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

The effects of periodic environmental fluctuations on population viabi lity are examined based on the use of environmental states. The approa ch is applied to the Florida population of the Snail Kite, an endanger ed wetland hawk that feeds almost solely on one species of snail. A pr eliminary assessment based on stochastic population fluctuations indic ated that populations became viable when initial size surpassed 300 in dividuals. However, changes in population size between consecutive yea rs, nesting success, and the length of the breeding season were all hi ghly and positively related to water level and rainfall characteristic s, which are highly periodic. Low water conditions cause Snail Kites t o disperse and result in low recruitment, increased adult mortality, a nd population declines. The effects of cyclic drought were explored us ing stage-based life tables for three different water conditions or en vironmental states (drought, lag years following drought, and high yea rs). Population sizes predicted by the model were closely associated w ith 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 sen sitive to estimates of survivorship. The use of the environmental stat e approach is compared to standard techniques for population viability analyses (PVA), and the implications of the model for Everglades wate r management are discussed.