Ryan is a theoretical ecologist based on the National University of Singapore. He has interests in tropical forest ecology and biodiversity. Current major research projects aim to understand the mechanisms responsible for large-scale patterns of tree diversity in tropical forests and to estimate recent extinction rates in the tropics. He also has active research projects on the evolution of co-operation in human societies and on the epidemiology of dengue. His work involves mathematical modelling, computer modelling, and statistical modelling of ecological systems.