Cross River Gorilla

Range countries
Cameroon, Nigeria

Population status
The Western Gorilla consists of two recognized subspecies: the Cross River Gorilla (Gorilla gorilla diehli) and the Western Lowland Gorilla (Gorilla gorilla gorilla). The Cross River Gorilla is classified as Critically Endangered on the IUCN Red List (Bergl et al. 2016), as it is considered to be facing an extremely high risk of extinction in the wild.

The range of the Cross River Gorilla is restricted to the hilly rainforest region along the Nigeria-Cameroon border (Dunn et al. 2014). Separated from the nearest population of Western Lowland Gorilla to the south by about 200 km, the subspecies is scattered across a surface area of approximately 12,000 km² (Dunn et al. 2014). About one-third of the Cross River gorilla landscape occurs in three areas of Cross River State in Nigeria: the Afi Mountain Wildlife Sanctuary, the Mbe Mountains, and the Okwangwo Division of Cross River National Park). In Cameroon, there are five key areas for Cross River gorillas. With the exception of Kagwene Gorilla Sanctuary, which extends a few kilometers into the Northwest Region, all other areas are located in Cameroon’s Southwest Region: Takamanda National Park and adjacent Mowambi Hills, Mone River Forest Reserve, the Mbulu-Oko forest, and the Bechati-Fossimondi-Besali forest (Bergl et al. 2012; Dunn et al. 2014).

The total population of mature individuals is estimated to be less than 250 (Dunn et al. 2014). Many of the sites where gorillas are found contain relatively small numbers of animals, commonly 20-30 individuals, and only two sites (Okwangwo and Takamanda) are estimated to contain larger groups up to a maximum of 50-65 individuals. Large areas of unoccupied potential Cross River Gorilla habitat remain, and these provide connectivity between many of the subpopulations (Bergl et al. 2012).

Threats
Due to their small population size, Cross River Gorillas are susceptible to poaching. Snares set for other species also pose a threat to gorillas, causing injuries or fatalities. Demographic modeling indicates that the level of hunting is unsustainable (Bergl 2006). Although legal protections against gorilla killings exist in Cameroon and Nigeria, enforcement of wildlife laws is inconsistent, leading to high poaching rates even within protected areas (Bergl et al. 2016).

The lack of effective management in protected areas across much of the Cross River Gorilla's range, coupled with the absence of protected status for a significant portion of their habitat in Cameroon, jeopardizes the availability of suitable habitat for the population's future (Bergl et al. 2016). Rapid conversion of forests for agriculture and grazing is widespread within the gorillas' range, including within the largest protected areas where human settlements encroach beyond legal boundaries. Expansion of these settlements, along with infrastructure development such as roads and bridges within protected areas, poses a further threat by fragmenting gorilla habitats. Road construction outside protected areas in Cameroon and Nigeria also contributes to habitat disturbance, intensifying forest resource extraction and increasing poaching pressure (Bergl et al. 2016).

The small size of the Cross River Gorilla population and its subpopulations raises concerns about inbreeding and genetic diversity loss. Studies have shown reduced genetic diversity in this population (Bergl et al. 2008, Thalmann et al. 2011), though the full implications for population viability remain unclear. Additionally, the population's small size heightens vulnerability to diseases. While Ebola, which has devastated other gorilla populations, has not been reported in the G. g. diehli population, their proximity to dense human populations and livestock increases the risk of disease transmission (Dunn et al. 2014).

Conservation Activities
The Western Gorilla is classified under Appendix I of CITES and falls into Class A of the African Convention.

A series of workshops outlined the priority measures for safeguarding Cross River Gorillas; these measures were consolidated into an IUCN Action Plan (Oates et al. 2007), which was updated for the 2014-2019 period (Dunn et al. 2014). The most urgent actions include protecting habitats and reducing illegal hunting. The establishment of new protected areas in Cameroon (such as the Kagwene Gorilla Sanctuary, Tofala Hill Wildlife Sanctuary, and Takamanda National Park), combined with existing areas in Nigeria, establishes the legal foundation for this endeavor. However, effectively managing these reserves remains a challenge. Illegal activities, including poaching, persist in all protected areas within the region. Adequate resources, equipment, training, and vigilant oversight are necessary for these areas to function efficiently. Beyond protected areas, law enforcement measures targeting the timber and bushmeat trades between the two countries are also imperative (Bergl et al. 2016).

The action plan identifies several other measures requiring attention throughout the range of the Cross River Gorilla, notably further research (e.g., more surveys on distribution and abundance, health monitoring, and assessing disease risks), capacity building, enhanced conservation education and awareness, and promoting greater community involvement in conservation efforts. The necessity for innovative approaches to increase community engagement is particularly urgent, given that around 30% of Cross River Gorillas inhabit areas outside protected zones, and important habitat corridors between subpopulations are unprotected. Preserving (or re-establishing) habitat connectivity is indispensable for future population expansion (Bergl et al. 2016).

References

Bergl, R.A., Dunn, A., Fowler, A., Imong, I., Ndeloh, D., Nicholas, A. & Oates, J.F. (2016). Gorilla gorilla ssp. diehli. The IUCN Red List of Threatened Species 2016. https://www.iucnredlist.org/species/39998/102326240

Bergl, R., Warren, Y., Nicholas, A., Dunn, A., Imong, I., Sunderland-Groves, J. & Oates, J. (2012). Remote sensing analysis reveals habitat, dispersal corridors and expanded distribution for the Critically Endangered Cross River gorilla Gorilla gorilla diehli. Oryx 46: 278–289.

Bergl, R. A., Bradley, B. J., Nsubuga, A., & Vigilant, L. (2008). Effects of habitat fragmentation, population size and demographic history on genetic diversity: The Cross River gorilla in a comparative context. American Journal of Primatology, 70(9), 848–859.

Dunn, A., Bergl, R., Byler, D., Eben-Ebai, S., Etiendem, D., Fotso, R., Ikfuingei, R., Imong, I., Jameson, C., Macfie, E., Morgan, B., Nchanji, A., Nicholas, A., Nkembi, L., Omeni, F., Oates, J.F., Pokempner, A., Sawyer, S. & Williamson, E.A. (2014). Revised Regional Action Plan for the Conservation of the Cross River Gorilla (Gorilla gorilla diehli) 2014–2019. IUCN SSC Primate Specialist Group and Wildlife Conservation Society, NY. https://portals.iucn.org/library/node/44661

Oates, J. F., Sunderland-Groves, J. L., Bergl, R., Dunn, A., Nicholas, A., Takang, E., Omeni, F., Imong, I., Fotso, R., Nkembi, L., & Williamson, E. A. (2007). Regional action plan for the conservation of the Cross River Gorilla (Gorilla gorilla diehli). IUCN/SSC Primate Specialist Group and Conservation International, Arlington, VA.
Thalmann, O., Wegmann, D., Spitzner, M., Arandjelovic, M., Guschanski, K., Leuenberger, C., Bergl, R. A., & Vigilant, L. (2011). Historical sampling reveals dramatic demographic changes in western gorilla populations. BMC Evolutionary Biology, 11(1), 85.

Last updated February 2024