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The distribution of bird species and the timing and extent of forest …


Biology Articles » Biodiversity » Forest losses predict bird extinctions in eastern North America » Conclusions and Some Implications for Tropical Deforestation

Conclusions and Some Implications for Tropical Deforestation
- Forest losses predict bird extinctions in eastern North America

These analyses of forest clearing strongly support simple predictions of consequent species losses based on species-area relationships. When there are discrepancies, the observed extinctions exceed the predicted ones.

Was habitat destruction the sole cause of these extinctions? Some consider that hunting or introduced diseases, not habitat destruction, exterminated the passenger pigeon and Carolina parakeet (39-41). More plausibly, hunting was so effective because the habitat fragmentation concentrated the birds. The pigeon was last collected in 1899, the parakeet in 1901, years when forest cover was near its minimum extent. Moreover, Bucher (42) argues that the extinction of passenger pigeons was a direct result of large-scale habitat destruction. Loss of breeding habitat is almost certainly the reason for the loss of the ivory-billed woodpecker. Its last stronghold in the United States was cleared in 1948 and there have been only sporadic sightings since (39). The loss of habitat is the reason for the endangerment of the red-cockaded woodpecker. The loss of winter habitat may have contributed to the extinction of the Bachman's warbler (36). However, its major decline was in the 1920s and could plausibly follow from the major forest clearings decades earlier.

Arguments about the causes of particular extinctions miss the point. There is nothing unique about the danger of hunting rare species-in fragmented habitats or of the loss of seasonally important habitats. These factors, plus the consequences of introduced competitors, predators, and pathogens, are typical explanations for extinctions in habitat fragments worldwide (43). Habitat loss is but one of many causes of extinction, causes that typically exacerbate its effects. Eastern forest extinctions are thus appropriate models of extinctions elsewhere. The failure of others, particularly Budiansky (5, 6), to draw the same conclusion is not just a matter of documenting the forest losses and listing the appropriate species. Of course, careful scholarship is essential, but the principal error is in choosing incompatible sets of rules. One can count all the species and all the extinctions for a region. The majority of the extinctions will be local ones and hard to document. Alternatively, one can count only the species restricted to a regionthat is, its endemics. The extinctions from this subset will be global by definition. To divide global extinctions by the total species list is obviously nonsense.

Calculations of extinctions following tropical deforestation usually consider the total number of species in an area, not its smaller number of endemics. Does this reliance on the total number of species inflate the concerns expressed about high extinction rates? It does not because many tropical areas are not only rich in species, they are also unusually rich in endemic species (44).

For example, the Hawaiian islands once held "135 species of terrestrial birds; all were endemic (45). The islands in the Old World tropics (Philippines, Indonesia) comprise an area half the size of eastern North America yet hold almost 20 times the number of endemic species of birds. Some islands have lost most of their forests and "30% of the endemic species are at risk of global extinction. The region allows the calibration of species loss from habitat loss using many species and we consider it elsewhere (44, 46). Tropical mainland regions are also rich in endemics. In Central and South America, regions smaller than the eastern forests house substantially more endemic bird species (47). For example, in the Atlantic coastal forest of Brazil and Argentina where >80% of the forest has been destroyed, some 70 of 199 endemics are at risk of extinction. The conclusion is that eastern North America lost few bird species because it had so few endemics to start with. This conclusion, combined with our knowledge of high tropical endemism in birds (and other taxa), supports the concerns about worldwide deforestation and species loss.

We thank Steven Beissinger, Thomas Brooks, David Ewert, Michael Rosenzweig, Edmund Telfer and three anonymous reviewers for comments. S.L.P. thanks the Pew Charitable Trust for support through his Scholarship in Conservation and the Environment. R.A.A.'s research was supported by funds provided by the U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station.


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