While Hollywood blockbusters focus on the threat of extraterrestrial invaders, a much quieter and potentially much more damaging series of invasions has been taking place in the southern United States for centuries. Exotic species, species which are introduced to and established in areas formerly outside of their natural range, are now recognized as a serious ecological and economic problem on nearly every continent on the globe.

When exotic species establish themselves in a new habitat, they may not have any natural predators to keep them in check. An introduced moth, for example, may be controlled by parasitic wasps or other predators in its natural habitat; but if it is introduced without its predators to a new environment it may spread virtually unchecked. As they spread, the newcomers may negatively affect other species. The huge leaves of the introduced miconia tree in Hawaii, for instance, cast a deep shade on the forest floor which prevents many native plants from growing. The aggressive European starling out-competes the native bluebirds for nesting sites in trees. Several fish which have been introduced into new environments to develop sport fisheries, have instead driven down the populations of native fish. The flathead catfish was introduced into the Colorado River as a sport fish but is driving out the native fish species which were already present. Exotic animals and plants may also transport new parasites, or diseases such as rabies, which may devastate native flora and fauna.

Exotic species, like the zebra mussel, may cause direct economic as well as ecological problems. This bivalve was introduced accidentally into the Great Lakes and has spread rapidly throughout the Mississippi and Ohio River watersheds. This mussel is a biofouler, that is it attaches in huge numbers to any exposed solid surfaces including the intake pipes for waterworks and industry. Treatment and cleaning of fouled pipes now costs industries in the Great Lakes millions of dollars a year, and the costs are likely to climb as the zebra mussel spreads across the southern United States.

As humans have become more mobile and spread over the earth, we have both deliberately and inadvertently transported a host of animal and plant species to new habitats. Wheat, corn and other major cereal crops have been transported around the world, while the grasses in the typical southern pasture are introducing species that were not found there a mere 200 years ago. It may be argued that the introduction of new food sources to other areas has had positive effects, but many of the other exotic species have no redeeming qualities. Rats introduced to islands may devastate ground-nesting birds; aquatic plants may choke waterways and lakes or exotic terrestrial plants may out-compete and displace native species. For example, the introduced chestnut and elm blights have changed the face of America's forests, perhaps forever.

The sheer number of exotic species in the US illustrates the magnitude of this problem. The Nature Conservancy estimates that there are 4,000 non-native plants and 2,300 non-native animals presently established in the United States. Some 205 new exotic species have been discovered in the United States since 1980, while 36 of the 300 weeds in the western US were introduced deliberately as crops or horticultural species. Exotic species cost the US economy 3 billion dollars in lost agricultural productivity and nearly 2 billion dollars in pesticides yearly. These aliens negatively affect about 42% of the species presently on the US threatened and endangered species lists. What is it about exotic species that make them so problematic from an ecological perspective?

The temperate climate of much of the southern United Sates has made this region an attractive environment for many alien invaders. The three examples detailed below are fairly representative of the broad array of exotic plant and animal species now making their homes in the southern US and the problems or benefits they provide.

Exotic species are both an economic and ecological problem for all countries. As transportation has become more efficient and democratic, and as the globalization of economies continues, the number of introductions and the damage they cause is likely to increase. It is impossible for most nations to seal their borders to alien species, but it is possible, through increased public education, to inform people about the threats posed by exotic species and to educate them about how they can help control the real alien invaders.

1. Florida's Transition Tree

The waterways of Florida have become a new home to several aggressive exotic plants. In the Everglades, the Australian melaleuca tree has spread over 450,000 acres of former wetland and is expanding its range by up to 50 acres a day. The melaleuca was deliberately planted in the Everglades earlier in this century as part of an aggressive campaign to turn wetlands into agricultural lands. The melaleuca removes a tremendous amount of water, drops litter which clogs the wetland, cannot be harvested for timber and is fire-resistant: in short, the perfect tree to "tame" the Everglades.

2. Use of Biocontrol

The use of natural predators to control pest species is called biocontrol, and it has a number of advantages and disadvantages. The insects are much more effective at finding plants than human spray operators are and they apparently feed only on the plant in question (such as the water hyacinth). Such specificity is rare for herbicides.

1. Melaleuca Problems

As water management strategies have changed in the Everglades and the conservation of biodiversity has become more important, the melaleuca has posed a special challenge to managers in the park. The tree has no insect predators in the US and insects are not attracted to the trees, so the huge stands of melaleuca trees support few of the insect-eating animals that the native vegetation does. The tree proved too difficult to harvest, so the timber production benefits never materialized. Park personnel and volunteers comb the huge stands killing the parents with herbicides, but the trees produce thousands of seeds which are coated with a thick shell which resists penetration by the herbicides. Although the parent tree can be killed, thousands of seedlings may sprout in its place.

2. Water Hyacinth Take Over

Another alien plant which invaded Florida in the last century is only now coming under control. The water hyacinth was introduced to Louisiana as part of the 1884 Cotton Exposition and spread rapidly to Florida. The water hyacinth produced dense mats of vegetation floating on the water and it greatly altered the aquatic environment by blocking out the sun, choking water channels and depleting oxygen levels. It also caused tremendous headaches for transportation and fishing in Florida. The huge mats of water hyacinth blocked barge and boat traffic on the rivers, literally immobilizing boats. When chemical herbicides became widely available after World War II, an intensive spraying program began. But despite the program, the hyacinth still covered thousands of acres of waterways by the end of the 1960s.

Today, however, the water hyacinth is only found in a tiny fraction of the waterways it once dominated. This is partly because of continued aggressive spraying of herbicides in some areas and partly due to the ravenous appetites of three species of insect introduced from South America, home of the water hyacinth. These insects are the predators of the water hyacinth in its native environment, and they have worked in conjunction with chemical treatments to control the water hyacinth. There is some concern about the introducing a species to control an exotic species; there is always the risk that the "cure will be worse than the disease."

3. Biocontrol Damage

An example of the back-fire effect of biocontrol is the mongoose which was introduced to control rats that had been released on tropical islands. While the mongoose did feed on the rats, it also fed on ground-nesting birds and other native species on these islands as well, doing more damage to the native animals than the rats might have. The Department of Agriculture researchers who released the biocontrol insects took special precautions to quarantine them to ensure that the right species were being released, but there is always the risk of unintended consequences.

Another example, the rat that ate Louisiana, as described by Annin, is a large (up to 15 pound) rodent native to South America who is spreading rapidly throughout the bayous of Louisiana. The nutria was introduced in the early 1900s to develop a fur industry in the state, but they escaped from their cages during a hurricane and rapidly spread through the state. At first, they were heavily trapped and helped support a bustling fur trade which peaked at about 1 million pelts per year; but fashion mores changed and fur was out. Without the control of trapping, the nutria populations exploded reaching densities of 6,000 individuals per square mile in places. The ravenous rodents can graze down nearly all of the bayou vegetation, leaving just open water. Without the floating vegetation present, habitat is lost and storm damage is worsened. The nutria grow so large that they are virtually immune to predation in the swamp except from the largest alligators. Damage has been so extensive in Lafitte National Park that park managers are poised to permit hunters to kill the nutria. This has led to a pubic outcry by some groups who oppose the hunting of the animals, especially within national parks. With no other viable means of control, however, the park service may be in the awkward position of sanctioning hunts to eradicate the animals. In a unique Louisiana twist, some chefs have been experimenting with various nutria dishes, trying to create a market for the meat. While new "haut-cuisine" may not be a universal answer to the problem of exotic species, it does highlight the fact that complex problems often require unusual solutions.

4. Translocating Non-native Species

"Fiery" Aliens insects have been among the most damaging of all exotic species. The gypsy moth, introduced in an abortive attempt to start a North American silk industry, has defoliated millions of acres of forest in the Northeast, while the Japanese beetle ravages fruit trees and home gardens over much of the southern US. In terms of direct interactions with humans however, perhaps no insect is more unwelcome than the fire ant.

The US had several native species of the fire ant, but these populations were relatively non-aggressive. In the early 1900s, however, ships from Paraguay docked in Mobile, Alabama and they unloaded two species of fire ants which were "hitch-hiking " in the cargo. The ants spread rapidly in their new home and they are now found across 250 million acres throughout the South, from Texas to Florida and north to Tennessee and Virginia. Aggressive and without natural predators here, the most aggressive of the fire ants, Solenopsis invicta, is now fifty times more abundant in some areas of the US as they are in South America. The fire ant gets its name from the intense burning sensation that accompanies its sting. In the last few years, an average of 2.5 million people a month have been stung by fire ants and 10,000 people a year have to receive medical attention because of their stings. The ants are capable of doing tremendous damage to potato and grain crops in fields. They also have a tremendous negative impact on native ant species; at one field station, they reduced the number of native species by almost 95%. They kill small mammals, such as mice, and may drive away birds by reducing the local insect populations. In Texas alone, a reported $750,000 a year was being spent to treat farm animals or pets injured by fire ant attacks.

The ants form huge colonies whose tunnels can reach six feet into the soil and which may contain 200,000 ants per colony. Some groups of fire ants may form multi-queen supercolonies which can reach 500,000 members. With each queen laying up to 2,000 eggs a day, the ants can quickly reach huge numbers and maintain their populations even if many are killed by control measures.

A variety of methods have been tried to control the ants. Some insecticidal baits will kill colonies if they are distributed by workers within the nest, but these baits may also harm non-target species as well. Direct-contact insecticides are not very effective at penetrating the labyrinthine tunnel system in the nest. Eradication attempts using pesticides have cost more than 200 million dollars and not accomplished any permanent control of the fire ant. In fact, large scale spraying of pesticides may have hastened the spread of the fire ant as they tended to recover much faster than their competitors and potential predators did.

Some biologists are convinced that one of S. invicta's natural predators from South America may be the control agent they have been looking for. The parasitic fly, Pseudoacteon litorlais, lays its eggs on the bodies of the ants. The eggs hatch and the larvae feed on the ant, eventually killing it. While the flies probably do not directly kill all of the members of an ant colony, their mere presence disrupts the ant colonies and reduces the colonies' feeding and reproductive success. Supporters of the introduction say that using natural predators is low-impact solution to the fire ant problems: no poisons, no effort spent locating every nest in an area, just release the flies. Critics of this plan counter that we do not know enough about the fly's biology. They are concerned that once the fly is here, it might just as easily target some of the native ant species as well, which are already losing ground in the face of competition from the fire ant. There is also a concern that the other pest parasites or diseases may "piggy-back" into this country in association with the flies, much as the ants came in association with shipping. Such disagreements are common among scientists when the possibility of biocontrol of exotic species is discussed.

Texas Fire Ant

This page provides background on efforts in Texas to control the fire ant.

National Park Service

This page details efforts by the National Park Service to control exotic species in our parks.

Impact of Exotic Species

This site has several article exploring the impact of exotic species.

References

Annin, P. "The rat that Ate Louisiana." Newsweek 121: 67, 1996.

Anonymous. "Dirty Dozen." US News and World Report 121: 12, 1996.

Carpenter, B. "Biological Nightmares." US News and World Report 119: 85-88, 1995.

Grisham, J. "The Attack of the Fire Ant." Bioscience 44: 587-592, 1994.

Lee, J. "Scuttling Water Hyacinths." Agricultural Research 44: 14-15, 1996.

Mills, E., Leach, J. et al. "Exotic Species and the Integrity of the Great Lakes." Bioscience 44: 666-669, 1994.

Raloff, J. "When this Fly Arrives, Ants' Heads Roll." Science News 146: 358, 1994.

Schmitz, D. "Diversity Disappears in Florida." Newsweek 125: 14, 1995.

Wesley, T. "Ant Advance." Horticulture Magazine 73: 16, 1995.