ATLSS Snail Kite Index Model Basic Model Description Jane Comiskey, John Curnutt, and Louis Gross The Institute for Environmental Modeling University of Tennessee, Knoxville Knoxville, TN 37996-1610 (Copyright University of Tennessee - 1998) Statement of Limitations The ATLSS Snail Kite Index (SKI) Model was developed as a crude indicator of potential habitat quality during the breeding season for snail kites in the Florida Everglades. All evidence suggests that the population dynamics for this species are influenced by environmental conditions occurring throughout its entire range in Florida. This model addresses only relative habitat quality within a limited area, ignoring larger spatial extent population dynamics that may have a much greater effect on this species than habitat quality in part of its range. Consequently, this model should not be interpreted to represent population dynamics or viability. The time scales at which evaluation of alternative scenarios are evaluated also are likely to be too short to encompass some long-term changes in habitat quality. Particularly, stabilized hydrologic regimes may result in a slow degradation of habitat that may be overlooked at the time scales evaluated with this model. In addition, very little verification of this model's performance has been performed and several of its parameter values are "best guess" approximations, for which data are either currently lacking or have not yet been fully analyzed. A spatially explicit full demographic model for snail kites based on available data is currently under development as part of the ATLSS project. Introduction The snail kite (Rostrhamus sociabilis) is an endangered raptor whose distribution in the United States is restricted to the South Florida Ecosystem, including watersheds of the Everglades, Lake Okeechobee, Kissimmee River, and Upper St. Johns River. Because snail kites feed almost exclusively on one species of aquatic snail (the apple snail, Pomacea paludosa), their survival depends directly on the hydrologic functioning of these watersheds. Each of these watersheds has experienced, and continues to experience, substantial degradation, resulting in the current planning for what probably will become the largest ecosystem restoration ever undertaken. Although other endangered species occur within the ecosystem, snail kites are probably the only species restricted to the watersheds within the South Florida Ecosystem and dependent on the entire network of wetlands within this ecosystem. Over half of the wetlands within central and southern Florida have been lost during the past century and those that remain have been highly fragmented and severely degraded (Weaver et al. 1994). This degradation has prompted planning for ambitious restoration efforts (e.g., the Central and South Florida Project Restudy, Kissimmee River Restoration, and the South Florida Ecosystem Restoration initiative). Because of the snail kite's restricted range and because their population is highly dependent on the success of restoration efforts, the snail kite is a key species to monitor throughout the restoration process. Model Development Temporal Constraints. - Although snail kites in Florida can potentially lay eggs in all months of the year, there is a very distinct seasonal distribution of nest initiations. Nest initiations begin as early as November, but in most years widespread initiations usually do not begin until January or February. During most years nest initiations decrease markedly after June, but may extend through July in some years. For this model we defined the primary breeding season as the period from January-July (reviewed by Bennetts and Kitchens 1997). Suitable conditions for any given year are required to persist for a minimum of 16 weeks during the primary breeding season (January-July). This is based on the time required to complete one breeding cycle, including nest building (10 days), egg laying (2-day intervals with incubation beginning with the 2nd egg), incubation (27 days), the nestling period (30 days), and a post-fledgling period (45 days) (Beissinger 1984, Beissinger and Snyder 1987, Snyder et al. 1989). Relative Habitat Quality - Available evidence suggests that suitable conditions for snail kite breeding are influenced by each of three aspects of hydrology that occur at different temporal scales (Bennetts et al. 1998). Suitable conditions at each scale are necessary, but none is sufficient alone to delimit suitable breeding habitat for snail kites. The hydrology at each of these scales regulates a different aspect of the environment important to snail kites. Thus our Snail Kite Index (SKI) takes on the values of 0 (unsuitable), 1 (marginal), or 2 (suitable) for each landscape grid cell. A value of 0 for any of the hydrologic measures results in a cell BPI value of 0 for that year. Otherwise, the cell is assigned the lowest non-zero factor value. The first hydrologic factor is daily water level (depth). The empirical relationship between snail kites' use of a given habitat and water depth has been well recognized and has been illustrated by the distribution of nests or foraging birds with respect to water depth (e.g., Stieglitz and Thompson 1967, Sykes 1987, Bennetts et al. 1988). The response of snail kites to changing water depth can be seen in shifts in spatial distribution (Bennetts and Kitchens 1997, Bennetts et al. 1998). For example, the spatial distribution of nesting kites within Water Conservation Area (WCA) 3A, a 237,000 ha impoundment used extensively for nesting during the past three decades, was similar for 1992, 1993, and 1994. During the 1995 breeding season, water depths were at record high levels throughout the Everglades as a result of tropical storm Gordon the previous fall. The distribution of nesting kites within WCA-3A shifted dramatically to the north during 1995 compared to observations for the previous three years. Birds moved from areas that were too deep to areas of higher elevation with correspondingly shallower water (Bennetts and Kitchens 1997). When water levels receded the following year, the distribution of nesting birds shifted back to the south where they had been prior to the high water event. Water depth is probably important for snail kites because of how it affects apple snail behavior and availability. Water depths that are too shallow (e.g., < 10-cm) may impede the movement of snails, as submergent vegetation is densely compacted within the water column (Darby et al. 1997). Shallow water during certain seasons also may result in water temperatures rising above the tolerance level of snails (Darby et al. 1997). Bennetts et al. (1988) suggested that a minimum of 20-cm at the time of initiation is required for suitable breeding conditions, with some drying expected during the nesting season. Our lower limit for suitable breeding conditions with respect to water depth is 20-cm at the time of initiation (i.e., during the primary breeding season), and depth must remain above 10-cm for at least the time required to successfully raise a brood (110 days). Water that is too deep may also be unsuitable for breeding snail kites. Water deeper than 1-m may lack sufficient oxygen to support apple snails (Hanning 1978) and/or sufficient vegetation that would enable snails to climb near the surface, where they are available to kites (Darby et al. 1997). The distribution of water depths in the Everglades typically ranges from 10 to 115-cm. Snail kite nests at alligator holes or other depressions are occasionally built over deeper water (Bennetts et al. 1994). Thus, we defined an upper limit of suitable depths to be 115-cm. The second hydrologic factor considered is the time since dry-down at a given location. This factor contributes both to apple snail population dynamics and to the maintenance of plant communities comprising snail kite habitat. Florida apple snails are aquatic and have a limited capacity to survive dry conditions (Little 1968), although the timing of drying may be more important to the overall population dynamics than just the occurrence of drying (Darby et al. 1997). However, drying events result in periodic reductions in the availability of snail kite food resources regardless of whether snail survival is significantly affected. Based on preliminary comparisons of numbers of kites counted during the annual survey before and after drying events in several wetlands, relative habitat quality on average is about 50% of pre-drying conditions the year following the drying event, 85% two years following and fully recovered by three years. Thus, we consider relative habitat quality to be unsuitable during the year that an area dried, marginal the following year, and suitable after two years. However, recent work by Darby et al. (1997) has indicated that the timing of a drying event may be a critical factor in how it affects the apple snail population. Snails hatched during the previous year undergo an almost complete die-off during May-July following reproduction. Thus the cohort that provides the breeding potential for the next year are those that hatched in the preceding year. Given that the peak of egg laying (for snails) occurs from March-May, a drying event that occurs before May can deplete the cohort of breeders for the following year. Consequently, if a drying event occurs before May, we consider habitat to be marginal for an additional year, while the breeding stock replenishes. Although the occurrence of drying events may affect apple snail populations, the absence of drying results in changes in plant communities. There is a considerable body of evidence regarding the tolerances to prolonged inundation of the plant species that comprise suitable habitat (e.g., Craighead 1971, U.S. Department of Interior 1972, McPherson 1973, Worth 1983, Dineen 1972, 1974, Gunderson 1994). Observable changes in plant communities in the absence of drying have occurred after 5-6 years (Ager and Kerce 1970, U.S. D.I. 1972), and some plant communities comprising kite habitat can be replaced by other communities in as little as 9-10 years (Milleson 1987). Thus, we consider habitat to be in the process of deterioration after 5 years of continuous flooding; it is considered unsuitable after 10 years of continuous flooding. The third hydrologic factor is a cumulative effect of the longer temporal pattern of repeated drying events. In particular, the frequency of drying events is expressed as a "hydrologic regime" and is measured as long-term (10-yr) hydroperiod (the proportion of time an area is inundated over a 10 year period). This long-term pattern is the primary hydrologic scale at which plant communities are regulated; although vegetation is also regulated by still slower processes that affect climatic regimes and sea-level rise (Gunderson 1994). Although rapid degradation of habitat occurs if a site is continuously inundated, most sites experience drying at intervals less than that which would result in direct transitions of plant communities. Habitat changes often occur slowly and incrementally, with periods of at least partial rejuvenation resulting from periodic drying. Because of the extreme lack of topographic relief across the central and southern Florida wetland landscape, relatively small changes in elevation correspond to relatively large changes in hydrology. Consequently, differences of a few centimeters in elevation can have profound effects on plant communities and ultimately on the quality of the habitat for kites. The response of snail kites at this scale also can be illustrated by changes in their spatial distribution over longer time periods. For this index model, cells which are inundated less than 80% or greater than 98% of the time over a ten-year are considered unsuitable as snail kite habitat; cells with inundation periods of 80-85% and 95-98% are considered marginal; and cells with 85-95% inundations periods are considered suitable. Acknowledgments The authors would like to thank Robert Bennetts for frequent and productive input into the development of this model and for reviewing our results. Literature Cited Ager, H.A., and K.E. Kerce. 1970. Vegetation changes associated with water level stabilization in Lake Okeechobee Florida. 24th Ann. Conf. of S.E. Assoc. Game and Fish Comm. 338-351. Beissinger, S.R. 1984. Mate desertion and reproductive effort in the Snail Kite. Ph.D. Diss. Univ. Michigan, Ann Arbor. 181 pp. Beissinger, S.R. and N.F.R. Snyder. 1987. Mate desertion in the Snail Kite. Anim. Behav. 35: 477-487. Bennetts, R.E., M.W. Collopy, and S.R. Beissinger. 1988. Nesting ecology of Snail Kites in Water Conservation Area 3A. Dept. Wildl. And range Sci., Univ. Florida, Florida Coop. Fish and Wildl. Res. Unit, Tech. Rep. No. 31. Gainesville, Florida. Bennetts, R.E., M.W. Collopy, and J. A. Rodgers, Jr. 1994. The Snail Kite in the Florida Everglades: a food specialist in a changing environment. Pages 507-532 in S. M. Davis and J. C. Ogden (eds.) Everglades: the ecosystem and its restoration. St. Lucie Press, Delray Beach, FL. Bennetts, R.E. and W. M. Kitchens. 1997. The Demography and Movements of Snail Kites in Florida. US. Geological Survey/Biological Resources Division, Florida Cooperative Fish and Wildlife Research Unit. Technical Report No. 56, Gainesville, Florida. Bennetts, R.E., W.M. Kitchens, and D.L. DeAngelis. 1998. Recovery of the Snail Kite in Florida: Beyond a reductionist paradigm. Transactions North American Wildlife and Natural Resources Conference 63: in press. Craighead, F.C. 1971. The trees of South Florida. Vol. 1., The natural environments and their succession. University of Miami Press, Coral Gables, FL. Darby, P.C., PL. Valentine Darby, R.F. Bennetts, J.D. Croop, H.F. Percival, and W.M. Kitchens. 1997. Ecological studies of apple snails (Pomacea paludosa, Say). Final Report prepared for South Florida Water Management District and St. Johns River Water Management District. Contract # E-6609, Florida Cooperative Fish and Wildlife Research Unit, Gainesville, Florida. Dineen, J.W. 1972. Life in the tenacious Everglades. In depth report. 1(5) 1-13. Central and Southern Florida Flood Control District. West Palm Beach, FL. Dineen, J.W. 1972. Examination of water management alternatives in Conservation Area 2A. In depth report 2 (3) 1-11. Central and Southern Florida Flood Control District. West Palm Beach, FL. Gunderson, L.H. 1994. Vegetation of the Everglades: determinants of community. Pages 323- 340. in S. M. Davis and J. C. Ogden (eds.) Everglades: the ecosystem and its restoration. St. Lucie Press, Delray Beach, FL. Hanning, G.W. 1978. Aspects of reproduction in Pomacea paludosa (Mesogastropoda: Pilidae). M.S. Thesis. Florida State Univ., Tallahassee 119 pp. Little, C. 1968. Aestivation and ionic regulation of two species of Pomacea (Gastropoda, Prociobranchia). Journal of Experimental Biology. 48: 569-585. McPherson, B.F. 1973. Vegetation in relation to water depth in Conservation Area 3, Florida. Open File Report, U.S. Geological Survey, Tallahassee. 62 pp. Milleson, J.T. 1987. Vegetation changes in the Lake Okeechobee littoral zone 1972-1982. Technical Publication No. .87-3. South Florida Water Management District. West Palm Beach, Fl. Snyder, N.F.R.,Beissinger, S.R., and R. Chandler. 1989. Reproduction and demography of the Florida Everglade (Snail) Kite. Condor 91: 300-316. Stieglitz, W.O., and R.L. Thompson. 1967. Status and life history of the Everglade Kite in the United States. Special Sci. Rept. Wildl. No. 109, U.S.D.I., Bur. Sports Fisheries and Wildl., Washington, D.C. 21 pp. Sykes, P.W., Jr. 1987. Snail Kite nesting ecology in Florida. Florida Field Naturalist 15: 57-70. U.S. Department of Interior. 1972. A preliminary investigation of the effects of water levels on vegetative communities of Loxahatchee National Wildlife Refuge, Florida. U.S.D.I. Bureau of Sport Fisheries and Wildlife. 20 pp. Weaver, J. And B. Brown (chairs). 1993. Federal Objectives for the South Florida Restoration. Report of the Science Sub-Group of the South Florida Management and Coordination Working Group. 87 pp. Worth, D. 1983. Preliminary responses to marsh dewatering and reduction in water regulation schedule in Water Conservation Area-2A. Tech. Publ. 83-6. South Florida Water Management District. 63 pp. =============================================================== Output associated with the ATLSS Snail Kite Index Model. In accordance with ATLSS file naming conventions, each file name will consist of the characters: UVXXYYZZ.EXT "U" or "_" => the Base, typically F for the F2050 base or E for the C1995 base "V" or "_" => the alternative scenario or base "XX" => "SK" for the ATLSS Snail Kite Index Model "YYZZ" => 4 character mnemonic, described below "." "EXT" = "PDF" or "TXT" or "DOC" => PDF, tabular text or documentation =============================================================== ATLSS Snail Kite Index Model 1. Maps Map outputs used to characterize results of the Snail Kite Index Model will consist of eight image files in PDF file format. Each map shows a shows a "Set" of model results, comparing one SFWMM hydrologic scenario to another, following the conventions for ATLSS comparisons of two model runs. Each map has three panels. The left panel displays index values for either an alternative or base scenario; the right panel displays index values for a base scenario (e.g., the Future without Project Conditions Case, or F2050). The middle panel displays the cell-by-cell difference between index values for the two compared scenarios (e.g., ALT-5 minus F2050). Grid cells in the left and right panels are color-coded to represent the (positive) values of the displayed index, which range between 0 and 1. Cell colors in the center panel represent either positive (shades of gold) or negative (shades of blue) differences between index values displayed in the left panel and those in the right panel. Color keys are provided at the bottom of each map. Each map depicts the model area at either a Fine (500-meter x 500-meter) or Coarse (2-mile) scale of resolution. For each of six selected years, images will provide a spatial display of index values for that year. In addition, an image file is provided for the mean of all simulated years. The selected years include years with high, low, and typical rainfall, and several additional years that serve to highlight differences between the compared scenarios. The mnemonic characters are composed according to the convention: "YY" = Last two digits of the year "ZZ" = CR - Coarse (2 mile) resolution, FR - Fine (500 meter) resolution Listing of ATLSS Snail Kite Index map files: File Name Time Period ------------ -------------------------------------------- UVSK69ZZ.PDF A High Rainfall Year (1969) UVSK70ZZ.PDF Highlight Scenarios (1970) UVSK77ZZ.PDF A Typical Rainfall Year (1977) UVSK83ZZ.PDF Highlight Scenarios (1983) UVSK90ZZ.PDF A Low Rainfall Year (1990) UVSK95ZZ.PDF Highlight Scenarios (1995) UVSKMYZZ.PDF Mean of All Years (1965->1995) ======================================================== 2. Time Series Time series sets associated with the ATLSS Snail Kite Index will display index values for five subregions : WCA-1, WCA-2A, WCA-2B, WCA-3A and WCA-3B. These show percentage of available habitat in which suitable habitat conditions occurred for each simulation year. File Name Description --------- -------------------------------------------------------- UVSKTSZZ.PDF Percentage of available habitat in which suitable habitat conditions occurred for each simulation year in each of three subregions. 3. Histograms None. 4. Tables None.