Possingham has made major contributions to four areas of mathematical and theoretical ecology. Early work focussed on the application of behavioural and life-history mechanisms to understanding ecological theory. Second, seminal work introducing a spatial dimension to marine population modelling has underpinned advances in marine conservation planning. Third, building and testing models of the population viability of species has changed the way in which Population Viability Analysis is used internationally. Finally, his current research program is applying decision theory tools to problems in population management and conservation. This has enabled him to spearhead an emerging interdisciplinary field – applied theoretical ecology.