Living on the edge 229


more empirical research into fine-scale species–environ- ment relationships. Tropical forests are heterogeneous environments with significant variation at finer temporal and spatial resolu- tions (Marsh et al., 2022). Landscape-scale models often do not include fine-scale variations in topography, logging intensity, forest structure and microclimate because they use categorical land units (e.g. primary forest, logged forest, agricultural land; Wearn et al., 2017). Microclimate varia- tions (i.e. sub-annual variations with spatial scales,1 km2) are important drivers of mammal distributions, behaviour and phenology (McCain & King, 2014; Buckley et al., 2018; Tamian et al., 2022). The inclusion of microclimate vari- ables in species distribution models has improved their performance for several mammal species (e.g. McCain & King, 2014; Varner & Dearing, 2014; Mathewson et al., 2020; Tamian et al., 2022), but the influence of microcli- mate on mammal habitat choice in tropical forests has not yet been tested. Microclimate variations are likely to be more extreme


in fragmented forests and near forest edges. Edge effects have been observed up to 1 km into forest patches (Laurance, 2004; Pohlman et al., 2007). Approximately 70% of forested areas are within 1 km of a forest edge (Haddad et al., 2015) and therefore a large proportion of forest habitat is likely to be subject to edge effects. Proximity to forest edges also leads to increased human disturbance from hunting and extraction of other re- sources. These changes are widely assumed to negatively affect mammals, with some studies reporting lower abun- dances of certain species at forest edges (e.g. Kinnaird et al., 2003). Species respond differently to these changes, and the most effective conservation strategies vary de- pending on the target species. Few studies to date have investigated how edge-related abiotic changes relate to mammal distribution and habitat use. In this study, we measured edge effects on forest struc-


ture and microclimate, and examined their impacts on forest mammals in a region of recovering secondary trop- ical forest near the edge of a protected area in Sumatra, Indonesia. Our data on fine-scale species–environment associations provide urgently needed information for the conservation of mammals in the region. We hypothesized that edge effects create an environment that is less favour- able for terrestrial forest mammals, resulting in them spending less time near forest edges. We also predicted that trees would be smaller and less well connected at the forest edge, resulting in increased light penetration through the canopy and higher temperatures. Our final prediction was that mammals would spend less time near forest edges, and that mammal detections would thus be asso- ciated negatively with temperature and light intensity, and positively with distance from the forest edge, larger trees and increased canopy connectivity.


Study area


We collected data from Aras Napal in the Sikundur region on the boundary of Gunung Leuser National Park in the North Sumatra province in Sumatra, Indonesia (Fig. 1). Sikundur comprises secondary lowland forest that was se- lectively logged before the establishment of the Park in 2004 (Orangutan Information Centre, 2009). Despite its protected status, illegal logging and hunting still occur in the area (Roth et al., 2020), although no data are available on the frequency and impacts of these activities. Aras Napal, which consists of c. 150 households, is adjacent to the Sikundur forest. Subsistence and smallholder agricul- ture are the predominant land uses outside the protected forest in Aras Napal, whereas the area to the north of the village comprises larger commercial rubber and oil palm plantations. Except for a few primate studies (Harrison et al., 2020; Roth et al., 2020; Hankinson et al., 2021, 2022), there are no published data on mammals in Sikundur.


Methods


We investigated fine-scale species–environment interac- tions and determined abiotic edge effects on terrestrial mammals by measuring forest structure, microclimate and mammal detections along four transects (Fig. 1). We liaised with landowners in Aras Napal to identify starting locations in orange plantations adjacent to the forest edge and the boundary of the National Park. We generated 2-km trans- ects with survey points at 0.5-km intervals from these start- ing locations using ArcGIS Pro (Esri, 2010). Where possible, we set up monitoring locations within 50mof these points. In some cases, monitoring locations had to be placed further apart because the intended survey points were inaccessible on foot.Weconducted camera-trap and climatemonitoring in all locations concurrently over 60 days during August– October 2019. To minimize disturbance-related changes in animal behaviour, we collected vegetation data only after we had completed the remote monitoring period.


Forest structure


Wemeasured forest structure in 25 × 25mplots at allmon- itoring locations. Within each plot, we recorded the total number of trees with a circumference at breast height of .31.4 cm. For each tree, we recorded circumference at breast height (cm), total height (m), bole height (m), north–south crown width (m), east–west crown width (m) and crown connectivity with neighbouring crowns (%). We used the tree circumference to calculate the diam- eter at breast height (cm), and estimated crown area (m2) from the crown widths in north–south and east–west directions.


Oryx, 2024, 58(2), 228–239 © The Author(s), 2023. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605323000212


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