The Role of Disturbances in Shortgrass Steppe Community and Ecosystem Dynamics

Abstract

The disturbance regime of an ecosystem consists of a number of different types of disturbance agents operating over a range of spatial and temporal scales (Pickett and White, 1985). Each type of disturbance has its own set of characteristics, including size, frequency of occurrence, intensity, and attributes associated with location, including soil texture, topographic position, and grazing intensity by cattle. These characteristics result in different short-term localized effects on ecosystems as well as long-term broad-scale effects as the disturbances accumulate through time. Disturbance effects occur at multiple levels of organization, from individuals to populations, communities, and the ecosystem. Effects of disturbance can also vary for different types of organisms or processes associated with plants, animals, and soils. Understanding interactions among the characteristics of a disturbance and the properties associated with the response variable is key to understanding and predicting recovery patterns through time and space. Although successional studies have been conducted in grasslands for more than a century, our understanding of the roles of different kinds of disturbances in generating and maintaining patterns in vegetation and in determining species dominance in shortgrass ecosystems has developed only since the 1980s. Referred to as gap dynamics, our current view of the role of disturbance is a dynamic one, in which the recovery of vegetation depends upon interactions among disturbance characteristics and the life history traits of plants. This gap dynamics conceptualization provides an alternative view of vegetation dynamics compared with traditional successional models based on Clements (1916, 1928). Much of the recent work on disturbances in the shortgrass steppe focuses on the relationships between disturbance characteristics and plant life history traits to test the different Clementsian-based models. Most successional studies of shortgrass communities prior to the 1980s focused on recovery after large-scale disturbances and, in particular, cultivation and subsequent abandonment of agricultural 1 elds (Costello, 1944; Judd, 1974; Judd and Jackson, 1939; Savage and Runyon, 1937). The earliest studies were based upon a Clementsian model (Clements, 1916, 1928) that formed the t raditional v iew of succession in these communities (Fig. 6.1). This model predicted that shortgrasses would dominate cover within 25 to 50 years after abandonment.