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.
        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.
        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
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        water level stabilization in Lake Okeechobee Florida.  24th Ann. Conf.
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        Snail Kite.  Ph.D. Diss.  Univ. Michigan, Ann Arbor.  181 pp.
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