Drought Frequencies

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Our results emphasize that several components of temporal variations in environmental conditions are critical to the persistence and abundance of snail kites. We found that simple modification of the amplitude of water level changes (while mean annual water levels were kept constant) could have dramatic effects on population growth. Surprisingly, it was not the scenarios with the largest amplitudes (largest differences between minima and maxima in water levels) that reduced the population growth rate by the greatest magnitude. The smallest amplitude fluctuations had the greatest effect, because they led to a decrease in the frequency at which wetland sites had drydowns, which had a long-term negative impact on wetland vegetation type and thus habitat suitability. Increases in amplitude also had a negative effect on population growth. This was mostly due to the resultant increased frequency of droughts, which are known to be directly detrimental to kites when they occur at a sufficiently high frequency (Beissinger, 1995; Mooij et al., 2002).

Thus, simple hydrological indicators based on a single factor, such as annual mean water levels (see, for example, the drought indicator developed by Bennetts and Kitchens, 1997) should be interpreted with caution. Indeed, such indicators may fail to identify drought events within a year when the amplitude of water level variation within a year is high relative to the yearly mean water level. Although scenarios with large amplitudes resulted in rapid decreases in intrinsic population growth rates, scenarios with substantially reduced variation (or amplitude) in water levels also led to even greater negative population growth rates over the long term, due to longer-term degradation of the habitat caused by prolonged hydroperiod and the near absence of dry downs (Kitchens et al., 2002; Mooij et al., 2002). Kitchens et al. (2002) hypothesized that occasional drying is critically important for maintaining the vegetation communities that provide the requisite habitat conditions for supporting both foraging and nesting activities of kites. Darby et al. (2005) also suggest that maximum egg clusters of apple snails occur when water depthfalls below a certain threshold (e.g., <40 cm at a study site in WCA3A that is heavily used by nesting kites). Hence, there is some moderate level of within-year variation in water level that is more favorable to the persistence of kites than is either extreme.

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