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Yiannis G. Matsinos, Wilfried F. Wolff, and Donald L. DeAngelis
changes in the Everglades. Several alternative hypotheses have been suggested in
an effort to explain these declines, with two of them seemingly more consistent
with observations: (1) development and/or drainage of former wetlands that has
reduced the total size of the Everglades and resulted in general habitat loss and
(2) an increase in the frequency of major drought events that has left insufficient
time for standing stocks of fish and aquatic macroinvertebrate prey to replenish in
some feeding areas.
The specific approach taken was to look at the endangered Wood Stork (Mycteria americana) by using a single-colony individual-based model. The principal
parts of the model include (1) a submodel of the spatially heterogeneous landscape (i.e., spatially varying elevation), (2) a submodel for the resources (predominantly fish), which are variable both in time and space, (3) models for the
behavior of the individual nesting adults, and (4) models for the energetics and
growth of each nestling until it fledges (or dies).
The heterogeneity of the landscape was taken into account by subdividing a 40
× 40-km region around the colony into 25,600 square spatial cells of 250 m ×
250 m each. Each cell had its own average elevation, so that the typical topography of the central part of the southern Everglades could be described. In the
model, the transition from the wet to dry season was modeled by daily changes of
water level that could also take into account rainfall events, which can cause
reversals in water levels during the dry season. Long-legged waders such as Wood
Storks usually require water depths within a certain range (for Wood Storks, 10–
40 cm) to feed successfully on their prey of macroinvertebrates and small fish.
Simulation runs were started at the end of the wet season, with initial prey
densities that are typical for the hydrological conditions simulated. Background
competition for resources from other species of wading birds was taken into
account by partitioning the resources based on local abundances and needs of the
competitors.
The wading birds feeding in a cell could reduce the fish biomass of that cell;
their foraging efficiency depended on resource levels and was reduced as resources became scarce. In the absence of foraging birds, cells previously depleted
of fish could replenish, provided that they continued to be flooded.
A set of rules determined the behaviors of the birds from one time interval to the
next. The time step for the simulation was taken to be 15 minutes because many
discrete activities of the birds occur on such short time intervals. Changes in water
depth were computed only once per day, however.
After nesting starts, the decisions made by the adults were guided by various
constraints. Each adult attempted to meet its daily energy maintenance requirement. Wading birds could decide whether to forage alone or to join preexisting
foraging groups (i.e., flocks). The location chosen by an individual in which to
forage was based on partial information concerning the system. It was assumed
that each individual had some knowledge, perhaps obtained by visual cues when
flying or soaring, about the water depths of various locations in its foraging area.
However, a bird was assumed not to know, a priori, if a cell also contained high
concentrations of prey. If an adult stork did not capture prey during a 15-minute
Yiannis G. Matsinos, Wilfried F. Wolff, and Donald L. DeAngelis
changes in the Everglades. Several alternative hypotheses have been suggested in
an effort to explain these declines, with two of them seemingly more consistent
with observations: (1) development and/or drainage of former wetlands that has
reduced the total size of the Everglades and resulted in general habitat loss and
(2) an increase in the frequency of major drought events that has left insufficient
time for standing stocks of fish and aquatic macroinvertebrate prey to replenish in
some feeding areas.
The specific approach taken was to look at the endangered Wood Stork (Mycteria americana) by using a single-colony individual-based model. The principal
parts of the model include (1) a submodel of the spatially heterogeneous landscape (i.e., spatially varying elevation), (2) a submodel for the resources (predominantly fish), which are variable both in time and space, (3) models for the
behavior of the individual nesting adults, and (4) models for the energetics and
growth of each nestling until it fledges (or dies).
The heterogeneity of the landscape was taken into account by subdividing a 40
× 40-km region around the colony into 25,600 square spatial cells of 250 m ×
250 m each. Each cell had its own average elevation, so that the typical topography of the central part of the southern Everglades could be described. In the
model, the transition from the wet to dry season was modeled by daily changes of
water level that could also take into account rainfall events, which can cause
reversals in water levels during the dry season. Long-legged waders such as Wood
Storks usually require water depths within a certain range (for Wood Storks, 10–
40 cm) to feed successfully on their prey of macroinvertebrates and small fish.
Simulation runs were started at the end of the wet season, with initial prey
densities that are typical for the hydrological conditions simulated. Background
competition for resources from other species of wading birds was taken into
account by partitioning the resources based on local abundances and needs of the
competitors.
The wading birds feeding in a cell could reduce the fish biomass of that cell;
their foraging efficiency depended on resource levels and was reduced as resources became scarce. In the absence of foraging birds, cells previously depleted
of fish could replenish, provided that they continued to be flooded.
A set of rules determined the behaviors of the birds from one time interval to the
next. The time step for the simulation was taken to be 15 minutes because many
discrete activities of the birds occur on such short time intervals. Changes in water
depth were computed only once per day, however.
After nesting starts, the decisions made by the adults were guided by various
constraints. Each adult attempted to meet its daily energy maintenance requirement. Wading birds could decide whether to forage alone or to join preexisting
foraging groups (i.e., flocks). The location chosen by an individual in which to
forage was based on partial information concerning the system. It was assumed
that each individual had some knowledge, perhaps obtained by visual cues when
flying or soaring, about the water depths of various locations in its foraging area.
However, a bird was assumed not to know, a priori, if a cell also contained high
concentrations of prey. If an adult stork did not capture prey during a 15-minute