Consequences of variation in male harem size to population persistence: Modeling poaching and extinction risk of Bengal tigers (Panthera tigris)

Background/Question/Methods

Poaching of tigers (Panthera tigris) and its prey base are considered important factors influencing wild tiger populations. Previous models, designed to understand the effect of poaching on the viability of wild tiger populations, either ignored their mating system or largely overlooked the possible effects of variation in male territory size on population persistence. We present a two-sex matrix model for the Bengal tiger in which either strict polygyny or partial polygynandry combined with either no variation, individual variation, temporal variation or both individual and temporal variations in the territory size of breeding males (i.e., harem size) occur. We parameterize our model using demographic rates based on extensive field data on tigers. We use our model to simulate four different poaching scenarios in the tiger population of the Kanha landscape in India, representing a healthy habitat that can support a viable population of tigers and their prey: (1) poaching-free environment, (2) annual poaching of two tigers, (3) annual poaching of four tigers, and (4) annual poaching of six tigers.

Results/Conclusions

We demonstrate that tiger populations in healthy habitats such as the Kanha landscape are very sensitive to unnatural additive or partially additive mortality factors. When we assume equal poaching probability among breeders and transients combined with strict polygyny, extinction probability increases from 2.2% chance of extinction under poaching of two tigers, to 62.6% chance of extinction under poaching of four tigers annually. When six tigers are being poached annually, in almost all cases the entire population goes extinct in 21.5 years. Similar results are also evident when relaxing the assumption of either equal poaching probability among breeders and transients, strict polygyny, or both. Introducing individual variation (within generations) in the territory size of breeding males (i.e., harem size) has only a minor effect on the results. However, when either temporal variation (between generations) or both individual and temporal variations in the territory size of breeding males are being introduced the extinction risk of the tiger population increases dramatically. Our results indicate that although female tigers are the limiting factor in sustaining the tiger populations in a given area, males also have a significant role in determining the ability of such populations to persist for a long period of time.