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The tiger trail

The camera-trapping method has become a reliable tool in estimating endangered animal populations, says SANJAY GUBBI.


Dr. Ullas Karanth with Samba Kumar setting up camera trapping equipment in Nagarahole.

THERE was a time when anyone who chanced upon a tiger would let the rest of the world know this by the pitch of the scream that followed. Strange are the ways of men, because, today, it is the tiger which roars when he sets sight on humankind; because it is the tiger who is being hunted out of his habitat, for money and for glory, or vainglory more likely. There may be a tiger heaven somewhere, however, because someone up there takes care of this magnificent, endangered beast by making sure that wildlife enthusiasts dot the globe.

Their numbers may be small, but the work they do is as wonderful as the animal they are trying to save. One method they use in their overall strategy of conservation is simply to count tiger numbers, thereby setting up a data bank from which other things naturally follow. What has turned into an almost crucial method of estimating tiger populations is the camera-trapping method.

Before we get into the technique which is as fascinating to the layman as it is important to the conservationist, it might be pertinent to learn the different types of methods that are used to count tigers. One name that crops up in any "tiger talk" has to be Dr. K. Ullas Karanth, of the Wildlife Conservation Society (WCS), India Program, who has made of the tiger a lifetime's work. Quoting Karanth extensively is inevitable. Karanth then, first of all, says the tools vary — from notebooks, pugmark tracers and plaster casts to radio-tracking gear and finally, camera traps.

Here is what happens in tiger habitats across the world.

In the Russian snow-track counts, the distance between geographical locations and size are used to assign different sets of tracks to individual tigers, without identifying them by using track shape.

In Nepal, trackers sometimes recognise tigers to a certain extent.

The Indian pugmark counts are inconsistent because of wrong identification of tracks and other reasons like soil substrate, etc. Although using tracks in conjunction with sampling techniques is a good option for knowing crude trends, routine management needs a working knowledge about tiger population trends in terms of increase, decrease or stability, rather than exact tiger numbers. That is an obvious common-sense approach. Karanth urges that tiger signs by field teams should be based on replicated sample surveys. Treating each data point as a count or frequency of tiger sign observed is more reliable.

However, what if you really want to count tigers or their prey accurately? As Karanth puts it, "You cannot go to the moon on a bullock cart." Formal estimation methods, then, are the order of the day; in the form of line transect surveys for prey species; photographic capture-recapture surveys for tigers. Another approach that is likely to gain popularity in the future is individual identification using DNA extracted from tiger scats or hair.

One perspective on the power of camera trapping states that it can be less prone to error if the technique is used for sampling rather than using it to carry out a census (which usually means a total count) of the tiger population. Some conservationists have pointed out that, sometimes, camera trapping is not practical, especially where the tiger densities are one to two tigers/100 sq. km. At such sites, radio telemetry offers the only option for estimating tiger densities. But this is a very expensive option.

Photographic capture-recapture estimates of tiger abundance were first obtained in the world by Karanth in Nagarahole in 1995. Karanth and internationally acclaimed quantitative ecologist Jim Nichols of the United States Geological Survey (1998) then analysed camera trap data from four sites in India using the CAPTURE programme and derived the first rigorous estimates of population densities for wild tigers by publishing their findings in the prestigious journal Ecology.

Instead of catching and marking tigers individually, Karanth innovatively used the "natural markings" in tigers for capture-recapture analysis. The stripe patterns in tigers are unique to each animal. Once an animal is photographically "caught" and "marked" with the help of its stripe patterns, the data obtained from subsequent "recaptures" are amenable to analysis under the well-established theoretical framework.

Thus, the essence of capture-recapture survey is to build "capture" histories for each individual animal caught during the sampling period and use this data to eliminate tiger population through rigorous statistical models.

A camera trap consists of a flash-equipped, simple aim and shoot, auto-focus camera wired to an electronic tripping device, not unlike those used in burglar alarms. When the tiger comes down the path where the camera has been set up, it breaks the invisible electronic beam that triggers the camera and takes its own pictures. The shape and arrangement of stripes on a tiger are distinctive enough for individuals to be identified. If clear photographs can be taken, then an efficient counting system is in place. Camera traps are mounted approximately 350 cm away on either side of a trail, with the infrared beam set at a height of 45 cm. The cameras are protected by a hood.

Tigers regularly travel along certain trails, communicating with other tigers through scent markings. The date, time and location of each photographic capture of a tiger are noted. Since stripe patterns vary from animal to animal, this is an excellent method for estimating tiger densities. An important point is that camera units are deployed from dusk to dawn as most tigers are nocturnal and this would also minimise tripping by other animals. Another crucial factor in assessment is that the capture-recapture sampling method assumes that individual tigers using the sampled area should have some probability of visiting these sites. There is no assumption that every tiger in the sampled area will and must visit the trap sites during the survey.

Only a partial count is assumed to be obtained during sampling and the total population, including "uncaught" animals, is calculated based on estimated capture probabilities. A better choice of sites would yield higher capture probabilities and a higher proportion of animals in the area may then be caught. The resulting estimates would then be statistically more precise. Since both cases measure probabilities, the proportion of animals caught in both instances remain valid. Camera-trap photography, therefore, becomes a truly useful tool in combination with the capture-recapture sampling method.

Till date, this method has been used by Karnath and his research team in 12 different sites across the country. Camera trapping has been carried out in various sanctuaries across a variety of habitats including the wet evergreen forests of Namdapha reserve on the Burmese border, to the semi-arid forests of the Ranthambore Tiger Reserve in Rajasthan to the flood plain swampy grasslands of Kaziranga, Assam. Trapping is currently on in the Tadoba and the Melghat tiger reserves in Maharashtra, and Panna in Madhya Pradesh. Results obtained are no less than stunning.

Of all the study areas, Kaziranga showed (mathematically calculated) the highest density of tigers of 16.8 tigers/100 sq. km. In Karnataka, figures for the Nagarahole National Park showed 11.9 tigers and for the Bandipur tiger reserve at 11.97 tigers/100 sq. km.


On film ... Tigress with cubs at the Bhadra Tiger Reserve.

Apart from these big cats, several other interesting and secretive fauna appear in camera traps. The Malayan Sun Bear that was thought to have been extinct showed its presence in the camera traps at Nampdapha. The ever-elusive Clouded Leopard took its self-portrait, also at Nampdapha. The rarely seen Striped Hyena appeared in photos in the Ranthambore Tiger Reserve.

Camera trapping has implications from the point of conservation. The research results indicate that tiger densities in optimal habitats in India are 10-20 times higher than those in poorer habitats such as the Russian Far East, where tiger densities are of the order of 0.5-1.5 tigers/100 sq. km.

This means that even relatively smaller reserves in India can support a large number of tigers. Even smaller tiger populations with six to 12 breeding females (approximately 24 to 48 tigers) may be demographically viable in a 100-year-time frame. It is likely that even small reserves that are of an area of 300-3,000 sq. km, in southern Asia, can potentially support such demographically viable populations of tigers.

Wildlife First Features

The writer is a wildlife expert.

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