Group Size Dynamics over 15+ Years in an African Forest Primate Community

Abstract

Group size affects many aspects of the ecology and social organization of animals. We investigated group size stability for five primate species in Kibale National Park, Uganda from 1996 to 2011 at three nested spatial scales. Survey data indicated that group sizes did not change for most species, with the exception of red colobus monkeys (Procolobus rufomitratus), in which group size increased at all spatial scales. Mangabey (Lophocebus albigena) group size increased in old-growth forest, but the sample size and increase were small. To augment this survey data, we collected several years of demographic data on three habituated groups of redtail monkeys (Cercopithecus ascanius), eight groups of black-and-white colobus (Colobus guereza), and one red colobus group. The red colobus group increased from 59 to 104 individuals, while redtail monkey and black-and-white colobus group sizes were stable, mirroring our survey results. To understand mechanisms behind group size changes in red colobus versus stability in other primates, we monitored forest dynamics at two spatial scales between 1990 and 2013, considered changes in predator population, and explored evidence of disease dynamics. The cumulative size of all trees and red colobus food trees increased over 24 yr, suggesting that changing food availability was driving group size changes for red colobus, while predation and disease played lesser roles. Overall, our results and evidence of changing primate densities suggest that the Kibale primate community is in a non-equilibrium state. We suggest future conservation and management efforts take this into consideration.Abstract in Swahili is available in the online version of this article.

Competing pressures on populations: long-term dynamics of food availability, food quality, disease, stress and animal abundance

“…(b) Red colobus diet and changes in forest composition and food quantity [47]. If the individual was feeding, the species and plant part (e.g.…”

“…A linear-mixed effects model (fixed effect ¼ sampling period, random effect ¼ vegetation plot identity) detected a significant increase in the cumulative log 10 DBH of all trees larger than 10 cm in both the 2006 and 2013 tree surveys relative to the initial 1989 survey (table 1a; [47]). In addition, when only those species consumed regularly by red colobus were included in the analysis, the linear-mixed effects model indicated that the availability of red colobus food (i.e.…”

“…In addition, when only those species consumed regularly by red colobus were included in the analysis, the linear-mixed effects model indicated that the availability of red colobus food (i.e. cumulative log 10 DBH) was significantly higher in both 2006 and 2013 than in 1989 (table 1b; [47]). …”

“…In eight tree species, the fibre content of young leaves increased 15%, and protein decreased 6% after 15 years. A 31% decline in monkey abundance was forecast when these nutrient values were placed into the model predicting folivorous primate biomass [64]; however, these results did not take into account the increasing habitat-wide food availability found by Gogarten et al [47] over a different time period. Thus, the observed increase in food availability and decrease in quality of food predict contrary effects on abundance.…”

“…To ensure accuracy, Table 2. Characteristics of census routes used at Kibale National Park, Uganda and the results of the censuses conducted over four periods in old-growth forest (statistics provided in the original studies) [47,85]. …”