Population status of black and white colobus monkeys (Colobus guereza) in Kakamega Forest, Kenya: are they really on the decline?

Abstract

Eastern black and white colobus monkeys, or guerezas (Colobus guereza), are among the few primate species that have traditionally been regarded as not being adversely affected by habitat degradation. This view was recently challenged by von Hippel et al. (2000) who, using data from short-term censuses in 1992 and 1998, reported a striking decline in guereza density over a six-year period of light to moderate habitat degradation at Isecheno study site in the Kakamega Forest, Kenya. In this paper, I present evidence from my own more intensive study during the same period that suggests that the guereza population at Isecheno is actually quite robust and may in fact be increasing. I provide evidence to suggest that the census methods adopted by von Hippel and his colleagues are prone to  overestimating density and that the decline in guereza density that they reported probably did not occur. My study suggests that brief censuses based on group counts over a given area, even when conducted by multiple observers, are not sufficient for accurately determining primate distributions and densities in rainforest environments. Data on animal distribution and density play a critical role in the development of conservation strategies and it is therefore important that these data be relatively accurate if biologists are to make informed conservation decisions.

Key words: population density, census, block method, group count method, birth rate, interbirth interval, forest degradation, conservation.