THE EFFECTS OF DISTURBANCE ARCHITECTURE ON LANDSCAPE-LEVEL POPULATION-DYNAMICS

Phenomena such as disturbance play a major role in structuring ecologi cal systems by producing a spatiotemporal mosaic of patches at differe nt successional states. The distribution of species within the resulti ng mosaic depends upon an interaction between species' life history tr aits and the spatial and temporal structure of the ecological processe s controlling species' distributions. We have used a spatially explici t simulation model (JASPER) Of a serpentine grassland to examine the i mportance of some of these relationships, focusing primarily on the ro le of disturbance. The model JASPER is hierarchical in design and was developed to simulate the population dynamics of three interacting pla nt species: Bromus mollis, Calycadenia multiglandulosa, and Plantago e recta. Population dynamics were modeled as occurring within local site s, which were then arranged in a square array to form a landscape. Con nections among sites within a landscape were made primarily through se ed dispersal. Several components of disturbance architecture were vari ed systematically among model runs to determine their impact on popula tion dynamics at the scale of the landscape. We considered three level s of organization in modeling disturbance: (1) overall rate of disturb ance, (2) size of individual disturbances, and (3) temporal and spatia l autocorrelation among individual disturbances. The results demonstra te that the impact of disturbance depends upon a complex interaction b etween the life history characteristics of the species making up the c ommunity and the spatial and temporal structure of the disturbance reg ime. For example, we found that the biggest impact on species abundanc e occurred in response to a shift in the temporal autocorrelation stru cture of the disturbance regime. Also, species diversity was found to increase at intermediate levels of disturbance (as has been shown in s everal other studies). However, what can be considered an intermediate level of disturbance depends as much upon the temporal autocorrelatio n structure of the disturbance regime as it does upon the absolute rat e of disturbance. These results suggest that predicting the impact of disturbance on ecological communities will require an explicit underst anding of at least some aspects of the spatial and temporal architectu re of the disturbance regime.