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.