Worldwide, humans have succeeded in establishing at least 330 species of nonnative birds and mammals. In many instances, exotics compete with natives for resources, often with serious consequences (Stilling. 2004). In freshwater, stocking of exotic fish for sport or (rarely) for food, has caused, at least, 18 fish species extinctions in North American rivers. Catastrophic changes in fish biodiversity of Lake Victoria (East Africa) resulted from introduction of Nile perch (Pimm, 1998).
Rich species diversity in various parts of world exist as they are separated by barriers like large rivers, seas, oceans, mountains and deserts from other land masses. These barriers could never be crossed except through continental drift. Ships and airplanes, now, bring into contact species on a time scale of days.
Widespread introduction of exotic species is a potent threat to biodiversity. When exotic species establish self-sustaining populations, endemic species which are predators, parasites, or simply aggressive species may not survive.
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Exotic species deprive indigenous species from nutrients, water and light. Exotic or invasive species often are competitive and makes endemic species defenseless and uncompetitive. As a consequence, world’s ecosystems would be dominated by relatively few, aggressive, cosmopolitan, highly fecund, ultra-competitive, “super-species”.
Two main reasons for biodiversity loss in the coming decades would be (i) habitat loss and landscape fragmentation and (ii) invasion of alien species. Species those modify resource availability, trophic structures or disturbance regimes, strongly influence ecosystem functioning. Widespread evidence suggests negative role of invasive species in wholesale reduction of biodiversity (vide Roemer et al., 2002).
“One of the greatest public policy challenges is the development of accurate, precise, generally applicable and widely accepted risk analysis protocols, that are capable of distinguishing invasive species form large pool of non-indigenous species”. To meet these challenges, a detailed understanding of ecological history is a requisite.
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Lodge and Shrader-Frechette stated that “The human induced rate not only of species extinction but also of species invasion has increased exponentially in concert with exponential growth of human population over the last few hundred years”.
Techniques and actions available to curtail the effects of invasive species is perhaps not realized, by either the public or the scientific community (Donlan et al., 2003). Further expansion of exotic species is expected as a consequence of changes in global socioeconomic systems, land use/ cover change and changes in abiotic environment.
Biological invaders can affect human population directly or indirectly. Important lessons can be learned from invasion events of the past. Comparative studies of past invasion in natural communities and monitoring current invasion events are important in this context.