Understanding long-term primate community dynamics: implications of forest change.

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.

Bigger groups have fewer parasites and similar cortisol levels: a multi-group analysis in red colobus monkeys.

If stress and disease impose fitness costs, and if those costs vary as a function of group size, then stress and disease should exert selection pressures on group size. We assessed the relationships between group size, stress, and parasite infections across nine groups of red colobus monkeys (Procolobus rufomitratus) in Kibale National Park, Uganda. We used fecal cortisol as a measure of physiological stress and examined fecal samples to assess the prevalence and intensity of gastrointestinal helminth infections. We also examined the effect of behaviors that could potentially reduce parasite transmission (e.g., increasing group spread and reducing social interactions). We found that cortisol was not significantly related to group size, but parasite prevalence was negatively related to group size and group spread. The observed increase in group spread could have reduced the rate of parasite transmission in larger groups; however, it is not clear whether this was a density-dependent behavioral counter-strategy to infection or a response to food competition that also reduced parasite transmission. The results do not support the suggestion that gastrointestinal parasitism or stress directly imposed group-size-related fitness costs, and we cannot conclude that they are among the mechanisms limiting group size in red colobus monkeys.

Temporal dynamics of nutrition, parasitism, and stress in colobus monkeys: implications for population regulation and conservation.

The need to develop conservation plans calls for the ability to identify ecological factors that influence population density. Because stress is known to affect fecundity and mortality, increasing stress may provide a warning of potential population declines. We examined the effects of temporal variation in nutrition and parasitism on stress in endangered red colobus monkeys in Kibale National Park, Uganda. First, we tested the hypothesis that parasitism and nutrition would individually affect stress levels. We found that periods of poor-quality diet corresponded with an increase in cortisol. Similarly, increases in parasite infections were associated with increased cortisol. Next, we predicted that a poor-quality diet would facilitate increased parasite infections, and that together, they would lead to amplified stress. However, we found no support for such amplification, likely because the quality of the diet had little effect on parasite infections. Third, we tested whether individuals in a larger group were subject to food stress due to greater within-group competition, which would intensify nutritional stress and parasitism, and lead to reduced reproduction. Although we found no evidence to support a group size effect on parasites, cortisol levels in the large group tended to be higher than those in the small group, and the large group had fewer infants per female. The results suggest that parasitism and poor nutrition lead to increased stress which, because they are known to be associated with reduced fecundity and increased mortality, may lead to population declines.