The tropical forest zone in West Africa has experienced centuries of human modification, leading to a substantial decline in forest cover and loss of forest biodiversity. Norris et al. (2010) identify agricultural expansion as the main cause of forest loss and degradation. In addition, the study examines biodiversity of plants, invertebrates, and vertebrates, as a measure of mean species richness across primary forest, logged forest, secondary forest, fallow, tree plantation, perennial crop, annual crop, and clearcut regions. The data show that across all biodiversity groups, there is a significant overall loss of forest species as tree cover is reduced and vegetation structure simplified. Forests are also intrinsically linked to the livelihoods of locals by providing food, fuel, fiber, and a range of ecosystem services to over 200 million people. Therefore, Norris et al. argue for multi-functional conservation solutions, which attempt to recognize and value the wide range of services forests provide. Further studies of this region can provide insights for developing policies that avoid comparable levels of degradation in other African forest regions, which are still largely untouched. — Anastasia Kostioukova
Norris, K., Asase, A., Collen, B., Gockowski, B., Mason, J., Phalan, B., Wade, A., 2010. Biodiversity in a forest-agriculture mosaic – The changing face of West African rainforests. Biological Conservation 143, 2341–2350.
Rapid population growth coupled with 4–5% per annum urbanization has turned West Africa’s lush tropical forest landscape into a forest-agriculture mosaic. From 1988 to 2007, agricultural expansion has been credited with an increase of 7.3 million ha in harvest area of commercial commodities and food staples, such as cassava (Manihot esculenta), plantain (Musa x paradisiacal), cocoyam (Colocasia spp.), oil palm (Elaeis guineensis), and cocoa (Theobroma cacao). In face of growing food demands, West Africa’s limited access to new technologies and poor investment in agricultural research has lead to yield stagnation. Subsequently, instead of increasing yields from existing agricultural practices, West Africans transformed forests into farms thereby increasing the overall amount of land under cultivation. As a result, numerous forest species became threatened with extinction.
Norris et al. aimed to estimate changes in biodiversity across forest and modified forests, using 25 plant, 44 invertebrate, and 18 vertebrate species for comparison. Although richness of endemic species tends to decline in modified habitats (i.e. logged and secondary forests), the data show that in some instances overall species richness increases. This can be explained by non-forest species and habitat generalists replacing many of the native forest species. For example, in cases of liberation thinning, which involve the cutting of lianas and climbers, and girdling of non-commercial trees, richness and diversity of non-native butterfly species increased. Considering that West Africa’s remaining dense forests are often fragmented, Norris et al. concluded that larger fragments held more species of trees, and also a higher proportion of rare species, than smaller fragments.
The authors suggest that further studies be conducted, since data collected in human-modified landscapes are patchy, and prone to a number of generic problems that affect biodiversity datasets. As a solution to forest degradation, Norris et al. encourage forest and agro-forestry systems to produce crops that store significant amounts of carbon, such as cocoa. These crops could be cultivated as carbon credit plants, and brought to either of the two carbon offset markets. In the larger, compliance market, companies, government, or other players buy such carbon credits to comply with caps on total amount of greenhouse gases they are allowed to emit. In the second, much smaller market, individuals, government, or companies voluntarily mitigate their own carbon dioxide emissions in an effort to be more sustainable. Participating in global carbon dioxide reduction markets provide West African farmers with financial incentives to protect local ecosystems by replanting and retaining tree cover.
Additional sources of West African land-use changes from forest to agriculture were attributed to logging, mining, and hydropower activities. These bring infrastructure (i.e. roads), which make the forest interior more accessible to farmers. West Africa’s severe forest degradation levels serve as potent reminder of what might happen to Central Africa’s undisturbed wild forest areas. Norris et al. suggest that studying West African forest loss would lead to smarter conservation efforts to combat environmental degradation foreshadowed in Central Africa’s future.