Introduced and exotic species

Definition: Species of plants and animals that have been inadvertently displaced or purposefully transplanted from their native geographic ranges to other areas

Exotic species often disrupt the functioning of the ecosystems into which they have been displaced or introduced.

Introduced and exotic species

An exotic species is one that has been displaced from its native or original geographic range to another area, generally by human activity. An introduced species is one that has been transplanted for some intended purpose. All introduced species may be considered exotic, but not all exotic species are introduced. Natural range extensions, although they may have been assisted indirectly by human activities, do not qualify for either category, and a species transported by a natural event (such as a storm or flood) may be considered exotic for a time while its fate is being determined by natural selection. Transplants may be accidental or purposeful. Accidental transport is usually suspected to occur by ship or other means of commercial conveyance.

Since most ecological communities have evolved containing unique assemblages of species with their own unique systems of interactions and interdependencies, seldom can new species be added to a community without causing one or more significant disruptions in some community pattern (such as competition, the food web, or predator-prey relationships). While some introduced species have been beneficial to their new communities, many have had one or more detrimental effects.

Asiatic Clam

The Asiatic clam (Corbicula fluminea) is an example of an exotic species that has had damaging impacts. The clam was apparently accidentally deposited from a ship’s bilge near the upper West Coast of the United States in the mid-1930s. From there, it spread rapidly into virtually all the river systems of the West Coast. By the late 1960s, it had spread over all major portions of the lower Mississippi River drainage, and by 1983 the Asiatic clam had infested nearly all rivers in the western and southern states and most of the eastern half of the United States.

The Asiatic clam has exhibited two significant detrimental effects. Its high reproductive capacity, its somewhat mysterious ability to disperse rapidly (even upstream), and its rapid particle-filtering rate give the Asiatic clam the ability to compete with native species of fingernail clams (Sphaerium spp.). In the Saline and Ouachita rivers of south Arkansas between 1976 and 1988, the Asiatic clam virtually eliminated the fingernail clam.

The second effect arises from the clam’s affinity for hiding in dark places, particularly water-intake structures. If eggs or small juveniles are drawn into water systems having some minimal flow, they may attach, filter particulates, mature, reproduce and cause occlusion of the pipes. During the 1970s and 1980s, this characteristic of the clam caused considerable consternation among industrial plant managers and necessitated significant monetary investment in the control and subsequent prevention of Asiatic clam reproduction.

Zebra Mussel

Another problematic exotic species is the zebra mussel (Dreissena polymorpha). Larvae of the mussel were apparently flushed from a ship’s ballast in St. Clair Lake (on the border between Michigan and Ontario, Canada) in the mid-1980s. Subsequent studies have revealed one of the most phenomenal dispersal rates of all time. By 1991 the zebra mussel had been found in the St. Lawrence River and the Hudson River, and by 1992 it appeared in the Mississippi River system. By early 1994 it had spread to virtually all tributaries of the Mississippi system navigable by commercial ships or barges.

Whereas the Asiatic clam is a more passive filterer and prefers smaller streams with sandy or gravelly substrate into which it can burrow, the zebra mussel is a more positive, pumping filterer that prefers large rivers and lakes with hard, rocky substrate to which it can attach. The zebra mussel’s dispersal pattern coincides with previously unoccupied freshwater niches in North America. Consequently, the zebra mussel disperses more rapidly than, and successfully competes with, native species of freshwater mussels, which are more passive filterers. Since zebra mussels must attach to hard surfaces, they often attach to each other, producing two or more layers of mussels. Adult densities up to 100,000 per square meter and veliger densities up to 350,000 per cubic meter have been reported.

Several concerns have arisen regarding the zebra mussel. Because of the species’ positive filtering mechanism and high densities, zebra mussels may remove a significant portion of the plankton and particulates from a water column, thus robbing other species of the resource. The zebra mussels’ need to settle and attach to hard surfaces has made the mussels that are native to much of the eastern two-thirds of the United States particularly vulnerable to being covered by zebra mussels because the partially exposed shells of mussels are often the only hard substrate in the lower reaches of most of the rivers of the region. When a native mussel is settled upon by zebra mussels, it is partially smothered, so it pulls itself farther out of the substrate, only to be covered more extensively by the zebras. Finally, the zebra mussels may completely cover the larger mussel and smother it. This is a source of grave concern in all aspects of the shelling industry and involves several nations.

The zebra mussels’ need to settle on hard surfaces also increases the probability that they will be transported on the hulls of sporting boats from large rivers to recreational lakes and reservoirs. Resource managers in North America are concerned that zebra mussels will significantly alter additional biological communities, reducing their value for sport fishing and possibly other types of water recreation.

Exotic Fishes

More than fifty fish species of foreign origin have been introduced into North American waters. Most of the introductions have been done intentionally to provide additional food or sporting opportunities. Among the most notable are the common carp (Cyprinus carpio) and the grass carp (Ctenopharyngodon idella). The common carp possesses such an omnivorous talent and high reproductive rate that it quickly became well-established in all types of aquatic habitats, preferring larger rivers and lakes. In its European homeland, it is prized for food, but in North America, people are more interested in other, more desirable native species as food sources. On the negative side, the common carp’s tendency to disturb sediments and destroy vegetation while feeding causes problems with high turbidity and community disruption. For a time, resource managers feared the carp would overpopulate and outcompete some native species.

Great caution accompanied the grass carp’s introduction in the late 1960s. There were still concerns about the common carp’s potential, and many feared that the grass carp would adapt and reproduce as well, if not better. This concern was grave enough to stimulate the legislatures of several states in the Mississippi River system to ban the grass carp’s import. The Arkansas Game and Fish Commission sponsored research to learn how to control the propagation and dispersal of the species; as a result, sterile grass carp are now produced and widely marketed as vegetation control agents. As it turned out, most of the concerns about overpopulation were unfounded.

Perhaps the best-known example of exotic fish introduction occurred when the Welland Canal was built to bypass Niagara Falls for shipping into Lake Erie and points beyond. Thesea lamprey (Petromyzon marinus) discovered the canal and invaded Lake Erie, displacing lake trout (Salvelinus namaycush) and whitefish (Coregonus spp.) populations. Canada and the United States have invested millions of dollars to discover a control for the lamprey. Among the approaches used have been the administration of a selective larval toxin and long-term community adaptation. The coho salmon (Oncorhynchus kisutch), another exotic species that is non-native to the Great Lakes, has replaced the lake trout in commercial and sport fisheries.

Aquaculture has matured as an industry in part because marine fisheries resources have significantly declined. Many different exotic species have been brought from other countries or transplanted from one basin to another within North America in the ongoing effort to provide variety and find more efficient subjects on which to practice aquaculture, to provide broader culinary and sporting opportunities, or to satisfy aquarium enthusiasts. The rainbow trout (Oncorhynchus mykiss) has been moved from its native range in western North America to many points east, particularly to dam tailwaters, for sporting purposes. Brook trout (Salvelinus fontinalis) have been transplanted from their native streams in the northeastern United States to numerous Rocky Mountain lakes. Brown trout (Salmo trutta) were carried from Europe to North America and successfully added to the native salmonid fauna.

Goldfish (Carassius auratus), guppies (Poecilia reticulata), variable platy (Xiphophurus variatus), oscars (Astronotus ocellatus), and many other species have been imported to the United States for the aquarium trade. The primary problem that has resulted from such importation has arisen from the ignorant and random release of exotic species simply because their owners have become tired of keeping them. The bighead carp (Hypophthalmichthys nobilis), ruffe (Gymnocephalus cernuus), and several cichlids (Cichlasoma spp. and Tilapia spp.) were introduced for possible food fish.

The importation of exotic species is sometimes done illegally, outside the supervision of any regulating agency. The potential for the disruption of biological communities by exotic species is serious enough that the American Fisheries Society was motivated to establish its Exotic Fishes Section, and many state game and fish agencies (or natural resource departments) have set up special committees or enacted regulations in attempts to control these importations. The National Park Service warned in 2020 that there were 250 non-native aquatic species from continents outside of North America present on the continent and more than 450 species native to North America but that had moved out of their native North American ranges.

Other Exotic Species

Most ecologists are aware of the detrimental effects of the chestnut blight and Dutch elm disease in the United States. It is still unknown whether the targeted species will eventually adapt to resist these diseases or finally succumb to them. Other problems include the overpopulation of rabbits (Sylvilagus sp.), in the absence of adequate predators, following their introduction into Australia. After mongooses (Herpestes sp.) were introduced into some of the Caribbean islands to control snakes, they apparently ran out of snakes and started eating non-intended prey items.

An often-overlooked introduced species that has had a profound impact on the history and development of the Americas is the domestic horse (Equus caballus). Although originally native to North America, horses migrated to Asia (presumably across the Bering land bridge during the final Pleistocene glaciation), became extinct in America, and were reintroduced by the Spanish during early explorations. During the horse’s absence from America, biotic communities would have certainly evolved horse-absent features so that reintroduction could well have caused ecological perturbations. On the other hand, the horse’s actual role in a natural community as a primarily domesticated species is debatable.

In addition to exotic species of shellfish, fish, mammals, and fungi, exotic insects, birds, and plants have also been introduced outside of their native ranges and then come to outcompete native populations. For example, the European starling (Sturnus vulgaris) was released in New York City’s Central Park in 1890 by a group that sought to have North America populated by birds found in the plays of William Shakespeare. The species is now spread throughout North America compromising the habitats of other birds. Burning bush (Euonymus alatus) was introduced in the US as an ornamental landscape plant in the mid-nineteenth century from Asia. Now considered invasive in parts of the US, it spreads rapidly when its seeds are ingested by birds, and it outcompetes native woodland and prairie species.

An important part of biogeographic studies includes the determination of the native range of a species, which is then used to help explain dispersal routes and mechanisms and possibly predict future range extensions. Biogeographic studies are also correlated with continental drift patterns and distributions of the fossil record. When transplant records are never made, or are lost or inaccessible, it may be difficult for biogeographers to determine the status of certain species.

Bibliography

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