Winged messengers: does habitat loss signal biodiversity's death knell? Destruction of ecological living spaces is the greatest threat to the survival and preservation of the globe's wide array of animal and plant species.

• Author: Youth, Howard
• Position: Ecology

DURING THE 20TH CENTURY, the human population mushroomed from 1,600,000,000 to more than 6,000,000,000. Settlers fanned out along spreading webs of roadway, chiseling settlements into frontier areas. Industries grew, increasing demand for natural resources. Commerce broadened among nations. The rapid changes transformed once-extensive wilderness into precarious habitat islands. Today, loss or damage to species' living spaces poses by far the greatest threat to the survival of birds and the preservation of biodiversity.

Looking at a current map of the world's biomes (major climate--influenced ecological communities, such as deserts and tropical rainforests) gives more a picture of how things were a few centuries ago than how they are now. Timber operations, farms, pastures, and settlements already have claimed almost half of the world's forests. Between the 1960s and 1990s, about 4,500,000 square kilometers of the globe's tropical forest cover (20%) were cut or burned. Estimates of deforestation rates vary, from 50,000 to 170,000 square kilometers per year. Perhaps easier to track are dwindling populations of creatures that must live in and beneath the trees: habitat loss jeopardizes 1,008 (85%) of the world's most at-risk bird species, with recent tropical forest destruction affecting 74% of these.

Ecologists herald the regrowth of temperate forests as an environmental success story, and in recent decades substantial reforestation has taken place in, for example, China, Europe, and the eastern U.S. Forest management profoundly affects diversity and natural balances, however, and satellite images of tree cover do not tell us how much of the new forests are quality habitat. In the southeastern U.S. over the last five years, for instance, more than 150 industrial chip mills have chewed up vast tracts of natural forest to produce paper, rayon, and pressboard. Foresters replace the clearcut area with rows of sameage, same-species pine saplings. For numerous native animals and plants, simplified plantation monocultures are no substitute for more complex natural forests, with their mix of old, young, living, dead, deciduous, and coniferous trees, along with lush, varied undergrowth. The consistent loss of some forest components can cause birds to abandon areas. Studies in intensively managed Finnish woods, where foresters remove older and dead trees, revealed marked declines in large birds such as the capercaillie, a peacock-sized grouse, and the crow-sized black woodpecker.

To perpetuate the wide array of bird species, conservationists need to preserve a multitude of habitats, including transitional stages of plant succession. Losses of these areas have been less publicized, but no less dramatic, than the destruction of mature forests. North America's eastern towhees, brown thrashers, yellow-breasted chats, and other thicket songbirds require shrubby growth that springs up after a field is idled. This tangled habitat prevails for less than a decade before forest closes in. Today, several shrubland species are disappearing as abandoned farm fields and other transitional areas are swallowed up by development or are fully reclaimed by...