But the once lush, dense mangrove cover is sparse now -- reduced to decaying branches -- and the big cats have now moved on in search of food and protection.

Wildlife experts say rising sea levels and coastal erosion caused by global warming are steadily shrinking the mangroves of Sunderbans, threatening the survival of the endangered tigers.

"We are very concerned at the erosion level in tiger habitat, and we are planning to increase mangrove cover in core areas to protect the tiger," said Kanti Ganguly, minister for the Sunderbans in India's West Bengal state.

The Sunderbans, a 26,000 square km (10,000 square mile) area of low-lying swamps on India's border with Bangladesh, is dotted with hundreds of small islands criss-crossed by water channels.

Once home to 500 tigers in the late 1960s, the Sunderbans may only shelter between 250 and 270 tigers now, wildlife officials say, although the Indian Statistical Institute recently suggested the numbers could be significantly lower.

The tigers of the Sunderbans regularly swim between islands in search of food and sometimes stray into villages. They are known to have killed at least 50 people over the last five years.

The area is the world's largest mangrove reserve and one of the most unique ecosystems in South Asia, recognised as a UNESCO World Heritage Site.

But as sea levels rise, two islands have already disappeared and others are vulnerable.

The destruction of the mangroves has also adversely affected numbers of estuarine crocodiles, fishes and big crabs, said Shakti Ranjan Banerjee, wildlife expert and former secretary of conservation group WWF.

That could leave the big cats hungry.

"We are very worried about the tiger's prey base which may not be breeding as we liked and also the fact that the tiger habitat is shrinking due to rising sea levels," Pradeep Vyas, the special chief conservator of forests, told Reuters.

"But you cannot fight nature and must accept the inevitable that the islands could submerge one day," he said.

As sea levels rise, mangroves have been overexposed to salt water. Many plants have lost their red and green colours and are more like bare twigs, exposing tigers to poachers who hunt them for their skin and bones.

Also, tigresses now have fewer places to hide their cubs from adult males, who seek to kill them in order to stem competition in the group, conservationists warn.