The effects of forest degradation on arboreal apes within Sikundur, the Gunung Leuser Ecosystem, Northern Sumatra.

Abstract

Tropical forests are being destroyed at a rate of 1.5 acres every second due to human activities, thereby accelerating climate change through impacts on the carbon cycle and causing the extinction of species dependent on these habitats. In the face of such immediate and globally significant issues, there is a lack of robust scientific knowledge on how tropical deforestation and degradation affects ecosystem stability and the fauna that inhabit tropical forests. As anthropogenic disturbance removes available habitat for rainforest species and degrades remaining forests, a multitude of species are threatened. There is a need to develop methods to rapidly assess tropical forest structure and relate this to habitat quality for keystone species, like primates. Only upon understanding the impacts of degradation on forests and their inhabiting animals can effective conservation methods be planned. This project aims to investigate the effects of forest degradation on primates over a large study site using innovative data collection methods, as well as enabling the identification of areas of conservation importance and the modelling of future predicted climate change effects on the well-being of primates inhabiting degraded forests, addressing the possible synergistic effects of forest degradation and climate change on primate species at a landscape scale. The findings of this project show that Sikundur in Northern Sumatra, a degraded tropical forest, is highly climatically variable. This climatic variability in turn alters how and when siamang range within the forest canopy. Due to the structural and climatic heterogeneity of the Sikundur landscape, different primate species are more abundant in different areas, with more morphologically and behaviourally specialist species dependant on specific structural elements with the forest. Although identifying historical forest degradation is problematic do to microtopography variation in Sikundur, modelling of future climate change shows that both anthropogenic disturbance and microtopographic variation may render some areas of Sikundur less suitable for primate species in the future. For species with narrower habitat requirements, climatic change is likely to have more impact, disproportionately effecting sympatric species. This thesis contains four data chapters with an introductory chapter and a discussion chapter. Chapter 1 reviews the available literature on the potential impacts of forest degradation on arboreal primates within the study site. Chapter 2 assess the effects of forest structure on microclimates within tropical rainforest canopy, with detailed recording of temperatures within the canopy. Both data collection and microclimate modelling indicate a highly diverse climate environment in the Sikundur forest canopy, with vertical temperature gradients potentially having a substantial impact on arboreal primates. Chapter 3 relates the synergistic relationship between forest degradation and microclimate on the behaviour and ranging of siamang, Symphalangus syndactylus. Results suggest that siamang are limited in their ability to behaviourally thermoregulate effectively in low cloud cover due to the limiting factors of near-exclusive arboreality and territorial defence. Chapter 4 assesses the abundance of three primate species, Thomas’s langur Presbytis thomasi, the lar gibbon Hylobates lar verstitus, and siamang, in relation to anthropogenic disturbance and forest structure at a landscape scale. In this study, the more behaviourally and ecologically specialist lar gibbons show clear habitat preferences. Thomas langur are seemingly adverse to anthropogenic disturbance whilst siamang habitat requirements, despite extensive vegetation surveys, remain unclear. Chapter 5 models the effects of future predicted climate change on the habitat suitability of siamang and Sumatran orang-utan, Pongo abelii, inhabiting the degraded forests of Sikundur. Results in this chapter indicate that areas of forest degradation and areas subject to intense solar radiation due to forest structure variation will be less suitable for arboreal primates in the future. This research contributes to a greater understanding of the effects of selective logging and climate change on tropical forests, vegetation structure and climate change on primate behaviour and ranging, and sheds light on the prospect of primate species survival in the face of anthropogenic disturbance. Additionally, it provides innovative, cost effective methods for the study of 3-dimensional forest structure and arboreal microclimate and the analytical techniques that apply these data to potential conservation actions.