Biological diversity faces many threats throughout the world. One of the major threats to native biological diversity is now acknowledged by scientists and governments to be biological invasions by alien species. The impacts of invasive species are immense, insidious, and usually irreversible. They may be even more damaging to species and ecosystems on a global scale than the loss and degradation of habitats. For millennia, the natural barriers of oceans, mountains, rivers and deserts provided the isolation essential for unique species and ecosystems to evolve. In just a few hundred years these barriers have been rendered ineffective. Human trade and travel, accompanied by intentional and unintentional introductions, have inadvertently ended millions of years of biological isolation and created major ongoing problems that affect developed and developing countries alike.
Nowhere is the problem more acute than on islands in general, and for small island countries in particular. Acute problems also arise in other isolated habitats and ecosystems, such as in Antarctica. The physical isolation of islands over millions of years has favored the evolution of unique species and ecosystems. As a consequence islands and other isolated areas usually have a high proportion of endemic species (those found nowhere else) and are centres of significant biological diversity. Isolation has also meant island species are especially vulnerable to more aggressive competitors and predators from continental areas.
The remaining fragments of the US coastal prairie ecosystem and efforts to restore it are currently threatened by Chinese tallow (Sapium sebiferum (L.) Roxb.), an invading exotic tree. With its capacity for rapid growth and prolific reproduction, tallow is capable of converting native prairie into a near monoculture forest in only a few years.
A number of endangered species have been seriously affected by the brown-headed cowbird, a species that parasitizes the nests of other birds by laying its eggs in their nests (at the expense of the host's own offspring). This behavior is thought to have originated in response to the cowbird's historic need to move frequently to follow the movement of bison herds. Unable to stay and rear its own young when the bison left an area, the cowbird conscripted other birds for that job. Because cowbirds are much more numerous and widespread today as a result of modern agricultural practices and forest fragmentation, their habit of nest parasitism severely reduces reproductive success.
The introduction of arctic foxes to the Aleutian Islands during the 1830s quickly eliminated the Aleutian Canada geese from most of the islands where they once nested. Wildlife biologists have eliminated the foxes from a number of the Aleutian Islands, enabling the geese to reclaim parts of their breeding range.
The sawgrass-dominated Everglades wetland ecosystem, adapted to a nutrient-poor environment, has been taken over by phosphorus-tolerant cattails. The cattails choke the aquatic ecosystem, disrupting the food chain and extinguishing species at all trophic levels, including the snails, shrimp, insects, crustaceans and fish. Higher on the food chain, the population of wading birds has already declined by 93 percent since the 1930s, for lack of food and nesting sites. With enough nutrient enrichment, a foul-smelling, anaerobic mat of green filamentous algae takes over, in which only cattails and few other species can survive.
Zebra mussels, spotted thistle, kudzu and spiny pigweed are just a few of the more than 2,000 alien species threatening biodiversity in the United States.
Many alien plant and animal species have been introduced into South Africa over the years. A large proportion of such introductions have been deliberate, for purposes of agriculture, forestry, or even conservation. Much of South Africa's agriculture and forestry production depends upon species that originated from other countries. These organisms provide important economic and social benefits, but many have become invasive, causing serious ecosystem degradation, disrupting ecological processes, and resulting in species extinctions and possible reductions in genetic diversity through hybridisation.
Introduced animals have also reduced South Africa's biodiversity, a few examples being the Argentinian ant, the Himalayan thar, the European starling, the house sparrow and the black rat, and on South Africa's islands, house mice, rabbits, and feral domestic cats.
The introduction of exotic species in South Africa over the past century contributed to biodiversity loss as some alien species 'out-competed' native vegetation. For example, parts of the fynbos in South Africa and eastern highland grasslands in Zimbabwe were invaded by exotic Australian Acacia and Pinus species, and threatened the survival of the indigenous Restio, Erica and Protea species (Geldenhuys 1996). Island species, such as those found in the Indian Ocean, are particularly vulnerable to extinction caused by competition or predation (WCMC 1992). However, the introduction of undesirable exotic species is declining, a positive trend that will probably continue as regulations on the importation of biological resources become increasingly stringent.
In Australia many species have been imported and are creating great damage. These include rabbits (approximately 200 million), foxes (5 million), cats (12 million), goats, Buffel grass, Rubber vine, Para grass, the giant sensitive plant, Siam weed, and the fungus Phytophthora cinnamoni — which is a pathogen threatening entire native plant communities in some areas of southern Australia. In addition at least 55 species of marine fish and invertebrates, plus several seaweeds, have been introduced, either intentionally for aquaculture or accidentally in ships' ballast water or encrusted on their hulls. These are damaging marine and coastal environments.