Sunderbans tiger count steady, but prey base not enough

Krishnendu Mukherjee, TNN | Feb 28, 2014,  
ISTKOLKATA: Even though there is euphoria over a minimum 103 tigers in the Sunderbans, a new study shows the prey abundance is supporting not more than 30 big cats in the mangroves.

While the foresters have pegged a presence of at least 13,000 spotted deer, considered a hoofed prey for tiger, experts feel this can at the most sustain a population of only 26 tigers. Though experts said there was no doubt about the tiger number since there's photographic evidence for each big cat, they sounded alarm over the prey count as it may not be enough for the tigers.

"We found 12,000 adult deer, 1,200 young ones and more than 2,000 wild boars, both young and adult, in the tiger reserve area. This is pretty good for the mangroves and is sufficient for 100 tigers or even more," said field director of Sunderbans Tiger Reserve (STR) Soumitra Dasgupta.

But, eminent conservation zoologist Ullas Karanth, who had conducted a study on the big cats' prey base across 11 different tiger habitats in the country between 1994 and 2004, said a full-grown tiger needs 50 hoofed prey, like deer, every year to survive. "And, a herd of 500 deer is required to provide a sustainable outcome of 50 deer per tiger. Though there is no proper methodology of estimating the density of prey animals in a terrain like the Sunderbans, 13,000 deer can support a population of only 26 tigers," he said, adding that his study on predicting carnivore densities from prey abundance had appeared in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) in 2004.

However, chief wildlife warden Ujjwal Bhattacharya said that not only deer and wild boar, tigers in the mangroves also feed on other animals like water monitor lizards, rhesus monkeys and even crabs. "We have also found 16,000 adult rhesus monkeys and 250 young ones apart from more than 1,600 water monitor lizards," Dasgupta said. Considering Karanth's methodology and taking into account the total estimation of all prey species - 33,000 - only 66 big cats can survive on this prey base in the tiger reserve area. Bhattacharya said the state would soon start a study on the status of prey animals in the mangroves with funds from the Japan International Cooperation Agency (JICA). This, according to him, will give a clear picture.

Echoing Karanth's views, Wildlife Protection Society of India's Belinda Wright said: "It is unlikely that the Sunderbans will have a high density of tigers since it doesn't have the prey species to support the number." Bangladesh-based tiger expert Monirul H Khan had earlier said that there was always a natural balance between the prey and number of tigers. "And 500 deer can provide sufficient food base to only one tiger, keeping in mind the fact that the breeding rate of deer is always balanced by the killing rate of tigers," he had said earlier.

The prey study in the mangroves was conducted a couple of months back covering an area of more than 80 square kms. Dasgupta said river transects were carried out for a total creek length of 137 kms. Later, the data was extrapolated to find out the prey abundance in the entire 1680 sq kms in the Sunderbans Tiger Reserve. STR is spread over 2,580 sq kms, of which 35% is water. An expert said extrapolation may not always give a reliable data for the Sunderbans as salinity, which decides the vegetation here, differs from place to place.

The camera trap study last year had predicted a minimum of 103 tigers in the mangroves, including 81 in the tiger reserve area and 22 in the South 24-Parganas forest division.

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