The Western Gorilla (Gorilla gorilla) has two recognized subspecies: the Western Lowland Gorilla (Gorilla gorilla gorilla) and the Cross River Gorilla (Gorilla gorilla diehli: Sarmiento and Oates 2000, Groves 2001). Genetic data suggest that the two subspecies of Western Gorilla diverged approximately 18,000 years ago (Thalmann et al. 2011) and that the Cross River Gorilla population can be clearly differentiated from Western Lowland Gorillas (Prado-Martinez et al. 2013). The taxonomic status of the Gorilla populations in Ebo (Cameroon) awaits clarification; however, measurements from a single Ebo Gorilla skull indicate this may be a relict population of a previously more widespread population living north of the Sanaga River (Groves 2005).

Gorilla gorilla diehli is listed as Critically Endangered since the total population of mature individuals is estimated to be less than 250 (Dunn et al. 2014). As a result of continuing low levels of illegal hunting and habitat loss from agriculture, logging and road construction, there is the strong probability that documented declines in the size of the population (Thalmann et al. 2011) may continue (Dunn et al. 2014) satisfying criterion C. Additionally, the population is sub-divided into approximately 11 subpopulations, each of which likely numbers less than 50 mature individuals (Bergl and Vigilant 2007, Bergl et al. 2008), satisfying the criteria for C2a(i).

Gorilla gorilla diehli (Matschie 1904) occurs across a landscape of approximately 12,000 km² on the Nigeria-Cameroon border, extending a short distance on either side of the border in the forests on the upper drainage of the Cross River. Within this landscape, the area currently known to be habitually used by G. g. diehli is approximately 700 km², concentrated in a number of rugged highland areas (Bergl et al. 2012, Dunn et al. 2014). Cross River Gorillas occasionally reach elevations of 1,900 m asl.

Although Gorillas in the Cross River region first became known to science in the early 20th century, little attention was paid to their conservation status until the late 1980s (Harcourt et al. 1989). Early reports had referred to their precarious situation, but little had been done to thoroughly examine their distribution and abundance, or to protect the remaining population and habitat (Anon 1934, March 1957, Critchley 1968). Intensive surveys in 1990–2005 suggested that approximately 250–300 G. g. diehli persisted in a forested area of roughly 12,000 km² (Oates et al. 2003, Sunderland-Groves et al. 2003, Bergl 2006, Oates et al. 2007). Ongoing monitoring of the Gorillas at multiple sites and genetic analysis at select locations (Arandjelovic et al. 2015) has confirmed the small size of the population, while also documenting a larger geographic range than previously recorded (Bergl et al. 2012, Dunn et al. 2014). Though confidence in subpopulation size estimates has increased for certain sites, overall the precise size of the population is somewhat uncertain, as it is primarily based on nest counts and estimated range size. Cross River Gorillas are found in at least 11 localities (Groves, J. 2001, Bergl et al. 2012, Dunn et al. 2014). Although the localities where the Gorillas are found are geographically distinct, the majority of these areas are connected by forested land. Genetic evidence suggests that these subpopulations have had reproductive contact in the recent past (Bergl and Vigilant 2007); however, field surveys paired with remote sensing analysis show that forest corridors connecting subpopulations may no longer be functional as routes for dispersal (Imong et al. 2014). Cross River Gorillas are found primarily in remote areas of high relief. This distribution appears to be directly related to greater levels of human activity (especially poaching) in lowland areas, rather than a result of ecological factors (Imong et al. 2013, Sawyer and Brashares 2013).

Studies of G. g. diehli have demonstrated flexible grouping patterns, overlapping home ranges, and re-use of nest sites, and group size varies from 2 to 20 individuals (McFarland 2007, Sunderland-Groves et al. 2009, Sawyer 2012). These grouping patterns likely occur for several reasons, including restricted habitat, food dispersal patterns, high hunting pressure, and limited opportunities for male migration between nuclei (McFarland 2007). Annual home ranges may be as large as 30 km² (McFarland 2007). The highly-seasonal nature of the Cross River Gorillas’ habitat (more markedly seasonal than that of any other Gorilla population) likely also contributes to grouping and ranging patterns. The Gorillas’ diet reflects the seasonality of their habitat, with fruits being preferred during periods of seasonal abundance and terrestrial herbs and bark acting as staples throughout the year (McFarland 2007, De Vere et al. 2011, Etiendem and Tagg 2013, Sawyer and Brashares 2013). While seasonal patterns in diet are generally consistent across the Cross River population, composition of the Gorillas’ diet can vary widely between sites, with only limited overlap of important food species (Sawyer 2012).

Cross River Gorillas range from lowland to submontane forest (Dunn et al. 2014). They occasionally use lowland areas between hills, but are presently mostly restricted to hilly areas (Oates et al. 2003, Bergl and Vigilant 2007). This distribution is largely the result of anthropogenic factors and several analyses suggest that habitat use by the Gorillas is driven by avoidance of human activity, rather than selection of areas that provide preferred foods (Bergl et al. 2012, Imong et al. 2013, Sawyer and Brashares 2013). A substantial amount of unoccupied potential Gorilla habitat exists in the intervening forest between areas currently used by the Gorillas (Imong et al. 2013).

The remaining population of G. g. diehli is small and fragmented, though the Gorillas were likely more numerous in the recent past (March 1957, Thalmann et al. 2011). Their habitat is surrounded by some of the most densely populated human settlements in Africa, and in Cameroon several subpopulations occur outside of protected areas. The small size of each subpopulation puts them at particular risk from poaching. While Gorillas do not generally appear to be targeted by hunters in the region, it is estimated that opportunistic hunting removes 1–3 individuals from the population annually (though this may be an underestimate; Dunn et al. 2014). Additionally, snares targeting other species may catch Gorillas, and cause injury or death. Demographic modelling suggests that the current level of offtake is unsustainable (Bergl 2006). Legal prohibitions against the killing of Gorillas exist in both Cameroon and Nigeria, but enforcement of wildlife laws is inconsistent, and even legally-protected areas suffer from high levels of poaching.

The lack of effective protected area management throughout much of the range of G. g. diehli, combined with the lack of protected status for much of the Gorillas’ habitat in Cameroon, threatens the future availability of suitable habitat for the population. Conversion of forest for agriculture and grazing is occurring rapidly in many parts of the Gorillas’ range and the largest protected areas in which Cross River Gorillas occur (the Okwangwo Division of Nigeria’s Cross River NP and the adjacent Takamanda NP in Cameroon) contain enclaves of human settlements whose farmlands have spread beyond their legal boundaries. Expansion of these settlements and associated development of access routes in the form of roads and bridges inside the protected areas threaten to exacerbate subdivision of the Cross River Gorilla population. The construction and improvement of roads outside protected areas in both Cameroon and Nigeria also threatens to increase subdivision of the population through habitat disturbance and intensification of the extraction of forest resources. Increased access is also likely to exacerbate poaching pressure. While most habitat loss in the region is the result of small-scale agriculture, commercial logging and industrial agriculture (e.g., for oil palm) are also impending threats.

The small size of the Cross River Gorilla population and its constituent subpopulations means that this subspecies is potentially at risk from inbreeding and loss of genetic diversity. Reduced levels of genetic diversity have been observed in this population (Bergl et al. 2008, Thalmann et al. 2011), though their implications for the future viability of the population are unclear. The small size of the Cross River population also means it is more vulnerable to disease. Ebola, which has caused significant mortality in G. g. gorilla populations, has not been reported in the G. g. diehli population, but their close proximity to dense human populations and livestock heightens the risk of disease transmission (Dunn et al. 2014).

Gorillas are completely protected by national and international laws in all countries of their range, and it is, therefore, illegal to kill, capture or trade in live Gorillas or their body parts.

Gorilla gorilla is listed under Appendix I of CITES and in Class A of the African Convention.

A series of workshops has identified priority actions for the conservation of G. g. diehli. These were formulated into an IUCN Action Plan (Oates et al. 2007), which was recently revised and updated (Dunn et al. 2014). The most urgently-needed actions identified are habitat protection and the control of illegal hunting. The recent creation of new protected areas in Cameroon (Kagwene Gorilla Sanctuary, Tofala Hill Wildlife Sanctuary and Takamanda National Park), in combination with existing areas in Nigeria, provides the legal framework for this. However, effective management of these reserves remains a challenge. Illegal activities, including poaching, occur in all the region’s protected areas. Resources, equipment and training, in addition to proper oversight are required in order for these areas to operate effectively. Outside of protected areas, law enforcement actions that control the timber and bushmeat trades between the two countries are also needed.

In addition to controlling poaching and habitat loss, the action plan identify several cross-cutting actions which require attention throughout Cross River Gorilla range, notably further conservation-relevant research (e.g., further distribution and abundance surveys, health monitoring and disease risk assessment), capacity building, increasing conservation education and awareness, and fostering improved community participation in conservation. The need for new modes of increased community engagement is particularly pressing, since approximately 30% of Cross River Gorillas occur outside of protected areas and many important habitat corridors between subpopulations exist as ungazetted land.

The key conservation need for the long-term survival of the Cross River Gorilla is to allow the population to expand. Since significant areas of unoccupied Gorilla habitat remain across the landscape, population growth is feasible if levels of hunting and other human activities can be reduced. Maintenance (or re-establishment) of habitat connectivity between areas currently occupied by Gorillas is essential for future population expansion.