An invasive species is one that disperses into a new range, become naturalized, and then undergoes a population explosion, reducing diversity in its adoptive ecosystem via exploitative interactions and hybridization with native organisms. Concurrent with our growing capacity for international exploration, travel and commerce, human-driven species distribution has transported numerous species over wider and more uninhabitable barriers than those species could transverse on their own- a quantitative, though not qualitative, facilitation of the phenomenon of biological invasion.

One or a few individuals, dispersed by forces such as ocean currents or wind, may represent the entire population with which an exotic species invades a non-native range. These individuals’ likelihood of ending up in a habitat in which they can survive is left entirely to chance, and in fact, the great majority of immigrants go extinct within a few generations after arrival (Mack et all 2000). In contrast, human dispersal of other species is often carried out with intent. The horticulture, agriculture, animal husbandry, and pet trade industries specialize in fashioning non-resource limited environments in which introduced species such as ornamental plants or predatory companion animals can acclimate to their new environment, only later to escape (Mack). Organisms intended to serve as biocontrol agents are typically introduced en masse and distributed in a systematic fashion, with humans breeding a given population in captivity until it is large enough to make sustainability likely. Biocontrol agents may also be chosen on the basis of insusceptibility to local biotic constraints.

Human activities make ecosystems more susceptible to invasion (Enserink and others 1999). In one instance, the exotic species prostrate knotweed (Polygonum arenastrum) out-competes other species on the Cornell University Arts quad, primarily in the region closest to the pavement, an area subject to frequent trampling by humans. This species also has a greater tolerance for pollution and mowing than native grasses do, and is therefore common in lawns and agricultural fields. On a larger scale, invasive species have the advantage in colonizing human built habitats, to which they are pre-adapted. Assume for a moment that university quads anywhere are relatively homogenous, being supplemented with the same fertilizer, mowed to the same height, and seeded with grass at the same frequency. If prostrate knotweed has adapted to university quads in the northeastern United States, and its seeds are spread inadvertently to a university quad in California, they will have a comparatively easy time invading that habitat, having to adapt only to changes in substrate-independant variables such as temperature.

Invasive species, whether due to human nurturing, pre-adaptation, or release from biotic controls, have reduced species diversity dramatically in many ecosystems. For instance, in Lake Victoria, the Nile perch preys upon native fishes and in a period of 7 years has reduced a community of 400 species to a community of three (Kaufman 1992). This species was introduced deliberately, presumably for aquaculture. It is debatable whether ecosystems such as Lake Victoria can be successfully restored; even the populations of native cichlids could recover from near extinction, such an effort would be futile should the human worldwide distribution of exotic species continue. On the other hand, were such distribution to halt today, than while native ecosystems would never return to their original conformations, they could generate new conformations of diversity, with remaining species evolving so as to exploit and contain the exotics, or resource partitioning partitioning resources so as to compete with them.

Works Cited:
  • Enserink, M et al. 1999. Biological invaders sweep in. Science 285:1834-1843.
  • Kaufman, L. 1992. Catastrophic change in species-rich freshwater ecosystems: the lessons of Lake Victoria. Bioscience 42: 846-858.
  • Mack, R. N., D. Simberloff, W. M. Lonsdale, H. Evans, M. Clout, and F. Bazzazz. 2000. Biotic invasions: causes, epidemiology, global consequences, and control. Issues in Ecology No. 5.