Invited Synthesis Paper: Spatial Components of Plant-Herbivore Interactions in Pastoral, Ranching, and Native Ungulate Ecosystems

Thespatialeomponent ofherbivory remainsenigmatic although it is a central aspect of domestic and native ungulate ecosystems. The effects of ungulate movement on plants have not been clearly established in either range or wildlife management. While livestock movement systems have been implemented to cope with increases in livestock density, restrictions on movement, and overgrazing, B large number of studies have disputed the effectiveness of different livestock movement patterns. Traditional pastoralism, particularly nomndism, has been perceived as irrational and even destructive, hut many studies have documented features of traditional pastoral land use that would promote sustainability. Disruptions of wild ungulate movements have been blamed for wildlife owrgrazing and population declines, but actual patterns and mechanisms of disrupted movement and population responses have been poorly documented. Models that integrate plant growth, ungulate movement, and foraging are suggested as a way to improve analyses of spatial plant-herbivore systems. Models must give due attention to nonforage constraints on herbivore distribution, such as topography. Models should assess the significnnee of movement 8s a mans Of coping with local climatic variation (patchy rainfall). Models that distribute an aggregate population over a landscape in relation to the distribution of habitat features deemphasize aspects of ungulate movements and population responses that inevitably cause nonideal distributions, particularly in natural ecosystems. Individual based models describe movement and foraging processes more accurately, hut these models are difficult to apply over large areas. Both top-down and bottom-up approaches to spatial herbiwry are needed. To model plant responses to movement, it is important to account for small scale phenomena such as tiller defoliation patterns, patch grazing, and grazing lawns as well as large scale patterns such as rotation and migration. Herbivory patterns at these difTerent scales are interrelated. Managers of wildlife and domestic livestock populations confront similar problems as they attempt to interpret ungulate spatial distribution patterns and their effects on plants. Overgrazing and subsequentecosystemdegradation onrangelandsorpastoralgrazing areas arc often attributed to inappropriate management of livestock spatial distributions. Overabundances of ungulates in wildlife preserves, and consequent overgrazing, arc often attributed to human interference with natural ungulate migrations or dispersal patterns. These management problems are analogous in that ungulate spatial utilization patterns determine how grazing impacts are distributed in space and in time. Ecosystem sustainability is affected by interactions among animal movement and abundance, plant growth, plant response to grazing, and the physical structure of the landscape. The consequences of herbivory for ecosystems depend, of course, upon herbivore abundance. However, herbivore abundance is expressed in terms of numbers of animals per unit of land, per unit of plant production, or per unit of land per unit time. These measures have been distinguished as stocking density, grazing pressure, and stocking rate, respectively (Heitschmidt and Taylor 1991). Until recently, research and management of ungulate herbivores have treated the denominators of these expressions as homogeneous. The aim of this paper is to explore heterogeneities in these denominators that are normally averaged out.