MODELING EXTINCTION IN PERIODIC ENVIRONMENTS - EVERGLADES WATER LEVELS AND SNAIL KITE POPULATION VIABILITY

The effects of periodic environmental fluctuations on population viability are examined based on the use of environmental states. The approach is applied to the Florida population of the Snail Kite, an endangered wetland hawk that feeds almost solely on one species of snail. A preliminary assessment based on stochastic population fluctuations indicated that populations became viable when initial size surpassed 300 individuals. However, changes in population size between consecutive years, nesting success, and the length of the breeding season were all highly and positively related to water level and rainfall characteristics, which are highly periodic. Low water conditions cause Snail Kites to disperse and result in low recruitment, increased adult mortality, and population declines. The effects of cyclic drought were explored using stage-based life tables for three different water conditions or environmental states (drought, lag years following drought, and high years). Population sizes predicted by the model were closely associated with actual kite population counts. Deterministic projections indicated that kite populations would increase when intervals between droughts exceeded 3.3 yr, but stochastic simulations found that populations did not become viable unless intervals exceeded 4.3 yr. The model was sensitive to estimates of survivorship. The use of the environmental state approach is compared to standard techniques for population viability analyses (PVA), and the implications of the model for Everglades water management are discussed.