Flawed Census Method May Mean There's No Rise of Tigers in India

Recently, a video was captured in China that showed a family of tigers in an area that has not been inhabited by the large cats for more than 65 years. Now, though, scientists have called the rise of the tiger population into question by pointing out flaws in a method that's common used in censuses of tigers.

The method, in this case, is called the "index calibration" method. This is the technique that is used by India for its national tiger survey, which recently claimed a surprise but welcoming 30 percent rise in tiger numbers in just four years.

Yet there may be a flaw in this survey. Index-calibration often relies on measuring animal numbers accurately in a relatively small region using reliable, intensive and expensive methods, such as camera trapping, and then relating this measure to a more easily obtained, inexpensive indicator, such as track counts, by means of calibration. The calibrated-index is then used to extrapolate actual animal numbers over larger regions.

In order to test the accuracy of this technique, the researchers created a mathematical model describing the approach and then tested its efficiency when different values, representing variations in data, were inputted. Under most conditions the model lost its efficiency and power to predict. The researchers then used the model on a real world example and found that the index-calibration model was unreliable again, with any high degree of success shown to be down to chance.

"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," said Arjun Gopalaswamy, lead author of the new study, in a news release. "Our empirical test with data from Indian tiger survey efforts proved that such calibrations yield irreproducible and inaccurate results."

The findings reveal that researchers may wish to use new methods to conduct their surveys. Not only that, but it shows that past surveys may be inaccurate and that the tiger population in India may not be recovering as much as expected.

The findings are published in the journal Methods in Ecology and Evolution.

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