Spotted deer nibbling Andaman archipelago’s mangroves are probably thwarting forest regeneration after the devastating 2004 Asian tsunami. Brought to the Indian Ocean islands by the British in the 1900s, the herbivore’s appetite risks stripping the mangrove ecosystem of its diversity, research¹ says.

Tree species such as Ceriops tagal and Lumnitzera racemosa that are not as favoured by the deer are more abundant in the Mahatma Gandhi Marine National Park, a protected area in South Andamans, says Nehru Prabakaran at the Wildlife Institute of India. “The tsunami and subsidence destroyed 50% of the mangroves in this part of the Andamans. The mangroves should have recovered but they didn’t,” he adds.

Prabakaran’s study is part of an expanding body of research on invasive species in the Andaman and Nicobar islands marking 20 years since the tsunami this December. There’s growing recognition of such introductions — such as avian malaria and mosquitoes to the Hawaii, or cane toads in Australia — being most damaging to islands, often sitting ducks for disasters.

“Our notion of island ecology in India is that islands are pristine, but it is no longer so,” says Karthikeyan Vasudevan, who along with Harikrishnan Surendran, first reported the Indian bull frog’s presence on the archipelago in 2013 as part of forest surveys.

The voracious bullfrog (Hoplobatrachus tigerinus), native to the mainland, devours everything from other frogs to chickens. Two years on when Nitya Mohanty picked up the trail of the bullfrog, the frogs had leaped onto five of the eight major Andaman islands. By the time the study wrapped up, the frogs had come to Little Andaman Island which is 55 kilometres out in the sea from the capital Port Blair, right at the southern tip of the archipelago.

“Incursion of non-native species is constant because there is no proper island-level quarantine,” Mohanty says. In addition to the deer and bullfrog, the Asian elephant brought from the mainland to help log the islands (also native to India but not to the islands), and the giant African snail (Achatina fulica), an agricultural pest, are listed among more than 600 animal and plant species alien to the islands. Several, including the elephants and snail, have turned invasive.

“But these numbers evolve; we need to look at door-knocker species on the islands,” says Vasudevan of mynahs and sparrows that have hitched rides on cargo ships and are now prolific. The current focus is usually on species that have already become abundant and noticeable.

Mammoth infrastructure projects such as the Great Nicobar seeking to boost international air and sea connectivity on the archipelago have come under intense scrutiny for environmental risks and impacts on the island’s indigenous communities, the Shompens and the Nicobarese. “There's no structured local governance. The islands must be given some importance in generating a response to the problems they face. It should come from there, organically,” says Vasudevan.

Prabakaran says one solution is to rid the islands of species that have caused maximum havoc, such as the deer which has also wiped out forest undergrowth in several islands thinning lizard populations. “It is very likely that the ecosystem will adapt and recover quickly if there’s no subsequent invasion.”

Eradicating invaders when they have just begun to establish is a once-in-a-lifetime opportunity, says Rohit Naniwadekar who looked at the food web affected by an invasive rat (Rattus cf. tiomanicus) on Narcondam, a remote island in the Andamans.