The Cross River gorilla is the most endangered of all great ape taxa, and its rare and increasingly fragmented populations continue to be threatened by habitat loss and, to a lesser degree, hunting for the bushmeat trade. The 11 or so sites in which the Cross River gorilla is still known are spread across approximately 12,000 km² of extremely rugged and forested terrain spanning the Nigeria-Cameroon border region, with remaining populations estimated at 70-90 individuals in Nigeria, and 150 individuals in Cameroon (Oates et al. 2003).

In the last decade a concerted effort has been mounted to learn more about the Cross River gorilla, and to implement conservation action in both Nigeria and Cameroon. As outlined in the Regional Action Plan for the Cross River Gorilla (Oates et al. 2007), this has been spearheaded by the Wildlife Conservation Society (WCS), working in collaboration with host governments and other partners. Considerable success has been achieved on a number of fronts, but efforts have been hindered by a lack of understanding of both the areas in which remaining populations are found, and the current and potential basic habitat requirements of the subspecies. Moreover, site-specific conservation plans are hampered by an absence of knowledge about habitats remaining in the area, as well as their current usage by gorilla populations.
With this in mind, we have recently undertaken a project to produce a habitat map for the newly formed (by Decree No. 2008/0634/PM issued 3 April, 2008) Kagwene Gorilla Sanctuary in Cameroon, in addition to an analysis of gorilla nesting preferences within the sanctuary. Such knowledge is essential to help guide practical field-based management activities linked to the protection, and possible expansion, of the current population.
The Kagwene Gorilla Sanctuary (KGS) covers an area of 19.44 km2, and reaches altitudes of roughly 2,000 m, lying between 06° 05' 55" and 06° 08' 25" North and between 09° 43' 35" and 09° 46' 35" East. It is divided by a provincial boundary, and encompasses the northern part of South West Province, and the southern part of North West Province, of the Republic of Cameroon.
Nine villages surround the KGS, and villagers have historically utilized land within the sanctuary boundary for farming and hunting to varying degrees. Bororo settlements (settled grazers) are dotted within and surrounding the sanctuary, and these pastoralists graze their cattle on areas of open grassland adjacent to the Kagwene Mountain forests. Much of this land is burned each year to encourage the growth of new grasses, often resulting in the unplanned burning of forest. Currently, farmers also continue to cultivate crops within the boundaries of the sanctuary, especially in the southern sector of the reserve, with cocoyams the crop most commonly grown beneath the forest canopy. WCS operates a research camp from within the KGS, which has a permanent staff presence.
To produce a working GIS habitat map for the KGS, we undertook fieldwork from mid-March to the end of May 2008. We aimed to map areas of farmland and grassland within the sanctuary, and then to determine the variety and distribution of vegetation within the undisturbed forested sections. Using GPS units set to track-log, we walked around the edge of farms and grassland patches. Farm areas were so numerous within the reserve that we mapped a sample of them in this way, while the remaining farm distribution was determined through conversations with gorilla trackers familiar with the area, resulting in the creation of a second habitat category for our map of "presumed farm under canopy". To assess habitat types in the remaining forested zones of the sanctuary we undertook "recce walks" (White & Edwards 2000), which involved using GPS to record movements through the sanctuary, documenting vegetation changes as and when they occurred. We recorded both top storey and understorey vegetation, in addition to other information, such as understorey density, the slope of the ground, and canopy cover. Data collected during this fieldwork were downloaded into ArcGIS 9.1 and manipulated in order to generate habitat maps for the forested areas, overlaying GPS points measured around farms and grassland patches. To investigate nesting habitat preferences of gorillas at KGS, data were used from WCS records collected between January 2006 and March 2008, and we re-visited these nest sites to reassess the top storey and understorey type and density. This assessment relies on the assumption that little change has occurred in the habitat between the time of construction and re-finding the nest, which seems likely. We also recorded a number of other aspects of the nest area, including measures of visibility, aspect, slope and canopy cover, using the same methods as were used during habitat mapping.

A preliminary analysis of the habitat map has shown that only a little over half of the sanctuary is forested (i.e. under tree cover and not farmland), and therefore potentially available to the gorillas. The other half of the sanctuary is roughly split between farmland under canopy and grassland patches. By overlaying nest site locations, we found that the gorillas at Kagwene do not nest in either farmed or grassland areas. As they are not known to crop-raid, farmland prevalent in the southern sector of the sanctuary is clearly avoided by the gorillas, while grasslands in the northern sector can be seen to fragment forest potentially suitable for the gorillas.
Further analysis and dissemination of gorilla nesting habitat preferences at the KGS are ongoing. This involves a full analysis of how the gorillas nest with respect to available top storey and understorey habitats in the sanctuary, and we hope to disseminate our results widely by publishing them in a scientific journal. These analyses will add valuable insight to our understanding of Cross River gorilla ecology. The habitat map will provide an extremely useful tool for the WCS, who are assisting the government with elaborating management strategies for the sanctuary, as well as for gorilla trackers working on a daily basis within the sanctuary. Thanks to the ongoing support of organizations such as the US Fish & Wildlife Service, WWF, The Gorilla Organization, and Margot Marsh Biodiversity Foundation, the conservation of the Cross River gorillas of the Kagwene Gorilla Sanctuary is increasingly assured. However, further support is needed to implement site-based action in all 11 known Cross River gorilla sites (including parks, forest reserves, sanctuaries and unclassified areas of forest), as well as in habitat corridors that provide connectivity between these core areas.

Ruth Wiseman, Ymke Warren, Aaron Nicholas, Mary Mackenzie and James P. Higham

References
_{Oates, J. F. et al. (2003): The Cross River Gorilla: Natural History and Status of a Neglected and Critically Endangered Subspecies. Pp. 472-497 in: Taylor, A. B. & Goldsmith, M. L. (eds.) Gorilla Biology. Cambridge (Cambridge University Press)
Oates, J. F. et al. (2007): Regional Action Plan for the Conservation of the Cross River Gorilla (Gorilla gorilla diehli). Arlington, VA, USA (IUCN/SSC Primate Specialist Group and Conservation International)
White, L. & Edwards A. (2000): Methods for assessing the status of animal populations. In: White, L. & Edwards A. (eds.): Conservation Research in the African Rain Forests: A Technical Handbook. New York (The Wildlife Conservation Society)}