The effects of disturbance architecture on landscape-level population dynamics

Ecology, March, 1996 by Kirk A. Moloney, Simon A. Levin

INTRODUCTION

The importance of spatial relationships and disturbance as factors affecting ecological systems has been recognized at least since Watt (1947) considered these influences together in his seminal paper on pattern and process. Although a large number of subsequent theoretical and empirical studies have examined the impact of disturbance on ecological systems, the spatial pattern of disturbance has in general been ignored, with attention being restricted to understanding only the effects of varying the rate and intensity of disturbance (e.g., Huston 1979, Hastings and Wolin 1989, Clark 1991a, b, Frelich and Lorimer 1991, Colasanti and Grime 1993, Turkington et al. 1993). Disturbance rate is generally represented only by the proportion of habitat affected by disturbance over a set time period or the temporal frequency with which disturbances occur (cf. definitions in Pickett and White 1985). Intensity is most often considered to be the proportion of damage (usually in terms of mortality or loss of biomass) caused by a localized disturbance. However, neither rate nor intensity of disturbance explicitly consider the spatial structure of the disturbance regime.

Recently, in the theoretical and empirical literature the traditional, non-spatial approach towards studying the impact of disturbance on ecological processes has been broadened to include an explicit consideration of the impact of varying the size of individual disturbances on ecological processes (Runkle 1982, Runkle and Yetter 1987, Armstrong 1988, Foster 1988a, b, Coffin and Lauenroth 1989, McConnaughay and Bazzaz 1990, 1991). This discussion has included an explicit consideration of the potential for disturbance size to influence community composition through an interaction with species' life history attributes (Brokaw and Scheiner 1989, Halpern 1989, Spies and Franklin 1989, Whitmore 1989). And, in a few of these cases, there has also been careful consideration of the joint distribution of frequency and size of disturbances in a broadly based spatial context (Levin and Paine 1974, Paine and Levin 1981). In these studies, disturbance is most often viewed as occurring at random within a homogeneous landscape: the effect is to reset the local successional clock, producing a spatiotemporal mosaic of sites within the landscape. However, the successional processes occurring after disturbance are in general not treated as being dependent on the spatial structure of the system or of the disturbance regime (although see Lawton and Putz 1988, Coffin and Lauenroth 1989, Frelich et al. 1993).

The spatial context of disturbance regimes has received some attention in the metapopulation dynamics literature, an offshoot of the theory of island biogeography. Here, habitat suitable for occupation by species is viewed as being composed of islands separated by a matrix of uninhabitable landscape (Lefkovitch and Fahrig 1985, Fahrig 1988, Fahrig and Paloheimo 1988). Disturbance acts to remove populations from individual islands of habitat at a specified rate and/or with a specified intensity, and recolonization is a function of spatial location and dispersal geometry. In this view, the spatial structure of the landscape is static (although see Fahrig 1992), with populations periodically going extinct, and the question of interest is to determine the impact of different spatial architectures and dispersal geometries among islands on survival probabilities of the metapopulation as a whole. In general, the literature does not consider the much broader class of ecological processes that occur within a highly interconnected landscape (the traditional concern of the disturbance literature).

A systematic approach needs to be taken towards understanding the impact of varying spatial and temporal structures of disturbance regimes on ecological patterns and process in continuous landscapes. What does this entail? First, a scheme must be developed that allows a classification of disturbance regimes according to their spatial and temporal structure. Once this is accomplished, a systematic exploration of the impact of varying the spatial and temporal structure of the disturbance regime on ecological systems can be undertaken.

There are essentially three levels of organization to consider in classifying a disturbance regime and in characterizing its impact upon ecological landscapes: (1) the basic, non-spatial components of disturbance: rate and intensity; (2) spatial components of individual disturbances: size and shape; and (3) spatial and temporal components of groups of disturbances: temporal and spatial autocorrelation among individual disturbances. The non-spatial factors - rate and intensity - determine the immediate impact a disturbance regime has upon an ecological landscape. The rate of disturbance, for example, determines the proportion of the landscape that is reset to an earlier successional state; intensity determines how far back the successional clock is set. However, the manner in which a landscape recovers from disturbance, and ultimately the longterm ecological dynamics of the system, may depend critically upon the temporal and spatial structure of the disturbance regime. The size and shape of individual disturbances and the correlation structure among individual disturbances will ultimately determine the rate at which disturbed sites can be recolonized by species of varying life history characteristics and will set the long term structure of the resulting spatiotemporal ecological mosaic.