The term alien, exotic, introduced, non-native, or non-indigenous refers to those species, which have arrived in an area outside their natural range. These introductions may be deliberate such as farm animals, pets, crop plants, ornamental plants, biological control organisms, or they could be accidental or unintentional; through contaminated trade commodities and by travelling as stowaways. The term invasive explains the exponential growth of species in an area outside their natural range by competing with the native species and usually bringing changes in the ecosystem of that area. There is considerable debate over the invasive nature of the native species, which have the potential to increase their populations after the human induced disturbance of their natural habitat. It is observed that some native species prefer to grow in human-made habitat such as gardens and cultivated lands and can increase their growth. However there is much argument on the issue of considering these native species as invasive species. (Randall and Marinelli 1996). All alien species are not capable of proliferating at a higher rate, some of them may remain in the foreign area for long and do not become invasive but some of them grow at an extensive rate thus outgrowing the native species. These alien aggressive species are called alien invasive or invaders. 

Naturally each species has its own niche where it lives, moves and reproduces. Species have their means to disseminate themselves such as plants spread through seeds and animals walk or run. The area occupied by the species naturally is known as its natural range, which may or may not relate to the political or geographic boundaries of the nations. Due to natural barriers such as high mountains, oceans, large rivers and great deserts, species cannot move from one place to another naturally, which has given rise to specific and unique biota of different parts of the world. Human beings have played an important part in transferring species from one area to another. Human migration in the early days shows the intentional introduction of exotic species for fulfilling their physical and social needs. The frequency of the early introductions was low but with growing communication technologies in the past few hundred years, the transport and passage of these species have increased many folds. 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). 

Biological diversity comprising of all the life forms is important for providing a balance to the lives on earth. It sometimes becomes difficult to determine which species are useful and vital for the ecosystem, which are non-essential and which species will be most important for the future of an area. It has been recorded that the introduction of alien species has caused serious problems for native species including human beings. The Nile Perch is one such example, which was introduced into Lake Victoria, Africa. It resulted in the extinction of more than 200 endemic species by competing for food and predation. In Pakistan examples of invasive species can also be seen such as the Prosopis juliflora (mesquite) native of West Indies and Mexico, introduced for afforesting deserts in Sindh, which later on became a highly invasive species and can be seen everywhere in Pakistan. 

Today alien invasive species account for the second important reason for habitat loss worldwide. The invasion of alien species poses problems for the native biodiversity by competing for food, territory and other facilities, which result in the reduction or sometimes extinction of the native flora and fauna. This also results in the damage of habitat and ecosystems such as the fresh and marine water resources, terrestrial, deserts and other vital ecosystems. Damage can also occur in the form of health risks for plants, animals and humans and can result in economic losses for the nation. The invasion of species also brings about the issue of global homogenization. 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). In addition to this it is difficult to estimate the risk caused by the introduction of invasive species. 

The consequences of invasion are often more dangerous than the other forms of chemical pollution where the effects of the pollution can be minimised by arresting the source of pollution where as the invasive species once introduced grow and spread at an alarming rate. The invasive species continue to proliferate and spread after being introduced, posing a more pervasive threat than chemical pollution (O'Konnen et al., 1999). 

Awareness raising about invasive species and their threats is not a common phenomenon especially in the Asia region including Pakistan. However scientist are working hard towards recognising the issue of invasive species and their impacts upon biodiversity in the world communities. 

Some major international initiatives have been taken such as the signing of conventions and treaties, which resulted in the move of global efforts towards the threats to the Earth's environment. In this connection the Earth Summit Conference was held in 1992 where161 countries of the world signed the Article 8th of the Convention on Biological Diversity (CBD). This calls upon the parties to "prevent the introduction of, control or eradicate those alien species, which threaten ecosystems, habitats or species." 

IUCN - The World Conservation Union also aims to enhance awareness and capacities to address the issue of Alien Invasive Species in the Asia Region. The Union thus started a Global Initiative on Invasive Species in 1999, in selected countries of the region vis-à-vis Pakistan, Sri Lanka and Vietnam. It is being co-ordinated by the IUCN Asia Regional Biodiversity Programme (RBP) based in Sri Lanka.