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
        Model Limitations
        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 
        Model Constraints
        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
	that year.
        Model Components
        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 
	WCA 1.
        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.  
        Literature Cited
	Fleming, D. M. 1991.  Wildlife Ecology Studies, Annual Report, South 
		Florida Research Center, Everglades National Park, Homestead
		Fl, V-10-1-52.

	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.

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