Endemism hotspots and floristics of Upper Guinea in the context of tropical Africa

This thesis documents patterns in plant species distribution across tropical Africa. Geographic patterns in the distribution of globally rare plants within Upper Guinea are emphasised, and correlates with these patterns are investigated. This thesis argues that globally rare species can and should be emphasised in conservation strategy.
Approximately 3.7 million global occurrence records of 28,803 tropical mainland African vascular plant species are compiled into a database framework. The database is used to propose an updated biogeographic framework for tropical Africa, which is sympathetic to previous chorological frameworks but maximises regional endemism within quantitatively defined boundaries. A definition of Upper Guinea as the forests of West Africa between Sierra Leone and Ghana is recovered.
The concentration of globally rare species in the tropical African flora (bioquality as measured by the GHI) is calculated and mapped at one degree square and half degree square resolution, revealing high bioquality of the horn of Africa. Bioquality is calculated by categorising the global area of occupancy of all tropical African taxa into a Star rating, with the result that a bioquality score can now be calculated at a local scale anywhere in tropical Africa to inform conservation strategy.
At the local scale, variation in bioquality is modelled in two areas of high global endemism within Upper Guinea: the Nimba Mountains and SW Ghana. Disturbance is the only significant variable retained in both models, and shows a strong negative relationship with bioquality. Bioquality scores in forest reserves of SW Ghana are shown to have been stable over 20 years, although our perception of the global rarity and identity of species within the area has altered substantially. This finding supports the GHI as a metric of conservation priority in the face of partial information.
At half degree square resolution, 55% of tropical African cells are estimated to have less than 2.5% of their likely species richness documented. A regression tree model is used to interpolate bioquality scores for cells lacking species distribution data, making use of a range of modern climatic, paleo climatic, geographic and biogeographic variables, found to be predictive of bioquality scores. Areas showing the most stable climates over the past 21,000 years are shown to have the highest modern-day bioquality across tropical Africa.
Areas with relatively stable climates during periods of past climate change have been hypothesised to show high modern endemism as the result of increased speciation in isolated refugia, and/or a lower rate of extinction within the refugia. Two dated phylogenies are estimated for African magnoliids and Rubiaceae species, which show that most of Upper Guinea’s globally rare species are young, vicariant species, although at least one species is likely to be a paleo-endemic, supporting both hypotheses.
Areas with historically stable climates showing present day climatic or topological isolation explain areas of high global endemism in tropical Africa, although the introduction of a high disturbance regime can remove this pattern. A conservation strategy which promotes the protection of globally rare species in tropical Africa is feasible, given what we now know of the African flora, and is wise, at least given the success of the Ghanaian programme over 20 years.

Advisor: Harris, S
Hawthorne, W