Assessing Bornean orangutan (Pongo pygmaeus) populations within the Sebangau National Park, Borneo: An analysis of current and emerging nest survey methods

Abstract

Escalating anthropogenic pressures exert a considerable burden on the natural world, leading to dramatic declines in wildlife populations globally. Gaining a comprehensive understanding of population trends is critical for the formation of effective conservation strategies. This thesis evaluates the value of nest surveys for quantifying populations of the critically endangered Bornean orangutan (Pongo pygmaeus). The first data chapter uses a long-term dataset from the Sebangau National Park, Central Kalimantan, to produce more accurate estimates for the parameters used to convert nest densities into population estimates. Overall, the proportion of nest builders in a population (‘p’) was estimated at 0.79 and the rate of nest building (‘r’) was estimated at 1.19. No significant effect of age-sex class on nest-building rates was found. The nest decay rate (‘t’) was estimated at 424 days and was found to be significantly impacted by nest height and annual weather patterns. All parameter estimates produced differ from previous estimates used for surveys at the site. Substantial temporal variation in nest decay rates was found, suggesting that population estimates produced from short sampling periods may be inaccurate, which has considerable implications for great ape population estimates using nest surveys. The second data chapter evaluates the potential of drones for conducting nest surveys by comparing nest counts from ground and drone transects. The drone surveys detected 17.2% of the total number of nests estimated from traditional ground surveys. Ground and aerial nest counts were not significantly correlated, and wide confidence intervals mean caution needs to be exercised if these methods are to be used for orangutan population estimates. Nest detection rates were significantly affected by nest age, canopy cover, and nest height. Overall, it is concluded that despite having higher inaccuracies than previously thought, nest surveys can still provide valuable insights into populations, and it may be possible to combine both aerial and ground nest surveys to increase spatial scale of surveys, so long as caveats are acknowledged.