Wading Bird Foraging Conditions Index Basic Model Description Jane Comiskey, John Curnutt, Lou Gross and Michael Huston The Institute for Environmental Modeling University of Tennessee Knoxville, TN 37996-1610 (Copyright University of Tennessee -- 1998) Introduction: Through the 1960's, the Everglades served as a major breeding center for wading birds in the eastern United States. Over the past several decades, wading bird reproduction has declined dramatically, although the area is still an important feeding ground (Robertson and Frederick 1994). Numerous theories have been offered to explain the decline, including reduction in extent of habitat, reduced prey availability, alteration of drying rates, loss of peripheral short-hydroperiod wetlands, increase in frequency of drydowns, mercury toxicity, effects of eutrophication, shifts in migratory patterns, and changes in storm frequency (summarized in Fleming, Wolff, and DeAngelis 1994). Changing water management strategies for south Florida, which have coincided with decreases in colonial wading bird populations, affect many of the processes implicated in these declines. As part of the Central and Southern Florida Comprehensive Study Review (Restudy), the ecological impacts of a series of proposed alternative water management regimes will be evaluated. Each scenario will affect potential foraging activity of wading birds across the landscape. The ATLSS Wading Bird Foraging Conditions Index Model uses knowledge of how hydrologic factors affect the concentration and availability of food resources during the breeding season to compute a Foraging Conditions Index (FCI) for wading birds. The FCI is a composite index of spatial and temporal patterns. We express the effects of proposed hydrologic scenarios as changes in the spatial pattern of foraging potential over the model area for the 31-year simulation period. Our sub-area reporting units are based on a combination of public area, drainage basin, and management unit subregion maps (see REPUNITS.PDF). Methods: For most wading bird species, small freshwater fish and invertebrates are the primary food brought back to rookeries to feed young birds. Both the amount and timing of prey availability are critical to breeding success during any specific nesting season. SFWMM restoration scenario hydrology output is used to make spatially explicit estimates of surface area with water in the depth range needed for successful feeding for group-foraging species in two categories: short-legged feeders such as white ibis and small herons and long-legged feeders such as wood storks and snowy egrets. The wading bird breeding cycle consists of courtship, nest-building/ breeding, incubation, feeding, and fledging. A breeding cycle is not initiated unless hydrologic conditions are appropriate. After nesting and feeding have been initiated, successful fledging will not occur unless adequate food is available over the period required for young to attain critical growth. The ATLSS "Wading Bird Foraging Conditions Index" is designed to evaluate whether the appropriate hydrologic conditions for successful wading bird feeding are present during the time period that includes nest initiation, feeding of young, and fledging of young. The primary criteria used in the index are (1) the occurrence of water in a specific depth range for each category (0-20 cm for short-legged wading birds and 5-35cm for long-legged wading birds) during a FALLING hydrograph, and (2) the length of continuous periods of local food availability. These criteria were developed by comparing alternative index definitions with published numbers for wading birds in WCA3A and ENP (Cramer et al. 1997, Ogden 1994, Bancroft et al. 1994). The critical feeding period ranges from 45 days for white ibises, snowy egrets, and small herons, to 90 days for wood storks (references in Frederick and Powell 1994, Bancroft et al. 1994). The best fits of index values to nesting bird numbers in WCA3A and ENP over a period of several years (1979 - 1989) were obtained when "reversals" (an increase in water depth during a period of falling water depths) were included as a negative factor in the index. In addition to this temporal component of the index calculation, there is also a critical spatial component, since "reversals" do not necessarily occur simultaneously across all of the modeled portion of South Florida. Specifically, the impact of "reversals" is calculated based on what proportion of the currently available foraging area is affected by the reversal. Thus, the index includes both the POSITIVE effects of having a large foraging area in comparison to a small area with appropriate water depths, and also the NEGATIVE effects of a decrease in the fish densities in the available foraging area due to dispersal that would result from a significant increase in water depth. While the "foraging index" does not include a mechanistic model of fish population dynamics, it does include a simple function for fish density based on the previous year's water depth. Thus, fish densities are assumed to be lower during a wet year following a dry year than during a wet year following a wet year. We calculate the hydrologically-based "foraging conditions index" to represent two different types of wading birds: 1) a "long-legged forager" type with a feeding depth range of 5-35 cm and a long nesting cycle (during which a major water level reversal would cause nesting failure and decrease the index value to zero); and 2) a "short-legged forager" type with a feeding depth range of 0-20 cm and a shorter nesting cycle (with potentially multiple opportunities for nesting during a single dry season). Additionally, we will work towards the addition of these categories to the fish model output. We compute the daily average area of water in the optimal depth ranges over 3 day periods for subregions of the model area. If this mean area in a subregion decreases by 30% or more from one averaging period to the next, the current cycle is terminated and calculations for a new cycle are initiated. Computations are currently made over the period from December 15 to May 15 for short-legged feeders and from December 15 to July 15 for long-legged feeders. The Foraging Conditions Index is computed as the number of periods of continuous availability of water in the selected depth ranges, scaled by the maximum potential number of continuous cycles in the simulation period. Calculations are performed for each subregion within the area covered by the SFWMM. Results are reported graphically and in tabular form as spatial averages over landscape subregions. Cycle lengths and other parameters are being evaluated as model calibration continues, and may be modified in future assessments. The wading bird FCI model is driven by input data from the South Florida Water Management Model (SFWMM). These data, which are provided as daily water depths for each 2 x 2 mile area in the region covered by the model, are processed by the ATLSS landscape model into finer resolution hydrology. Water depths at the 500-m scale of resolution for ATLSS index models are based on a pseudo-topographic map which incorporates information from a 28.5-meter resolution vegetation map (see HYDRO___.DOC for a more detailed description of high resolution hydrology and pseudotopography). This resolution captures the fine-scale spatial structure of the South Florida wetlands that creates the shallow depressions and ponding areas that are critical for wading bird feeding. ADDENDUM For assessment of Alternative 5 (and subsequent alternative scenarios), spatial FCI patterns are computed at the 500-m scale of resolution using ATLSS High Resolution Hydrology. Results are not averaged by subregion. We compute the daily average area of water in the optimal depth ranges for cells within a set distance of each 500-m grid cell of suitable habitat type within the model area. Cells within a set radius of each central cell are considered for this moving spatial average for each wading bird model (long-legged and short-legged feeders). If the mean suitable area surrounding a cell decreases below 20% of the total area, the current cycle for that cell is terminated and calculations for a new cycle are not initiated until the area mean rises above 20%. Cycle lengths, foraging radii, suitability thresholds and other parameters are being evaluated as model calibration continues, and are likely to be modified in future assessments. References: Bancroft, G.T., A.M. Strong, R.J. Sawicki, W. Hoffman, and S.D. Jewell. 1994. Relationships among wading bird foraging patterns, colony locations, and hydrology in the Everglades. In Everglades: The Ecosystem and Its Restoration, S.M. Davis and J.C. Ogden (Eds.), St. Lucie Press, Delray Beach, Fla., chap. 25. Cramer, P., K.M. Portier and D.M. Fleming, D.M. 1997. Systematic Reconnaissance Flights, Wading Bird Study, ENP. www.stat.ufl.edu/~arcs/enp/. Fleming, D.M., W.F. Wolff, and D.L. DeAngelis. 1994. Importance of Landscape Heterogeneity to Wood Storks in Florida Everglades. Environmental Management 18(5):743-757. Frederick, P.C. and G.V.N. Powell. 1994. Nutrient transport by wading birds in the Everglades. In Everglades: The Ecosystem and Its Restoration, S.M. Davis and J.C. Ogden (Eds.), St. Lucie Press, Delray Beach, Fla., chap. 23. Ogden, J.C. 1994. A comparison of wading bird nesting colony dynamics (1931-1946 and 1974-1989) as an indication of ecosystem conditions in the southern Everglades. In Everglades: The Ecosystem and Its Restoration, S.M. Davis and J.C. Ogden (Eds.), St. Lucie Press, Delray Beach, Fla., chap. 22. =============================================================== Output associated with the ATLSS Wading Bird Foraging Conditions Index Model. In accordance with ATLSS file naming conventions, each file name will consist of the characters: UVWXYYZZ.EXT "U" or "_" => the Base, typically F for the F2050 base or E for the C1995 base "V" or "_" => the alternative scenario or base "WX" => "WS" for the ATLSS Wading Bird Foraging Conditions Index Model for Short-legged Feeders => "WL" for the ATLSS Wading Bird Foraging Conditions Index Model for Long-legged Feeders "YYZZ" => 4 character mnemonic, described below "." "EXT" = "PDF" or "TXT" or "DOC" => PDF, tabular text or documentation =============================================================== ATLSS Wading Bird Foraging Conditions Index 1. Maps Map outputs used to characterize results of the Wading Bird component of the ATLSS Foraging Conditions Index Model will consist of sixteen image files in PDF file format, eight for each of two categories of feeders (short-legged and long-legged wading birds). Each map shows a "Set" of model results, comparing one SFWMM hydrologic scenario to another, following the conventions for ATLSS comparisons of two model runs. Each map has three panels. The left panel displays index values for either an alternative or base scenario; the right panel displays index values for a base scenario (e.g., the Future without Project Conditions Case, or F2050). The middle panel displays the cell-by-cell difference between index values for the two compared scenarios (e.g., ALT-5 minus F2050). Grid cells in the left and right panels are color-coded to represent the (positive) values of the displayed index, which range between 0 and 1. Cell colors in the center panel represent either positive (shades of gold) or negative (shades of blue) differences between index values displayed in the left panel and those in the right panel. Color keys are provided at the bottom of each map. Each map depicts the model area at either a Fine (500-meter x 500-meter) or Coarse (2-mile) scale of resolution. For each of six selected years, images will provide a spatial display of foraging condition index values for that year. In addition, an image file is provided for the mean of all simulated years. The selected years include years with high, low, and typical rainfall, and several additional years that serve to highlight differences between the compared scenarios. The 4 mnemonic characters are composed according to the convention: "YY" = Last two digits of the year "ZZ" = CR - Coarse (2 mile) resolution, FR - Fine (500 meter) resolution Listing of ATLSS Wading Bird Foraging Conditions Index map files: File Name Time Period ------------ -------------------------------------------- UVWX69ZZ.PDF A High Rainfall Year (1969) UVWX70ZZ.PDF Highlight Scenarios (1970) UVWX77ZZ.PDF A Typical Rainfall Year (1977) UVWX83ZZ.PDF Highlight Scenarios (1983) UVWX90ZZ.PDF A Low Rainfall Year (1990) UVWX95ZZ.PDF Highlight Scenarios (1995) UVWXMYZZ.PDF Mean of All Years (1965->1995) ======================================================== 2. Time Series None. 3. Histograms None. 4. Tables None.