POPULATION-DYNAMICS, DISTURBANCE, AND PATTERN EVOLUTION - IDENTIFYINGTHE FUNDAMENTAL SCALES OF ORGANIZATION IN A MODEL ECOSYSTEM

We used auto- and cross-correlation analysis and Ripley's K-function a nalysis to analyze spatiotemporal pattern evolution in a spatially exp licit simulation model of a semiarid shrubland (Karoo, South Africa) a nd to determine the impact of small-scale disturbances on system dynam ics. Without disturbance, local dynamics were driven by a pattern of c yclic succession, where ''colonizer'' and ''successor'' species altern ately replaced each other. This results in a strong pattern of negativ e correlation in the temporal distribution of colonizer and successor species. As disturbance rates were increased, the relationship shifted from being negatively correlated in time to being positively correlat ed-the dynamics became decoupled from the ecologically driven cyclic s uccession and were increasingly influenced by abiotic factors (e.g., r ainfall events). Further analysis of the spatial relationships among c olonizer and successor species showed that, without disturbance, perio ds of attraction and repulsion between colonizer and successor species alternate cyclically at intermediate spatial scales. This was due to the spatial ''memory'' embedded in the system through the process of c yclic succession. With the addition of disturbance, this pattern break s down, although there is some indication of increasing ecological org anization at broader spatial scales. We suggest that many of the insig hts that can be gained through spatially explicit models will only be obtained through a direct analysis of the spatial patterns produced.