Ecology of Everglades Alligator Holes

Authors

Frank J. Mazzotti, Mark R. Campbell, Michelle L. Palmer, Jocie A. Graham, Karen Minkowski, Laura A. Brandt, Kenneth G. Rice

Description

The American alligator (Alligator mississippiensis) creates small ponds, called alligator holes, by excavating substrate and vegetation. These holes are common in the Everglades and Frank Craighead in 1968 characterized alligator holes by these three components:

A depression in the muck or limestone bedrock;
Water to fill the resulting basin; and,
Alligators to create and maintain the hole.

Function

Provide water necessary for mating
Act as dry season refugia for aquatic organisms
Provide concentrated area for birds and mammals to forage
Provide disturbance site for plant establishment through soil enrichment and removal of vegetation
Increase overall diversity of Everglades

Questions

Where are alligator holes?
What do alligator holes look like?
How are alligator holes arranged in space?

Methodology Flow Chart

Mapping Summary

Holes are classified into three categories by vegetation type.
Color, infrared aerials (1:24,000) are adequate to map alligator holes.
Minimum 5 m across, 20 cm deep
Spatial accuracy 60 m
Commission accuracy depends on size, location, and vegetation. Holes in sloughs are more difficult to identify than in wet prairies and sawgrass.

Surveying Alligator Holes

Ecological Characterization

The image above is a digital enlargement of an alligator hole at 1 meter resolution. These field maps assist in classifying vegetation types, identifying alligator holes, and mapping vegetation. Yellow dot indicates location of picture on right.

Small holes are structurally and vegetatively distinct from larger holes.
Larger holes surrounded by woody vegetation provide upland habitat and increase the diversity and richness of the surrounding vegetation (Figure 4).
Larger alligators (and hatchlings) are found in larger holes, while juveniles are found in surrounding smaller ponds - holes act as social refugia (Table 1).

General Vegetation Zones

Type 1 Vegetation Zone: Woody
Type 2 Vegetation Zone: Marsh
Type 3 Vegetation Zone: Artificia

	#Active-Spring	#Active-Fall	Trails	Adult	Hatch.	Juven.	Nest	Other*
Type 1	5 (56%)	7 (78%)	9	9	7	0	1	3
Type 2	5 (55%)	8 (73%)	11	0	0	5	0	3
Type 3	7 (78%)	8 (89%)	9	3	5	1	2	6
Total	18 (62%)	23 (79%)	29	5	12	6	3	12
*Other signs include: tail drags, footprints, sunning spots or gator dens.

Table 1. Number of Alligators Observed at Different Hole Types

Figure 4. Composition of Vegetation in Three Meter Rings from Edge of Open Water.

Figure 5. Example of transect data for an alligator hole showing water and muck depths in the North to South direction. Note vertical scale exaggeration.

Spatial Analysis

Holes are clustered, but clusters are not.
Clusters are 3 km apart.
Vegetation and canals have an effect on overall hole distribution.
Fewer holes than expected are found within 1 km of a canal.

Figure 6. Density Map: All Holes

Figure 7. Number of Alligator Holes as compared to distance from canal.

Figure 8. Canal Influence. Number of Alligator Holes as Compared to Distance from Canal at 1 km increments.

Acknowledgements

This research was supported in significant part by the Everglades Agricultural Area Environmental Protection District and the National Fish and Wildlife Foundation.

Citation

Pearlstine, L.G., F.J. Mazzotti, E.V. Pearlstine and G. Mann. (2004, December). Integrating Urban Growth Models and Habitat Models for Ecological Evaluation of Landscape Impacts. Poster presented at the National Conference on Ecosystem Restoration, Orlando, Florida.