Primate diet and biomass in relation to vegetation composition and fruiting phenology in a rain forest in Gabon

To test the hypothesis that primate populations are limited by food resources, we studied the feeding ecology of three cercopithecines and one colobine in a rain forest in central Gabon. Simultaneously, we monitored the fruiting phenology of trees and estimated the biomass of the monkey community. The Makande Forest is dominated by Caesalpiniaceae and characterized by a lack of secondary vegetation and of trees species producing fleshy fruits. Fruit production was irregular intra- and interannually. Fruiting peaks of dry fruits (mainly Caesalpiniaceae) and of fleshy fruits occurred at the same period. However, interseasonal and interannual variability was greater in Caesalpiniaceae than in other families. As a result, the Makande forest is subject to bottlenecks when food is scarce. On an annual basis, seeds (primarily Caesalpiniaceae) dominated the diet of all monkeys. On a seasonal basis, cercopithecines preferentially consumed fleshy fruits as long as they were available, whereas colobines increased consumption of young leaves when seed availability declined. The consumption of mature leaves was low. The monkey community biomass (ca. 204 kg/km(2)) is one of the lowest in Central Africa. We suggest that both cercopithecine and colobine populations are limited as a result of the combined effect of the dominance of Caesalpiniaceae, which provide dry fruits according to a mast-fruiting pattern and mature leaves of low quality, and the lack of seral successional stages, which provide fleshy fruit on a more regular pattern and leaves of better quality. During the period of food scarcity, cercopithecines should suffer from the low availability of fleshy fruit, which are their favorite food. At the same period, colobines should be limited by the low availability of edible leaves. Similar low primate biomasses are found in forests dominated by Caesalpiniaceae or Lecythidaceae in South America and in Dipterocarpaceae forests in South Asia, which suggests that their biological characteristics, in particular dry fruits and mast fruiting, are unfavorable to monkey populations. Our results confirm that habitat mosaics may support larger populations of primary consumers than homogeneous primary forests can.