Forest losses predict bird extinctions in eastern North America.

Abstract

Claims that there will be a massive loss of species as tropical forests are cleared are based on the relationship between habitat area and the number of species. Few studies calibrate extinction with habitat reduction. Critics raise doubts about this calibration, noting that there has been extensive clearing of the eastern North American forest, yet only 4 of its "200 bird species have gone extinct. We analyze the distribution of bird species and the timing and extent of forest loss. The forest losses were not concurrent across the region. Based on the maximum extent of forest losses, our calculations predict fewer extinctions than the number observed. At most, there are 28 species of birds restricted to the region. Only these species would be at risk even if all the forests were cleared. Far from providing comfort to those who argue that the current rapid rate of tropical deforestation might cause fewer extinctions than often claimed, our results suggest that the losses may be worse. In contrast to eastern North America, small regions of tropical forest often hold hundreds of endemic bird species.As forests or other habitats are destroyed, the remaining habitat may be too small to hold viable populations of all the species that require it (1). Consequently, extinction follows habitat reduction. The often unmistakable destruction of habitat is vital to arguments about the global loss of species (2). With important exceptions, the species losses themselves are hard to document. We can estimate only imprecisely the total number of species an area holds. Indeed, we have names for only a tiny fraction of the planet's species (3). Our confidence in predicting species loss from habitat reduction stems from the relationship between the number of well-known species an area holds and its size. Those who point to the extensive reductions in the forests of eastern North America during the nineteenth century (4, 5) challenge this confidence. Historically, few of the region's '200 terrestrial bird species have gone extinct. Birds are well-known and we cannot plead ignorance of their extinctions. Do these observations refute the predictions of the species-area calculations (6) and so call into question fears about massive loss of species on a global scale?We review the history of deciduous and coastal plain coniferous forests in the eastern United States from European settlement to the present. Forest losses have been extensive, but they were not concurrent. In New England, for example, forests began to recover as deforestation-and many of the people who caused it-moved into the Ohio Valley. At the period of lowest forest cover, about half of the forest was gone. We also list the species of birds that became extinct and those that remain. Of the species found only in eastern North America, the species losses have been higher than we predict from forest losses. This region has surprisingly few rangerestricted bird species, however, and most species could survive elsewhere as the forests were cleared. Many tropical fore...

Species–area relationships always overestimate extinction rates from habitat loss

“…Currently, no such theory is available. The EAR curve is consistent with the concept of 'imminent extinction', which states that predictions of near-term extinctions due to habitat loss should focus on species endemic to the area of destroyed habitat 4,7,27,28 .…”

“…The most widely used indirect method is to estimate extinction rates by reversing the species-area accumulation curve, extrapolating backwards to smaller areas to calculate expected species loss. Estimates of extinction rates based on this method are almost always much higher than those actually observed [2][3][4][5] . This discrepancy gave rise to the concept of an 'extinction debt', referring to species 'committed to extinction' owing to habitat loss and reduced population size but not yet extinct during a non-equilibrium period 6,7 .…”

“…Most predictions of species extinction rates, including those in the Millennium Ecosystem Assessment, are inferred from applying the SAR to rates of habitat loss [8][9][10][11][12][13][14] . The wide discrepancy between the rates of species extinction predicted by this method and the extinction rates actually recorded, has fuelled a continuing debate about how to explain the discrepancy 2,4,[15][16][17][18][19][20] . The main issue is that, almost always, more species are left after a given loss of habitat than the number of species predicted to remain, based on the SAR.…”

Species-area relationships underestimate extinction rates

“…Thus, we selected two examples reporting well documented extinctions in well known taxa/areas. Forest losses and bird extinctions in North America are well documented (Pimm and Askins, 1995). Forest clearing in North America reached a peak in 1872, when only half of the area covered by the eastern forest at the time of European settlement (1620) was still wooded (after 1920, the amount of deciduous forest showed a steady increase in the Northeast and the South).…”

“…With a 50% loss of habitat, a value of z ¼ 0.15 predicts a 10% loss of species. Out of the 28 species of birds restricted to the North American forest habitat, 4 have gone extinct (Pimm and Askins, 1995), which means a 14% loss of species. Madagascar, one of the most important biodiversity hotspots at global scale, has lost about half of its forest cover since 1953.…”