Assessment of the effects of water regimes
2000 Base Version 1 executed at a 2-mile resolution (2000B1 (2 Mile))
Natural System Model Version 4.6 Final (NSM_v4.6F) on
Foraging Conditions for Short-legged Wading Birds in South Florida
Jane Comiskey and Louis J. Gross
The Institute for Environmental Modeling
University of Tennessee, Knoxville
Knoxville, Tennessee 37996-1610
(Copyright University of Tennessee - 2003)

Hydropattern effects on wading birds are a function of water depth, duration and areal extent of inundation, and timing of water flows in areas that provide suitable foraging habitat. The ATLSS Short-legged Wading Bird Foraging Potential Index (BPI) Model uses SFWMM restoration scenario hydrology output to make spatially explicit estimates of the occurrence of water in the depth ranges that restrict wading bird foraging and express depth effects as changes in the spatial pattern of breeding potential over the model area. In this evaluation, output from the Natural Systems Model (NSMv4.6F) is compared to that of the SFWMM baseline scenario 2000B1.

Because of differential spatial effects of proposed hydrologic modifications for South Florida, when we compare the results of two hydrologic scenarios some regions of the model area will show changes favorable to wading bird foraging for a given year, while other regions for the same year will reflect a deterioration of conditions. The spatial and temporal dynamics of rookery formation involve the interplay of many factors and are not well understood; therefore, it is difficult to ascertain the effects of predicted foraging potentials on breeding activity. For example, because suitable wading bird rookery sites are not distributed uniformly across the South Florida landscape, a direct comparison of the magnitude of suitable foraging area available under alternative scenarios would be misleading. Therefore, interpretation of current results should focus on areas where most birds have traditionally nested. Those areas are located (1) along the eastern part of WCA-3A, WCA-3B, and Northeast Shark River Slough (the Northeastern Rookeries), and (2) in the transition zone between Shark River Slough and the mangrove estuaries (the Southwestern Rookeries) (see rookery figure).

SESI result comparisons are presented for the SFWMD Indicator Regions as well as ATLSS 3-panel maps. Note that the Ridge & Slough indicator regions have been subdivided for these analyses into two groups: (1) those that fall in the WCAs (WCA-2 and WCA-3) and (2) those in Everglades National Park. Considering these regions as a single group masks divergent trends in the WCAs versus the Slough system to the south. Model years are ranked by mean water depth (ponding) over the model area (see year ranking graph and table).

Assessment of Comparison Maps

Wet Years (as measured by ponding)
FCI values for the wet years of 1995 , 1970 , and 1969 indicate that suitable foraging conditions are somewhat more likely to exist under NSM in what are currently the deeper water areas of the WCAs and in the southern portion of North Taylor Slough, while conditions are more likely to be favorable under 2000B1 through Shark River Slough and, as deeper water precludes foraging in the Slough itself, along the periphery.

Dry Years (as measured by ponding)
FCI values for dry years ( 1989 , 1990 , 1971 ) indicate that foraging conditions become somewhat localized in the wetter areas of the WCAs and the Shark River Slough system in the driest years. FCI values tend to be higher under NSM in northern and NE Shark River Slough, and along the periphery of the Slough, while those under 2000B1 are sometimes higher in deeper water portions of the WCAs. However, there is variation from year to year in these effects.

Average Years (as measured by ponding)
In the average ponding years of 1972 , 1991 , and 1984 , patterns of foraging conditions indicated by FCIs and patterns of relative FCI differences are variable. More extensive areas of positive FCIs are seen in the WCAs and in some years in the Southwestern Rookery area under NSM, while higher values are often seen under 2000B1 in the flowway of Shark River Slough and in NE SRS.

Average of All Years
The figure showing the average of all years masks the divergent trends seen in wet vs. dry years. Small advantages in foraging conditions are seen in Shark River Slough and NE SRS under 2000B1, while values are roughly equivalent to those under NSM in most other areas where a difference is seen.

Assessment of Line Graphs For Indicator Regions

The FCI time series graph for the LNWR indicator regions shows substantially higher foraging index values under NSM for all but the driest years, when values are slightly higher under 2000B1.

Ridge & Slough (north)
The FCI time series graph for the Ridge & Slough (north) indicator regions shows slightly higher foraging index values under NSM in average to wet years and roughly equivalent (low) values for the two scenarios in dry years.

Ridge & Slough (south)
The FCI time series graph for the Ridge & Slough (south) indicator regions shows consistently higher foraging index values under 2000B1 in all but the driest years.

Marl Prairie
The FCI time series graph for the Marl Prairie indicator regions shows a pattern of low values for both scenarios in dry years and higher values in wetter years. FCI values for NSM are somewhat higher than those under 2000B1 in intermediate years and slightly higher under 2000B1 in wet years, with few differences between scenarios.

Big Cypress National Preserve
The FCI time series graph for Big Cypress National Preserve indicator regions shows minimal foraging conditions under both scenarios, improving somewhat in wet years.

In relatively dry years, FCI values under NSM are generally greater relative to 2000B1 in Shark River Slough and Northeast SRS, while FCI values under 2000B1 are sometimes slightly higher in wetter portions of the Water Conservation Areas encompassing the Northeastern Rookeries. In intermediate to wet years the pattern is reversed, with generally higher values under NSM in the WCAs and higher values under 2000B1 in Shark River Slough and Northeast SRS.

Considering years in a sequence of dry to wet hydrologic conditions, as water is added to the system it can be seen that advantages diminish for both (a) 2000B1 in deep water areas of the WCAs and (b) NSM in the flowway and periphery of Shark River Slough. Under higher surface water conditions, short-legged wading bird foraging is significantly curtailed under both scenarios, but in years when foraging is not precluded by high water NSM produces higher FCI values relative to 2000B1 for most of the area encompassing the Northeastern Rookeries, while relatively higher values are seen under 2000B1 in the Southwestern Rookery area and along the periphery of Shark River Slough

As with other index model comparisons, loss of spatial extent of suitable foraging habitat for wading birds under 2000B1 relative to conditions under the Natural Systems Model cannot be adequately reflected in these comparisons.

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