Effective island area: Disentangling species-area relationships in the Galápagos Islands

Background/Question/Methods

One of the most widely observed biogeographic patterns is the species-area relationship, where species richness increases with area. Although the overall pattern is usually consistent, its underlying drivers are varied and less certain. In the context of island biogeography, proposed mechanisms for why there are more species in larger areas, such as greater resource availability, higher speciation rates, and more diverse habitats, imply that the increase in species richness may not be driven by total island area itself, but rather the area that provides resources to maintain populations and enable speciation, or hosts diverse habitats. In addition, different species groups may have different probabilities of colonization, establishment, and extinction as a function of habitat availability. Here, we describe the "effective island area" as the size of an island as it is actually experienced by the relevant organisms. We explore this concept using plant species and area types from the Galápagos Islands. In each scenario, we compare species-area relationships that differ either in the type of area (e.g. total island, non-barren, specific habitat) or in the species groups considered (e.g. habitat specialists, endemic, introduced, or by dispersal mode) and interpret the model parameters in the context of effective island area.

Results/Conclusions

We found that overall, the strongest relationship among Galápagos plant species and area measurements was between the all species and the non-barren island area. Most other relationships had similar parameters, with the exception of those with introduced species, humid specialists, and ferns. These species are missing on smaller islands that have less human development and lack sufficient elevation for humid habitat. Using and expanding this approach will help to disentangle the many connotations of term "area" to see which driving increases in which species, and inform predictions about how species may be affected by landscape changes.