768 K. Syxaiyakhamthor et al.
(Duckworth, 2008). Thus, Nam Et-Phou Louey National Protected Area represents the best opportunity for the long- term survival of this species. However, its status is poorly known; reliable national population estimates are not avail- able because most remaining habitat patches have not yet been surveyed. Reliable population estimates are important for deter-
mining threat levels and prioritizing conservation actions (Rawson, 2010). Understanding the relationship between gibbons and their habitats is essential for effective conserva- tion planning (Hamard et al., 2010). As gibbons are strictly arboreal and mainly frugivorous, their abundance and den- sity in a particular area have typically been associated with ecological characteristics that support feeding, vocalizing, sleeping and sheltering (Hamard et al., 2010). Many gibbon species inhabit primary tropical forests (Geissmann, 2007; Gray et al., 2014) containing a high density of flowering and fruiting food plants (Wich & Van Schaik, 2000). Although they are typically associated with evergreen for- ests, they can also be observed in deciduous and mosaic for- est patches (Phoonjampa et al., 2011; Light, 2016). Gibbons tend to be sensitive to human presence, preferring undis- turbed habitats with a continuous canopy of tall trees (Phoonjampa et al., 2011). The aims of this study were to estimate the abundance
and density of N. leucogenys and identify the variables influ- encing the species’ spatial distribution in Nam Et-Phou Louey National Protected Area, Lao. Our findings will provide managers with the baseline data on gibbon status necessary for designing priority conservation areas and improving the management of suitable forests, ensuring that gibbon habitat stays intact and that connectivity is maintained, and will support more effective patrol planning.
Study area
The 5,950km2NamEt-Phou Louey National Protected Area of north-eastern Lao (Fig. 1) is one of the country’s largest National Protected Areas. The 3,000 km2 core area forms a totally protected zone in which access and harvest are pro- hibited. The remaining 2,950km2 are in amanagement zone in which sustainable harvest of specified animals and plants for local subsistence is permitted (Johnson et al., 2012). Altitude is 400–2,257 m, with .60% of the area above 1,000 m and 91% on slopes of .12% incline. The tem- perature ranges from ,5 °C (December–January) to 30 °C (May–June). Annual rainfall is 1,400–1,800 mm (Vieng Xai weather station data from 2003), with a rainy season (May– October), a cold dry season (November–January), and a hot dry season (February–April). The landscape of the Protected Area has a long history of human settlement, resulting in many patches of secondary forest, bamboo stands, and an- thropogenic grasslands traditionally burned for hunting and cattle grazing (Johnson, 2012). The majority (72%) of
the Protected Area is covered by mixed evergreen and de- ciduous forests up to 1,500 m, transitioning into evergreen forest at 1,500–1,800 m that is interspersed with Fagaceae (primarily Castanopsis and Lithocarpus)and Rhododendron species above 1,800m(Davidson, 1999). These forested areas are embedded in a mosaic of old shifting cultivation fallow and bamboo groves (Johnson, 2012). Approximately 50 spe- cies of mammals and 290 species of birds have been recorded (Davidson, 1999).
Methods
Field survey We used an auditory sampling survey to count the number of gibbon groups within a defined area, utilizing fixed radius point counts of gibbon vocalization. We focused on duets, the species’ most distinctive vocalization, during which the bonded adult male and female sing simultaneously.Wesur- veyed 34 sites during May–August 2014 and six sites in May 2015,totalling 157 days and covering 125.6 km2.We set up lis- tening points in 40 locations separated by at least 2,000 m, in four of the Protected Area’smanagement sectors (Fig. 1), covering moist evergreen (18 sites) and mixed deciduous forest areas (22 sites). To maximize gibbon audio detection, we preselected listening points using a topographical map, placing them at high altitude on mountain ridges or tops, where the gibbons’ calls can be heard from a greater dis- tance. The field survey was conducted simultaneously by two teams working separately at different listening points, with each team consisting of a main observer and two or three assistants trained in gibbon survey techniques. At a given listening point, the team surveyed gibbons for four consecutive days, starting from c. 5.00 until c. 10.00,or until gibbons had ceased vocalizing for at least 30 min (Cheyne, 2008; Hamard et al., 2010; Coudrat et al., 2015). Gibbon duets can be heard from a distance of up to 2 km under favourable conditions (Brockelman & Srikosamatara, 1993). However, to avoid counting the same group multiple times simultaneously from different listening points, and to minimize errors in groups singing beyond the typically audible distance, only groups detected within a 1-km radius from the listening point (as determined by estimated distance and triangulation) were included for estimating abundance (Brockelman & Ali, 1987). Thus, we defined the area within a 1-kmradius (3.14 km2) around each listening point as the ef- fective listening area for calculating gibbon density (number of groups per km2).To count the number of groups on any day at each point, observers recorded all duets.We considered duet start and end times, as well as compass angles and distance between the observer and the calling gibbon, to determine the location of calling animals and thus avoid double counting. We regarded duets with different start and end times, and plotted at locations .500 m apart, as different groups.
Oryx, 2020, 54(6), 767–775 © 2019 Fauna & Flora International doi:10.1017/S0030605318001515
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