On the tiger's trail

WHEN Joseph Niepce invented photography in 1816, little did he realise that among the millions of uses his technology would be put to, counting tigers would be one of them! That is precisely what Ullas Karanth has begun to do at Nagarhole National Park near Mysore in Karnataka. (Biological Conservation, No 71, 1995).

In a 15 kilometre area, Karanth set up automatic cameras at 15 sites during nine different periods in 1991-92, for a cumulative period of 387 camera trap-nights. The cameras were so designed as to take pictures when any large object moved in front of them. After exposing 15 rolls of film, he obtained 31 usablephotographs of 10 unique tigers. Remarkable as it may seem, Karanth has been able to use this data to estimate that Nagarhole has 13.3 to 14.7 tigers per 10,000 kms. This estimate fits impressively well with an independent estimate of 15.1 tigers per 10,000 kms that Nagarhole can be expected to support, based on the availability of prey in the area.

Now how does Karanth get such an apparently precise and accurate (accurate because of the close fit to the independent estimate of tigers that can be supported at Nagarhole) estimate? That is where the power of statistics comes in. Mark-recapture or capture-recapture isa standard technique for estimating thedensity of mobile animal populations.For example, of Karanth's 10 tigers, twowere captured (by the camera of course)only once each, five were captured twiceeach, two were captured five times eachand one tiger was captured six times.Karanth was able to individually identify the tigers by studying the pattern ofstripes on their bodies.Reliable methods areavailable to fit a statistical distribution to the observedand/or expected distributionof individuals captured once,twice, thrice, four times andso on. Once this is done, thenumber of individuals captured zero times, such thatthose not captured at all canbe estimated. From the number not captured and thenumber captured, the totalpopulation can be estimated.

Karanth's effort at usingcamera trap data and application ofcapture- recapture models:for estimating population densities is the first suchattempt for tigers. If used widely, itpromises to yield an independent andmore reliable estimate of tiger populations in India, compared to the presently available, rather unreliable estimates,based as they are on the traditionalmethod of pug-mark census.