Impacts of Alien Invasive Species 

Biodiversity loss

Biodiversity loss is one of the most negative results of introduction of alien invasive species and could lead to marginalization or even elimination of the native species. The invasive species grow fast as they do not have any natural predators and as such draw more nutrients and moisture from the ground than the native species. Due to the frequent presence of toxic metabolites they also keep away the native species through allelopathy. As a consequence, they occupy more space and form purer populations. Whenever alien plant species invade a habitat, they also eliminate the natural fauna, since fauna always depends upon natural flora. The invasives can pave the way for further invasions by other alien species of plants, animals or microorganisms, which may include agricultural pests or human pathogens. 

They may also change the structure and function of an ecosystem completely, resulting in large-scale changes in the environmental conditions of that area. This happens when; the invasive species change the soil composition 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 also alter the nutrient cycle in the ecosystem. When fast growing species invade a certain catchment area, they 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. This, coupled with high loss of water through increased transpiration ultimately converts the wetland into dry land, etc. Due to these features, Clout & Lowe (1996) define invasive species as "An invasive species is an alien species, which colonizes natural or semi-natural ecosystems, is an agent of change, and
threatens native biodiversity". 

Economic loss 

Loss in biodiversity also leads to economic loss. Invasive species may cause major financial losses to a 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 the future (Randall & Marinelli 1996). The loss could be the result of the direct problems caused by invasive species, loss of land's living resources, and losses  due to any pest brought with the invasive species. 

However, occasionally invasive species also bring financial benefits. Even so a point of concern is whether significant biodiversity values may be lost due to their introduction. Generally, the damage caused to biodiversity through them is far more than any benefits that might be associated with them. Thus when indigenous biodiversity is at stake, it should always be given preference over alien species (Clout & Lowe 1996). 

It is therefore necessary that ecologists and economists work together 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

Invasive microorganisms can be human pathogens, or invasive plant or animal species may bring with them human pathogens or allergins. For example, the pollen of certain invasive plants can cause severe allergies of the respiratory system. The money spent on the treatment of invasive species-related human ailments also falls under economic losses. 

Alien Invasive Species in Pakistan 

In the context of Pakistan there is no information and has been no cataloguing of alien invasive species or their impact on the local environment. Rough estimates suggest that about 700 alien species of vascular plants are present in Pakistan, compared with about 4500 indigenous species (Khatoon & Ali, The Herald annual, January 1999). Of these 700 alien species, 5 or 6 could be recognized as high-impact invasives. There is no comprehensive cataloguing even of indigenous species, or of other organisms including animals, algae, fungi, bacteria, etc.; therefore the number of alien species in these groups is uncertain. However some that are known include sheep and cows from Australia and New Zealand, fin and shellfish for aquaculture and aquaria, like silver carp, china grass carp, tilapia, rainbow trout etc., and Australian bees for apiculture. Of these introductions, tilapia has escaped aquaculture and has established on its own in many wetlands in Sindh (Q.M. Ali - personal communication), while China-Grass Carp introduced in 1970's into Haleji Lake for controlling weeds has reportedly deprived the native herbivorous fish of their food. Accidental introductions include the insect pests of agriculture (e.g. American bollworm, Heliothis armigera) or horticulture. 

Fortunately, there are not as many alien species in Pakistan as in some other countries. Most of these date back to the colonial era however, new intentional introduction are also going on unchecked. It is unfortunate that there is virtually no awareness on this issue among the general public, policy-makers, or even among most of the forest department personnel. Nursery-owners frequently import seeds of ornamental plants, while in many cases, the official tree-plantation drives involve alien species. The 1999 monsoon tree plantation drive in Sindh was initiated by planting Lignum (Guaicum officinale - a native of S. America), while spring tree plantation of 2000 involved planting Eucalyptus. In August 1999, an anti-pollution rally organized by various government departments and NGOs planted 1000 saplings of Eucalyptus, neem, and Conocarpus around Keenjhar Lake to "save the lake" (DAWN Aug. 29, 1999). They were unaware of the damage being caused to the lake surroundings by planting these alien species. 

The Eucalyptus tree, a native of Australia lowers the water table due to its fast growth and excessive transpiration, besides having allelopathic effects on vegetation. It also contains highly inflammable volatile oil contents, which can trigger forest fires in hot and dry seasons. Its plantation in Sindh has not only changed the characteristic landscape of the Province but in the long run would also have adverse effects on the water resources. Both Eucalyptus and neem are home to fruit-eating bats, a potential threat to orchards, which serve as the seed-dispersal agents for the latter. Although these species have not become invasive so far, given their large scale and repeated introduction along with destruction of native vegetation, they 
may become so in future. 

Legislation/Prevention Measures 

Besides the Biodiversity Action Plan (BAP) of Pakistan that deals with this problem at a superficial level, there is no other legislation. Action 6.6 of BAP reads: "Take measures to control invasive alien species of fauna and flora, and to prevent further introductions". While the Quarantine Department checks the plant or animal species being imported for any pests or pathogens accompanying it, it does not check for the possible impacts of imported species.