Professor Ralph Mac Nally is Australia’s principal expert and a globally significant leader in spatial ecology, making enduring contributions in the related themes of the ecology of whole catchments and geographic ecology. He unifies research in terrestrial, aquatic and transitional (floodplains and estuaries) ecosystems to bring understanding to how integrated catchments work ecologically. Mac Nally has contributed important new findings to the challenging problem of the concurrent management of carbon sequestration through landscape reforestation, water yields, and terrestrial and aquatic biodiversity. Professor Mac Nally has focused on south-eastern Australia, where he has provided keystone contributions to the understanding of the ecological dynamics of the nation’s most important catchment, the Murray-Darling Basin. His pioneering method for measuring forest health over vast areas is the standard adopted by the Murray-Darling Basin Authority to report on vegetation condition for the entire basin (> 1 million km²), which is updated annually. Mac Nally has linked that work with climate-change projections to estimate the viability of bird and frog species over the next 50 years. Mac Nally has internationally recognized leadership that has led to long-term, significant collaborations in geographic ecology, specifically in major research programs in western North America on birds and butterflies and in the dynamics of US west coast estuaries, with a United Kingdom–Brazil collaboration to understand and respond to the impacts of land-use change on the Amazonian terrestrial and aquatic biota, and with catchment dynamics in response to human pressures in Spanish landscapes.