Line Transect and Triangulation Surveys Provide Reliable Estimates of the Density of Kloss’ Gibbons (Hylobates klossii) on Siberut Island, Indonesia

Abstract

Estimating population densities of key species is crucial for many conservation programs. Density estimates provide baseline data and enable monitoring of population size. Several different survey methods are available, and the choice of method depends on the species and study aims. Few studies have compared the accuracy and efficiency of different survey methods for large mammals, particularly for primates. Here we compare estimates of density and abundance of Kloss’ gibbons (Hylobates klossii) using two of the most common survey methods: line transect distance sampling and triangulation. Line transect surveys (survey effort: 155.5 km) produced a total of 101 auditory and visual encounters and a density estimate of 5.5 gibbon clusters (groups or subgroups of primate social units)/km2. Triangulation conducted from 12 listening posts during the same period revealed a similar density estimate of 5.0 clusters/km2. Coefficients of variation of cluster density estimates were slightly higher from triangulation (0.24) than from line transects (0.17), resulting in a lack of precision in detecting changes in cluster densities of <66 % for triangulation and <47 % for line transect surveys at the 5 % significance level with a statistical power of 50 %. This case study shows that both methods may provide estimates with similar accuracy but that line transects can result in more precise estimates and allow assessment of other primate species. For a rapid assessment of gibbon density under time and financial constraints, the triangulation method also may be appropriate.

Population mapping of gibbons in Kalimantan, Indonesia: correlates of gibbon density and vegetation across the species’ range

“…Data on tree species are not available for all areas due to a lack of qualified botanists. Data from Sabangau are from Hamard et al (2010), data from Bawan and Mungku Baru are from Harrison et al (2010Harrison et al ( , 2012. Data from Tujang and Borah are from Cheyne et al (2012), and data from Sungai Wain are from Gilhooly et al (2015).…”

Ecological and Behavioural Factors Influencing Territorial Call Rates for the Bolivian Titi Monkeys, Plecturocebus modestus and Plecturocebus olallae

“…Besides the logistic advantage of this technique that direct observation of every group in a survey area is not required, its accuracy relies on knowing the daily rate of calls to be used as a correction factor. Nevertheless, this correction factor is often unknown, and some studies have deduced or extrapolated call rates from other species [Brockelman and Ali, 1987;Nijman and Menken, 2005;Höing et al, 2013]. For example, the territorial calls of the Beni titi monkey (Plecturocebus modestus) and Olalla brothers' titi monkey (P. olallae) , primates endemic to Bolivia (formerly: Callicebus modestus and C. olallae , respectively [Lönnberg, 1939;Hershkovitz, 1990;Felton et al, 2006;Martinez and Wallace, 2007, 2013Byrne et al, 2016]), both categorized as Endangered [Veiga et al, 2008a, b], were used to assess population density and size, but due to a lack of behavioural information the correction factor was estimated from short observations [López-Strauss, 2008;López-Strauss and Wallace, 2015].…”

Population density of Ecuadorian mantled howler monkeys (Alouatta palliata aequatorialis) in a tropical dry forest, with information on habitat selection, calling behavior and cluster sizes

“…This information was collected to estimate population density using distance sampling methods and assess habitat selection based on triangulated call locations. Call triangulation allows information on animal locations to be collected from environments where surveys might otherwise be difficult, due to impenetrable undergrowth or sharply varying topography (Höing et al 2013). Call locations were analyzed to investigate use of replanted and naturally regenerating areas, and selection for other habitat characteristicsaltitude, slope, aspect, distance to roads and streams and land cover type.…”

“…Visits to a single location were separated by a minimum of 12 h and sample locations were at least 1 km apart. Triangulation, which uses multiple observations from different locations to infer the position of calling animals (Buckland et al 2008; for an example in primates see Höing et al 2013), was used to determine the locations of howler monkeys heard during the sampling period. At each sampling location, three listening points were designated 100 m apart (black points on Figure 2).…”