Ecology of Everglades Alligator Holes
Frank J. Mazzotti, Mark R. Campbell, Michelle L. Palmer, Jocie A. Graham, and Karen
Minkowski
University of Florida, Davie, FL
Laura A. Brandt
US Fish and Wildlife Service
Boynton Beach, FL
Kenneth G. Rice
US Geological Survey
Davie, FL
As a keystone species, the American Alligator (Alligator missippiensis) plays a critical
role in shaping the ecosystem of the Florida Everglades. One of the many ways they
impact this ecosystem is through construction and maintenance of deeper water areas, or
alligator holes. The Everglades is subject to seasonally fluctuating water levels, and
crocodilians, requiring water for reproduction and survival, often wallow out organic
muck as water levels drop. These holes serve many functions and are characterized by
three major components; a depression, water to fill the depression, and an alligator(s) to
maintain the hole. Not only do these holes provide water necessary for mating, they also
provide foraging habitat for both female alligators and their young. Alligator holes also
increase the overall diversity and productivity of the Everglades by 1) acting as a dry
season refuge for aquatic organisms when the surrounding marsh dries down; 2) serving
as an area of concentrated organisms to provide a foraging site for wildlife; and, 3)
providing a disturbance site for plant enrichment through soil enrichment and removal of
vegetation.
Until recently, only one alligator hole has been quantitatively studied and no systematic
inventory or mapping effort has been made. Consequently, this project aimed to map
alligator holes throughout the Everglades ecosystem, ecologically characterize them,
determine relative distribution and abundance of alligators, and spatially analyze how
alligators and alligator holes are arranged (Figure 1).
Mapping of alligator holes was accomplished in Water Conservation Areas (WCA) 2 and
3 using color infrared (CIR) aerial photographs at a scale of 1:24,000, and is underway in
Arthur R. Marshall Loxahatchee National Wildlife Refuge. In WCA 2 and 3 alligator
holes greater than 5 meters in diameter appeared as clearly defined black spots on the
photographs. Locations of the holes were marked on acetate overlays and the overlays
were then scanned and spatially referenced using image processing software, and
imported into a geographic information system. Each point, line and/or area was
identified, digitized, and tied to data collected on individual holes. Once in a database,
holes were easily compared to others in the landscape with three types: those with a
surrounding zone of woody vegetation, those without a surrounding zone of woody
vegetation, and those artificially formed from the construction of tree islands.
To perform an ecological characterization of alligator holes, three types of data were
analyzed; morphology, vegetation, and wildlife. Morphology was characterized using
attributes such as size, shape, and basin contour. Water and muck depths at half-meter
intervals were also determined to help distinguish any variations or patterns among
alligator holes. Vegetation was evaluated using CIR field-map enlargements as well as
collecting field data using the line-transect method. Wildlife occurrence was determined
through observations of alligators and other wildlife, as well as dip netting and setting
minnow traps. Here we report the results of mapping and characterizing alligator holes in
WCA 3.
We mapped 845 alligator holes larger than 5 m diameter in WCA 3. Six hundred and
forty holes were natural and 205 were created incidental to the construction of tree
islands for deer. Holes were mapped with a spatial accuracy of 60 m and 83 % were
correctly classified. Holes ranged form 5 to 15 m in diameter and 20 to 150 cm in depth.
Alligator holes surrounded by trees (both natural and man-made) were larger, deeper, and
created a greater disturbance in the marsh matrix than holes that only consisted of a
central pond. The open water of the pond increases habitat diversity in the Everglades
landscape and increases plant diversity by providing hydrological conditions different
from the surrounding marsh. Contrary to expectation more alligators were observed in
ponds at the end of the wet season than in the dry season. Larger alligators and
hatchlings occurred in the larger tree lined ponds and juvenile alligators were found in the
smaller marsh ponds. The separation of adults and juveniles is common in crocodilians
and suggests that alligator holes may serve as social refugia as well as aquatic refugia.
Mapping
Ecological Characterization
morphology
Air-photo interpretation
Computer processing
GIS data
classification
scanning
vegetation
Water chemistry
wildlife
Alligator Surveys
Spatial Analysis
Database Synthesis
Perform layer/data analysis
Figure 1. Methods for mapping and ecologically characterizing alligator holes.
Frank J. Mazzotti, University of Florida Fort Lauderdale Research and Education Center,
3205 College Ave., Ft. Davie, Florida, 33314,
Phone: 954-577-6304, FAX: 954-475-4125, e-mail: fjma@mail.ifas.ufl.edu