Our simulation work shows the preponderance of temporal variation of water level fluctuation in determining the dynamics of the snail kite. Any management actions aiming at helping kites recovery should, therefore, carefully examine the frequency, duration, and timing of dry downs. Interestingly, the hydrograph presented in Figure 9.6, indicates that drought duration and drought frequency have been considerably reduced during the last 13 years (1993-2005), which may have initially promoted higher snail kite population growth. On the other hand, prolonged hydroperiods and increases in flooding frequency observed during the 13 year period (Fig. 9.6) may also have led to degradation of the habitat and to lower apple snail abundances.
In order to understand the causes for the more recent population decline and lack of recruitment, especially in WCA3A during the last eight years (Martin et al., unpublished data), we may need to focus more attention on the effect of flooding events and extended hydroperiods on kite demography and habitat dynamics.
1964 1968 1972 1976 1980 1984 1988 1992 1996 2000 2004
Figure 9.6. Average monthly water levels in Water Conservation Area 3A (WCA3A, station 3A-28 and 3-65 combined; location for 3A-28 was N 25° 48' 44" W 80° 43' 19"; location for 3A-65 was N 25° 48' 53'' W 80° 43' 11''). Solid fluctuating curve indicates monthly water levels; the gray dotted fluctuating curves correspond to the 95% upper and lower percentiles. The dark horizontal line (a) indicates ground elevations at the recording stations. The black long-dashed horizontal line (b) corresponds to the water level threshold used in Everkite that would determine drought conditions in WCA3A (drought conditions result in substantial kite movement and mortality). The black short-dashed horizontal line (c) corresponds to the water level threshold used in Everkite that would determine low water conditions in WCA3A (low water conditions result in moderate movement and mortality of kites). The horizontal, gray short-dashed line (d) corresponds to the minimum water levels in WCA3A in 2001 that triggered a typical kite response to drought conditions (movement and mortality).
The principal constraint for not including this factor in the model is the lack of adequate empirical evidence relating flooding effects on vegetation and snails and mechanistic information regarding the impacts of changes in vegetation and snail abundances to kite population dynamics. Given the stakes involved in the enormous Everglades ecosystem restoration project and the Federally Endangered status of the kite, it is apparent that this challenge will need to be addressed. Our study, therefore, emphasized the deep practical implications of carefully considering temporal explicitness in key environmental variables, an aspect which is too often neglected in studies trying to uncover the ecological dynamics of natural populations inhabiting heterogeneous landscapes (Jonze et al., 2004). In many cases this important component of environmental variation is ignored because of the level of complexity it may add to the study of already complex systems. The integration of detailed demographic studies into flexible mechanistic models (e.g., individual-based models) may therefore provide a powerful framework to unravel these critical issues.
Acknowledgments. Financial support was provided by the US Fish and Wildlife Service, the US Army Corps of Engineers and St Johns River Water Management District. DLD's contribution was supported by the DOI's Critical Ecosystem Studies Initiative and the USGS Florida Integrated Science Centers.
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