I stood on the roof of a tall hotel near the Atlanta airport recently, trying to photograph airplanes as they descended on the runways of Hartsfield International. My shoes crunched in the loose gravel that coated and protected the hotel's roof. If you spend time on top of skyscraper roofs, you usually find a mix of stone and tar, along with the maintenance man's carelessly discarded pieces of hardware and spent cigarette butts. This day something unusual caught my eyea small clam shell the size of my thumbnail. I reached down and picked up the shell of an Asiatic Clam, Corbicula manillensis, the scourge of America's waterways. This small exotic animal is responsible for a variety of crimes in the U.S., from interfering with a nuclear power plant in Alabama to outcompeting native mussel species. Here, the nuisance had even somehow managed to reach the top of a 16-story building.
Where there's man, there's going to be a higher than usual number of introductions of exotics, animal or plant. Because of man's influence, intentional or unintentional, the urban environment is an experimental farm for exotic species, usually resulting in a mixed bag of effects on the environment, many poorly understood.
Technically, what is the difference between natives and exotics? "Native" species are species found dispersed in a region before the arrival of humans, according to Dr. William W. Bushing, an expert on the subject. Exotic or nonnative species are species that are known with reasonable certainty to have dispersed to a region or location due to direct or indirect human actions.
Because introduction of exotic species into the environment initially increases biodiversity, the general public often questions why environmentalists are worried about them. One way to understand scientists' concerns is to consider the effect of viruses on the human body, says Dr. Bushing. "Those viruses which are native to an area ... have been with us for decades or millennia. Our bodies have developed defenses against many of them, so they pose less of the health threat. However, when a passenger stepping off an international flight brings with them a virus not found naturally here, it may have serious effects. Our bodies have not adjusted their defenses to these new pathogens. We are far more seriously affected by these exotic viruses than those living where they naturally occur. An excellent historical example is the transmission of new diseases between early European explorers and the Native American populations in the New World."
Not all invasions are successful. The alien plant or animal may encounter what some refer to as "ecological resistance." New birds might find that the most suitable places to raise their young may already be taken by successful existing species.
But humans aid successful introduction of exotics by reducing native diversity wherever they settle, thus reducing natural competition. It's easier for Bermuda Grass to survive in your yard if you remove all the native plants, "weeds," and grasses that might compete with it.
Exotic species are introduced in a wide variety of ways. The Great Lakes region has been invaded by the Zebra Mussel, which came in ship ballasts from Europe around 1985. The ships fill their ballast holds with water at their departure site to increase stability. When they reach their destination, they empty their ballast, which contains many exotic organisms, including (in the case of the Great Lakes) the Spiny Water Flea and European Ruffe.
Zebra Mussels have spread widely in the Great Lakes and into other freshwater rivers and streams. The mussels, which filter a liter of water a day, have cleared up the murky water of the Great Lakes. On the surface, this appears to be a positive effect. But they consume tremendous amounts of phytoplankton and zooplankton, which make up the base of the food chain for the Great Lakes, and scientists are worried the mussels' appetites may cause a crash in the native fish populations that depend on plankton when the fish are juveniles. Also, many of the Great Lakes' poisons are filtered out of the water and concentrated in the mussels' bodies and feces pellets. Despite the Great Lakes seeming to be less polluted, experts worry that as the concentrated pollutions work their way into the food chain that native fish may absorb more toxins into their flesh than ever before.
Some exotics are introduced intentionally. One hundred Starlings were brought into New York City in the 1890s and released as a special effect during a Shakespeare play. Since then, their population has exploded, and Starlings are now one of the most commonly seen birds in the U.S. They have been blamed for a decline in many native bird species such as the Bluebird. Also, many pets, especially birds and fish, are released into the wild when their owners tire of caring for them, causing harm to the environment. Nonnative game animals, introduced intentionally through government programs and by private individuals in promotion of hunting and fishing, can also have a negative effect on native species.
The Asiatic Clam that I found on top of a building invaded Georgia's rivers and lakes in 1972. Today, if you find a small clam at your favorite fishing spot, chances are good that it's the Asiatic Clama Georgia immigrant for several decadesand not one of Georgia's threatened and endangered nativesbeing shoved out after thousands of years.
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