HABITAT FRAGMENTATION AND POPULATION EXTINCTION OF BIRDS

It has not been established that a major cause of extinction in birds or any other taxa is failure of metapopulation dynamics: the collapse of a network of ephemeral but discrete populations as movement between them becomes increasingly infrequent. The few data on who goes where and who mates with whom suggest that most species are structured as ei ther a single large population or a small set of source populations an d a larger set of sinks. The extinction of the latter is irrelevant to the persistence of the species. However, regional decline of a specie s in the face of habitat destruction and fragmentation can mimic a fai lure of metapopulation dynamics, because distinct aggregations of indi viduals will disappear much as they would if populations in an interac ting network were eliminated one by one. Any species with highly restr icted range is at great risk of extinction from spatially localized fo rces, such as cyclones or deforestation, Restricted range rather than inherent weakness is the main reason that so many island species have gone extinct or are endangered, Species with small populations in cont act with much larger heterospecific ones with which they are interfert ile are threatened with extinction by hybridization. Finally, the disa ppearance of a species from a site may be due to subtle habitat change , even if this observation seems superficially consistent with some ge neral population theory, such as the dynamic equilibrium theory of isl and biogeography. Current theory is an inadequate substitute for inten sive field studies as a means to address the conservation problems of individual species.