The demography of the Tana river red colobus colobus badius rufomitratus

Abstract

This study on spatial and temporal changes in the demographic composition and distribution of the Tana River red colobus, Colobus bad i u s rufomitratus. was conducted for 9 months (September, 1988 - June, 1989). The main objectives were to determine the current population status of this primate species and to investigate the distribution and composition of colobus groups. Environmental variables such as forest size, tree species diversity and density per forest were also studied to show how they varied among different forest patches. Various models and theories were applied to estimate the effective population size and, hence, the viability of the species. An annual census was carried out in the May-June period throughout the 52 km long geographical range of the species while demog raphic variables were investigated by carrying out monthly censuses in 16 sample groups for 9 months. The results show that the colobus population consisted of, at least, 264 individuals. The effective population size was estimated as 94. There were at least 22 extant colobus groups living in 17 different forests. The majority of these groups, apparently, were on the western bank of the river. Demographic composition varied in space and time among the groups although adult females dominated most groups. The sex ratio was highly biased in favour of females possibly because some solitary males were missed during the censuses. The forest refuges inhabited by the colobus were of varying sizes, tree species diversity and density and, of all the environmental variables considered, only forest size correlated significantly with the total number of colobus per forest. The forests were fragmented to an extent that, although the groups appeared to constitute a panmictic population, in some cases inter-forest migration would be difficult due to the presence of geographical and man-made barriers like the Tana River, as well as intervening farmland and villages. The Tana River red colobus is still endangered because such a small population is jnore prone to genetic drift and loss of genetic diversity than a large population. This may lower their adaptability to environmental changes. The subspecies may be saved from extinction if active and effective conservation and management strategies are adopted now.