Cast away on Mindoro island: lack of space limits population growth of the endangered tamaraw (Bubalus mindorensis)

Abstract

Endangered species, despite living at low population density, may undergo 2 density-dependent feed-backs in case of successful recovery or marked reduction in range. When at work, density-dependence dynamics increases extinction risks 4 and hamper conservation efforts. The last population of the critically endangered tamaraw (Bubalus mindorensis) lives on a < 3, 000ha in Mounts Iglit-Baco 6 Natural Park (MIBNP) with very limited expansion possibilities. Tamaraw abundance has been monitored on a yearly basis from animal counts by 8 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 10 rate of +6% per year, which we found to be relatively low compared to other similar-sized Bovinea species. Population growth was strikingly spatially 12 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 14 periphery of the species range inside MIBNP. Highly concerning is the fact that the annual population growth rate progressively decreased significantly over the 16 years since 2008, which we interpreted as an evidence of density-dependence. This isolated tamaraw population is currently experiencing a contraction of its 18 range at MIBNP, likely caused by anthropogenic pressures forcing large herbivores to live at relatively high density in the core zone of the monitoring 20 where protection is most effective. Our study highlights that beyond the encouraging results of a continuous growth over the last two decades, the 22 MIBNP tamaraw population remains subject to uncertainty of its long term viability. 24 14 is a decrease of the population growth at low population abundance (i.e. demographic component, see Stephens et al. (1999)). On the other hand, 16 classical density-dependent effects, the decrease of the population growth rate with density (Nicholson 1933), on population dynamics are often overlooked in 18 conservation because they are a priori expected to occur at high population abundance. Increasing density in populations of endangered species may, 20 however, occur incidentally following a range reduction caused by habitat loss or successful protection measures, and in turn triggers undesirable ecological 22 consequences for its conservation.Density-dependence is a pervasive demographic responses among large