India's tiger success story may be based on inaccurate census, says study

The accuracy of a recent census suggesting India’s tigers have increased by a third has been called into question by a new study. The Indian government had trumpeted the rise in tigers from 1,706 in 2011 to 2,226 in 2014 as a sign that state-led conservation programmes were working. “This is a proof of India’s biodiversity and how we care for mitigating climate change. This is India’s steps in the right direction, which the world will applaud,” environment minister, Prakash Javadekar, said at the time. But the new University of Oxford paper concluded that the statistical model used by India is a poor way to accurately predict tiger numbers. The team said that they were not disputing the rise in Indian tigers but the methodology behind the census was not robust enough to measure population changes. Dr Ullas Karanth, co-author and a member of India’s National Tiger Conservation Authority, said: “This study exposes fundamental statistical weaknesses in the sampling, calibration and extrapolations that are at the core of methodology used by the government to estimate India’s numbers, thus undermining their reliability. “We are not at all disputing that tigers numbers have increased in many locations in India in last eight years, but the method employed to measure this increase is not sufficiently robust or accurate to measure changes at regional and country wide levels.” They are not the first to criticise the accuracy of India’s tiger count. K Ullas Karanth, the director of the Centre for Wildlife Studies in India, said shortly after its publication that it did not use the “best currently available methodology for this task” and could not accurately measure changes. Those involved in the census have defended the method underpinning it. The new study questions the efficacy of index-calibration, the technique behind the Indian census. The method estimates wildlife populations at large scales by combining accurate but expensive measurements of small animal numbers in a small area – using camera traps, for example – with cheaper but less reliable measures such as counting tiger print marks. The authors used a mathematical model to test the technique, employing tiger fieldwork data, and concluded it had poor predictive power. Such modelling also assumes high and unvarying animal detection rates, said Arjun Gopalaswamy, the study’s lead author, based at Oxford University’s Department of Zoology. In India tigers make extensive use of manmade roads where the movement of vehicles, people and cattle can make tiger tracks less detectable. Gopalaswamy said: “Index-calibration relies on the assumption that detection rates of animal evidence are high and unvarying. In reality this is nearly impossible to achieve. Instead, there are many flexible approaches, developed over the past decade by statistical ecologists, which can cut through noisy ‘real world’ data to make accurate predictions.” The research is published in the journal Methods in Ecology and Evolution.