Usefulness of the concepts of carrying capacity and species substitution ratios in natural resource management has been limited by single objectives and single management options implicit in their definitions. When applied to livestock, they have been further limited by poor conceptualisation of the animal-unit concept often used to quantify them. A systematic approach to the animal-unit concept logically leads to concepts of livestock carrying capacity and livestock substitution ratios compatible with the multiple objectives and multiple management options chamcteristic of range livestock systems. This paper develops simplified multi-objective concepts of carrying capacity and substitution ratios, and discusses their interpretation and application in agroecological systems. Also discussed is the applicability of these concepts to describe components other than livestock in managed and unmanaged systems. Some thoughts are presented on abstraction and simplification of concepts.

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Expressions such as stocking density, stocking rate, grazing pressure, herbage allowance, grazing intensity and stocking pressure have long been used to describe animal-pasture systems. These expressions describe relationships among the basic variables of animal demand, forage quantity, pasture area, and grazing duration. Our objective was to develop a dynamic, mathematical framework of expressions summarizing all of the meaningful relationships among these variables. The resulting expressions have dimensional validity, and are useful in describing dynamic animal-pasture systems. The expressions should also prove useful in future efforts to model these systems. Efforts to describe basic quantitative relationships between grazing animals and pasture have produced numerous expressions, including stocking density, stocking rate, grazing pressure, herbage allowance, grazing intensity, stocking intensity, stocking pressure, and others (Booysen 1967, Society for Range Management 1974, Hodgson 1979). In general, four variables are involved in describing animalpasture relationships. These variables are pasture area (A), usually expressed in ha; forage dry matter (DM) usually expressed in kg; duration of grazing (t), usually expressed in days or months; and the animals' forage demand rate (D), here expressed in animalunits (au). The variable which is functionally important in describing the potential effect of the animal component on a pasture is neither animal live-weight (LW) nor animal numbers alone, but rather the animals' rate of demand for forage, which might be termed the potential rate of forage intake. The potential rate of intake is a function of animal numbers, weight, physiological state, and other animal-related factors. Literature Cited Booysen, P. De V. 1%7. Grazing and grazing management terminology in South Africa.

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This paper analyzes the rangeland condition concept, and discusses how the search for a general concept has been part of the larger search for the identity of range science. It distinguishes between the concept and the assessment of rangeland condition, and distinguishes between the concept and ecological theories used in condition assessment. It proposes a general condition concept of modular character in which different ecological theories and field data are interchangeable components applied locally on appropriate, specific areas. It discusses past distinctions between range management and range science, implores the development of range management science, and discusses efforts needed in research, education, and administration to pursue its development. It interprets past and current events related to range science, including the advent of rangelund health, and discusses their relationships to range science's unfulfilled development as a management science. The paper encourages systematic design of concepts needed to allow range science to fulfill its philosophical potential as a management science.