Any species which is not a natural inhabitant of the locality but is deliberately or accidentally introduced into the system may be designated as an exotic species. There are many instances when introduction of exotic species has caused extensive damage to natural biotic community of the ecosystem.
Organisms adapted to life in a stressful environment conditions under which there is a hard struggle for existence – become tough, tenaceous and resistant. When introduced into locality with gentle mild and hospitable living conditions such organisms thrive well.
The original inhabitants being unable to compete are gradually suppressed and forced out of existence. The introduction of Nile perch from north in Lake Victoria, Africa’s largest lake, for example, has driven almost half of the 400 original fish species of the lake to near extinction. Both Eucalyptus (Myrtaceae) and Casurina (Casurinaceae) are plants introduced on the Indian subcontinent from outside. Chief centres of distribution of Eucalyptus are Australia arte Tropical America, while Casurina is a native of Southern hemisphere, mainly Australia.
The remarkably fast growth of these plants has made them valuable as a source of rough timber However; these plants appear to be ecologically harmful as they tend to suppress the original species of the locality.
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While economically useful plants are deliberately introduced a large of number of exotic weeds are transferred from one locality to another accidentally. The wheat imported to India from USA under PL-480 scheme was contaminated with seeds of Parthenium hysterophorus, the congress grass and Agrostemma githago, the corn cockle. Both of these plants have spread throughout India as a pernicious weed in wheat fields. Parthenium was first observed growing on a rubbish heap in Pune in 1960.
It is an aggressive plant which matures rapidly and produces thousands of seeds. The indegenous grasses and other herbs are crowded out of existence. Cultivated crops suffer as it depletes the nutrients from the soils. An enormous quantity of pollens are produced by Parthenium which produced allelopathic effects on cultivated plants like brinjal, tomato, chillies and inhibit development of fruits. The weed causes extensive damage to wild as well as cultivated plants.
Lantana camara, probably one of the ten most common weeds of the world was introduced in India as an ornamental hedge plant from Sri Lanka in nineteenth century. After cutting trampling or burning this plant regenerates itself quickly to form dense thickets. Prolific growth and adaptability has enabled this plant to overrun large tracts of land including pastures, cultivated fields, waste land and forests. In the process indegeneous plants are-suppressed or eliminated altogether from the locality. Water hyacinth, Eichornia crassipes, was introduced for thefirstime in 1914 at Narayanganj in West Bengal. The first appearance of Alligator weed, Alternanthera philexeroides, was reported near Calcutta airport in 1965, while Salvinia molesta was brought to India by an aquarist.
These plants vigorously produce stolons and regeneration of detached parts is quick which result in development of thick mats floating on the water surface. The damages caused by these plants are extensive. They impede run off in streams and promote water logged conditions.
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The decay and decomposition of dead plant part places a heavy demand on dissolved oxygen in the aquatic system. Mat-like spread cuts of air and sunlight and sub-surface biota is almost completely eliminated. Due to active transpiration both alligator weed and water hyacinth accelerate water loss by 130-250% as compared to the evaporation from clear water surface.
In West Bengal alone the annual loss of fisheries due to exotic weeds has been estimated to be about 8.5 million tons. Mats of water hyacinth break loose and are pushed by winds to adjacent paddy fields which are rendered unproductive. A number of useful water plants are displaced by these vigourous but useless weeds. There is an overall reduction in biological diversity wherever these exotic weeds migrate.