Ecological correlates of chimpanzee (Pan troglodytes schweinfurthii) density in Mahale Mountains National Park, Tanzania

Abstract

Understanding the ecological factors that drive animal density patterns in time and space is key to devising effective conservation strategies. In Tanzania, most chimpanzees (~75%) live outside national parks where human activities threaten their habitat's integrity and connectivity. Mahale Mountains National Park (MMNP), therefore, is a critical area for chimpanzees (Pan troglodytes schweinfurthii) in the region due to its location and protective status. Yet, despite its importance and long history of chimpanzee research (>50 years), a park-wide census of the species has never been conducted. The park is categorized as a savanna-woodland mosaic, interspersed with riparian forest, wooded grassland, and bamboo thicket. This heterogeneous landscape offers an excellent opportunity to assess the ecological characteristics associated with chimpanzee density, a topic still disputed, which could improve conservation plans that protect crucial chimpanzee habitat outside the park. We examined the influence of fine-scale vegetative characteristics and topographical features on chimpanzee nest density, modeling nest counts using hierarchical distance sampling. We counted 335 nests in forest and woodland habitats across 102 transects in 13 survey sites. Nests were disproportionately found more in or near evergreen forests, on steep slopes, and in feeding tree species. We calculated chimpanzee density in MMNP to be 0.23 ind/km2, although density varied substantially among sites (0.09-3.43 ind/km2). Density was associated with factors related to the availability of food and nesting trees, with topographic heterogeneity and the total basal area of feeding tree species identified as significant positive predictors. Species-rich habitats and floristic diversity likely play a principal role in shaping chimpanzee density within a predominately open landscape with low food abundance. Our results provide valuable baseline data for future monitoring efforts in MMNP and enhance our understanding of this endangered species' density and distribution across Tanzania.

Fig 1. Map of MMNP and its position within the GME and Tanzania.

The 13 survey sites visited during the current study are indicated with their letter name. Land cover classification courtesy of Holly E. Copeland (University of Wyoming) who sourced the data from USGS/NASA Landsat imagery.

Fig 2. Graph showing vegetation type percentages observed at each site.

We distinguish forests according to canopy cover (open forest <50% coverage; closed-canopy ≥ 50%). “Other” includes non-wooded vegetation types (e.g., grassland, swamp).

Fig 3. Predictor variable plots from the top-ranked model of nest density.

(a) Plot of coefficient estimates (circles) presented with 95% CI (vertical lines), confirming their significance (because CI does not cross zero); (b) response curves of predicted nest density against topographic heterogeneity and (c) total basal area.

Fig 4. Selection of vegetation types observed in MMNP, illustrating its mosaic landscape.

(a) lowland closed canopy forest; (b) miombo woodland; (c) grassland; (d) lowland bamboo woodland (Photos courtesy of A.C.).