Wildlife: Animal species impacted by changing forests
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Last week I flew from Pittsburgh to North Carolina via New York City. The view from a window seat revealed much about man's impact upon the land, especially large tracts of forest.
From 30,000 feet, towns, farms, schools, malls, parking lots, rights of way and highways dot and bisect the forested landscape. For more than 30 years, ecologists have warned that fragmenting large chunks of forest was ecologically perilous to plants and animals that require large tracts of unbroken forest.
Scarlet tanagers, wood thrushes, pileated woodpeckers, cerulean warblers and worm-eating warblers, for example, are just a few of the deep forest birds that decline or even disappear when tracts of forest are broken into many small pieces. Ecologists call this "forest fragmentation."
Fragmented forests create conditions that cause lower nesting success among birds. Predators such as raccoons, coyotes, cats, and crows thrive along edges and are frequent nest predators, but they are less common deep inside large forests. Fragmentation creates "edge habitat," discrete lines along the borders of woodlands and fields.
Brown-headed cowbirds also prefer edges. Though not predators, female cowbirds lay their eggs in nests of other species such as warblers, vireos and indigo buntings. Cowbird hens can "parasitize" up to 35 nests during a single nesting season.
Furthermore, cowbird eggs hatch before host eggs, and cowbird hatchlings are larger than host chicks. This gives nestling cowbirds a head start on their nest mates. Cowbird chicks grow faster and usually fledge successfully while host chicks often starve.
To be safe, forest birds must nest well inside the edge. From the air, it is apparent that tracts of forest large enough to provide this buffer zone are increasingly uncommon.
On the other hand, many familiar species thrive in edge habitat. Robins, cardinals, song sparrows, and many common backyard species thrive in both directions from edges. Thus, edges benefit common species, while forest-dwelling species decrease in abundance as edge habitat increases.
The latest threat to large tracts of unbroken forest is the natural gas fracking industry that is proliferating in parts of Pennsylvania, West Virginia and Ohio. Drilling pads and access roads require clearing blocks of land. Pipeline rights of way required to transport the natural gas are the greater threat. Increasingly, they crisscross the landscape like giant spider webs ready to snare deep forest dwelling species. I wonder if 50 years from now, when the gas boom is over, it will have been worth the ecological toll.
First Published September 23, 2012 12:00 am