As opposed to native species, which are indigenous and found naturally in an environment, animals and plant species introduced from other countries and which are not otherwise found locally are termed exotic. These introduced or exotic species can adversely affect the ecosystem. In India, large variety of exotic animal and plant species have been introduced from other parts of the world through the ages. Some exotic plants have turned into weeds, multiplying fast and causing harm to the ecosystem, e.g. water hyacinth and lantana.
Exotics are invariably introduced without their natural enemies that control and balance their spread in their native land, and hence, grow and flourish without any hindrance and cause harm to the environment. Therefore, when planting saplings, remember to choose that only those that form a part of the natural ecosystem of an area. In a stable ecosystem, all species -animals, plants, and microbes- are in healthy co-existence. Any disturbance in one gives rise to imbalance in others and this is what happens when an exotic species is introduced.
Introduced species can often negatively affect native species. While they are selected specifically for their adaptability and in the long run often out number the native species and compete with them for the resources. This results in the expansion of the introduced species and the decline of the native species.
Plants from all over the world have been brought to India and grown here. Some have proved beneficial while others have not. Vegetables such as the chillie and the onion have been brought from South America and Persia (modern day Iran) respectively.
Some quick growing plant species were brought from Australia for afforestation programmes such as the acacia and the eucalyptus. The demand for wood in different industries led to a growth of forest area under these species. These trees shade the ground and the leaves that are shed do not allow other plants to grow nor do they decompose easily. During the rains there is heavy erosion and poor percolation in these areas. Thus, the introduction of these species has caused more harm than good to the forests and the soil in general.
Some weeds have not been intentionally introduced but have come accidentally as for instance the Mexican weed came along with American wheat that came as PL480 aid from the USA in the 1960s when quarantine rules were not so strict. In fact all plants and seeds that come from another country should be quarantined to ensure that no other foreign material has come with it. Quarantine facilities must be made available at all entry and exit points, at the airports, border crossing on land and the harbours. This will prevent further damage, and the existing plants and animals can then and should be allowed to flourish in their natural surroundings and habitat.