The tropical rainforests of Andaman islands are under threat – not from illegal felling of timber or clearing of forests for development activities – but from being finished off by an invasive species of deer and elephant, a research says.

Spotted deer, also known as Chital, were introduced to the islands in the 1930s by the Britishers as a game animal.

In the absence of natural predators and competitors, and being good swimmers, they have now spread over to every island in the Andaman, with the exception of Little Andaman.

According to the report prepared by Rauf Ali from the Foundation for Ecological Research, Advocacy and Learning in Puducherry, the herbivore species is causing the vegetation to decrease significantly.

The islands have no native herbivores except for wild pig, which is omnivorous while elephants were brought over from mainland India in the 1880s.

To assess the change in vegetation cover across Interview Island, Mahatma Gandhi Marine National Park, Jarawa Reserve and Little Andaman, a comparison of Normalised Difference Vegetation Index (NDVI) trends between sites over time was made by the researchers.

“Results indicate that areas with deer have faster rates of degradation than those without them. The maximum rate of degradation occurred at sites with both elephants and deer, and the minimum where neither of the two animal species occurred,” says the report.

Elephants have damaged the vegetation badly in recent years, and have created a situation where forest regeneration rates are higher than normal. Elephant damage is seen in the form of trees that have been knocked down, or damaged because their bark has been stripped.

The chital, on the other hand, prevent regeneration by browsing on the seedlings.

“Besides direct observation on this, there is an abundance of stumps of browsed seedlings that can be seen,” conservationist Ali says adding that the degradation observed over relatively short time span is worrying.

Stressing on the need to urgently tackle the problem of invasive in the Andaman Islands, he says the internationally accepted scientific principle is to remove species which cause economic and environmental damage.

“If a policy change allowing hunting to take place is made, then the removals would actually generate revenue for the Andaman & Nicobar Administration,” the report says pointing out that India is one of the very few countries in the world that does not have an invasive species policy.

On the menace caused by elephants, it says culling is impossible because of popular sentiments.

“Periodic translocation of the younger animals to areas in mainland India, or overseas appears to be a solution,” recommends the researchers.

Inhabited by a number of indigenous tribes, major parts of the Andaman Islands were damaged in the 2004 tsunami.