trial vertebrate groups (birds, mammals, reptiles, and amphibians; 258 species) and plants (87 species) were considered further. Species accounts were used to classify each species as being from a mainland or an island. Only species restricted to islands were listed as “island” species. Included in these tallies are three marine mammals, which we listed as “mainland” for the purposes of these analyses. Species accounts were also used to tally the factors listed as having contributed to species extinction. To do so, both the annotated list of contributing factors, and the text description were examined for each species. We classified extinction factors into three categories: predation, competition, and other factors. Predation includes pairwise species interactions that are positive for one species and negative for the other; this included any of the following: human hunting (including any form of direct human exploitation of a species), carnivorous predation, herbivorous predation, parasitism, and infectious disease. “Competition” included interactions where species competed for resources. All other factors, such as habitat loss and pollution, were considered to be “other factors.” Tallies were compiled for species listed as (i) only being impacted by predation, (ii) impacted by predation together with other factors, and (iii) impacted by competition together with other factors. In no case was competition listed as the sole factor causing a species extinction. These same tallies were repeated for the subset of extinct species listed as having been impacted by an “alien,” i.e., nonnative or exotic, species.
Island characteristics are recorded in Appendix Table 5A.1. Prehistoric occupancy, date of European settlement (for uninhabited islands) or first date of trade with Europeans (for islands already inhabited), latitude, island elevation, island area, human population size, and island occupation history are taken from references described in Sax et al. (2002) and as cited in Appendix Table 5A.1. Native and naturalized richness were tallied from the literature after applying a standardized set of criteria to published work. These criteria defined native and naturalized plant species as those believed to have self-supporting populations, such that species believed to be ephemeral were not included. Further, species that are “cryptogenic,” i.e., possibly native or exotic, were excluded from these tallies. See Sax et al. (2002) for a complete discussion of these criteria and their application. Native and naturalized richness recorded in Appendix Table 5A.1 reflect the most up-to-date values available; these values differ slightly from those recorded in Sax et al. (2002), particularly where a more detailed accounting of historical records has provided additional information on species status.