Divergent views of the influence of livestock grazing on rangeland ecosystems are largely the result of a narrow focus that compares grazed landscapes to grazing exclosures. Our study systems are the rangelands of the Great Plains, where the evolutionary history of grazing is long and grazing is accepted by grassland ecologists as a keystone process of the grassland ecosystem (Milchunas et al. 1988, Knapp et al. 1999). The long evolutionary history of grazing in these prairie ecosystems suggests that biodiversity can be enhanced by mimicking temporal and spatial grazing patterns that occurred before European settlement. To accomplish this, one must first understand the structural heterogeneity of rangelands and the effects of different spatial and temporal grazing patterns on landscape patterns and ecosystem processes. The term heterogeneity can have many meanings (Kolasa and Pickett 1991), but the relevant parameters in the present context derive from variability in vegetation stature, composition, density, and biomass. This type of heterogeneity influences species diversity, variety of wildlife habitats, and ecosystem function (Christensen 1997, Wiens 1997, Bailey et al. 1998). Heterogeneity is therefore the precursor to biological diversity at most levels of ecological organization and should serve as the foundation for conservation and ecosystem management (Christensen 1997, Ostfeld et al. 1997, Wiens 1997). Rangelands have been described as inherently heterogeneous because composition, productivity, and diversity are highly variable across multiple scales (Ludwig and Tongway 1995, Patten and Ellis 1995, Fuhlendorf and Smeins 1999). A heterogeneous patchwork on rangelands can result from differential timing of disturbances and corresponding out-of-phase succession among patches, spatial variability of resources associated with topographic and edaphic patterns, or competitive interactions among plant species (Fuhlendorf and Smeins 1998).

Ecologists understand that many rangeland ecosystems evolved with disturbances, including fire and grazing, but until recently the importance of the spatial patterns and heterogeneity evidenced by these disturbances was not widely recognized. Recent descriptions indicate that disturbance patterns on the Great Plains led to a shifting mosaic in which, at any point in time, the landscape included areas that had been recently burned or grazed (or both), as well as areas that had not been disturbed for years or even decades (Kay 1998). Many of the species that are declining on grasslands today most likely evolved on rangelands best described as heterogeneous across many spatiotemporal scales. The evolutionary importance of heterogeneity on rangelands is evident from the variability in habitat requirements of grassland birds. The structure of grassland avian