Introductions and Survival of Nonindigenous Species in Freshwater Systems

Pathways of Introduction

Most evidence of the pathways of introductions of exotic species is circumstantial and based on the opinion of the individual who first discovered that the species was released. Usually, the date and site of an introduction are questionable but may sometimes be estimated by locating the center of the present range, the relative abundance of the species throughout its range, and the rate of the species' expansion. Many species may exist in an area for many years in small, token populations that suddenly expand rapidly for poorly understood reasons. It is also possible that after many unsuccessful introductions the species may finally become established and flourish.

Introductions are either planned, incidental, accidental, or unintentional or caused by a natural disaster. Several attempts have been made to identify the possible pathways of introduction of nonindigenous species (Carlton 1989,1990,1992a,c,d,e; Mills et al. 1993). Intentional introductions are either preceded by research and by careful consideration of the suitability of life history traits of the species to the environment of the receiving waters or releases on the spur of the moment with little consideration for the welfare of the species or the receiving environment (e.g., aquarium dumps). Carefully planned introductions are rare.

The American Fisheries Society (1973) developed a position statement about the intentional introduction of exotic aquatic organisms by governmental agencies. The reasons for intentional introductions of nonindigenous species are (1) establishment of wild populations for recreation or for later commercial harvest; (2) establishment of a forage fish population; (3) use as biological control agents for nuisance plants and insects (Avault et al. 1968); (4) recreational hunting and trapping; (5) disposal of unwanted pets or unprofitable animals or avoidance of prosecution; (6) landscaping, soil stabilization, or commercial harvest; and (7) bait. The incidental introductions are caused by (1) escapes from holding facilities (federal, state, private) such as fish farms, hatcheries, pet dealers, and research facilities because of poor designs of facilities, employee error, or operational mismanagement of the facility; (2) releases from natural disaster such as flooding, hurricanes, earthquakes, etc.; (3) escape of specimens from display pools, aquariums, gardens, zoos, etc.; (4) natural dispersal; (5) movement by predators, and (6) ballast-water discharges. The use of nonindigenous species for bait, recreational fishing, commercial crops, aquaculture, landscaping, and soil stabilization almost always guarantees establishment in new areas-- especially if its release could result in profit.

Many plants are introduced for aesthetic or ecological reasons. For example, botanist David Fairchild imported large quantities of nonindigenous plants into Florida in the 1800s and early 1900s (Sailer 1983). Since that time, the two most cataclysmic, intentional terrestrial-plant introductions are melaleuca (Melaleuca quinquenervia), a tree introduced for its ability to dry out the swamps of southern Florida, and Brazilian pepper (Schinus terebinthifolius), a tree noted for its ornamental value. Both species are spreading rapidly through southern Florida. Hydrilla (Hydrilla verticillata) and water hyacinth (Eichhornia crassipes) are two examples of nonindigenous aquatic plants that currently cause considerable ecological and economic damages. Another pathway of introduction is the indirect importation of plant pathogens and other species that occur on or around foodstuffs and plants (Denmark and Porter 1973). Although routes of entry for many species have been identified, the exact pathways of introductions of most nonindigenous plants and some animals into the state are unknown.

Theoretically, all undesirable introductions could be avoided if the proper precautions were taken before the species are imported into the area. Many believe that, if the professionals in charge of the resources fully recognize and appreciate the problems by the introduction of exotic species and take the necessary precautions, the number of nuisance introductions could be significantly reduced (McCann 1984). The current debate between those for and those against new introductions relates to the necessary level of precaution that slows the introduction of harmful species without hindering the use of beneficial species. Another issue is a failure by some individuals to separate the act of importation from the act of introduction. In this respect, whether the nonindigenous species is an exotic species (from another country) or a transplanted species (from another part of the same country) makes little difference to the impact on the receiving ecosystem. A good example of the latter is the impact of the transplanted brown darter (Etheostoma edwini) that was introduced as a baitfish on the endangered Okaloosa darter, E. okaloosae, (Burkhead and Williams 1990). The change from the introduction does not depend on the origin of the species but on the characteristics of the species in relation to the characteristics of the receiving ecosystem.