Introduced Species – Effect on Biodiversity

Introduction of an exotic, non-native species into the distributional range, either deliberately or accidentally by human activity many-a-times poses a serious threat to the local habitat. A habitat containing all indigenous varieties tries to balance and control each species living within its limits. An introduced species has no enemies to control it; hence they slowly out number the native species and compete with them for the available resources.

Threats to biodiversity are many like habitat destruction by forest clearing, release of poisonous gasses due to industrialization etc. Out of all these threats introduction of exotic species ranks second. In fact, native biodiversity is affected more by introduced species rather than by pollution, harvest, and disease combined.

Some of these species are also being used as biological weapons as they cause mass destruction of ecosystem.

Examples of Damage to Biodiversity

Species are habitat-specific and can thrive only in a particular habitat but introduced species change the entire habitat and cause a major threat to biodiversity.

• Australian paperbark tree was introduced which replaced native plants, such as sawgrass, over a wide range of about 400,000 acres of south Florida. Australian paperbark tree had a combination of traits like flammable leaves, spongy outer bark and litter which can increase fire frequency and intensity. The uncontrolled spread of paperbark made many birds and mammals extinct as they were adapted to the native plant community.

Some other invasive species, do not change a habitat, but pose a threat to a single species. Sometimes they even destroy the entire groups of them in various ways.

• Brown tree-snake, is a predator which was introduced from Admiralty Islands in cargo. This brown tree snake has affected forests of Guam by eliminating ten of the eleven native bird species.

Some times the impact of invaders is subtle but destroys the native species at the same time.

• In North America native red squirrels are being driven to extinction by gray squirrels. Both these species compete for nuts and gray squirrels are more efficient than the native species. Such competition for resources is difficult to observe, but loss of a native species is the end result.

An interesting example amongst plants is that of acacia and the eucalyptus. These species were brought from Australia as a part of afforestation programmes. These are quick growing plants and were encouraged as the demand for wood had also increased. But these plants are known for their large number of disadvantages like:

• These trees shade the ground and the leaves that are shed do not allow other plants to grow.
• These trees do not decompose easily
• During the rains there is heavy erosion and poor percolation in the areas rich in these trees.

Only a few Introduced species are beneficial. Like most of our food crops (rice), onion, chilies and pets. The statistics of damaging introduced species is startling. The solution to this problem is stringent quarantine rules at all entry and exit points like airports, harbours, border crossing areas etc. 

Such proactive measures to check this problem will allow the existing plants and animals to flourish in their natural surroundings and habitat.