ABSTRACT: We surveyed over 50 populations of the gopher tortoise, Gopherus polyphemus, in Florida to evaluate how the apparent demographic “health” of each population is influenced by area reduction over time, vegetation structure, and geographic isolation. The influence of isolation was evaluated by comparing demographic profiles of populations on islands and on large mainland sites. A few of the populations studied had been surveyed previously by other researchers, and we were able to relate changes in tortoise demography to changes in vegetation structure. A long-term study on a single population gave us insight into how management practices can influence growth and time to sexual maturity of gopher tortoises.
For all gopher tortoise populations surveyed, the same basic methods were used. We used transects to locate burrows, from which population sizes of tortoises were estimated. Gopher tortoises spend most of their time inside their burrows, and the width of a burrow is known to correlate strongly with the carapace length of the resident gopher tortoise. Therefore, we assessed the condition of burrows and used measurements of burrow widths to estimate both population sizes and the size class distribution of each population. The extent of gopher tortoise habitat area was estimated from the positions of burrows, and amount of area reduction was estimated from a temporal series of aerial photographs. We determined the density of plant cover at ground level and in two levels of canopy.
Our findings indicate that gopher tortoises respond in both obvious and subtle ways to the quality of their habitat. Area reduction and habitat degradation force tortoises into ever-fewer patches of suitable habitat and narrow their choices of locations for burrows. The size distributions of tortoises illustrate the detrimental effects of area reduction and habitat degradation on tortoise demography. The demography of tortoise populations on habitat remnants, therefore, does not necessarily resemble the demography of tortoise populations on equal-sized subsamples of large areas. Gopher tortoises in relatively small, but highly managed areas can maintain healthy populations. One key to successful management is periodic controlled burning of the habitat. Without intensive management, most small, remnant populations are doomed to extinction, but nevertheless may persist for substantial periods of time.
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Demography, Ecology, and Genetics