Subdivision of nature reserves and the maintenance of species diversity

Abstract

The roles of area—its size, shape and possible subdivision—in the conservation of species is an important but controversial topic^1–5. The fundamental assumption is that nature reserves act as habitat islands in an inhospitable sea of environment that has been modified by man, and thus that the empirical and theoretical findings of island biogeography are pertinent. Empirical studies of oceanic islands and continental habitat islands show that larger areas hold more species. The relationship between these two variables is such that a 100% increase in area produces roughly a 25% increase in species. Thus, two small habitat islands which have more than this 25% difference in their species composition, hold more species than a single habitat island of the same total area. This is probably due to microhabitat differences between these two smaller habitat islands. Yet, even against a constant ecological background, stochastic fluctuations (‘turnover’) of species due to random colonization and extinction⁶ will cause inter-island variation in species composition.