This unit focus on students developing a mechanistic understanding of the hydrologic and carbon, nitrogen and phosphorus cycles in both terrestrial and aquatic ecosystems as a basis for developing solutions to fundamental questions in ecology and environmental science. For example, how do we predict what happens to rates of nitrogen transformation as climate changes? How do plant communities respond to alterations in groundwater distribution and availability? What role do termites and ants play in driving nutrient cycles? How do we determine impacts of catchment land use on estuarine food webs? Lectures are complemented by a series of problem-based laboratories where students have the opportunity to learn cutting-edge techniques, in particular the application of stable isotopes to ecological studies. There is also an opportunity to get involved in a variety of field-based research projects to gain experience in field sampling methodologies. The unit poses as many questions as it provides answers. The idea is that, by trying to answer these questions, students develop a mechanistic understanding of biogeochemical cycles and develop the skills to apply this knowledge to real-world problems. A core theme for the unit is the way stable isotopes are used as a tool for developing both fundamental and applied understanding of the functioning of ecosystems.