Response of Roseate Spoonbills Nesting
in Florida Bay to Hydrologic Restoration
A cooperative study with:
The Everglades ecosystem has suffered extensive degradation over the past century, prompting the largest wetland restoration effort ever undertaken. Changes in water management have contributed to widespread loss of ecological integrity, including an 85-90% decrease in the numbers of wading birds (C&SF Restudy 1999). Previous monitoring of Roseate Spoonbills (Ajaia ajaia) by the Audubon of Florida in Florida Bay over the past 50 years have shown that this species responds markedly to changes in hydrology and corresponding changes in prey abundance and availability (e.g., Powell et al. 1989, Lorenz et al. 2002). Shifts in nesting distribution and declines in nest success have been attributed to declines in prey populations as a direct result of water management (Lorenz et al 2002). Consequently, the re-establishment of spoonbill colonies in northeast Florida Bay is one change predicted under a conceptual model of the mangrove estuarine transition zone of Florida Bay (Ogden and Davis 1999).
Based on this history, the USGS and Audubon of Florida have recently joined forces in collaborative project that will serve to monitor changes in nesting distribution and success as a performance measure for success of Everglades restoration efforts. This project will further enable evaluation of hypotheses for the causal mechanisms of observed changes.
Testing Hypotheses about Water Management
Past research (e.g., Lorenz et al. 2002) has shown that the spatial distribution of nesting and nest success of spoonbills are both strongly related to the hydrologic regime, but the particular response may be expressed at different spatial and temporal scales. The spatial distribution of nesting is expressed at a larger spatial scale encompassing all or most of Florida Bay and at time scales on the order of several years to decades. As such, long-term monitoring of the entire bay is required to detect systematic changes in nesting distribution. In contrast, the response of nest success to hydrologic change may be expressed at more local scales related to the proximity of specific colonies to their foraging grounds, and at an annual time scale. The combination of these responses at different spatial and temporal scales provides a comprehensive assessment of the response of spoonbills to hydrologic regimes. Further, because of the annual time scale and local spatial scale of nest success, there is an opportunity to test the following hypothesized mechanisms for these responses:
Hypothesis 1. Wet season hydroperiod and water levels are positively associated with prey biomass.– This hypothesis is based three primary, but not mutually exclusive, factors. First, longer wet- season hydroperiods increases the generation time of fishes (i.e., enabling increased time for reproduction). Secondly, longer hydroperiods enable additional time for growth and thirdly, more three dimensional space promotes secondary productivity by providing increased habitat size (Heald et al. 1974, Kushlan 1976, Kushlan 1978, Carlson and Duever 1979, Kushlan 1980, Haake and Dean 1983, Loftus and Kushlan 1987, Loftus and Eklund 1994, DeAngelis et al. 1997, Lorenz 2000, Trexler et al. 2002).
Hypothesis 2. Lower salinity resulting from freshwater flow results in increased prey-base fish stock.– It has been shown that mangrove fish production is related to salinity regimes whereby increased freshwater volumes is positively correlated with prey-base fish stock (Weinstein et al. 1980, Petterson and Ross 1991, Ley 1992, Ley et al. 1994, Ley et al. 1999, Lorenz 2000, Lorenz et al. 2002).
Hypothesis 3. Low dry-season water levels concentrate mangrove fishes in creeks making them more available to spoonbills during nesting.– The concentration of fish as a result of drying has been previously hypothesized in the Everglades (e.g., Kushlan 1978, Loftus and Eklund 1994, DeAngelis et al. 1997); however, the topography of the mangroves (i.e., the creek formations) enables a more precise prediction of where concentrations will occur (Lorenz et al. 1997, Lorenz 1999).
Hypothesis 4. Substantial "reversals" (i.e., rapid increases of water levels during the dry season), whether they be from extreme rainfall events or anthropogenic water releases, will "dilute" prey concentrations causing systematic nesting failure of spoonbills.– Substantial reversals can offset the advantages of prey concentration (Kushlan 1978, Frederick and Collopy 1989, Bjork and Powell 1994, Bjork and Powell 1996, DeAngelis et al. 1997, Lorenz 2000), such that systematic nesting failure of spoonbills would be predicted immediately following the reversal, regardless of the antecedent conditions.
Variation of hydrologic conditions among years and locations is virtual certainty. Consequently, we can treat this variation in a quasi-experimental framework where the variation in wet and dry season conditions constitutes a series of "natural experiments". Further the specific hypotheses that we have outlined enable us to make predictions independently for both mangrove fish responses and spoonbill nest success based on a combination of wet-season and dry-season conditions (Table 1).
Mangrove Fish/Spoonbill Model
This hypothesis testing framework can also be viewed as the basis for a simple conceptual model that links mangrove fishes to spoonbills. These hypothesized relationships and predictions can therefor be linked with the existing ATLSS mangrove fish model. As such, the testing of hypotheses within this framework also constitutes a validation of this conceptual model. The validation process, then also serves as a mechanism by which this simple model can be refined and/or provide the scientific foundation for the development of a more appropriate model should our conceptual framework prove invalid.
Management Utility and Relevance to Everglades Restoration
This project has several aspects which in combination provide a comprehensive assessment of the responses of spoonbills to restoration efforts. First, this project has a monitoring component, enabling us to detect and track changes in Roseate Spoonbill colonies as a measure of restoration success. Secondly, it has a quasi-experimental component enabling us to evaluate alternative hypotheses about the causal mechanisms of observed changes. Lastly, it supports a modeling component enabling us to make predictions about future responses to hydrologic restoration.
The reestablishment of healthy wading bird populations in the Everglades system has been at the forefront of restoration goals since its inception (Science Subgroup 1993, C&SF Restudy 1999), and the Roseate Spoonbill has been frequently identified as one of the key indicator species for restoration of the estuarine system (e.g., Science Subgroup 1997, Hobbie et al. 1999). Thus, monitoring changes in the distribution and abundance of this species will provide a valuable measure of successful restoration.
This study goes beyond a measure of success in that the methods we have proposed enable us to evaluate alternative hypotheses for the causal mechanisms of observed changes. Unlike most monitoring efforts, we have outlined specific hypotheses related to how hydrologic changes will affect prey species, such that we are able to make specific predictions for the response of spoonbills for any given year. Thus, we will be able to use annual variation in spoonbill response in a quasi-experimental context to test hypotheses, while simultaneously using the cumulative changes over longer time scales to evaluate the success of restoration efforts.
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Dr. Robert E. Bennetts
USGS - Biological Resources
7920 NW 71st Street
Gainesville, FL 32653
Dr. Jerome J. Lorenz
Audubon of Florida
Tavernier Science Center
115 Indian Mound Trail
Tavernier, FL 33070
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