Environmental Effects on a Coastal Population of Gopher Tortoises

Abstract

--Gopher tortoises (Gopherus polyphemus) typically occur in upland xeric habitats, where long-term stability is thought to be required for population persistance. The southernmost population on Cape Sable, Florida, occupies 113 ha of unstable coastal dunes. This isolated population is relatively dense, estimated at 11.3 tortoises/ha, and consists of over 1200 tortoises. Burrows are randomly distributed over the coastal grassland habitat. The size distribution of burrows suggests the occurrence of larger animals than in other populations studied, probably owing to the long-term absence of human predation. Population density seems to have increased substantially in the 16 years since the last hurricane passed over the area. Environmental conditions, particularly storms and drought, may explain the recent population trend, size structure, and distribution of this coastal population. The gopher tortoise (Gopherus polyphemus) is widely distributed along the southeastern coastal plain of North America. Its population core is in southern Alabama and Georgia and in northern and central Florida, where it inhabits inland xeric habitats, especially pine scrub (Ernst and Barbour, 1972). Nearly all published information on the population ecology and status of gopher tortoises is from these upland habitats (Auffenberg and Iverson, 1979; Alford, 1980; Iverson, 1980; Auffenberg and Franz, 1982; Landers et al., 1982). Auffenberg and Franz (1982) noted that tortoises are generally absent or scarce in low-lying areas near the coast, particularly in the poorly drained southern half of the Florida peninsula. Gopher tortoises do occur in coastal dune habitats (Auffenberg, 1978), although little is known about their ecology there. The southernmost population occupies the coastal beach dunes of Cape Sable, Florida (Fig. 1). The gopher tortoises of Cape Sable are disjunct from currently and previously occurring populations further north. 1 Present address: Department of Biology, Pennsylvania State University, University Park, Pennsylvania 16802, USA. The population is also notable in its being protected from human predation for decades, owing to its location in Everglades National Park. The dune environment of Cape Sable is unstable because of periodic tropical storms, a factor that may affect tortoise population characteristics. In this respect Alford (1980) has suggested that long-term habitat stability may be required for the persistence of tortoise populations. The literature contains only the barest mention of the existence of the Cape Sable population (Carr and Goin, 1955; Auffenberg, 1978) and no analysis of its status or population structure. The purpose of the current study was to analyze the size, structure, and distribution of this isolated island population of gopher tortoises and to consider possible environmental factors affecting population characteristics.