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.,