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Southeast Ecological Science Center


Modeling Alligator Production Probabilities in the Everglades

Alligator hatchlings - click to enlargeFemale Alligator guarding nest - click to enlarge
M. R. Palmer1, K. G. Rice2,  L. J. Gross1

1ATLSS, The Institute for Environmental Modeling
University of Tennessee, Knoxville
Knoxville, TN 37996-1610

2USGS, University of Florida Field Station
3205 College Ave, Ft. Lauderdale, FL 33314

Presented at the15th Working Meeting of the Crocodile Specialist Group, IUCN, Varadero, Cuba. 2000.

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INTRODUCTION

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. 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.

Alligators and Water Management

Historical Trends

  • Summer water levels were predictable.
  • Nests built at appropriate height to avoid flooding.
  • Abundant in prairie habitats of the eastern floodplain.
  • Occupancy of the deep water, central sloughs was relatively low.

Current Trends

  • High and unpredictable rate of nest flooding.
  • Natural water level predictability has been lost.
  • 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 relatively high on levees and associated spoil islands.
  • Low survival of young.
  • Decrease in the number of alligator holes or possible brood habitat proximal to canals (F.J. Mazzotti, Univ. of FL pers. comm.).

Future Trends with Restoration

  • Modified hydrological conditions might be expected to increase nesting effort, nesting success, and abundance of alligators in the aforementioned edge habitats.
  • A corresponding increase in the number and occupancy of alligator holes to serve as drought refugia may be seen.
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MODEL COMPONENTS

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Nest Construction

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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 construction.

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Nest Flooding

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The probability of a nest being flooded 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 egg incubation period (July 1 through September 1) and the mean water level during nest construction to give the probability of nest flooding.

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Relative Habitat Quality

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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.

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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.

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Breeding

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OUTPUT

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Output consists of a map of the predicted index values for a restoration alternative (Alternative D13R4 in the figures below), the base condition (F2050 in the figures below), and a central map describing the difference in performance between the alternative and the base.

Alligator Production Index 1970 (High Rainfall) - click to enlarge
Alligator Production Index 1990 (Low Rainfall) - click to enlarge
Alligator Production Index Mean - click to enlarge

API MODEL
ATLSS American Alligator Production Index

  • Coarse indicator of yearly production potential.
  • Based on local habitat and hydrologic conditions. 
  • Production potential is directly influenced by unique environmental conditions.
  • Addresses only the effects of relative local habitat quality and hydrological dynamics. 
  • Should not be interpreted as providing estimates of population dynamics or viability. 
  • Not likely to encompass long-term changes in habitat quality.  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 in Everglades National Park, WCA2, and WCA3. 
  • Density of essential landscape features (e.g. tree islands) within WCA1 (ARM Loxahatchee National Wildlife Refuge) are "best guess" approximations since data are currently lacking.
     

MODEL CONSTRAINTS

Spatial Constraints

  • Spatial resolution - 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

  • Temporal resolution
    • One day for all water data (height and depth).
    • Static for the vegetation habitat types.
  • The model produces a single yearly value for each spatial cell that incorporates the daily water data affecting nesting and offspring production during that year.

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. 4(2):176-184.

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.
 

Acknowledgements

This research was supported in significant part by DOI's Critical Ecosystem Studies Initiative, a special funding initiative for Everglades restoration administered by the National Park Service; and in part by USGS's Florida Integrated Science Center.

T. Foster assisted with original poster formatting.

We appreciate the support of the U.S. Geological Survey, Biological Resources, through Cooperative Agreement No. 1445-CA09-95-0094 with The University of Tennessee. The statements, findings, conclusions, recommendations, and other data in this report are solely those of the authors and do not necessarily reflect the views of the U.S. Geological Survey.

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