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

Abstract

The ecology and population dynamics of the endemic tamaraw on Mindoro island, Philippines, remains poorly documented so far. A better knowledge of its growth rate and abundance trend would potentially improve our ability to assess its status and long-term viability, and the feasibility of envisioned conservation actions such as translocation of individuals for reinforcement or captive breeding. Tamaraw abundance in Mounts Iglit-Baco Natural Park (MIBNP) has been monitored on a yearly basis from animal counts by Philippine authorities since the year 2000. Consistent with its protected status by law, the MIBNP tamaraw population has been increasing in size at an average rate of +6% per year, which we found to be relatively low compared to other similar-sized Bovinea species. Population growth was strikingly spatially structured within MIBNP with a population growth close to +16% in the core area of protection, while a reduction of abundance of -27% was measured at the periphery of the species range inside MIBNP. Highly concerning is the fact that the annual population growth rate progressively decreased significantly over the years since 2008, which we interpreted as an empirical evidence of density-dependence. This isolated tamaraw population is currently experiencing a contraction of its range at MIBNP, likely caused by anthropogenic pressures forcing large herbivores to live at relatively high density in the core zone of the monitoring where protection is most effective. Our study highlights that beyond the encouraging results of a continuous growth over the last two decades, the MIBNP tamaraw population remains subject to uncertainty of its long term viability.