The interaction of fire and grazing is an important ecological process in the Great Plains grasslands of North America. The fire-grazing interaction promotes a shifting mosaic of patches that support a diverse array of grassland flora and fauna. This ecosystem variability, or heterogeneity, has been identified as critical to the maintenance of biological diversity and therefore should serve as the foundation for conservation and ecosystem management. Landscape heterogeneity has been a primary focus of the ecological management plan at The Nature Conservancy's Tallgrass Prairie Preserve in northeastern Oklahoma. Since 1993, an expanding bison (Bison bison) herd has been interacting with randomly selected burn patches that reflect the historical seasonality and frequency of fire. The fire-bison interaction produces vegetation structural and compositional heterogeneity in an ever-shifting landscape patch mosaic. Research and monitoring has confirmed that this heterogeneity provides for the full array of tallgrass prairie biodiversity. The larger-scale conservation challenge is to develop cattle management regimes that incorporate some of the same "biodiversity friendly" elements as fire-bison. To that end, The Nature Conservancy is in the sixth year of a research partnership with Oklahoma State University to investigate "patch-burning" with cattle. Results thus far are very encouraging: increased heterogeneity and biodiversity can be realized with little or no decrease in livestock production.