Of forests and farms: Species and functional diversity patterns of herbaceous and shrubby plant communities in neotropical countryside landscapes.

摘要

The research I report on in this dissertation was developed in the framework of countryside biogeography, which focuses on patterns of biodiversity in human-altered (countryside) landscapes. I examined three aspects of plant community diversity in forested and deforested tropical habitats of human-dominated landscapes: (1) species diversity, (2) functional trait diversity and (3) community assembly rules (phylogenetic patterns). All projects were based on plant diversity data collected during my survey of herbaceous and shrubby plants in 85 sites in three forested and three deforested habitat types in three areas of southern Costa Rica. In total, I surveyed the richness and abundance of over 750 plant species. For studies of functional diversity, I also collected data on six functional traits from 668 species. These traits were pollination mechanism, dispersal mechanism, growth form, fruit type, fruit size and seed size. I found that only 16--20% of native plant species were present in both forested and deforested habitats but total species richness did not differ between forested and deforested habitats on a per site basis. Community composition differed greatly by habitat type, with riverbanks in forest most floristically similar to deforested habitats. Functional diversity patterns were largely trait dependent. There were more dispersal mechanisms represented on average in deforested than forest habitats but the reverse was true for growth form and seed size. Pasture and understory consistently had the lowest levels of functional diversity while road verges and tree-fall gaps were consistently functionally rich. Studies of community assembly revealed that for all six traits, ecological filtering was the dominant filtering pressure acting on focal plant communities, but the extent of these pressures varied between forested and deforested habitats. Phylogenetically, only understory communities were under-dispersed indicating intense ecological filtering on suites of unstudied traits in these communities. Few of my studied trait states, however, correlated with patterns of phylogenetic clustering. This thesis is one of the most in depth examinations of how tropical herbaceous and shrubby plant communities are altered by human activities and has advanced our understanding of the ecology of complex human-altered landscapes.