The primate community of the Lopé Reserve, Gabon: diets, responses to fruit scarcity, and effects on biomass.

The diets of all diurnal primates (Gorilla g. gorilla, Pan t. troglodytes, Mandrillus sphinx, Colobus satanas, Cercocebus albigena, Cercopithecus nictitans, C. pogonias, C. cephus) in the Lópe Reserve, central Gabon, are described from qualitative and quantitative data collected over 10 years. A total of 397 foods were recorded, of which 91% were from plants. The diet of seven of the eight species were numerically dominated by fruit, the exception being Colobus satanas with a diet dominated by seeds. When proportion of feeding time was examined, fruit remained the dominant food class for six species, while Cercocebus albigena, like the black colobus, spent most time feeding on seeds. The number of foods recorded per species ranged from 46-220, but dietary breadth of the four species that have not been subjects of detailed study was certainly underestimated. Almost half of the foods (46%) were eaten by only one species, and only four (1%) were recorded for all eight species. At Lópe, fruit is scarce for 2-3 months during the annual dry season, and fruit crop failures can prolong this period of scarcity to as many as 8 months in some years. When fruit is scarce, the diets of all the species changed to include more nonfruit foods, but chimpanzees and the three species of guenons maintained a fruit dominated diet. The total biomass of the Lópe primate community is 374 kg/km2, which is low compared to other sites in tropical Africa. Within the Lópe community, biomass correlates negativity with degree of frugivory, suggesting that fruit availability may be critical, but this does not explain the low total biomass compared to other sites. Neither floristic diversity nor the composition of the primate community at Lópe can account for the biomass. While historical events such as climate-induced changes in forrest cover, disease, or impact of human activities cannot be ruled out, we conclude that the most likely explanation of low primate biomass is the occurrence of an ecological "catastrophe" in the fairly recent past from which populations have not yet recovered. We believe that the most likely scenario was climate-mediated disturbance of flowering of important fruit food species. Prolonged fruit scarcity would have caused mortalities and affected the most frugivorous species more severely.