Land use, food production, and the future of tropical forest species in Ghana

Agriculture is arguably the greatest threat to tropical forest species. Conservation scientists disagree over the relative importance of two opposing strategies for minimising this threat: enhancing on-farm biodiversity, through wildlife-friendly farming practices, or sparing land for nature by using high-yielding farming methods on the smallest possible area to reduce the need to convert natural habitats. Previous theoretical work shows that understanding the relationship between population density and yield for individual species is crucial for determining whether one of these strategies, or a mixed strategy, will maximise their populations for a given food production target. In this thesis, I aim to identify what land-use strategy will permit increases in food production with least impact on species in the forest zone of Ghana. Farm-fallow mosaic landscapes with shifting cultivation and native canopy trees produced only around 15% as much food energy per hectare as the highest-yielding oil palm plantations. In farm mosaics where perennial tree crops dominate, food production and profits were higher, but did not reach those of oil palm plantations. I surveyed birds and trees in forest, farm mosaic, and oil palm plantation, and combined these data with information on yields to assess the likely consequences of plausible future scenarios of land-use change. My results provide evidence of a strong trade-off between wildlife value and agricultural yield. Species richness was high in low-yielding farming systems, but there was considerable turnover between these systems and forests, with widespread generalists replacing narrowly endemic forest-dependent species. Species most dependent on forest as a natural habitat, those with smaller global ranges and those of conservation concern showed least tolerance of habitat modification. For virtually all species, including even widespread generalists, future land-use strategies based on land sparing are likely to support higher populations of most species and minimise their risk of extinction compared to land-use strategies based on wildlife-friendly farming. If food production is to increase in line with Ghana‘s population growth, a combination of efforts to improve forest protection and to increase yields on current farmed land is likely to achieve this at least cost to forest species. Efforts to better protect forests, which require further restrictions on human use, might be most effective if they can be closely linked to support for farmers to improve their yields. In the long term however, this strategy will only delay and not avert biodiversity loss, unless global society can limit its consumption.

This work was supported by the Robert Gardiner Memorial Trust, St. John‘s College, and a Domestic Research Studentship, with additional fieldwork, travel and equipment grants from Denis Summers-Smith via the Royal Society for the Protection of Birds, the British Ornithologists‘ Union, the Smuts Memorial Fund and the Cambridge Philosophical Society.