Long- and Short-legged Wading Bird Breeding Colonies
Foraging Potential Index (FPI)
Empirical basis and model assumptions:
The wading birds in a wading bird breeding colony exploit an area of many square kilometers around the colony site. A colony site, to be successful, should be surrounded by sufficient foraging habitat.
* To construct the FPI, all spatial cells within a 1.5 km radius (short-legged wading birds) and 3.0 km radius of the colony site (cell or pixel) are considered to be in the primary "foraging areas" and are evaluated. Only cells that have sufficient surrounding cells that qualify as habitat (> 50%) and less than 25% urban area are considered a potential colony sites. Others are excluded (index set to zero).
Wading birds can only exploit fish from an area when water depths are within certain ranges.
* In the FPI long-legged wading birds are assumed to require water levels in the range 5 to 35 cm and short-legged wading birds are assumed to require water levels in the range 0 to 20 cm.
Wading bird breeding can occur roughly between December and July and wading birds require a continuous supply of available food for the entire period they are caring for eggs and young. This requires that a high enough fraction of their foraging area be in the correct depths, with water depths going down, so that fish are concentrated.
* In the FPI the breeding period for short-legged wading birds was chosen to be January 1 to May 15, while the breeding period for long-legged wading birds was chosen to be from January 1 to July 15. The foraging potential index for the region around a particular colony site or cell is calculated to reflect that a sufficient fraction of the cells in the primary foraging areas are in right depth conditions with decreasing water depth. In the index model "foraging cycles" of 54 and 21 days are assumed for long-legged and short-legged wading birds, respectively. The index keeps track of the number of cycles of these lengths for which at least 20% of the cells in the primary foraging area are in appropriate depth regimes with decreasing depths. These "foraging cycles" do not correspond to the actual breeding cycles, which are much longer. However, they are assumed to be reasonable surrogate measures of the goodness of conditions. If the mean suitable area surrounding a colony decreased below 20% of the total area, the current wading bird breeding cycle for that pixel was terminated and calculations for a new cycle not initiated until the area mean increased above 20%. If a reversal of water depth takes place over a sufficiently large fraction of the foraging area (> 80%), the cycle is terminated.
Wading bird breeding success depends on the number of continuous days during the breeding season in which breeding conditions are suitable, as well as the percentage of cells in their foraging area that are habitat.
* The index for a cell is computed based on the predicted number of breeding cycles multiplied by a factor relating to the fraction of habitat cells in the foraging area. The maximum possible index value is 1.
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.
The flow chart shows the steps in computing an index value for a cell:
Variables of index computation (top box):
To evaluate the index for a cell, the conditions of all the cells with a radius of 1.5 km for short-legged wading birds and 3.0 km for long-legged wading birds are considered. Those cells are considered to be the "primary foraging area" of the wading birds. Only 500-m cells with the listed FGAP habitat types are considered. Habitat cutoff (%) means that only those pixels with 50% of their primary foraging area being of those habitat types can be considered as sites for colonies. Similarly, Urban cutoff (%) means that only pixels with less than 25% urban area can be sites for colonies. Reversal threshold (%) means that if there is a 20% in the water depth of a cell from one time step to the next, that cell is defined as being in reversal. If a fraction of 100% - Foraging area reversal (%) of the cells in the primary foraging area of the colony undergo water level reversal, then the current foraging cycle is ended. The primary criteria used in the index are (1) the occurrence of water in a specific depth range for each wading bird type; that is, between LoWaterlim and HiWaterlim (0-20 cm for short-legged wading birds and 5-35cm for long-legged wading birds). The foraging cycles for the two wading bird types are given by Cycle length. These respectively have foraging cycles of 21 and 54 days.
Cycle through days of year to determine breeding conditions (middle):
The model tracks hydrologic conditions from start season date until end season date. Starting at the beginning of the reproductive season, the model checks the status of all 500-m cells within the primary foraging radius of the colony (only those cells that qualify by being of the right habitat type and having had hydroperiod > 150 days the preceding season. It checks two things, (1) whether a given cell is within the correct water depth range, and (2) whether water depth is decreasing. If conditions are satisfied over at least 20% of the cells in the primary foraging area, then the current foraging cycle is continued. If these good foraging conditions hold for 21 or 54 days, for small or large wading birds, respectively, then one complete foraging cycle is recorded and a new one is started. If the mean suitable area surrounding a pixel decreases below 20% of the total area, the current wading bird breeding cycle for that pixel is terminated and calculations for a new cycle not initiated until the area mean increases above 20%.
A reversal is defined as a greater than 20% depth increase over the previous time step. If a reversal of water depth takes place over a sufficiently large fraction of the foraging area, the current foraging cycle is terminated. If more than 100% - Foraging area reversal (%) is in reversal (currently, 100% - 20% = 80%), then the current foraging cycle is terminated, and a new one begun when foraging conditions are good again.
Calculation of total BPI (bottom):
The index for a cell is composed of two factors. The first factor, NC(x,y)/MaxNC is found by summing the number of predicted foraging cycles for the colony and dividing by the maximum possible number of foraging cycles. The second factor, FeedingRadiusFactor(x,y), is the fraction of cells in the primary foraging area that are of suitable habitat type.
For more information, see Original Model Description