The snail kite (Rostrhamus sociabilis) is a wetland hawk whose distribution in United States is limited to the freshwater marshes of southern and central Florida (Bennetts and others, 1994). It is listed as an endangered species in the United States. Because the snail kite feeds almost exclusively on the apple snail (Pomacea paludosa), it is an example of an extreme prey specialist, and would, therefore, be expected to be at high risk from environmental variations that affect apple snail dynamics. Apple snails occur in areas of extended inundation and their availability to kites is greatly reduced during droughts. Frequent droughts may also reduce the long-term densities of apple snails.

Bennetts and Kitchens (1997) have suggested that snail kites undertake exploratory movements and that individuals may have mental maps of potential habitats over wide areas. The question then is how much the snail kite's ability to move and find good conditions compensates for temporal variability in habitat condition. This depends on whether the array of potential habitats is large enough to ensure the viability of the snail kite population. There is some reason for concern, because reports from earlier times indicate that snail kites are not as widespread in Florida now as they were in the 1800's up through the 1930's.

Predictions concerning the viability of the snail kite under altered conditions require that the population be modelled in a spatially explicit manner, using realistic models for the hydrology over the population's range. A spatially-explicit, individual-based model of the dynamics of the snail-kite population has been developed within the ATLSS project. The dynamics of the kite population under different management regimes will be analyzed by means of a spatially-explicit individual-based model. Given the total size of the kite population (order of magnitude 100-1,000), it is possible to represent each individual kite in the model.

Empirical information for the model comes from a detailed study performed by Bennetts and Kitchens (1997) on the demography and movements of the kites. Fecundity and fledgling survival have been estimated through nest studies. Radio telemetry (282 birds) and mark-resighting of banded snail kites (913 birds) are being used to estimate survival, to evaluate the influences of environmental conditions (for example, hydrology) on survival, and to evaluate the movement patterns of snail kites in Florida, including what environmental conditions are correlated with these movements. At present, density dependence has not been incorporated in any of the model parameters, as there is no evidence yet of density-dependent limitations on the snail kite.

The snail kite model aims at predicting the viability of the snail kite population in southern and central Florida under a range of future hydrologic scenarios. It is hypothesized that the viability critically depends on the frequency of droughts, but also on the spatial extent of these droughts. The model snail kites inhabit an array of fifteen major wetlands of southern and central Florida, represented on a GIS map layer. The habitat quality within each of these areas is assumed to be relatively uniform, and maximum carrying capacities are assigned to each area. There is also one aggregated "peripheral" habitat, representing areas of inferior quality that snail kites may use temporarily. Hydrological status and the availability of apple snail prey in these wetlands are based on information from hydrologic models.

A system-wide drought is likely to result in an increased mortality, but local droughts are unlikely to have such an effect, because the birds can respond to a local drought by migration. The specific aim of the present model is to study the dynamics of the snail kite in relation to the following two parameters: the interval between droughts and the spatial correlation between the droughts.

The model shows that high drought frequencies lead to reduced numbers of snail kites. The effect of habitat degradation after a prolonged period of inundation, however, had no effect within the range of drought intervals that was studied. The most interesting aspect of the current version of the model is that it allows for the evaluation of spatial correlations between droughts and the unpredictability in the number of kites over longer time spans. When the spatial correlation between droughts is low, the model shows narrow ranges of predicted numbers of kites. When the droughts tend to be system-wide, the effect of environmental stochasticity strongly increases the unpredictability of future numbers of snail kites and, in the worst case, the viability of the kite population is threatened.

REFERENCES

Bennetts, R.E., M.W. Collopy, and J.A. Rodgers, Jr. 1994, The snail kite in the Florida Everglades: A food specialist in a changing environment, in S. M. Davis and J. C. Ogden (eds.)., Everglades: The System and Its Restoration, St. Lucie Press, Delray Beach, Florida, pp. 507-532.

Bennetts, R.E., and W.M. Kitchens, in press, The demography and movements of snail kites in Florida. U.S. Geological Survey, Biological Resources Division, Florida Cooperative Fish & Wildlife Research Unit, Technical Report.

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