The Meaning of z in Species/Area Regressions and the Study of Species Turnover in Island Biogeography

Abstract

A simple model system involving 10 species capable of colonizing seven islands of different area was set up under various restrictions of the species' ability to persist in terms solely of island area. The slope z of the log species/log area regression is -0.25 when five of the species can persist on the smallest island, and -0.13 when seven of the species persist on the smallest island. Use of additional model data on dispersal abilities, habitat requirements and competitive interactions can also generate slopes commonly found in the island biogeographic literature. I conclude that z has no unique biological meaning and therefore adds little to our understanding of species richness of insular floras and faunas. A more fruitful approach would involve studying the biology of individual species, establishing their requirements for minimum area and habitat and determining which factors restrict their overwater dispersal. The difficulties of detecting genuine species turnover are outlined. These include bias and thoroughness of each compiler or collector, assumptions about how 'missing' species should be treated, scale (the taxon in relation to island size), the necessity for providing information on the accumulation of species per unit of collecting time, and the need to restrict attention to breeding species. Most genuine species turnover of birds and mammals seems attributable to human effects, though this is not so with plant species.