The Louisiana Environment
In the late 1930's, Tabasco tycoon and naturalist Edward McIlhenny brought thirteen adult Nutrias (Myocastor coypus) from Argentina to his home in New Iberia, LA. He released the animals into a large pen, hoping to introduce them into the Louisiana marsh after they had multiplied. Two years later, one hundred and fifty descendants somehow found their way out of the pen, ostensibly escaping during a storm. The nutrias reproduced at an astounding rate, increasing by the thousands every year. Estimates of nutria numbers in the 1960's ranged as high as twenty million -- and the population kept growing. By the time the government instituted a control program, the nutria was systematically eating Louisiana marshes to the ground, ravaging wetlands and causing widespread erosion. Today, sixty years later, the nutria is still one of our most common and despised pests.
The story of the nutria is not unique. Many
species of birds, mammals, fish, and plants have been introduced into the
Louisiana environment in the past two centuries. Exotic species, or species
that have been introduced to areas outside their native range, take heavy
tolls on the ecosystems they colonize. Some invaders, such as the leafy
vine kudzu (Pueria lobelia), destroy the habitat for resident wildlife.
Other species fiercely compete with native plants and animals for resources.
By some estimates, exotic species pose the second most serious threat to
endangered species after habitat loss. The Red Fire Ant
(Solenopsis invicta), for example, has replacd nearly half the native insect species in some areas it has colonized.
Why are invaders so incredibly successful? Exotic species tend to run rampant in their new environments partly because they lack the predators that keep them under control in their native range. Plants and animals that thrive in disturbed habitats are able to survive in most parts of the world. Aggressive and resilient, these species are often tolerant of climate changes that kill less adaptable native species. Additionally, the most successful exotic species are those with high reproductive rates. One parent tree of the genus Melaleuca, for example, is capable of producing thousands of seeds yearly.
Though ecologists have tried many methods of eradicating exotic species, most have proven to be ineffectual. Against plants, herbicides are apparently the most potent weapon. However, it is very difficult to poison one plant without rendering its entire ecosystem toxic. The use of pesticides against insects, mammals, or birds usually creates a deadly domino effect as the chemicals travel through the food chain. Physical removal of plants or animals is costly, time-consuming, and only temporarily effective.
Increasingly, scientists have come to depend on biocontrol -- essentially, the introduction of an exotic predator to control a pest. Often this method backfires; the predator ignores the target species and wreaks havoc on populations of native species. In some cases, however, biocontrol efforts have yielded unexpected successes. Though biocontrol is capable of doing more harm than good, it may prove to be the only solution to the complicated problem of introduced species.
Louisiana, like every state, has a host of exotic pests. Because of its tropical climate and its proximity to Mexico and Central America, Louisiana is able to support many invasive species that cannot survive elsewhere in the U.S. One such example is the Red Fire Ant (Solenopsis invicta). Native to South America, the Red Fire Ant has flourished beyond control in many southern states since its introduction in the 1930's. Superficially similar to most other ants, the fire ant is a vicious predator, attacking birds, rodents, and larger mammals in swarms. One study of White-tailed Deer (Odocoileus virginianus) found that death rates for young deer were twice as high in areas with fire ants than in uninfested areas. In Louisiana, the spread of fire ants has been linked to the decline of Loggerhead Shrikes (Lanius ludovicianus) and some species of warblers.
Some introduced species are able to survive only under certain conditions and so have difficulty in spreading over a wide range. Such is the case with the Monk Parakeet (Myiopsitta monachus). An agricultural pest in its native South American range, the Monk Parakeet was first brought to the U.S. as a cage bird. They were so popular that over 60,000 were imported between 1969 and 1972. By the 1980's it had already been released in many parts of the country and had established small breeding colonies.
Twenty years later, Monk Parakeet numbers have increased exponentially but their distribution remains spotty. Monk Parakeets tend to be restricted to urban areas where they feed and nest in ornamental palm trees, occupying a niche that no indigenous bird holds. So far, their distribution in Louisiana has been limited almost exclusively to New Orleans, where they have had no adverse effects on local wildlife. If their numbers increase, however, Monk Parakeets could pose a serious threat to agricultural areas, possibly becoming as much of a pest here as they already are in their native range.
One of Louisiana's most harmful exotic plants is the Water
Hyacinth (Eichhornia crassipes), which readily takes hold in
manmade structures such as irrigation ditches and artificial lakes. In
natural waterways near agricultural communities, it thrives on the constant
supply of nutrients from fertilizer runoff. The water hyacinth can reproduce
asexually, breaking into small pieces which can each form a complete organism.
This allows it to reproduce exponentially and disperse over vast areas
of the water surface. The plant forms thick green mats which fill reservoirs,
dam drainage canals, and infest rivers. This in
turn depletes the water's supply of oxygen, asphyxiating fish and other aquatic organisms.
The water hyacinth was first brought to the U.S. as part of the 1884 Cotton Exposition held in New Orleans, when the Japanese delegation distributed several plants imported from Venezuela. They proved to be popular gifts and were transported to garden ponds around the city. The hyacinths reproduced and quickly spread to neighboring waterways. It soon covered immense areas of Louisiana, clogging canals used for boating and fishing. Though still a common nuisance, water hyacinths have been considerably reduced by the introduction of insects of the genus Neochetina. However, in the case of the water hyacinth, biocontrol is not very effective when used alone. In most areas, heavy doses of herbicides are also needed.
Another abundant exotic plant is the Chinese Tallow (Sapium sebiferum), an Asian tree that escaped cultivation in the U.S. sometime this century. It is one of the most common trees in Louisiana -- a recent study of Bayou Sauvage found that tallows far outnumbered all native trees combined, with an average of nearly four thousand tallows per hectare. It grows fast and flourishes in disturbed areas such as roadsides, suburbs, and drainage ditches. Chinese Tallows are shade-tolerant and can grow in almost any damp environment (like Louisiana's bottomland forests and wetlands). Although individuals of the species are short-lived, the tallow's adaptability and resistance to floods will probably allow it to dominate Bayou Sauvage and other Louisiana ecosystems.
One of the most recent local invaders is the Eurasian Collared-Dove (Streptopelia decaocto). The doves were introduced to North America in 1974, when several captive-bred birds escaped from a breeder in the Bahamas. Descendants of these individuals appeared in southern Florida in the early 1980's, and spread quickly over the rest of the state. Eurasian Collared-Doves have since swept over the southeast so rapidly that until last year they were not included in most field guides to North American birds. Today they are most numerous along the Gulf Coast, with isolated populations in the Midwest and along the Atlantic Coast.
As of yet, little is known about the natural history of Eurasian Collared-Doves in North America. Their invasion took place so recently and happened so quickly that not even their food habits on this continent have been studied. No one knows, for instance, whether they compete with native doves such as the Mourning Dove (Zenaida macroura). Louisiana birders have taken the pessimistic view that all exotic species are harmful to the environment and have given the Eurasian Collared-Dove the unfavorable nicknames of "scuzdove" and "Eurotrash". Eurasian Collared-Doves are currently expanding their range north and west and increasing their populations. It seems likely that they will continue to colonize new areas of the U.S. and eventually become as common here as they are in Europe
Though most exotics are brought to the U.S. by people, a few species have managed to invade without human help. The Cattle Egret (Bubulcus ibis) has colonized the world, Louisiana included, in an amazingly short time. Native to Africa, this stocky heron first invaded South America before invading the U.S. via the tropics. The first U.S. specimen was photographed and collected in 1952. Nesting was reported in 1953, and by 1954 it was common over the Southeast and still increasing. It was first discovered in Louisiana on October 17, 1955, in Jefferson Parish. Three days later there was a flock of over 100 in the same place. Today, the Cattle Egret is abundant in Louisiana and in much of the rest of the United States.
Cattle Egrets have been less harmful than other exotic species, partly because of their unique foraging behavior. Unlike other herons, Cattle Egrets are not tied to aquatic habitats. Though they nest in swamps with native herons, for the rest of the year they lead a highly terrestrial existence. As indicated by their name, Cattle Egrets follow ungulates to catch insects flushed by the larger animals. This nearly eliminates feeding competition between Cattle Egrets and native herons.
During the nesting season, however, Cattle Egrets nest in colonies of thousands, usually with several species of native herons. In the northern U.S., the aggressive Cattle Egrets compete somewhat with resident species for nesting sites and nest material. In the southern states such as Louisiana, the breeding season is longer and Cattle Egrets nest late in the year, neatly avoiding conflict with early-nesting native herons.
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Alien Invaders of the South Region
Biological Control: A Guide to the Natural Enemies in North America
Biological Control of Exotic Aquatic and Wetlands Plants
Center for Aquatic and Invasive Plants
Exotic Species in the Great Lakes Region
Exotic Species in the National Parks System
Expansion of the Eurasian Collared-Dove
Fact Sheet: Invasive Species
MIT Exotic Species Web Page
Monk Parakeets as an Introduced Species
Nonindigenous Nonaquatic Birds of Special Interest
The Red Imported Fire Ant Fact Sheet
Water Hyacinth Information Page
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This page was last updated on 1/04/00