exotic species to more taxonomic groups and different types of SARs (e.g., nested, contiguous, non-contiguous) will facilitate a better understanding of how native and exotic species richness scale with area.