Population size and habitat preference of the Omo River guereza (Colobus guereza guereza) in a multi-habitat matrix in the central highlands of Ethiopia.

Given the current rate of habitat degradation and loss in the tropics, data on primate population densities and habitat use are indispensable for assessing conservation status and designing feasible management plans for primates. The Omo River guereza (Colobus guereza guereza) is a subspecies of the eastern black-and-white colobus monkey endemic to the western Rift Valley forests of Ethiopia. Their restricted distribution along with habitat loss and hunting within their range render them vulnerable to local extirpation and extinction. Furthermore, there are no published data available on the population status and habitat use patterns of the Omo River guereza. We therefore aimed to assess the population size of Omo River guerezas in different habitats (Erica-Juniperus mixed forest, mixed plantation forest, undisturbed natural forest, disturbed natural forest) using transect surveys at Wof-Washa Natural State Forest (WWNSF) in central Ethiopia. Our surveys covered a cumulative distance of 88.5 km in four different habitats, during which we recorded a total of 140 Omo River guereza groups. The average group density was 14.3 groups/km2, average individual density was 94.4 individuals/km2, and we estimated the total population size within WWNSF to be 2549 individuals. The sex ratio of the population was split evenly between males and females, though the age classes skewed strongly towards adults. Of the habitats surveyed, the highest group encounter rate (1.83 groups/km) occurred in the disturbed natural forest. However, the highest individual density (110.1 individuals/km2) was recorded in undisturbed natural forest. Still, sizable densities (group and individual) were recorded in three of the disturbed habitats (disturbed natural forest, mixed plantation forest, and to a lesser extent Erica-Juniperus mixed forest). Our study offers the first baseline information with which to compare future population density estimates and habitat use in the range of Omo River guerezas.