Simulation of the Effect of Spatial and Temporal Variation in Fire Regimes on the Population Viability of a Banksia Species

Populations of plants that rely on seeds for recovery from disturbance by fire (obligate seeders) are sensitive to regimes of frequent fire. Obligate seeders are prominent in fire-prone heathlands of southern Australia and South Africa. Population extinction may occur if there are successive fires during a plant's juvenile period. Research on the population biology of obligate seeders has influenced the management of fire in these heath and shrublands, but work on the effects of the spatial variability of fires is lacking. We hypothesize that extinction maybe avoided under an adverse fire frequency if fires are patchy. We present a model that simulates the effects of spatial and temporal variations in fire regimes on the viability of a plant population in a grid landscape. Seedling establishment, maturation, senescence, and seed dispersal determine the presence or absence of plants in each cell. We used values typical of serotinous Banksia species to estimate probability of extinction in relation to fire frequency and size. We examined the sensitivity of predictions to dispersal, senescence, fire frequency, spatial burning pattern and size variance, and the size of the grid. Simulations 200 years in length indicated that extinction probability was lowest when mean fire frequency was intermediate and mean fire size was large. When fire frequency was high, extinction probability was high irrespective of fire size. Senescence was more important than high-frequency fire as a cause of extinction in cells. Interactions between dispersal, fire frequency, and size were complex, indicating that extinction is governed by intercell connectivity. The model indicates that fire patchiness cannot be assumed to ensure avoidance of extinction of populations. Conservation of populations is most likely when fire patchiness is relatively low—when the size of fires is moderate to large and when burned patches are contiguous.