Most of us have a vague idea of how tigers are counted, though we are pretty sure the process is very different from conducting a human census. Unlike human beings who can be reached at homes and numbered, tigers are not waiting to be counted. Left to themselves, they would rather not have anything to do with the Homo sapiens variety. The fact that they are generally nocturnal, do not move in large groups and are largely unseen, makes the exercise that much more difficult. So, if conducting a countrywide human count is a long drawn out complicated exercise, arriving at tiger numbers is several times more challenging. Finally, even after applying the best available techniques, one can only get an approximate figure. How close that approximation is to the actual numbers depends entirely on the scientific execution of the census process.
A beginning is made by identifying the potential habitats of the striped cat. For a huge- sized country like India with several forest reserves spread across all four directions, this is an essential prerequisite because it tells us where to locate the tigers and then number them. Not doing so would be as foolish as arriving on Mars to count human beings. The exercise of identifying tiger habitats may sound unnecessary since we already have well defined tiger reserves, but the fact is that habitats keep changing their form, and any change in their composition impacts both the flora and the prey that are essential to tiger survival. Thus, theoretically we may have so-called tiger habitats that have run out of tiger population for a variety of reasons and we might also have new regions that could be inhabited by the big cat. A preliminary survey is, therefore, essential. “Monitoring Tigers and their Prey’, an excellent video guide on tiger census and edited by noted wildlife experts Dr Ullas Karanth and James D Nichols, offers us a scholarly insight into the process.
Written, filmed and directed by another wildlife enthusiast, Shekar Dattatri, the video takes us through the gamut of conducting a tiger count. And, although the identification of the feline’s habitat comes towards the end of the video presentation, it really sets in motion the procedures for tiger enumeration that are detailed earlier on in the guide. The purpose of the exercise, called Occupancy Sampling (or OS), is not to arrive at tiger numbers but to merely understand what proportion of the country’s landscape is occupied by the tigers and their prey, and how that occupancy has varied over a period of time. The important thing to remember here is that understanding the habitation of not just the tigers but their prey as well is essential at arriving at the final numbers. But, because the number is not the immediate issue in OS, it is not necessary to actually ‘see’ the animals – tigers or prey – either directly or with the help of camera images. Field workers have to depend on scat, scratch or other signs that indicate the possibility of tiger or prey habitation. As the video tells us, once a state’s forested region is identified as potential habitation for tigers, it is broken into several grid cells, each of roughly 200 square kilometres for the tiger. The idea is that the cell size should be greater than the average home range of the tiger: with the home range of the Indian big cat around 150 square kilometres, the video editors settled for the slightly higher figure. Each of these cells is then equally segmented and handed over to the field workers who look for tiger signs and detail them in the data sheets.
If Occupancy Sampling helps in identification of tiger habitats, the Camera Trap Sampling and the Capture-Recapture Sampling are direct methods to count the striped cats, and they can be put to good use in regions confirmed as tiger habitats through the method discussed earlier. Automatic cameras are placed on either side of a tiger trail or path (a trail is identified through signs like scratches, pug marks, scat etc.) at a height of some 45 centimetres from the ground. The cameras are fitted with a transmitter and a receiver that works on infra red technology to capture images. The need for such covert methods to photograph the tiger is simply because the big cats are extremely elusive and rarely spotted by enumerators. The numbers are arrived at through comparison of the stripes of the tigers, since no two tigers have identical stripes. The cameras, one each on either side, are essential since the stripe patterns on the two sides are different for a tiger, and the two images need to be clubbed together as that of a single tiger.
In the Capture-Recapture technique, repeated camera trap results are used to check what proportion of the tigers have been re-photographed. All the data so collected is then fed into the computer where especially designed software then wades through the material and arrives at a close approximation of the number of tigers.
But the most fascinating method of arriving at tiger numbers is an indirect one. The video calls it Line Transect Sampling (LTS). Working on the assumption that there is a direct correlation between tiger population and the number of prey available, this system estimates the prey population in a given habitat. The line transects can take the shape of either a straight line from point A to B, or a square or triangle. They should be randomly selected and not just close to locations like waterholes, otherwise we may end with overestimation of prey population. Some 20-40 such transects – each of them around four kilometres in length – would suffice for a habitat, according to the video.
Once the line is finalised, the field workers equipped with binoculars, range finders and compass walk straight along it and scan either side to sight prey animals. Binoculars will help them see those animals, range finders measure the distance between the workers and the prey first sighted and the compass will confirm the direction that will later help them calculate the perpendicular distance through its angle measurement. All of these statistics are then fed into the computer where again a software takes charge and churns out the prey estimate. Experts equipped with the prey population in a habitat can quite accurately estimate the number of tigers it has been supporting in the region.
In the end, these are all sampling methods and are, therefore, prone to error. It is good enough if these errors are within acceptable margins. So, the next time we are presented with tiger population figures, let us for a moment appreciate the complexity involved and pause to admire the various unknown and faceless field workers who make the exercise possible.