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.
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: