A SPATIAL PATCH DYNAMIC MODELING APPROACH TO PATTERN AND PROCESS IN AN ANNUAL GRASSLAND

Landscapes are hierarchical mosaics of patches that differ in their age, size, shape, content, and other aspects. The Jasper Ridge serpentine grassland exemplifies hierarchical patchiness and pattern-process interactions that are common features of natural ecosystems. Gopher mounds formed each year destroy all the plant individuals underneath and result in conspicuous spatial pattern in the landscape. A snapshot of the system is, therefore, a reflection of the patch mosaic of gopher mounds that are different in age and species composition and abundance. Based on a patch dynamics perspective, we have developed a spatially explicit patch-based modeling approach to studying landscape pattern and process dynamics. The simulation model (PATCHMOD) has two major components: a spatially explicit, age- and size-structured patch demographic model and a multiple-species plant population dynamic model. We use this simulation model to examine the spatiotemporal dynamics of the disturbance patches and of populations of two species on the local and landscape scales. The spatial patch dynamic model can relate spatiotemporal dynamics of plant populations to the age- and size-structured disturbance patch population, taking into account variability in microhabitats, complexity in patch overlap, and patch-based plant competition. The localized gopher disturbances can significantly structure the vegetation dynamics at the landscape level. Local populations at the patch level may go extinct frequently, though metapopulations may show little fluctuation. Disturbance promotes coexistence of Bromus mollis and Lasthenia californica by divorcing local competitive exclusion and global extinction. The functional representation of an ecological relationship such as density-dependent fecundity at the local patch scale may be transmuted by patchiness at the landscape scale.