Impacts of Alien Invasive Species

Biodiversity loss:
The invasive species invariably exert negative effect on the indigenous biodiversity by marginalizing or eliminating the native species. Due to their fast growth rate, the invasive species draw more nutrients and moisture, out-competing the native species; and due to the frequent presence of toxic metabolites they keep away the native species through allelopathy thus occupy more space and form pure populations. Whenever alien plant species invade some habitat, they marginalize or eliminate the natural fauna also, because fauna always depends upon natural flora. The invasives can pave way for further invasions by other alien species of plants, animals or microorganisms, which may include agricultural pests or human pathogens.

In worst cases, the invasives may entirely modify the structure and function of an ecosystem resulting in large scale transformation of environmental conditions of that area. This may occur in various ways; the invasive species may change the soil chemistry due to allelopathic substances released from their roots or leaf litter, the chemical nature and decomposition rate of their leaf litter and other dead parts may alter the nutrient cycle in the ecosystem, invasion by fast-growing tree species in catchment area may drastically reduce the flow of streams and rivers due to high rate of evapotranspiration; the invasive species in aquatic habitat accumulate greater amount of sediment due to their gregarious nature which, coupled with high loss of water through increased transpiration ultimately converts the wetland into dryland, etc. Due to these features, Clout & Lowe (1996) define invasive species as: "An invasive specie is an alien specie which colonizes natural or semi-natural ecosystems, is an agent of change, and threatens native biodiversity".

Economic loss:
The biodivesity loss is not without economic loss. Invasive species may cause colossal monetary losses to national economy. It is estimated that just 79 invasive plant species in USA caused $97 billion loss from 1906-1991, and another 15 potentially high-impact species may cause $134 billion loss in future (Randall & Marinelli 1996). The loss could be the net result of the direct problems caused by invasive species, the loss of land's living resources, and the losses due to any pest brought with the invasive species. These points will be elucidated by the forth coming examples in the next section.

In many cases, financial benefits of some kind are attached with the invasive species, as without it, these species could not have been introduced. The main question is whether significant biodiversity values may be lost? Generally the damage caused to biodiversity through species extinction and disruption of natural ecosystems far outweighs any benefits of invasive species. Thus when indigenous biodiversity is at stake, it should always be given preference over alien species (Clout & Lowe 1996).

Harvesting invasive species can result in conflict with biodiversity values. The management options in such cases may include harvesting for a sustained yield in certain area, so that the native species can co-exist with moderate densities of invasives (Clout & Lowe 1996). Nevertheless, such an arrangement is certainly not without risk and the invasive species populations need to be closely monitored.

For a viable control or eradication program, the basic need is to show that the resultant benefits would outweigh the cost of such a program. For example, Diamond et al. (1991) estimated the total benefits from controlling Melaleuca infestation is to be $168.6 million/year mainly from tourism, while the value in terms of honey obtained from Melaleuca would be just $15 million.

An effective partnership between ecologists and economists is thus imperative and essential to quantify the costs and benefits involved in dealing with invasive species. Economic analysis can provide a useful and rigorous structure to guide the policy makers (McNeely 1996).

Risk to human health:
In case of invasive microorganisms, they themselves can be human pathogens; or the invasive plant or animal species may bring with them human pathogens or allergins, such as the pollen of certain invasive plants can cause severe allergy of respiratory system. The money spent on the treatment of invasive species-related human ailments also falls under the economic losses.