Amazonian small mammal abundances in relation to habitat structure and resource abundance

Previous studies in tropical rain forests suggest that most small mammal species reach their highest densities in disturbed habitats; however, only a few sites have been examined. Consequently, habitat and resource use for many species is poorly understood. This is especially true in the Amazon Basin, where no studies of microhabitat associations of small mammals have been undertaken. We studied relationships with habitat variables and resource abundances for 5 species of marsupials and 9 species of rodents at a site in southeastern Amazonia. Small mammals were sampled with traps placed both on the ground and in the understory. Eight habitat variables were measured to quantify habitat structure. Measures of insect biomass were collected by the use of sticky traps, and fruit abundance was quantified. Patterns of habitat use were examined using logistic regression, multiple regression, and ordinations. Many species showed increased abundances with habitat features indicative of edge-affected or disturbed habitats, showing negative relationships with understory openness, understory woody-stem density, tree density, and tree size; and positive relationships with number of vines per tree, mean log size, number of logs, and volume of downed wood. We obtained support for the hypothesis that the cause of this pattern is increased resource abundances in these areas, because both insect biomass and number of fruiting trees showed similar relationships. However, for many species, measures of resource abundance were not important once habitat features were entered into the models, indicating that the relationship to resources is an indirect one.