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)
        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).
        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 great blue 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

        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.

        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.
        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.

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