Assessing tiger population dynamics using photographic capture-recapture sampling

Abstract

Although wide-ranging, elusive, large carnivore species, such as the tiger, are of scientific and conservation interest, rigorous inferences about their population dynamics are scarce because of methodological problems of sampling populations at the required spatial and temporal scales. We report the application of a rigorous, noninvasive method for assessing tiger population dynamics to test model-based predictions about population viability. We obtained photographic capture histories for 74 individual tigers during a nine-year study involving 5725 trap-nights of effort. These data were modeled under a likelihood-based, "robust design" capture-recapture analytic framework. We explicitly modeled and estimated ecological parameters such as time-specific abundance, density, survival, recruitment, temporary emigration, and transience, using models that incorporated effects of factors such as individual heterogeneity, trap-response, and time on probabilities of photo-capturing tigers. The model estimated a random temporary emigration parameter of γ^"=γ^'=0.10±0.069 (values are estimated mean±se). When scaled to an annual basis, tiger survival rates were estimated at S^=0.77±0.051, and the estimated probability that a newly caught animal was a transient was τ^=0.18±0.11. During the period when the sampled area was of constant size, the estimated population size N^_t varied from 17±1.7 to 31±2.1 tigers, with a geometric mean rate of annual population change estimated as λ=1.03±0.020, representing a 3% annual increase. The estimated recruitment of new animals, B^_t, varied from 0 ± 3.0 to 14 ± 2.9 tigers. Population density estimates, D^, ranged from 7.33 ± 0.8 tigers/100 km² to 21.73 ± 1.7 tigers/100 km² during the study. Thus, despite substantial annual losses and temporal variation in recruitment, the tiger density remained at relatively high levels in Nagarahole. Our results are consistent with the hypothesis that protected wild tiger populations can remain healthy despite heavy mortalities because of their inherently high reproductive potential. The ability to model the entire photographic capture history data set and incorporate reduced-parameter models led to estimates of mean annual population change that were sufficiently precise to be useful. This efficient, noninvasive sampling approach can be used to rigorously investigate the population dynamics of tigers and other elusive, rare, wide-ranging animal species in which individuals can be identified from photographs or other means.