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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 » Pattern of Forest Clearing

Pattern of Forest Clearing
- Forest losses predict bird extinctions in eastern North America

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 forests are rich in such species and thus are likely to lose many of them following deforestation.

Although agricultural fields and human-created grasslands occupied hundreds of square kilometers in some areas of the eastern forest biome before European settlement (7, 8), clearing probably was not extensive enough to cause the extinction of forest bird species. More extensive deforestation followed European settlement in the early 1600s. To assess the complex patterns of forest destruction after 1600, we divide the eastern forest into four regions (Fig. 1) and examine each on three spatial scales.

At the smallest spatial scale, we report forest cover for individual counties or townships. These may not be typical of the region as a whole, but they provide the temporal detail missing from regional summaries. At an intermediate scale, we report on individual states that are geographically typical of the region. At the largest scale, we compile summaries for each of the four regions. Estimates of forest cover at this largest scale can only be approximate, especially for the nineteenth century. Yet, if the estimates of forest cover generally do show similar patterns across all three scales-and if we understand the reasons when they do not-then we will have confidence in our final estimates of forest cover for the entire eastern forest.

During the eighteenth and nineteenth centuries, regions near the Atlantic Coast were almost completely cleared for agriculture, leaving only small patches of forest in the form of farm woodlots (18, 19). Foster (19) provides detailed data for Petersham County, Massachusetts, that are similar to estimates for the entire state in showing how quickly the forest was cleared. The pattern of forest loss was similar for Concord, Massachusetts (20), and Onondaga County, New York (21). In the early nineteenth century, much of the Ohio Valley was deforested (9, 10). Again, the process was rapid and estimates for Wayne County, Ohio, closely match those for the entire state.

Destruction of the eastern deciduous forest was neither simultaneous nor necessarily permanent. An accelerating wave of deforestation spread from the Atlantic Coast to the edge of the western prairie, followed by a wave of forest regeneration caused by farm abandonment (Fig. 1). By the time >50% of the original forest had disappeared from Ohio, forests in New England had already shown substantial recovery. Later, when hardwood forests of Minnesota and Wisconsin had been reduced to a small proportion of their original area, forests in Ohio had begun to grow back (Fig. 1). For example, Cadiz County (on the local scale) and the hardwood forests of Minnesota (on the state-wide scale) did not lose much forest until the end of the nineteenth century (11, 12). The regional scale shows a different pattern, probably because of the growth of aspen and oak following extensive logging of coniferous forests in the northern parts. of the three Lake States (8).

In summary, forest clearing reached a peak in the late nineteenth century, when logging and agricultural clearing were particularly intense in the Lake States and the South (7, 8, 22). Between 1850 and 1909, 22% of the eastern deciduous forest was destroyed. (We derive this estimate from ref. 7.) Yet even during the peak period of deforestation in the late nineteenth and early twentieth centuries, there were large forest refugia that provided habitat for forest birds. After 1920, the amount of deciduous forest showed a steady increase in the Northeast (19, 23) and the South (7, 24), resulting from the conversion of primarily agricultural landscapes into landscapes dominated by forest.

According to our best estimates of changes in forest cover for the four regions (Fig. 2), we estimate that 48%-or roughly half-of the area covered by the eastern forest at the time of European settlement (1620) was still wooded at the low point in 1872.

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