Great apes across Africa are facing a severe threat: the loss of suitable habitats essential for their reproduction and survival. This decline is largely due to human activities impacting their distribution range. One subspecies particularly affected is the Nigeria-Cameroon chimpanzee [Pan troglodytes ellioti (Matschie, 1914)], with limited information available about its habitat suitability, especially for populations within forest reserves in North-West Cameroon.
To address this critical knowledge gap, a study was conducted in the Kom-Wum Forest Reserve (KWFR) in North-West Cameroon. This study employed a common species distribution model (MaxEnt) to map and predict suitable habitats for the Nigeria-Cameroon chimpanzee, based on environmental factors that potentially affect habitat suitability. By relating these environmental factors to a dataset of chimpanzee occurrence points, researchers aimed to understand and prioritize potential chimpanzee habitat for developing site-specific conservation plans.
Patrols cooperate to protect endangered apes on the Cameroon-Nigeria border
Study Area: Kom-Wum Forest Reserve
The Kom-Wum Forest Reserve (KWFR) is a crucial conservation site for the Nigeria-Cameroon chimpanzee and primate diversity. The KWFR stretches across Boyo and Menchum divisions in the North-West region of Cameroon (Fig. 1a; latitude 6°9′39.47″N and longitude 10°13′9.16″E to latitude 6°19′39.42″N and longitude 10°13′3.93″). It has a surface area of about 8029 ha with an elevation of 584-1654 m above sea level (Fig. 1b). The vegetation is dominated by lowland-montane tropical forest species including Khaya ivorensis, Triplochiton scleroxylon, and Milicia excelsa. The temperature ranges from 15 °C to 38 °C, with a mean yearly rainfall of about 2400 mm and a humidity of 82%. The area has two main seasons, a rainy season (March to October) and a dry season (November to March). The rivers Meteh, Tschuh Akooghe and Mughom flow within the reserve and join the Menchum River that flows at the boundaries of the reserve towards Nigeria.
Hunting and forest degradation are the major threats to forest resources in KWFR. This reserve harbors seven diurnal and six nocturnal primate species. About 10-83 chimpanzees live in the reserve.
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This chimpanzee subspecies is highly threatened by habitat loss and poaching, with approximately 6000 individuals remaining in the wild.
Previous surveys at this site focused on estimating chimpanzee abundance and habitat preference using the line transect method. So far, data on the availability of suitable habitat for chimpanzees in and around the reserve has yet to be measured.
Considering the limited information on suitable habitat for chimpanzees in this reserve, it is important to map and prioritise potential chimpanzee habitat to develop site-specific conservation plans for long-term monitoring of chimpanzee populations.
Nigeria-Cameroon Chimpanzee Range
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Methodology
The study combined field surveys with species distribution modeling to assess chimpanzee habitat suitability in the Kom-Wum Forest Reserve.
- Data Collection: Chimpanzee occurrence points were recorded during line transect and reconnaissance surveys in the forest reserve and surrounding forests.
- Environmental Variables: Nine environmental variables, including elevation, forest density, and distance to human settlements, were analyzed for their impact on habitat suitability.
- Modeling: The Maximum Entropy Distribution Model (MaxEnt) was used to predict suitable chimpanzee habitat in relation to the environmental variables.
- Validation: The model's performance was evaluated using the area under the curve (AUC) of the receiver operating characteristic (ROC).
Field Surveys
Researchers conducted systematic surveys within the study area to gather data on chimpanzee presence. Geo-referenced 2 × 2 km2 grids were produced in ArgGis10.6 and superimposed them on a map of the study area (100 km2) following a systematic random design.
Spatial thinning was done using the thin function in the spThin R package.
We sampled 23 grids (92 km2) with spatial line transects (each 2 km in length) linked by 42.09 km recces. In the field, the first author, two experienced forest guides and two community eco-guards repeatedly surveyed recce-transects 16 days every month from May to September 2018 and from November 2019 to March 2020 for signs of chimpanzee activity (chimpanzee occurrence points). We marked the locations of chimpanzee signs using a handheld Global Positioning System (GPS). At nesting sites, we searched for chimpanzee nests within a 50 m radius.
In total, we recorded 653 chimpanzee occurrence points including nesting locations, tool used sites, dung, feeding locations, direct observations, tracks and footprints over a survey effort of 700.9 km for the two survey periods.
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Environmental Variables
To model the habitat suitability of chimpanzees within the study area, we used nine environmental variables, including aspect, the density of bare land, density of primary forest, density of secondary forest, elevation, distance to roads, distance to villages, distance to rivers, and slope derived from a variety of sources.
First, we obtained land cover data (primary forest, secondary forest, bare lands and water bodies) for the study area from a land cover classification map generated by Fotang et al. (2021a). Second, we obtained topographic data by calculating aspect and slope in ArcGIS using elevation data from a Shuttle Radar Topography Mission 30 m resolution Digital Elevation Model (Jarvis 2008). Third, we re-scaled the raster layers of all environmental variables at 50 × 50 m grid cells (pixels). Lastly, we converted the raster layers to points (number of pixels) and used the Kernel Density interpolation method in ArcGIS to calculate the densities of primary forest, secondary forest and bare lands per km2 (Fig. 2; Tarjuelo et al.
Additionally, we used Google Earth images to digitize roads, rivers and villages, and then measured the distance to each feature as Euclidean distance.
To check for collinearity between variables, we used the package usdm in R (Naimi et al. 2014). When the correlation between two variables was greater than a threshold value of 0.7, the variable with the greater VIF was removed, and the correlation matrix was calculated again until the threshold condition was satisfied (Naimi et al. 2014). We only discarded distance to rivers after analyzing for collinearity.
MaxEnt Modeling
We employed the Maximum Entropy Distribution Model (MaxEnt) version 3.1.4 to predict suitable chimpanzee habitat in relation to eight environmental variables in the study area (Phillips et al. 2006). MaxEnt has many advantages. First, it uses only presence data of a species and produces accurate prediction even with an incomplete dataset and small sample size (Phillips et al. 2006). Second, it uses environmental data from the whole study area rather than only from parts of the area (Phillips and Elith 2013). Third, Maxent uses presence data plus background data (pseudo-absence data) from the study region because true absences are very difficult to obtain, especially for mobile species, and require higher levels of sampling effort to ensure their reliability compared with presence data (Mackenzie and Royle 2005). Fourth, MaxEnt can also be integrated inside other presence-only species distribution models such as Wallace (Kass et al. 2018).
We used 198 chimpanzee occurrence points for modelling (178 points for training and 20 for testing). We added 1000 generated background points to the 198 training points, resulting in 1198 points in the final model.
To quantify the habitat suitability of chimpanzee in the reserve, we classified potential chimpanzee habitat into four habitat suitability index scores: highly suitable (> 0.6-1.0), moderately suitable (> 0.4-0.6), low suitable (> 0.2-0.4) and unsuitable habitat (> 0-0.2), following Yang et al. (2013).
We then used a jackknife test to measure each environmental variable percentage contribution to chimpanzee habitat suitability (Phillips et al. 2006).
Key Findings
The MaxEnt model fit was very good, with an AUC value of 0.958 (SD ± 0.009). The results revealed a concerning situation:
- Habitat Suitability: Up to 91% of the study area is unsuitable for chimpanzees.
- Suitable Habitats: Only 9% of the study area is suitable, with a high proportion of highly suitable habitats located outside the forest reserve.
- Important Predictors: Elevation, secondary forests density, distance to villages and primary forests density were the most important predictors of habitat suitability for the Nigeria-Cameroon chimpanzee.
- Chimpanzee Occurrence: The probability of chimpanzee occurrence increased with elevation, secondary forest density and distance from villages and roads.
These results highlight the degraded state of suitable chimpanzee habitat in the reserve, indicating that current protected area efforts are insufficient.
Habitat Suitability Map
Environmental Factors Influencing Habitat Suitability
Elevation, secondary forest density, distance to villages, and primary forest density were identified as the most critical factors influencing chimpanzee habitat suitability. The probability of chimpanzee occurrence increased with elevation, slope, and primary and secondary forest density. In contrast, chimpanzee occurrence decreased closer to villages, roads, and bare land.
These findings underscore the need for targeted conservation strategies that address these specific environmental factors.
Threats to Chimpanzee Habitats
Human activities such as deforestation and forest degradation are causing continuous declines in the habitat suitability for terrestrial mammals worldwide. This decline is due to the conversion of forest to farmlands, commercial logging, industrial agriculture and road infrastructure development. The availability of suitable habitats for chimpanzees (Pan troglodytes) and gorillas (Gorilla gorilla) has declined over time, especially outside protected areas. The related human activities resulted in a decrease in chimpanzee densities as well as behavioural change of primates.
Despite these previous studies, information is still missing for some protected areas across the distribution range of the Nigeria-Cameroon chimpanzee. Junker et al. (2012) reported that the availability of suitable habitat for the Nigeria-Cameroon chimpanzee decreased slightly between the 1990s and 2000s. Sesink Clee et al. (2015) conducted an additional assessment, and their results predicted that suitable habitat for this subspecies in the ecotone region of Cameroon would decline drastically by 2080, while habitat availability in the rainforest region in North-West Cameroon is predicted to remain stable. Onojeghuo et al. (2015) reported that suitable habitats of the Nigeria-Cameroon chimpanzee are facing severe threats from deforestation and forest fragmentation in the Afi Mountain Wildlife Sanctuary, Afi River Forest Reserve, Mbe Mountains and Cross River National Park in the northern part of Cross River State in Nigeria.
Illegal hunting and the bushmeat trade also pose a grave danger to these primates.
Conservation Implications
The study provides evidence that suitable chimpanzee habitat in the reserve is degraded, suggesting that efforts to maintain protected areas are insufficient. These results are alarming, as the proportion of suitable habitat for this rare chimpanzee subspecies in KWFR is one of the lowest compared with values reported at other sites across its distribution range. Although suitable chimpanzee habitat in the North-West region of Cameroon are predicted to remain intact in the next six decades (Sesink Clee et al. 2015), chimpanzee habitats in KWFR are already scarce, and may no longer be suitable in the near future.
The area of suitable chimpanzee habitat is small in the study region, and a large proportion of highly suitable chimpanzee habitat occurs outside the reserve. Although the natural vegetation cover is much larger, chimpanzees are confined to less than 10% of the area because of illegal gun hunting.
Suitable chimpanzee habitat is degraded in the study area and this population could go extinct unless immediate action is taken. Elevation, secondary forest density, distance to villages and primary forest density were the most important predictors for habitat suitability in the study area.
The Nigeria-Cameroon chimpanzee (Pan troglodytes ellioti) is a subspecies of the common chimpanzee which inhabits the rainforest along the border of Nigeria and Cameroon. The subspecies inhabits tropical rain forests and montane forests at elevations of up to 2,750 m (9,020 ft) above sea level.
Nigeria-Cameroon chimpanzees face significant conservation issues that threaten their populations. Habitat loss and fragmentation due to deforestation and expanding human activities pose a grave danger to these primates.
Conservation Strategies
Our conservation strategy focuses on halting habitat loss and fragmentation. We establish and protect critical chimpanzee habitats, working closely with local communities and stakeholders to promote sustainable land-use practices and restore degraded areas. At the same time we tackle illegal hunting and the bushmeat trade through law enforcement and community outreach. By strengthening anti-poaching measures, supporting local community management, and raising awareness about the importance of conserving chimpanzees, we aim to reduce hunting pressure.
WCS fully recognizes that it is essential to involve local people in conservation efforts to save Nigeria-Cameroon chimpanzees. Conservation education and raising levels of awareness within local communities is essential for long-term success and a vitally important part of our work.
