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.
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 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
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.
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