Cast away on Mindoro island: could density dependence threaten tamaraw (Bubalus mindorensis) on the long term?

Bonenfant, C.; Rutschmann, A.; Burton, J.; Boyles, R.; García, F.; Tilker, A.; Schütz, E.

Endangered species, despite often living at low population densities, may undergo unexpected density-dependent feedbacks in the case of successful recovery or marked reduction in range. Because density-dependence dynamics can increase risk of extinction, these effects can hamper conservation efforts. In this study, we analyse the dynamics of the largest population of the tamaraw (Bubalus mindorensis), a Critically Endangered species endemic to the Philippines. The population is located within a < 3,000ha area in Mounts Iglit-Baco Natural Park (MIBNP), with limited expansion possibilities. We took advantage of a 21 year time series of tamaraw counts to estimate annual population growth rate and possible density-dependence using state-space models in a Bayesian framework, hence accounting for sampling errors in the counts. We found that the MIBNP tamaraw population has been increasing in size at an average rate of +5% per year, as would be expected given its protected status by law. However, this growth rate is relatively low compared to other similar-sized Bovinea species. Population growth showed strong spatial structuring, with a population growth close to +14% in the core area of protection, and a reduction of abundance of −27% at the periphery of its range inside MIBNP. That the annual population growth rate has progressively decreased since 2008 is concerning, and we interpret this result to be a sign of density-dependence. The isolated tamaraw population in MIBNP is currently experiencing a contraction of its range, likely caused by anthropogenic pressures forcing tamaraw to live at relatively high densities in the core zone where protection is most effective. Our study highlights the fact that, despite the encouraging results as indicated by continuous population growth over the last two decades, the long-term viability of the MIBNP tamaraw population remains uncertain.