  Within community ecology, philosophers have lately paid considerable attention to the theory of island biogeography and the controversies surrounding its relevance for the design of networks of biological  reserves.[56]  The basis for this theory is the species-area relation: larger areas of the same habitat type usually contain more species than smaller  ones.[57]  Thus there is a monotonic relationship between species richness and area. But, what is the form of this relationship?  Moreover, what is the mechanism responsible for it? In spite of sporadic work over almost an entire century, these remain open questions. Perhaps the most popular answer to the first question, but one that gives no hint of the operative mechanism, is a power law going back to  Arrhenius:[58] S = cAz, where S is the number of species, A the area, and c and z constants. This power law represents what is often called the “species-area curve.” Turning to the question of mechanisms, traditionally, the species-area relation was attributed to environmental heterogeneity. Larger areas were presumed to have greater habitat heterogeneity and could, therefore, host a larger number of species each with its own specific needs. In recent years the relation is more often attributed to the belief that larger areas can support larger populations of any  species.[59]  Thus fewer populations are likely to become extinct in a larger area than a smaller one in any specified time interval.  Consequently, on the average, more species are likely to be present in larger areas than smaller ones even if both started with the same species richness.