Human activities are triggering some of the most rapid losses of biodiversity in the history of life on Earth. Eutrophication, overexploitation, habitat destruction and fragmentation are the main drivers of this decrease in species richness. There is increasing evidence that this reduction of diversity will have dramatic impacts on the functioning of the natural ecosystems of the world, and on their ability to provide society with a variety of essential ecosystem services. It is therefore urgent to understand the causal mechanisms responsible for the
maintenance of diversity and their potential use for restoration to develop effective conservation policies. In this thesis, I examine several potential mechanisms for the maintenance or loss of plant diversity. These include
resource competition (competition for light and nutrients) and the effects of natural enemies (plant hemiparasites and seed predators).