What are the impacts of exotic species?

Predation
Native species of animals are much more vulnerable to alien predators because they have been thriving without the introduced species prior to the latter’s entry into the habitat. Coupled by a slow reproductive rate that was sufficient to maintain the species, alien predators can wipe out native species extremely effectively. Entire species of plants and animals that are endemic to the area, which is defined as species that are found only in the particular geographical location, can perish very quickly as a result. Endemism is a crucial factor in extinction as global extinction is said to have occurred if an endemic species on an isolated habitat is exterminated (Primack, 1998). For example, the introductions by the Polynesians in the 5th century into Hawaii, together with the clearing of forests for agriculture, caused the extinction of an estimated 50 of the 98 endemic bird species in the archipelago (Olson, 1989; Pimm et al., 1995). Following that, the animals introduced by the Europeans brought the extinction toll up to about 70% of Hawaii’s endemic bird species, with the remaining not far from extinction.

One of the most dramatic events in species decimation occurred in the late 19th century at Stephen Island, an atoll off the coast of New Zealand. A single cat brought into the island by the lone lighthouse keeper killed the entire species of a native rare flightless bird, now known as the Stephen Island Wren (Diamond, 1984). This could well be the only documented elimination of an entire species by a single animal predator, but this surely highlights the grave danger faced by native animals in any habitat in the face of alien animal introductions.

Herbivore Grazing
Native plants are vulnerable to the introduction of herbivores. Of the many exotic herbivores documented to cause extinctions or near-extinctions of plant species, goats have been found to be the most voracious alien grazers. One of their many undesirable effects have been the grazing and reduction to near-extinction of an endemic shrub, Hebe breviracemosa, in the Kermadec Islands off New Zealand (Atkinson, 1989).

Competition
Exotic species compete with native plants and animals primarily for food and space. Competition occurs when there are overlaps in the requirements and preferences for food and space, along with other factors such as water source and also light, especially in the case of plants and immobile organisms. Resources have to be split among the two species to reduce the overlap and the native species is eventually forced to compromise its dietary and spatial requirements (Schoener, 1974). If this is partition is not achieved, the competition may result in decreased reproductive capability and reduced growth in either the exotic or the native species, or even both. The social wasp of European origin, Vespula germanica, introduced into southern Australia in the late 1970s (Spradbery and Maywald, 1992), was found to have an adverse effect on the previously predominant native wasp, Polistes humilis, due to a food overlap. This overlap is only slight but because the native species has a more specific diet preference, this gives the exotic wasp the advantage as it can utilize both the exclusive resource as well as the shared resource, leading to its invasive success (Chase, 1996; Kasper et al., 2004). This example highlights the detrimental ecological effects of invasive competition due to the highly delicate interspecies relationships in the ecosystem.

Hybridization
When closely related species are present in the native habitat, the introduced species can hybridize with native species to give rise to hybrids that will result in the mixing of genetic information between the 2 related but distinct species. The genetic information of the native population, also known as the gene pool, becomes diluted by the alien gene pool and this leads to the elimination of the unique features present in the local population. The populations of the Apache trout (Oncorhynchus apache) and the Gila trout (O. gilae), both of which are residents of the rivers of Arizona in southwestern U.S., have been significantly reduced by the introduction of the rainbow trout (O. mykiss). Hybridization with the alien sport fish has been found to be an important cause of this population and range reduction (Dowling and Childs, 1992; Loudenslager et al., 1986).

Diseases and Pests
Pathogens introduced by alien plants and animals can have devastating effects on the ecological environment as well as on human health. In the example of the native birds of the Hawaiian archipelago, it has been found that avian malaria also had a part to play in the reduction and extinctions of native bird species. The disease was believed to have been brought into the region by alien birds introduced from Asia (van Riper et al., 1986). Disease-bearing insects have also found entry into Singapore by way of international aircrafts landing at the Singapore Changi Airport (Goh et al., 1985). If not monitored closely, the exotic pathogens and insects may cause outbreaks of infectious diseases across the country. Physical injuries may also result from the bites of exotic killer bees, fire ants, sandflies and tropical biting midges. It has been predicted that the increase in the incidence of diseases and injuries caused by these exotic pests can lead to the diminishing appeal of the outdoors and the subsequent development of biophobia that might undermine current research efforts in biological conservation (Soulé, 1990). Eventually, the greatest lost to the nature-loving community will be the strong supporters of biological conservation. What lies beyond is seriously unthinkable.