Several prominent articles have recently revived the debate on how to advance and reconcile two pressing global issues: conservation of biodiversity, and food production for an increasing human population. These discussions contrast a 'land-sparing/intensive agriculture' strategy with a 'biodiversity-friendly' agriculture approach. We propose that swidden or shifting cultivation should be an important component of the latter approach in the tropics because many swidden systems maintain very high levels of biodiversity while providing livelihood for populations in tropical forest areas worldwide. We suggest further that when many swidden systems are viewed without prejudice and in broader spatial and longer temporal perspectives, the conservationist aspects of the systems become evident.

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.

Bioenergy crop plants that function as solar energy collectors and thermo-chemical energy storage systems are the basis for biological systems that are expected to contribute to renewable energy production, help stabilize the rising levels of green house gases (GHG), and mitigate the risk of global climate change (GCC). Wide genetic resource bases, especially of wild and semidomesticated perennial grasses and woody species of starch-, oil, and lingocellulose-producing plants, are available to select, breed, genetically-modify, and develop environmentally-friendly bioenergy crops. Plant species, with fast growth, tolerance to biotic and abiotic stresses, and low requirements for biological, chemical or physical pretreatments, are being evaluated as potential bioenergy crops. Currently, bioenergy systems based on traditional sources and first generation bioenergy crops, are not sustainable and their exploitation may contribute to environmental degradation. New genetic resources and technological breakthroughs are being employed to develop dedicated bioenergy crops (DECs) with better GHG profiles and with a suite of eco-physiological traits to maximize radiation interception, water- (WUE) and nutrient-use efficiencies (NUE), improved lingocellulosic accessibility to enzymatic degradation, and to confer environmental sustainability. Large-scale bioenergy crop plantations pose both opportunities and challenges, and will inevitably compete with food crops for land, water, nutrient resources and other inputs; whereas, biodiversity consequences of increased biofuel production will most likely result in habitat loss, increased and enhanced dispersion of invasive species, and pollution. Recent genetic modifications and breeding efforts of bioenergy crops aim at improving biomass yield, quality, and conversion efficiency. Improvements in composition and structure of bio-chemicals in bioenergy crops will enable the production of more energy per ton of biomass and will improve its caloric value, GHG profile, and GCC mitigation potential.