The Issue

The term invasive species refers to those species which establish themselves in an area outside their natural range, exponentially increasing their population thus out-competing the native species of that area and usually bringing change in the ecosystem functions.

Every specie has its own natural means to spread over certain area, such as plants spread through their seeds and animals can actively move or walk. The area thus occupied by a specie is known as its natural range, which may not necessarily coincide with the political boundaries of the countries. Through their natural means, species generally cannot cross geographic barriers like oceans, high mountains and great deserts. This is why the biotas of different parts of the world have evolved differently and have their own unique species of plants, animals and microorganisms.

However, during past few hundred years, with the development of better travelling means, humans have become instrumental in the transport of species beyond their natural ranges. The human-assisted travel of species crosses all natural geographic barriers. A specie so arrived in a new area much away from its natural range, is known as an alien, exotic, introduced, non-native, or non-indigenous species for that area. The introduction of exotic species may be either intentional, such as crop plants, ornamental plants,farm animals, pets, biological control organisms, etc.; or the introductions could be unintentional or accidental, such as contaminants with trade commodities or the organisms travelling as stowaways.

Although every alien specie is not capable of establishing itself in the new habitat on its own, but certain fraction of such species not only establish but become gregarious and aggressive thus out-competing the native species of that area for nutrients and space. This results in the extinction of more vulnerable species, which are generally already rare or endemic to that area. Such aggressive alien species are called alien invasive species or invaders.

According to Clout and Lowe (1996), mixing of floras and faunas across bio-geographic boundaries through human-assistance has been a major cause of species extinction throughout the world in past few hundred years. The invasive species, or bio-invasion as it may be called, pose one of the greatest dangers to Earth's overall biodiversity. With the improvement in humankind's transportation technology, the travel of species across their natural boundaries has exponentially increased (Randall & Marinelli 1996) and virtually every type of ecosystem on Earth is now affected by invasive species (Clout & Lowe 1996). In many cases, the alien species equal or even outnumber the native species. With the present fast means of travel and globalization of international trade, alien species are now present in every part of the world and belong to all taxonomic groups, i.e. plant, animals and microorganisms. According to McNeely (1998), the biggest danger from alien species is the global homogenization of biota. The invasive species not only damage the natural ecosystems and native biodiversity, but may also become severe agricultural pests or pose danger to industries, and even human health (Randall & Marinelli 1996, O'Konnen et al., 1999).

The human assisted travel of species is now about a million times greater than that through their natural means in certain cases. The number of alien species world over is in thousands. Some parts of the world are faced with greater proportions of alien species as compared to others. In the USA, there are 4500 exotic species of which 20% have caused serious economic and ecological harm. Out of the total harmful alien species, 81% have arrived from 1981 to 1993 through unintentional introductions, (McNeely 1998) indicating the risks of increased global communications and trade.

Islands are specifically prone to infestation by aliens, such as New Zealand has 1570 alien vs 1790 native, Hawaii has 861 alien vs 956 native, and Tristan de Cunha has 97 alien vs 70 native plant species.

More problematically, it is not a short-term concern. Unlike chemical pollution which usually begins to dilute and subside after blocking the pollution source; the invasive species continue to proliferate and spread after being introduced, posing a more pervasive threat than chemical pollution (O'Konnen et al., 1999). The impacts of invasive species are generally irreversible. Prevention of further biological invasions is therefore an extremely urgent priority.

As a note for clarification, some native species also have a tendency to increase their population disproportionately after human-induced disturbance of a natural habitat. Some native species particularly prefer man-made habitats such as cultivated fields and gardens. However, there is considerable disagreement whether these could be regarded as invasive species (Randall and Marinelli 1996). The point is that these species are part of indigenous biodiversity and they become aggressive or gregarious only under particular ecological condition, i.e. human-induced disturbance; otherwise they remain in equilibrium in the undisturbed natural ecosystems. Therefore, the indigenous or native species occurring as weeds in agriculture or gardens are generally not treated under the category of invasive species in a global perspective as well as in this report.