Abundance and distribution of sympatric gibbons in a threatened Sumatran rain forest

Abstract

Agile gibbons (Hylobates agilis) and siamangs (Symphalangus syndactylus) are sympatric small apes inhabiting threatened forests of Sumatra, Indonesia. We censused both species in the 3,568-km2 Bukit Barisan Selatan National Park, at the southern limit of their ranges, over a 7-mo period in 2001. First, we monitored daily calling rates from known populations to develop probabilities of calling during a specified number of days and used the probability of calling at ≥1 time during 3 days to convert calling rates to abundance. Next, we used 3-day calibrated call count censuses (n = 31) stratified by distance from forest edge and across a range of elevations to estimate species-specific group densities. We used group size from the known populations as well as data collected ad libitum during the census to convert group density to individual density. Agile gibbon group density averaged 0.67 km-2 (SE = 0.082) and group size averaged 2.6 (SE = 0.73) for a population estimate of 4,479 (SE = 1,331) individuals. Siamang group density averaged 2.23 km-2 (SE = 0.245), and group size averaged 3.9 (SE = 1.09) for a population estimate of 22,390 (SE = 8,138). Agile gibbon and siamang densities are negatively correlated, with agile gibbons more abundant in mid-elevation forests and siamangs most abundant in lowland and submontane forests. The small group sizes of agile gibbons indicate potential survival problems in infant and juvenile size classes. Although neither species is presently threatened by direct human disturbance, continued deforestation will jeopardize the long-term viability of both species in Bukit Barsian Selatan National Park and on Sumatra.