In Ethiopia, most primate populations exist in small, isolated forest fragments surrounded by a mosaic of agricultural fields, exotic plantations, and human settlements (Hurni et al. 2005; Kifle and Bekele 2020; Mekonnen et al. 2020; Kifle 2021; Yazezew et al. 2022). Although most of these primates are expected to exist in such human-modified landscapes, there has been very little information on their current distribution and population abundance in the country. This study was the first empirical assessment of black-and-white colobus monkey population abundance and distribution in the Awi Zone, Ethiopia. The information generated here from our surveys expands on the knowledge of the geographic distribution of this Ethiopian primate, and the habitats in which they occur for implementing conservation plans and strategies in the region.
The results of our study demonstrate that black-and-white colobus monkeys were present at 27 surveyed forest patches. The results also show that these forest patches support considerable number of black-and-white colobus monkeys. However, the populations of this primate species are patchily distributed in rapidly deforesting tropical forests. It is not surprising, because, in many other studies, most tropical primates are found in fragmented forests in human-modified landscapes (Chapman et al. 2007; Arroyo-Rodríguez et al. 2017; Estrada et al. 2017; Helenbrook and Valdez 2021; Kifle 2021). Habitat fragmentation forced black-and-white colobus monkeys to be distributed in different forest patches where firewood collection, livestock grazing, and selective logging are not restricted from occurring. Thus, like most other primates, human disturbance is the major ongoing threat to black-and-white colobus monkeys in the Awi Zone. Likewise, studied by Rovero et al. (2012) on primate species in the Udzungwa Mountains, Tanzania, Hoffman & O’Riain (2012) on chacma baboons in the Cape Peninsula, South Africa, Cavada et al. (2016) on arboreal primates in the Udzungwa Mountains National Park, Tanzania, and Linder et al. (2021) on Preuss’s red colobus in southeastern Nigeria and western Cameroon also show similar patterns.
The average group encounter rate of black-and-white colobus monkeys along transect lines was 5.5 groups/km. As the forest habitat shrinks, the monkeys were forced to concentrate in a particular area by reducing range sizes, and hence the encounter rate becomes higher. Fashing (2002) reported that C. guereza is resilient to habitat degradation, often reaching medium to high densities. On the other hand, the densities of many primate species are primarily related to food quality and availability (Fashing et al. 2002). The relatively high encounter rate of black-and-white colobus monkeys may be related to the higher availability and productivity of food sources in the Awi Zone. But this needs further research to evaluate the availability and productivity of food sources concerning encounter rate and population abundances for black-and-white colobus monkeys in the region.
The mean group size of black-and-white colobus monkeys was 8.8 intermediate in the region. Contrary to this, Yazezew et al. (2022) reported that the mean group size of black-and-white colobus monkeys was low (6.8) at Wof-Washa Natural Forest, Ethiopia (Table 2). In contrast, the mean group size of black-and-white colobus monkeys was 14.6 in Kakamega Forest, Kenya (Fashing et al. 2012). Variation in group size might be a sign of the ecological flexibility of guerezas across study sites (Anderson et al., 2007; Nowak & Lee, 2013; Tesfaye et al. 2021; Fashing 2022; Yazezew et al. 2022) as well as variation in sampling efforts among researchers.
Table 2
Mean group sizes, encounter rates, and study sites of Colobus guereza across study sites.
Country | Study site | Mean group size | Mean encounter rate/km | Elevation | Sources |
Ethiopia | Awi Zone | 8.8 | 5.5 | 1859–2557 | This study |
Ethiopia | Wof-Washa Natural State Forest | 6.8 | 1.5 | 2510–3100 | Yazezew et al., 2022 |
Ethiopia | Nech Sar National Park | 6.9 | 1.6 | 1108–1650 | Chanie et al., 2021 |
Kenya | Kakamega Forest | 14.4 | - | 1580 | Fashing et al., 2012 |
Uganda | Kyambura Gorge | 8 | - | 1150 | Krüger et al., 1998 |
D.R. Congo | Ituri | 13.9 | - | 600–1200 | Bocian, 1997 |
Uganda | Kibale Forest | 11.4 | - | 1500 | Oates, 1977 |
When anthropogenic disturbance increases, the available forest patch can be reduced in size and increase resource competition among primate populations. Human disturbance tends to have a consistently negative effect on forest primates which depend on trees for food, movement, and sleeping sites (Anderson et al. 2007; Pozo-Montuy et al. 2011; Almeida-Rocha et al. 2017). These lead to negative impacts on primate abundance (Harris, 2006; Anderson et al. 2007; Chapman et al. 2013). There are high levels of anthropogenic disturbances (e.g., deforestation, habitat degradation, firewood collection, livestock grazing, human settlement, exotic plantation, and agricultural expansion) occurring almost in and around all surveyed forest patches (Z. Kifle pers. comm. with local people, and obs.). There are few but vital fragmented primary forests remaining in the Awi Zone. Thus, the distribution of black-and-white colobus monkeys is highly fragmented and reduced in the region.
Finally, black-and-white colobus monkeys in the Awi Zone were most abundant in the dry evergreen Afromontane forest ecosystem between 1859–2557 m asl. The most dominant flora in this altitude of the region includes Albizia schimperiana (Apocynaceae), Prunus Africana (Rosaceae), Albizia gummifera (Fabaceae), Apdoytes dimidiata (Icacinaceae), Croton macrostachyus (Euphorbiaceae), Ekebergia capensis (Meliaceae), Deinbollia kilimandscharica (Sapindaceae), Erythrococca trichogyne (Euphorbiaceae), Vepris dainellii (Rutaceae), Celtis Africana (Ulmaceae), Maytenus arbutifolia (Celastraceae), Maytenus obscura (Celastraceae), Canthium oligocarpum (Rubiaceae), Rytigynia neglecta (Rubiaceae), Discopodium penninervium (Solanaceae), Combretum paniculatum (Combretaceae) Tiliacora troupinii (Menispermaceae), Rothmania urcelliformis (Rubiaceae), Bersama abyssinica (Melianthaceae), Landolphia buchananii (Apocynaceae), Ehretia cymosa (Boraginaceae), Teclea nobilis (Rutaceae), Vernonia auricullifera (Asteraceae), Millettia ferruginea (Fabaceae), Maesa lanceolata (Myrsinaceae), Lepidotrichilia volkensii (Meliaceae) and Justicia schimperiana (Acanthaceae) (Berhanu, 2017). Many of the these tree specimens of the forest patches in this area are remarkable in that they are likely primary growth stems, with extremely notable trunk width and height, and are important for the survival of black-and-white colobus monkeys in the region. This altitudinal range of Awi Zone might be one of the suitable places for the ecology and conservation of this primate species.