In this module we look at the theory behind our understanding of ecological systems and how that theory can be applied to ecological problems in the real world. Starting with populations of a single species we will progress to understanding two species interactions including predation, competition and parasitism and then to whole communities of interacting organisms. We will then study how ecological theory, used in concert with population genetics and evolutionary theory, can be applied to understanding ecological issues such as the conservation of small populations, harvesting natural populations and predicting responses to environmental change.