Monday, February 23, 2015

Indias 30% Tiger Population Increase in Question

This is important, though only partly related to this blog due to the species involved-- the Bengal tiger-- which is frequently involved in human-wildlife conflict in India.

Image from recent tiger census

Recently, numerous articles came out touting that India's tiger population has increased by 30% in four years due to successful conservation efforts. The moment this news was posted I'm pretty sure many conservation biologists were happy, though with reservation (and a little disbelief-- 30% in four years?), myself included. Sadly, this increase might have been due to faulty statistical methodology, and not an actual increase at all. Here is a quote from the WCS press release, posted today:

...Wildlife Conservation Society exposes, for the first time, inherent shortcomings in the ‘index-calibration’ method that means it can produce inaccurate results. Among recent studies thought to be based on this method is India’s national tiger survey (January 2015) which claimed a surprising but welcome 30 percent rise in tiger numbers in just four years

...Arjun Gopalaswamy, lead author of the report from the Wildlife Conservation Research Unit at Oxford University’s Department of Zoology, said: ‘Our study shows that index-calibration models are so fragile that even a 10 percent uncertainty in detection rates severely compromises what we can reliably infer from them. Our empirical test with data from Indian tiger survey efforts proved that such calibrations yield irreproducible and inaccurate results.’
Arjun added: ‘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.’

So far, no word on a recalculation of tiger numbers, though it is very likely that the increase will be much lower than initially reported.

This is a great example of the importance of solid statistical methodology. Well thought out conservation efforts are of the utmost importance, but just as important are the methods we use to quantify their performance.

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