Perhaps the most underrated human threat to nature is our propensity to move organisms to where they do not belong. The result, sometimes called biological pollution, tends to homogenize and deplete ecosystems everywhere.

A classic example here in the Tennessee Valley is the zebra mussel. A fingernail-sized mollusk native to Russia, the zebra mussel is so-named because its shell is striped by alternating dark and light bands. In 1985 or 1986, a few zebra mussels were inadvertently dumped into Lake St. Clair near Detroit when a European freighter flushed its freshwater ballast. From there, their descendants hitchhiked down the Illinois River to the Mississippi, then up the Ohio and the Tennessee. Within the last few years, Zebra mussels have been observed as far upriver as Knoxville.

Zebra mussels reproduce at an astonishing rate, blanketing the muddy bottoms of rivers or lakes and coating any hard object they find. In Lake Erie, less than a decade after their introduction, researchers were finding as many as a hundred thousand per square meter of lake bottom. Since they also build colonies on top of other mussels, competing for their food and smothering them, they may virtually eliminate other mussels from the Tennessee River.

The zebra mussel explosion is not a natural phenomenon, but a disquieting side effect of rapid global transportation systems. It is, moreover, an instance of a larger pattern: unique native plants and animals are increasingly being pushed aside and replaced by tougher imports from around the world. This is happening not only in the water but on the land.

Many of the most prominent plants and animals in the Southern Appalachia are nonnative—and they are the same plants and animals that now occur in climatologically similar regions around the world.

Among these, perhaps the most familiar is kudzu. Kudzu was introduced into the United States at the 1876 World’s Fair in Philadelphia as a way to fertilize and loosen soil worn out by overfarming. Now thoroughly naturalized, it grows rapidly on disturbed land, smothering all other plants, from the tiniest mosses to the tallest trees.

More damaging still are the imported insects, fungi, and microorganisms that attack native plants. The Japanese beetle, now a prominent lawn and garden pest, is a familiar example.

Southern Appalachian forests are facing a mounting accumulation of these biological assaults. Logging, road-building, and other forms of human disturbance facilitate the spread of disease and invasive organisms by opening routes of infection into the heart of the forest.

The first massive disease outbreak was the chestnut blight, which was caused by a fungus imported early in the twentieth century with chestnut trees from China. By the late 1930s this blight had killed virtually all the mature chestnut trees of Southern Appalachia, depriving many forest creatures, including black bears, of the nuts that were until then a staple of their diet.

Many dogwood trees have been killed by dogwood anthracnose, a fungus thought to have been introduced with imported Chinese dogwood trees in the late seventies. This is a foreboding loss, since dogwoods are prime soil builders and important sources of high-protein fruit for migratory birds.

Beech trees are ailing from two invaders, an insect and a fungus, which work together. The beech scale (the insect) feeds on bark, leaving tiny holes which provide entry for the Nectria coccinea fungus. Both were introduced into Nova Scotia around 1890 and progressed southward, reaching the Smokies in 1993. Stands of dead beech are now common there.

Many elms have fallen to the Dutch elm disease, which was introduced into the United States in 1930.

Butternut trees have been decimated by the butternut canker, a disease that first appeared here in 1967. In the years since, it has killed ninety percent of the butternut trees in the Southern Appalachians.

After the chestnuts died, oaks took their place. Now the oaks too are suffering from invasive pests. The Asiatic oak weevil has infected many, and all are likely soon to face the devastating attack of the gypsy moths.

Introduced from Europe into Massachusetts sometime between 1867 and 1869, gypsy moths have advanced gradually down the Appalachian Mountains, defoliating forests as they come. They are especially fond of oak leaves. Infected forests literally crawl with caterpillars, which at times drop from the trees like rain. By the time the caterpillars spin their cocoons, few leaves remain. A few gypsy moths have already reached the Smokies, but there have been no serious infestations yet. When the main "front" moves in, probably within a decade, yet another major stress will be added to our already beleaguered forests.

In 1993, a related invader, the Asiatic gypsy moth, was inadvertently introduced into North Carolina by a munitions ship docked near Wilmington. More voracious and faster-spreading than its European counterpart, this moth could do even greater damage.

Sometimes an invasion has ramifying effects across whole ecosystems. The balsam woolly adelgid, an insect carried here on plants from Europe earlier in the century, has almost eliminated Fraser fir trees from the high Southern Appalachian mountains, which are their sole habitat. Ninety-one percent by volume of the mature Fraser firs are dead. Only a few small stands are left, and these are infested.

Because of the complex interdependence of other species with the Fraser firs, their elimination has initiated a cascade of further declines. The most prominent side effect is the blowdown of red spruce trees, some over two centuries old, which used to be shielded from the wind by the Fraser firs. Other effects are less obvious. At least eight specialized species of moss and liverworts grow mainly or only on the bark of Fraser firs. As the firs die, these tiny plants die with them. Moreover, as the trees die, sunlight penetrates the forest canopy, drying out the moss mats that once dappled the moist forest floor. And the loss of the moss in turn threatens other species, including the spruce-fir moss spider. Where this cascade of effects will end remains to be seen.

The balsam wooly adelgid has recently been joined by the hemlock woolly adelgid, an import from Asia so destructive that it may kill every eastern and Carolina hemlock tree that is not actively protected by pesticides. The loss of the hemlocks is expected to initiate further cascades, since the eastern hemlock is crucial habitat for already-stressed neotropical migrant birds and an important component of streamside ecosystems, providing cooling shade and vital nutrients to the flowing waters.

These are merely examples. The problem is global in scope. It is as if we had thrown all the world’s species into a blender and turned on the juice, mixing competitors, diseases, parasites, and predators from each unique ecosystem into all the others. Delicate, unique, and rare species are too fragile to survive this treatment and are being lost. And, across the earth, tough, aggressive, and weedy species are moving in to replace them. Thus we blur the outlines of Creation, leaving a homogenized, standardized, scrambled, and depleted world in its place.