Tennessee's Rich, Yet Fragile Web of Life
Dr. Daniel Simberloff
Tennessee contains an extraordinary wealth of species, reflecting its many distinctive habitats. Due to its temperate climate and lack of glaciation, Tennessee is considered by scientists to be the biologically richest inland state in the nation. Remarkably, little of this staggering biodiversity has been studied scientifically.
Our high biodiversity rests not only on the diversity of our habitats - floodplains of large rivers, high mountain ranges, glades, lakes, forests, caves, grasslands - but on the fact that most of them are patchily distributed, either by the nature of the habitat or by the geography and history of Tennessee. Thus the state is a patchwork quilt of habitat "islands," which is precisely the landscape that scientists recognize as fostering high biodiversity. Sites isolated from others of similar habitat can allow the evolution of distinctive features in their populations. And, if sites are large enough, they can allow these populations to persist indefinitely in the absence of human activities.
Tennessee is best known for its high diversity of freshwater animals - fishes, mussels, crayfish, amphibians. Our state encompasses 19 river basins, and we are particularly blessed with small, high-gradient streams, such as those in the Smoky Mountains and Cumberlands. These are again isolated from one another in their headwaters. This insularity and the absence of great geological change over the last 100 million years probably accounts for the great species diversity. This region is a center of evolution with many endemic species, although for some groups, like many invertebrates, over 50% of species are not yet described by scientists. Unusual red algae and diatoms are often prominent members of the plant community, but fishes are best known. Minnows, darters, and catfish (including madtoms and bullheads) comprise the most species, but trout, especially brook trout, are often the ecological dominants. Despite the widespread attention given to the aquatic biodiversity of Tennessee, some entire habitats are poorly known.
Consider, another example, caves. The Southeast has more caves than any other part of the United States. Tennessee has 7,800 known caves scattered largely in the center of the state. Although some caves are linked by either subterranean passages or water courses, for the organisms inhabiting a cave, it is usually as isolated as an oceanic island, and populations trapped in them may have little or no contact with other populations of the same species. The cave environment - limited food, constant chemical and physical features - leads to the evolution of distinctive species found nowhere else, combined in unusual biotic communities.
Similarly, limestone and sandstone rock outcrops are common in Tennessee and constitute rocky islands in a sea of forest for the plants and animals that inhabit them. Some cedar glades on limestone in central Tennessee are well studied for plants and several animal groups and are remarkably diverse, with over 400 species of invertebrates, six salamander species, eight species of frogs and toads, 12 snake species, five lizard species, and two turtle species. One small glade alone had 79 species of birds. Among the plants, many are endemic - found only in this region and presumed to have evolved there.
It is not widely recognized that Tennessee contains many isolated prairies and other small grasslands, and these have plant species typical of the Great Plains, Ozarks, and other distant regions. Some are remarkably rich - one 50-acre prairie on the Highland Rim has 331 plant species and subspecies, of which 100 are highly characteristic prairie plants. Small grassy barren areas near Oak Ridge and on the Cumberland Plateau contain about 200 plant species and subspecies. Grassy "balds" in the Smoky Mountains have almost twice as many plants. Although animals have not been well-studied on most of these grassy habitats, they are diverse and sometimes important. For example, on balds on Roan Mountain, 13 mammals and 38 birds of national conservation significance were recorded, including the meadow jumping mouse, Bachmann's sparrow, and the grasshopper sparrow.
High elevation forests, especially in the eastern part of Tennessee, are another isolated habitat typified by high biodiversity of certain types of species. High ridges, separated by valleys and passes from other ridges, are dominated by red spruce and Fraser fir (though the latter is disappearing everywhere), and superficially they appear both species-poor and similar. However, over 200 species of mosses and a similar number of lichens occur in these forests, some of them endemic to the Appalachians. Among animals, the endemic imitator salamander and pygmy salamander are found only in these high elevation forests.
Though Tennessee is among the richest of all states in diversity of many groups of plants and animals, it is, perhaps, a leader among the continental states in the degree of threat to this biotic wealth. We have more than 300 globally imperiled species. Tennessee has the highest percentage of declining breeding birds - the rapid loss of neotropical migrants on the Cumberland Plateau is particularly striking. We are one of the four states with the highest proportion of threatened aquatic species. At least 17 plant and animal species are known extinct in recent times, and another 15 are feared lost as well. This is a significant fraction of the approximately 500 species believed to have disappeared from the entire United States recently.
The very features that make Tennessee a biodiversity leader also make its biota especially vulnerable to many threats. Patchily distributed, insular habitats are key to the great number of species. But such habitats are also often inherently fragile, and humans seem particularly prone to disregard their beauty and biological significance. Freshwater is easily polluted by chemicals or overwhelmed by sedimentation. Dams, ditching, drainage, and dikes can wreak havoc with waters and wetlands. Caves, and highly fragile ecosystems they harbor, are prone to disturbance and vandalism. The mosaic of barrens and glade habitats exist on shallow soil which protects them from timbering and agriculture. However, they are now targeted for residential and industrial development. Massive clearcutting of hardwood forests is occurring in much of eastern Tennessee, even on steep mountain sides. To the extent that reforestation occurs, it often consists of conversion to pine plantations, with unknown impacts of the animals and plants that typified the original communities. Air pollution and acid rain are major threats to many of the richest montane areas of eastern Tennessee.
Finally, exotic species have invaded Tennessee as they have many other states and rival habitat destruction and pollution as threats to biodiversity. Only in Hawaii and Florida is their impact more severe. All habitats are affected. The zebra mussel converts soft-bottom habitats to masses of shells, and it filters massive amounts of water, removing food particles that other species would have used; it threatens the very existence of dozens of native mussel species. Feral hogs ravage areas of the Great Smoky Mountains National Park and environs, destroying many plant populations, generating erosion, and changing the course of plant community succession. The balsam woolly adelgid, an-aphid like insect, has destroyed most of the Fraser fir trees that used to dominate high elevation forests, leaving scenes of destruction that resemble forest fires even in the Great Smoky Mountains National Park. Our large hemlocks are now threatened by the invading hemlock woolly adelgid, while the gypsy moth has recently reached the northeastern corner of Tennessee, a particularly ominous threat to oaks. Introduced plant pathogens such as dogwood anthracnose, butternut canker, and beechbark disease threaten other important trees.
We have a priceless biological heritage, far surpassing that of most other states. But this heritage confers on us a great responsibility, one that demands increasing efforts in the face of burgeoning threats. The great number of threatened species and ecological communities can easily induce a feeling of despair, and there is little doubt that some of our species will be lost despite our best efforts. However, solutions are possible for all of these problems, with sufficient research, resources, and commitment. If we want Tennessee to retain its distinctive ecological wealth, we will have to fight for it.
by Daniel Simberloff
Department of Ecology and Evolutionary Biology
University of Tennessee, Knoxville
and Tennessee Chapter Board of Trustees Member of the Nature Conservancy
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