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Heitschmidt's objectives in this review are to (1) examine the potential impact of livestock and other large herbivores on rangeland vegetation; and (2) relate these concepts to the concept of "improved" range vegetation. There are 3 fundamental mechanisms whereby herbivores may impact rangeland vegetation: (1) defoliation; (2) physical (trampling, burrowing, etc.); and (3) excretion. The primary short-term impact of defoliation on individual plants is that it frequently reduces absolute growth rates, which in turn reduces aboveground net primary production (ANPP). The primary long-term impact is related to the effect that discretionary levels of defoliation have on the competitive abilities of individual plants. There are, however, notable exceptions to the above generalizations. For example, Sims and Singh (1978) showed shifts in species composition, due to livestock grazing, increased ANPP in the tallgrass regions of the U.S. However, universal evidence in support of these findings is at best inconclusive particularly with regards to livestock grazing. The direct impacts of livestock are related to the destruction of tissue as a result of trampling whereas the indirect effects are related primarily to its effects on vegetation structure and soil properties as they relate to water infiltration rates and soil erosion. Based on the evidence Heitschmidt reviewed, it is extremely doubtful that any grazing scheme will improve a hydrologic circumstance over that found under ungrazed conditions. Free-ranging herbivores can dramatically alter patterns of nutrient distribution over landscapes and nutrient cycling processes. As for improving range vegetation, Heitschmidt notes there are two problems. The first problem stems from the ambiguity associated with the term "improve". Thus, for a given act or procedure (management practice) to be judged as improvement, directional movement toward a specified goal must occur. This can be interpreted many different ways depending on if it is a rancher, hydrologist, wildlife biologist, etc. The second problem stems from the titled linkage between livestock and other herbivores. It is difficult to imagine that most current livestock management tactics generally benefit range ecosystems. Support for this conclusion stems from the fact that most of the evidence in support of the grazing optimization hypothesis has been garnered from grazing studies conducted in natural ecosystems or environmental chambers. In regards to the grazing optimization theory, there is little doubt that herbivory often enhances ANPP in the more productive indigenous grasslands and/or intensively managed tame pastures of the world. But there is little if any evidence in support of the grazing optimization hypothesis in terms of management of large herbivores, particularly livestock, on most arid and semi-arid rangelands. Even if it is assumed that graminoids and herbivores co-evolved, it is doubtful current livestock management tactics mimic pre-historic evolutionary environments in many regions of the world, particularly in the inter-mountain regions of the western U.S. Heitschmidt does not believe that we have sufficient evidence to support a broad grazing management optimization hypothesis which is: current livestock grazing tactics frequently enhance ANPP (improve range vegetation).