Understanding the causes of population declines often involves comprehending a complex set of interactions linking environmental and biotic changes, which in combination overwhelm a population's ability to persist. To understand these relationships, especially for long-lived large mammals, long-term data are required, but rarely available. Here we use 26-36 years of population and habitat data to determine the potential causes of group density changes for five species of primates in Kibale National Park, Uganda, in areas that were disturbed to varying intensities in the late 1960s. We calculated group density from line transect data and quantified changes in habitat structure (cumulative diameter at breast height [dbh] and food availability [cumulative dbh of food trees]) for each primate species, and for one species, we evaluated change in food nutritional quality. We found that mangabeys and black-and-white colobus group density increased, blue monkeys declined, and redtails and red colobus were stable in all areas. For blue monkeys and mangabeys, there were no significant changes in food availability over time, yet their group density changed. For redtails, neither group density measures nor food availability changed over time. For black-and-white colobus, a decrease in food availability over time in the unlogged forest surprisingly coincided with an increase in group density. Finally, while red colobus food availability and quality increased over time in the heavily logged area, their group density was stable in all areas. We suggest that these populations are in nonequilibrium states. If such states occur frequently, it suggests that large protected areas will be required to protect species so that declines in some areas can be compensated for by increases in adjacent areas with different histories.