ATLSS American Alligator Production Index Model
Basic Model Description
Mark R. Palmer and Louis Gross
The Institute for Environmental Modeling
University of Tennessee, Knoxville
Knoxville, TN 37996-1610
(Copyright University of Tennessee - 1998)
Kenneth G. Rice
USGS Biological Resources Division
Everglades National Park Field Station
Homestead, FL 33034
The ATLSS American Alligator Production Index (API) Model was developed
as a coarse indicator of the yearly production potential (probability of
producing nests and offspring successfully) for the American Alligator in
South Florida based upon local habitat and hydrologic conditions. The
production potential of this species is directly influenced by unique
environmental conditions occurring throughout its range in Florida. The
API model addresses only the effects of relative local habitat quality and
hydrological dynamics. Consequently, this model should not be interpreted
as providing estimates of population dynamics or viability. Further, the
temporal extent of the model is not likely to encompass long-term changes
in habitat quality. Particularly, stabilized hydrologic regimes may result
in slow degradation or improvement of habitat not included in this model.
Little verification of the model's performance was possible except for those
populations in Everglades National Park and Water Conservation Areas 2 and 3.
In addition, the density of essential landscape features (e.g. tree islands)
within Water Conservation Area 1 (ARM Loxahatchee National Wildlife Refuge)
are "best guess" approximations since the data are currently lacking.
The American Alligator (Alligator mississippiensis) is a keystone species of the
South Florida Ecosystem. Population growth and survival depends directly on the
hydrologic functioning of South Florida watersheds. Each of these watersheds has
experienced, and continues to experience, substantial degradation. In fact, 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). Currently, planning is underway for what may
become the largest ecosystem restoration ever undertaken. Although other
endangered and keystone species occur within the ecosystem, the American
Alligator's role as a top predator and its effect on the structuring of plant
communities and associated aquatic animals (Mazzotti and Brandt, 1994) make it
an ideal indicator of ecosystem health. The response of alligator populations
to spatio-temporal changes in hydrological conditions throughout the South
Florida Ecosystem are integral to the evaluation of any restoration alternative.
Current water management practices have resulted in a high and unpredictable rate
of nest flooding. Historically, maximum summer water levels were positively
correlated with water levels during alligator nest construction. This natural
predictability has been lost. For instance, in some areas/years, water levels
may be relatively low during nest construction and result in minimal nest height
or placement of nests in lower elevation habitats. Late summer rain events or
scheduled water releases may then lead to increased nest flooding.
Historically, alligators were abundant in prairie habitats of the eastern
floodplain, along the edge habitats of the central sloughs. Pre-drainage
occupancy of the deep water, central sloughs was relatively low. Marsh
alligator densities are now highest in the central sloughs and canals (Kushlan
and Jacobsen, 1990; Fleming, 1991) and relatively low in the edge habitats.
Canal habitats contain high concentrations of adult alligators. Nest
densities are also relatively high on levees and associated spoil islands. Less
flooding of nests occurs on these higher elevations. However, survival of young
may be very low due to a decrease in the number of alligator holes or possible
brood habitat proximal to canals. In Water Conservation Area 3A, densities of
alligator holes are reduced adjacent to canal systems (Mazzotti pers. comm.,).
Presumably, the adult animals do not maintain holes since the canals provide
adequate deep water habitats. Alligator telemetry data suggest that canal
influence extends a kilometer or so into the surrounding marsh (Rice pers.
comm.,). Therefore, the remaining canals and levees will provide a sink to adult
animals but may not contribute to an increase in the overall population. In
fact, if alligator holes are reduced in surrounding habitats, refugia for other
organisms and immature alligators are also reduced. Modified hydrological
conditions might be expected to increase nesting effort, nesting success, and
abundance of alligators in the aforementioned edge habitats. There may also be
a corresponding increase in the number and occupancy of alligator holes to serve
as drought refugia.
This degradation of habitat and hydrological conditions 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). The American alligator is both highly dependent on the success of
restoration efforts and indicative of the restoration's effect upon other species.
Continued monitoring of this species throughout the restoration process is
Spatial Constraints. - The spatial resolution for the model is 500 meters by 500
meters. Historical observations suggest that this roughly corresponds to the
home-range of nesting female alligators. All data (water depth, vegetation type,
ground elevation, breeding indices) represent values for a 500x500 meter area.
Temporal Constraints. - The temporal resolution for the model is one day for all
water data (height and depth) and is static for the vegetation habitat types. The
model produces a single yearly value for each spatial cell that takes account
of the daily water data affecting the nesting and offspring production during
Breeding. - Water levels encountered during the period ranging from May 16 of
the current nesting year to April 15 of the previous year are used as an indicator
of the probability of breeding occurrence in an area. The probability that nesting
will occur correlates positively with the amount of time spent in flooded
conditions during this period. This model component is defined to be the
proportion of this period for which there was water depth greater than 0.5 feet.
Biologists at ARM Loxahatchee NWR have suggested that a static value of 1.0 for
this model component is appropriate for WCA 1.
Nest Construction. - The mean water depth during the peak of the mating season
from April 16 through May 15 is used as an indicator of the probability that
mating and nest construction will occur in a given area. Two linear functions are
applied to indicate the value of this model component such that the highest
probability of nest construction occurs at a mean level of 1.3 feet. Mean water
depth values higher or lower than this reduce the probability of nest
Nest Flooding. - The probability of a nest being flooding is calculated from a
combination of the mean water level during nest construction and the maximum
water level during egg incubation. Field observations indicate that the mean
water level between June 15 and June 30 will determine the elevation at which
a nest will be constructed. A linear function is applied to the difference
between the maximum water level during the the egg incubation period (July 1
through September 1) and the mean water level during nest construction to give
the probability of nest flooding. Biologists at ARM Loxahatchee NWR have
suggested that a static value of 0 for this model component is appropriate for
Relative Habitat Quality - Available evidence suggests that the type of vegetative
cover and elevation within an area greatly influence the probability of nesting.
This model uses a static ranking of the dominant vegetation type within a 500 meter
spatial cell as a measure of habitat quality.
The overall API is calculated as a weighted product of the above described model
components. This uses (1 - the probability of nest flooding) in the product and
applies highest weight to the nest flooding component, a lower weight to the
breeding and nesting components, and the lowest weight to habitat quality factor.
All output is produced as maps in the standard ATLSS format, comparing one
hydrologic scenario to another and displaying a map of the differences between
the two scenarios.
Fleming, D. M. 1991. Wildlife Ecology Studies, Annual Report, South
Florida Research Center, Everglades National Park, Homestead
Kushlan, J. A. and T. Jacobsen. 1990. Environmental Variability and
Reproductive Success of Everglades Alligators. J Herpetol.
Mazzotti, F. J. and L. Brandt, 1994. Ecology of the American Alligator
in a Seasonally Fluctuating Environment. Pgs:485-505 in S. M.
Davis and J. Ogden (eds.) Everglades: The Ecosystem and Its
Restoration. St. Lucie Press, Delray Beach, Fl.
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.