Population dynamics of the Manyara monkey (Cercopithecus mitis manyaraensis) and vervet monkey (Chlorocebus pygerythrus) in Lake Manyara National Park, Tanzania

Abstract

Estimating population densities and their trends over time is essential for understanding primate ecology and for guiding conservation efforts. From 2011 through to 2019, we counted two guenon species during seasonal road transect surveys in Lake Manyara National Park: the Tanzania-endemic Manyara monkey Cercopithecus mitis manyaraensis (International Union for Conservation of Nature and Natural Resources, IUCN, Red List category of "endangered") and the vervet monkey Chlorocebus pygerythrus (Red List category of "least concern"). To account for imperfect detectability, we analysed the data in a line distance sampling framework, fitted species-specific detection functions, and subsequently estimated seasonal densities. To test for seasonal differences and yearly trends in the species-specific density estimates, we fitted generalized additive models. Seasonal point density estimates fluctuated considerably during the 9 years (2011-2019) of our study, ranging from 3 to 29 individuals km ${}^{-2}$ for Manyara monkeys and from 19 to 83 individuals km ${}^{-2}$ for vervet monkeys. Densities of both taxa did not differ seasonally, and we did not detect marked directional population trends. Our study illustrates the utility and limitations of line distance sampling for long-term primate monitoring. Beyond informing primate ecology and management, our results highlight the conservation importance of Lake Manyara National Park for primate conservation.

Figure 1

Map of Lake Manyara National Park (LMNP): thin black lines denote the border of the national park; dashed lines denote the border between the old park and the park extension (Marang Forest); thick black lines represent transects; light green polygons show the approximate position and size of forests, extracted from Loth (1999) and CCI Land Cover (2016); (a) blue dots represent sightings of Manyara monkeys during 2015–2019 surveys; and (b) green dots represent sightings of vervet monkeys during 2015–2019 surveys. Base map by Esri.

Figure 2

Frequency of sightings (blue histogram) at each perpendicular distance bin as well as the fitted detection functions (red line) for (a) Manyara monkeys (conventional distance sampling, hazard rate key function with cosine series expansions) and (b) vervet monkeys (conventional distance sampling, negative exponential key function with cosine series expansions) along road transects in Lake Manyara National Park, Tanzania.

Figure 3

Seasonal (LR – long rains, Dry – dry season, and SR – short rains) density estimates (error bars indicate 95 % confidence intervals) of (a) Manyara monkeys and (b) vervet monkeys in Lake Manyara National Park, Tanzania, during 2011–2019 (one survey in 2011, three surveys per year from 2012 to 2018, and two surveys in 2019), and seasonal trend lines based on a general additive model.

Figure 4

Box plots (bold midline indicates the median, and the upper and lower limits of the box represent the respective third and first quartiles) of (a) Manyara monkey and (b) vervet monkey cluster sizes during seasonal (LR – long rains, Dry – dry season, and SR – short rains) line distance surveys in Lake Manyara National Park, Tanzania. The grey dots represent individual data points.