Modelling spatial turnover patterns in tree species diversity for conservation.

Abstract

The planet’s biodiversity is increasingly threatened by the effects of rapid environmental change. Biodiversity loss led by expanding anthropogenic activity, combined with the effects of changing climatic regimes, pose a threat to natural communities and the ecosystem services that they underpin. Understanding how communities of species vary across space and the factors that drive such variation is essential to predicting the impacts of human and climate-induced changes on biodiversity and ecosystem services. This knowledge can be used to plan conservation areas that effectively represent the underlying ecological processes that maintain biodiversity and ensure the provision of ecosystem services crucial to human wellbeing. Beta diversity has been variably defined since the concept was first introduced as a measure of variation in species assemblages. This has given rise to the development of different measures of beta diversity, and to varying conclusions regarding the variation of species composition across space. In this thesis, I aimed to investigate the spatial structuring of tree species turnover patterns (beta diversity) and the potential drivers of these patterns, within the highly environmentally heterogeneous Mesoamerican region. In addition, the effect of habitat loss on tree diversity up to the year 2000 was estimated, and the representativeness of the protected area system was assessed in an effort to highlight potential areas of conservation concern across the region. The efficacy of ecoregions in representing turnover patterns was also assessed. The spatial structuring of areas of low similarity (high beta diversity) was clustered in the north and south of the study region, as well as within the Central American mountainous regions. Areas of low similarity mostly fell within dry, pine-oak, and montane ecoregions. Moist forests were indicated as having high similarity. The congruence of the modelled pattern of beta diversity in trees with patterns of high beta diversity in other taxa provides insights into areas of potential conservation efforts. Habitat loss in the past decade was high in the southern areas of the isthmus indicating that countries like Nicaragua, Costa Rica, and Panama had lost high proportions of their original habitat and species. However, habitat loss was intensive throughout the region, and areas of high beta diversity were found to be among the least represented in the current protected areas system. Classification based on beta diversity broadly resembled the WWF ecoregional classification for the region. This suggests that models based on broad climatic variables can help elucidate beta diversity patterns at broad spatial scales, although conservation planning also requires robust information at finer scales.