Abstract

Conserving biodiversity in the face of deforestation, climatic change and other threats is one of the foremost challenges facing ecologists. Effective conservation policy relies on understanding both the mechanisms that maintain biodiversity and the ways in which human activity threatens its persistence.

There are many theories that seek to explain the coexistence of species in diverse natural communities, such as tropical rainforests. The Janzen-Connell hypothesis provides one, widely supported explanation for the extraordinary diversity of tropical trees. It suggests that seeds and seedlings surrounded by high densities of conspecifics will be discovered more easily by natural enemies, and subsequently have higher mortality. I will show that both fungal pathogens and insect herbivores attack seedlings of locally common species disproportionately. This reduces dominance by certain species and promotes greater local seedling diversity. In particular, our experiments establish a link between natural-enemy mediated density dependence in individual species and community diversity.

While we are starting to understand these mechanisms which permit species coexistence within communities, anthropogenic environmental change is radically changing the distribution of biodiversity on a global scale. Climate is an important determinant of species distributions, and as the climate changes the distributions of species may change accordingly. One task is to assess how effectively present reserve networks will protect suitable habitats for species in the future. We used species distribution models in combination with projected future climates from General Circulation Models to estimate the future distributions of 400 bird species in Asia. Our results suggest that in order to protect species in the face of climate change, we might have to protect areas that are not considered important to conservation at present. However, these models do not account for interspecific interactions. Extrapolating from models of species interactions and coexistence within communities to the global scale remains an exciting avenue for future research.