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The Influence of Forest Structure on Sumatran Orangutan Nesting and Densities in the Lowland Dipterocarp Forests of North Sumatra

Abernethy, J (2019) The Influence of Forest Structure on Sumatran Orangutan Nesting and Densities in the Lowland Dipterocarp Forests of North Sumatra. Doctoral thesis, Liverpool John Moores University.

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Prior studies on Bornean orangutans have suggested that orangutans were primarily selecting nest sites that provided more stable and sheltered platforms thereby offering greater comfort rather than selecting for nests which reduced the risk of predation by limiting access to ground predators (Ancrenaz et al. 2004a; Cheyne et al. 2013). The aims of this study were to answer three questions: why are nests built in a particular tree, why was the nest in that position and height within the tree and why build the nest in that part of the forest and not another? Nest and forest structure data were recorded from transects conducted at two sites in North Sumatra; Sikundur (a naturally recovered site) and Sei Betung (reforested site). Selection patterns for tree and forest structure variables showed no significant difference across sites when modelling nest trees against forest trees. Nest trees had properties that both increased both comfort through nest stability and shelter from adverse weather as well as attributes which reduced the threat from predators. However, a preference for trees with narrow trunks was also found, which would limit access to ground predators but also reduce stability, suggesting that predator avoidance may take precedent over comfort. A greater proportion of Position 2 and 3 nests were found in Sei Betung where tree height and connectivity are lower, further suggesting that predator avoidance has a greater influence than comfort or stability on Sumatran orangutan nest placement. The lower connectivity in Sei Betung was associated with increased rarity of tree-tied nests (Position 4) compared to Sikundur which went against our expected results based on those of Prasetyo et al. (2009). Over 60% of nest trees in Sei Betung held multiple nests, suggesting that there is significant pressure upon preferred nest sites in Sei Betung. Nest densities were most closely associated with variables linked with forest recovery such as higher canopy density, fewer gaps, increased rugosity and higher stem density. Nest densities were also associated with reduced distance to the edge of the forest, further research is needed to determine whether this is due to orangutans utilising neighbouring plantations. Our results show the need for protecting habitat not only for preferred food species but also nest trees.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: Orangutan; Sumatran orangutan; Pongo abelii; Forest structure; Nest ecology; Nest site selection; Predator avoidance; Reforestation; Sumatra
Subjects: Q Science > QL Zoology
Divisions: Natural Sciences & Psychology (closed 31 Aug 19)
Date Deposited: 07 Jun 2019 09:20
Last Modified: 28 Nov 2022 13:02
DOI or ID number: 10.24377/LJMU.t.00010841
Supervisors: Wich, S, Koyama, N and Nowak, M
URI: https://researchonline.ljmu.ac.uk/id/eprint/10841
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