Nesting and Population Ecology of Western Chimpanzee in Bia Conservation Area

Methods use to Estimate Chimpanzee Abundance

In the early days, estimates of abundance of species were derived at large geographic scales through the administration of questionnaires to local people (Sugiyama and Soumah 1988). Scientists have unanimously agreed that for successfully conservation of wild animals, dependable estimates of population size are indispensable (van Schaik et al., 2005; Plumptre & Cox, 2006; McKechnie et al., 2007). Absence of such reliable baseline data for wild animals especially forest mammals make it difficult to unerringly predict changes in population trend over time (Teleki, 1989; Oates, 1996). Additionally, species that occur at low density and are very elusive such as great ape and other large mammals are impracticable to survey by total counts of individuals within a landscape (Reynolds & Reynolds, 1965). In cognizance of this, large forest mammals including chimpanzees are often survey using indirect signs (Kouakou et al 2009) including dungs (Barnes, 2001) and sleeping nest (Ghiglieri, 1984; Sanz et al., 2007).

In the late 1960s, techniques were developed for primate surveys (Plumptre and Cox 2006), including ground and aerial surveys techniques (Ancrenaz et al. 2004), relying on sleeping nest counts to estimate abundance and home range (Plumptre and Reynolds 1996). The elusive nature of apes including chimpanzees compelled researchers to rely heavily on indirect signs, especially sleeping nests for population estimates (Plumptre & Reynolds 1996). Sampling techniques such as line transect distance sampling (Buckland et al., 2001, 2010), reconnaissance trail surveys, i.e. recces (Walsh and White, 1999), dung sampling for genetic analysis (Arandjelovic et al., 2010, 2011) and remote camera trapping (Head et al., 2013; Nakashima et al., 2013) are basically used depending on the objectives of the study and resources available. Aerial surveys using helicopter (Ancrenaz et al., 2004) and Unmanned Aerial Vehicle (UAV) are now increasingly being employed to survey wildlife including chimpanzees (Van Andel et al, 2015).

Generally, chimpanzee population density and size estimates are derived using Marked Nest Count (MNC) and Standing Crop Nest Count (SCNC) survey methods applying the principles of distance sampling (Kouakou et al., 2009; Van Andel, 2015). The SCNC is the most common approach use to estimate density of chimpanzee as it requires a single visit to each transect to record all nest detected (Ghiglieri, 1984; Marchesi et al., 1995). However, this approach is limited by site specific calculations of nest production rate and the mean lifetime of nests to provide unbiased estimates. The nest factors are very difficult and problematic to calculate as this require studying habituated chimpanzee over a period of time and accounting for geographic area and time-based heterogeneity in decay rates (Kuehl et al., 2007) . Consequently, several studies normally adopt these factors from other sites where it was calculated but unfortunately do not often incorporate sampling errors associated with them, resulting in questionable population estimates with wide confidence limits which is usually as a factor of high difference in mean lifetime of nest (Plumptre, 2000, Tutin & Fernandez, 1984).

MNC approach was proposed to avoid problems associated with nest decay studies (Hashimoto, 1995; Plumptre & Reynolds, 1996). This approach requires a higher transect sampling effort as involved repeated counts of only fresh nests on transects within a short period to help reduce temporal bias of the density estimate (Furuichi et al., 2001; Kuehl et al., 2007). Kouakou et al 2009 has validated both nest count method as reliable for estimating chimpanzee population by comparing their estimates on a known population size of habituated chimpanzees in the Tai National Park. Both SCNC and MNC methods usually use the distance formula (Dc = Dn/r.t) to calculate ape densities. Where; ‘Dc” is the estimated chimpanzee density, “Dn” is the estimated nest density, “r” is the nest production rate and “t” is the nest mean lifetime.

Habitat Use in Chimpanzees

In depth knowledge of habitat requirement and use of any species are very essential in effective conservation planning for the species. Multiple factors can influence habitat selection in animals; these may include the distribution of predators, competitors, food resources and abiotic properties of the environment (Tews et al. 2004). Of all the great ape species found in Africa, chimpanzees are recognized to quickly adjust to changing environments and use a wide range of habitats with different ecological conditions (Hockings et al 2015). According to Balcomb et al., (2000), habitat composition and structure affect the species’ abundance as it influences the presence of food and nesting resources for the species. Bryson_Morrison et al (2017) noted that chimpanzees preferentially utilized forest habitat types for traveling and resting, and highly disturbed habitat types for socializing. Generally, chimpanzees spends ample time in or close to trees which provide them with a source of food (Matsuzawa et al. 2011), protection from predators (Pruetz et al. 2008), sleeping sites (Stewart et al. 2007), or shade in drier climates (Pruetz & Bertolani 2009). Hence, their habitant use is strongly influence by the presence of trees.

In forest habitats where there are higher tree density, the species use a smaller home range (about 20 km2) than in the savanna woodland (70-200 km2) (Suzuki, 1969; Baldwin et al., 1982). Several studies have established that changes in food availability across seasons affect habitat use of chimpanzees within a landscape. Lehmann & Dunbar (2009) argued that there is a very low probability that chimpanzees can persist in areas with low forest cover due to their diet requirement and large body mass index. Hence, woody habitats are very important ecological requirement for chimpanzees (McGrew et al. 1988; Lehmann & Dunbar, 2009; Pruetz & Bertolani, 2009).

Comparatively, relatively cooler, wetter and more forested regions provide more comfort and food resources for chimpanzees than extreme savanna habitats (McGrew et al., 1981; Baldwin et al., 1982; Moore, 1996; Pruetz & Bertolani, 2009). For example in Kahuzi–Biega National Park, Terada et al., (2015) established that chimpanzees rely heavily on small pockets of primary forest that provide them more fruits than other habitat types. However, Basabose (2005) also documented that the subspecies prefer to use secondary forest habitats when particular fruits like figs are in season.

Nesting Ecology of Chimpanzees

Chimpanzees generally construct arboreal nests every night for sleeping and sometimes during the day for resting. Terrestrial day or night nests have also been reported at Bossou, Guinea (Sugiyama, 1981, Humle, 2004) and at Taï (Boesch, 1995) and in the Nimba mountains (Matsuzawa & Yamakoshi, 1996; Koops et al., 2012) in Côte d’Ivoire. Chimpanzees construct their nests by making a foundation of hard side branches or forks, curving, breaking and inter weaving side branches crosswise (Fruth & Hohmann, 1996). Nest re-use has also been recorded among chimpanzees at some sites whereby a chimpanzee would add new materials to old nest remains to construct a new nest (Humle, 2004).

Several studies have revealed that chimpanzee are selective in choosing nesting sites and accumulation of nests in a particular area may be influenced by habitat type and nearness to water and food resources (Baldwin et al. 1982; Kortlandt 1992; Sept 1992; Humle, 2004), predation levels and human disturbance (Wrogemann, 1992; FleuryBrugiere, 2001). Habitats and tree species use for nesting varies across chimpanzee populations and communities within and between regions (Wrogemann, 1992; FleuryBrugiere, 2001). According to Wrogemann (1992) and FleuryBrugiere (2001), chimpanzees use some nesting tree species more than others irrespective of their density in the habitat and also have preferred heights at which they build their nest. For example, Erythrophleum suaveolens was documented as the most utilized and preferred species in Fouta Djallon, and constitute 26% of the total nests. At Bossou in Guinee Forrestiere, 44% of total nests encountered were built in oil palm tree Elaeis guineensis (Humle, 2004). According to Fruth and Hohmann (1996), chimpanzees construct nests between the heights of 0-45 m but show preferences for heights at 10-20m. Chimpanzees can also construct a single nest by integrating a maximum of four tree species (Fruth, 1995).

Also, Goodall (1962) observed 10 nests built in a single tree in Gombe serving as the largest ever number of nests recorded in a single tree species. Several studies have indicated that chimpanzees nest in groups; however, some chimpanzees may nest alone; e.g. mother and infant. A comparative study on nesting behavior of chimpanzees in Rio Muni, Equatorial Guinea and Mt. Assirik, Senegal by Baldwin et al. (1981) showed significant differences in nesting pattern of the two populations. Chimpanzees in Assirik and Fongoli in Senegal have different preference for nesting tree species and height for nest construction (Stewart et al., 2007). Assirik chimpanzees exhibit nesting site preference for gallery forest and construct nests are relatively higher heights in tree crowns (Pruetz et al., 2008).

Human Impact on Chimpanzee Populations and Distribution

Human disturbance quantifiable through infrastructure development, such as settlements or roads, logging, industrial agricultural activities, slash-and-burn agriculture, poaching, pet trade, and other threats are drivers of biodiversity loss and habitat fragmentation worldwide (Carvalho, 2013). For example, poaching and illegal hunting affect primate populations directly whilst deforestation and slash-and-burn agriculture cause indirect impacts (Oates, 1996; Hashimoto, 1995; Devos et al., 2008). In Africa, forest resource utilization and conversion of forest via logging and agricultural activities has significantly reduced chimpanzee habitats (Marchesi et al., 1995; Oates, 2006). Disintegration of continuous habitats into patches has disconnected most chimpanzee populations, ensuing reduced survival (Goodall, 1986; Butynski et al., 2000). Chimpanzee habitat areas that are dominated by human population are typically highly fragmented (Campbell et al. 2008), resulting in changes in population and distribution of the species within a landscape. Slash-and-burn agriculture in landscapes account for low chimpanzee density estimates (Carvalho, 2013). Durant et al., (2011) pointed out that low-intensity selective logging is compatible with primate conservation whilst high-intensity logging is a great threat to species’ survival. For example in Kalinzu Forest, Uganda, chimpanzees comfortably live in selectively logged areas that were contiguous to unlogged areas (Hashimoto, 1995). Studies have also documented that about 3% of bushmeat sold in urban markets constitute chimpanzee meat (Whites et al., 2002; Caspary et al., 2001). This hunting pressure is resulting in local extinction and very low population densities of the species in some regions.