Examples of some Worst Impacts of Invasives

Chestnut blight:
In early 1900s, the most important hardwood species in eastern American forests was chestnut (Castanea dentata). The chestnut blight brought in with imported horticultural stock from China killed nearly a billion trees, leading to profound change in the hardwood forest ecosystem (McNeely 1998).

Melaleuca quinquenervia:
This fast-growing tree (a native of Australia) was introduced into Everglades (Florida) in 1906 to drain this wetland, which was later recognized as a national treasure and declared as National Park in 1940. However, Melaleuca proved to be highly invasive. Till 1987, it had formed pure stand on 19000 hectares, converting the wetland into woodland, depriving the original bird fauna of its habitat. The cost on removing this specie would be enormous. (Bush 1997).

Nile perch:
To enhance fish catch, Lates niloticus was introduced into Lake Victoria in 1960. Being a voracious predator, Nile perch totally eliminated about 200 endemic species of cichlid fish, and diminished another 100 species to just remnants. This is the worst recorded example of vertebrate extinction in recent history (Simon 1993).

Zebra mussel (Dreissena polymorpha):
It reached America from Russian lakes in mid 1980's through ship ballast. Till 1994, Zebra mussel spread throughout Great Lakes and invaded eight major rivers. Being colonial, it forms thick crusts clogging the intake ducts and pipes of power stations and various industries, drains, etc. The cost for removing this pest could be upto $5 billion in the Great Lakes alone till 2000 (Bush 1997).

Water hyacinth (Eichhornia crassipes):
This specie was introduced into China from S. America in 1930's and was spread through mass campaigns in 1950's to 1970's as an ornamental plant, as livestock food, and as a pollution control by absorbing heavy metals. But gradually, it became the worst weed in aquatic habitats leading to the loss of plant and animal species. In Dianchi Lake, the number of fish species declined from 68 to 30. Reduction in lake area due to Eichhornia infestation also caused noticeable climatic changes in the area around lake (McNeely 1998).

Fynbos vegetation:
A number of fast growing woody species were introduced into South Africa in middle of 19th century for timber, fuel wood and tannin extraction. As a result of these introductions, Fynbos area which is one of the world's six floral kingdoms with 8574 plant species (68.2% endemic), is under severe threat with 26 species already extinct and another 750 are threatened. Besides thus, the watersheds are becoming less productive due to greater requirement of water by the biomass of fast growing invasive trees as compared to original fynbos (shrubland) vegetation, as well as high rate of transpiration from these invasives. It is estimated that this would result in 30-50% reduction in the supply of water to Cape Town in next 100 years, leading to increase in the cost of water. Besides this, other losses would include the loss of revenue from cut flowers, dried flowers and thatching grass which are presently harvested from native fynbos vegetation ($18-19 million/year), and loss of tourism value of the area due to modified landscape (van Wilgen et al. 1996, Le Maitre et al. 1996).