Effects of habitat quality and hunting pressure on arboreal folivore densities in neotropical forests: A case study of howler monkeys (Alouatta spp.)

This large-scale geographic comparison examines the effects of subsistence hunting pressure and several indirect indicators of habitat quality on the abundance of howler monkeys (Alouatta spp.), the best studied and the most folivorous of all platyrrhine primates. Alouatta population densities were obtained from a standardized series of line-transect censuses undertaken at 23 Amazonian forest sites (1987 to 1995), including annually flooded (várzea), unflooded (terra firme), and eastern Amazonian transitional terra firme forests. A total of 83 density estimates from elsewhere were compiled from a comprehensive review of field studies from southern Mexico to northern Argentina. Howler density and biomass in Neotropical forests is profoundly affected by degree of hunting pressure, structural heterogeneity of the forest canopy, and to a lesser extent, the rainfall seasonality, total rainfall, and latitude of a given site. Mild gradients of forest elevation were a poor predictor of howler densities. In evergreen forests of lowland Amazonia, distance to major white-water rivers, which covaries with forest heterogeneity and soil fertility, is the best single predictor of howler densities. In Amazonia, habitat-specific patterns of distribution and abundance of Alouatta and other arboreal folivores conform to geochemical gradients determined by watershed types and their consequences for soil fertility. Results presented here confirm suggestions for Old World generalist herbivores that howler monkeys face strong dietary constraints which are most likely related to quality, phenology and productivity of digestible foliage during periods of resource scarcity.