Estimation of tiger densities in India using photographic captures and recaptures

Ecology, Dec, 1998 by K. Ullas Karanth, James D. Nichols

INTRODUCTION

The tiger (Panthera tigris) is an endangered big cat whose demographic status is uncertain across its entire distributional range, spanning 13 Asian countries. Because of their large body size and carnivorous diet (Eisenberg 1981), tigers naturally occur at low population densities. Further, wild tiger populations are now being affected by adverse factors such as prey depletion due to overhunting (Karanth and Stith, 1998), tiger poaching (Kenny et al. 1995, WCS 1995), and habitat shrinkage and fragmentation (Wikramanayake ctal., 1998). Although recent field surveys, combined with forest cover maps, have generated more accurate distribution maps for tigers (Wikramanayake et al., 1998), their utility for assessing the status and viability of tiger populations is limited by the absence of reliable data on population densities.

Tigers are adapted to a wide range of environments (Schaller 1967, Sunquist 1981, Seidensticker and McDougal 1993) through a social organization that permits considerable behavioral plasticity (Sunquist 1981, Smith et al. 1987, Smith 1993). based on their studies of prey selection in Nagarahole Park, India, Karanth and Sunquist (1995) suggested that ecological densities of tigers and other syntopic predators may be governed primarily by how their prey community is structured, in terms of abundances of different size classes. According to their predictions, where tigers and leopards occur syntopically, if both large and medium-sized prey are abundant, tigers would select large prey, enabling the coexistence of leopards at high densities. Where large prey are scarce, tigers would switch to medium-sized prey and reduce leopard densities through competition, as hypothesized for Chitwan Park, Nepal (Seidensticker et al. 1990). On the other hand, if both large and medium-sized prey are scarce, leopards would be relatively more abundant because of their ability to survive on smaller prey, as recorded in Huai Kha Khaeng, Thailand (Rabinowitz 1989). In the absence of reliable data on tiger and prey densities, however, it is difficult to test these predictions. Therefore, establishing theoretically sound and practically feasible population sampling methods to estimate densities of wild tiger populations is critically important for both scientific and management reasons.

Most prevailing methods of counting wild tigers appear to fail, because they are unable to dent with three important ecological characteristics of the species: scarcity, extensive range, and secretiveness. Because of their secretive behavior, tigers cannot be visually counted under usual field conditions. Consequently, most methods depend on counting tiger tracks. In India, the official "censuses" of tiger populations are based on the assumptions that each individual tiger can be identified by its unique track shape, and that track prints of every tiger can be simultaneously found and recorded (Panwar 1979). Because both of these fundamental assumptions are demonstrably erroneous (Karanth 1987, 1988, 1995), the results are neither total counts nor valid sample statistics. Although methodologically better, the snow-track counts used by Russian scientists also have problems that may result in undercounts (E. N. Smirnov and D. G. Miquelle, unpublished manuscript). In Chitwan Park. Nepal, counts of long-term resident tigers are made by experienced naturalists from individual identification of tracks, based primarily on injury-related differences (McDougal 1977; C. McDougal, personal communication). This site-specific technique still does not solve the general problem of density estimation at other sites or of counting tigers including transients and juveniles. The application of radiotelemetry to estimate tiger densities (Sunquist 1981, Smith 1993) is constrained by the small number of animals that can be tagged simultaneously, uncertainty about numbers of untagged tigers, and the high costs and effort involved.

Based on the fact that tigers are individually identifiable from their stripe patterns (Schaller 1967, McDougal 1977), Karanth (1995) has demonstrated the potential for estimating their population size using photographic "captures," within the theoretical framework of formal capture-recapture theory (reviewed by Nichols 1992). However, his pilot study (Karanth 1995) was constrained by the small size of the population sampled and the arbitrariness associated with the sampling design. In this paper, we report results of a detailed study in which we overcame these problems and estimated tiger densities using photographic capture-recapture sample surveys at four ecologically representative sites in India. Our study had the following objectives:

1) To develop field methods and sampling designs appropriate for camera-trapping tigers, and to evaluate the suitability of different capture-recapture models (see Otis et al. 1978, White et al. 1982, Pollock et al. 1990, Nichols 1992) for estimating tiger population size and density under field conditions.