Studies of species coexistence within communities and food webs depend on knowing how species use varying resources. Diet has been asserted as a partitioned resource and an important proxy for measuring ecological similarity between species. Diet, like any aspect of an organism's ecology, varies over space and time, which may diminish the generality of conclusions made about how species interact. Few studies have examined diet variation across two or more dimensions, but here we evaluate diet variation over space and time for four lizard species within the genus Ctenotus (Scincidae). Samples were collected at three field sites in the Great Victoria Desert of Western Australia over the course of 16 years. Diet varies widely over both space and time. However, changes in diet over time overwhelm variation over space at the scale of our study. Despite diet variation within species, distinct differences exist between species in fundamental and realized dietary niche space. Limited overlap between species in dietary niche space implies fundamental ecological differences between species that may not be overturned by environmental variation.