Environmental Variability and the Reproductive Success of Everglades Alligators

The reproductive success of the American alligator in the Everglades is constrained primarily by egg mortality, due to flooding by seasonally fluctuating water levels. The timing of nesting is positively correlated with spring air temperatures, and nesting during the summer rainy season places eggs at risk to flooding. Water level at the time of nesting is positively correlated with the elevation at which eggs are deposited above the marsh bottom, which involves nest size, egg placement, and site elevation. Alli- gators respond and to some extent accommodate to higher water levels at the time of nesting. The percent of nests lost to flooding depended on clutch elevation and on the maximum water depth during the incubation period. Clutch elevation frequencies were normally distributed. Regression models permit estimation of the extent of nest flooding over a 30-yr period. Flooding has increased in recent years, coincident with the institution of water management in the southern Everglades. Prior to water manage- ment, the increment of water level rise during the incubation period was predictably correlated with water level at the time of nest construction. This predictability disappeared during the current period of water management, thereby altering the pattern of environmental variation. These results suggest that conser- vation of alligators in the Everglades, and by extension of the Everglades wetland ecosystem, depends on restoration of more predictable hydrological fluctuations. A primary factor regulating tropical and sub- tropical wetlands is the seasonal fluctuation of water depth caused by variation in rainfall and