PS 47-128 - Impacts of increasing Colobine density on tree phenology and survival: Analysis of folivore pressure over 20 years in Ghana

Background/Question/Methods

One of the main threats to animals is habitat change, including habitat loss, fragmentation, and degradation. This is especially true for primates, as many populations live in small, isolated fragments. In the short term, habitat loss can lead to high population densities if surviving populations become compressed within the remaining habitat. These rapid changes in population density can change the dynamics between plant communities and animal populations. We investigated whether the increasing density of a highly folivorous black-and-white colobus (Colobus vellerosus) population living in a forest fragment was associated with phenological changes in food trees at Boabeng-Fiema, Ghana. The population density increased from 108 individuals/km² in 2001 to 235 individuals/km² in 2020. We collected monthly phenology data during 2001-2020 from 662 trees of 73 species in the forest. First, we tested if weather (monthly number of days with rainfall and mean monthly temperature) and colobus density as a proxy of feeding pressure were associated with leaf cover on individual trees using a generalized linear mixed model. Second, we analyzed whether phenology trees that are important food species for the colobus were more likely to die during the study period by running a chi-square test with Yates’ continuity correction.

Results/Conclusions

Leaf coverage decreased with increasing colobus population density, suggesting the possibility that high colobus population density may be having harmful effects on food trees in the study area. In addition, leaf coverage increased with the monthly number of rainfall days and decreased with mean monthly temperature with the generalized linear mixed model explaining 22% of the variation in leaf coverage. Out of the 662 individual phenology trees included in the study, 31 died. The observed number of these dead trees that were of an important food species (N=16) was significantly lower than expected (N= 22.24; χ2 = 5.51; df = 1; p-value = 0.02), suggesting the impact of colobus foraging may not be driving patterns of tree death of their important food trees. However, it is possible that the colobus foraging pressure is having more subtle negative effects on individual food trees. If herbivory is causing plant productivity or composition to change, these effects can cascade throughout the trophic web and may lead to changes in community dynamics. By investigating the links between foraging behaviors, plant community dynamics, and population persistence, we may be able to improve conservation strategies to promote the long-term survival of threatened animal populations.