Natural enemies, climate, and the maintenance of tropical tree diversity

At least 50% of earth's plant and animal species is found in tropical rainforests, but this rich biodiversity is under threat from deforestation and climate change. Ecologists are interested in understanding why these habitats are so diverse, and how their diversity will change in the future. One leading explanation for high plant biodiversity in tropical forests is the Janzen-Connell Effect. This theory suggests that pests such as plant-feeding insects and fungal diseases can help maintain tropical biodiversity if (1) they specialise on particular plant species, and (2) they cause 'density-dependent' mortality (i.e., they kill more seeds and seedlings where these are locally abundant). This pest pressure acts as a negative feedback mechanism, putting locally rare plant species at an advantage and preventing any one species from reaching high abundance. Recent research shows that this form of density-dependence from both insects and fungi plays a key role in the maintenance of plant diversity in the tropics. We now want to discover how this process changes under different climatic regimes. Wetter tropical forests have more plant species than drier forest, and we will test the theory that more intense density-dependent pest pressure in these places is a factor behind these differences. We will also investigate whether future changes to the climate (higher or lower rainfall) are likely to alter the strength of the Janzen-Connell Effect, and consequently plant diversity. Our work will take place in Panama, where we will take advantage of a steep gradient in rainfall and soil humidity from the dry (Pacific) coast to the humid (Atlantic) coast to test our hypotheses. We will carry our experiments in the field and in controlled nursery conditions that manipulate the density of seeds and seedlings and the presence of fungal pathogens and plant-feeding insects, and we will analyse long-term data and build mathematical models to explore whether and to what extent climate change will alter tropical plant diversity.

We identify three categories of Specific users:

1. Researchers in a range of disciplines
There is renewed interest in the possibility that density dependence caused by plant natural enemies is maintaining and structuring diversity in tropical forests, and more widely. This is an area of research which crosses taxonomic boundaries and applies to a wide range of ecosystems beyond tropical forests.

2. Policy Makers
Our proposal represents fundamental science and therefore we are reluctant to claim direct and immediate impacts of our research on policy. However, within the realm of climate change policy and advocacy we believe that our research might be influential in providing a clear demonstration of the potential consequences of climate change for biodiversity mediated through mechanisms of coexistence rather than through direct effects on individual species. This is part of an emerging realisation that the effects of global environmental change cannot be understood fully without considering the interconnectedness of ecological communities and the processes structuring and maintaining them.

3. Members of the Public and Society at Large
Because of widespread public interest in tropical biodiversity our previous tropical forest research has generated considerable interest to members of the public and enthusiastic amateur naturalists.
We propose three specific knowledge-exchange activities to engage with each category of user identified above:

1. Engagement with the community of researchers will be achieved through publications in the scientific literature, seminars at STRI and in the UK, and through making our quantitative networks of plant - seed-predator food webs freely available to the relevant user community by archiving them in the appropriate international repository (the open-access interaction web database hosted by the US National Centre for Ecological Analysis and Synthesis (http://www.nceas.ucsb.edu/interactionweb/)). We will also host three Panamanian MSc students on 6-month internships within our research programme, one in each year of the project.

2. Engagement with policy makers will be achieved under the aegis of the Oxford Centre for Tropical Forests (OCTF; http://www.tropicalforests.ox.ac.uk/). Owen Lewis serves on the Member's Council of OCTF which links a network of Oxford University departments and neighbouring NGOs, consultancies and businesses in the Oxford area. OCTF provides an excellent forum for collaborative research and communication on issues related to tropical forest governance, management and conservation. Through OCTF we will host a workshop on 'Tropical Forest Biodiversity in a Changing Climate' in the final year of the project. This is a topic where Oxford hosts world-leading researchers (e.g. Prof. Yadvinder Malhi's group within the School of Geography and the Environment) highly influential Private Sector (e.g. ProForest) and Third Sector (e.g. Earthwatch and the Global Canopy Programme) organisations, as well as organisations devoted to communicating the need for tropical forest conservation to wider audiences (e.g. The Living Rainforest). The proposed 1-day workshop will provide a forum to present the results and wider implications of our project to this audience alongside selected invited speakers and external participants (from a variety of sectors) whose work is relevant to the meeting theme. We will place a particular emphasis on consequences of climate change mediated through interacting species and ecological processes rather than through modelled effects on component species considered in isolation.

3. Engagement with members of the public will be achieved through talks by the PIs and the two PDRAs in schools and to local naturalists' groups, Cafe Scientifique and other fora in the UK. In Panama, we will take advantage of opportunities for outreach and communication through STRI.