Spatial point pattern analysis of gorilla nest sites in the Kagwene Sanctuary, Cameroon: Towards understanding the nesting behavior of a critically endangered subspecies

Dissertation submitted in partial fulfillment of the requirements for the Degree of Master of Science in Geospatial Technologies.Gorilla nest site data from the Kagwene sanctuary, Cameroon were analyzed to foster an understanding of the nesting behavior of Cross River Gorillas. The main objective of the study was to verify the pattern of nest site distribution in the sanctuary, the influence of environmental covariates and possible interaction between nest sites and between nest sites of two gorilla groups – the Major and Minor groups. Spatial point pattern analysis methods were implemented in the R software environment for this purpose. Overall, we sought to fit models that could best estimate an intensity function for nest site distribution in the sanctuary. Resulting models revealed that nest site distribution does not conform to a Poisson process, and that the data can be better described by a combination of environmental factors and interaction between nest sites. Univariate models fitted to different nest site categories proved to be more useful than bivariate models in defining intensity functions for nest site distribution. The final model category chosen for the data therefore constituted a combination of the effect of covariates and higher-order interaction between nest sites. This set of models, chosen through their AIC values, showed that nest site distribution in the sanctuary exhibits characteristics of attraction so that clustered patterns are observed. Gorillas tend to create hotspots for nest site location, with southern parts of the sanctuary proving to be very much avoided. Intensity tends to increase with increasing distance to the centre of the sanctuary. Coefficients obtained from the models also showed that gorillas prefer constructing nests close to transition vegetation, on steep slopes and generally on east-facing slopes. In the dry season however, colonizing forests exert a greater attraction on nest sites, which can be attributed to the fact that transition zones experience such edge effects as bushfires, and plants that provide food (such as Zingiberaceae) do not bear fruit in this season. These can be assumed to be specific habitat requirements of this subspecies of gorillas. Also, intensity drops with increasing elevation. Interaction parameters obtained from the bivariate models also suggested that there is attraction between nest sites of the Major (sites with more than 6 nests) and the Minor groups. This analysis is the first of its kind for this subspecies, and we recommend that further models can be fitted to include a wider range of covariates (both anthropogenic and natural) as they may be available to expand the scope of the models