General EcologyRead full entry
BURROWING AND BURROW ECOLOGY: Gopher tortoises excavate deep burrows that provide shelter from climatic extremes and refuge from predation. Adult burrows average approximately 4.5 m in length and about 2 m deep (Diemer 1989; see Guyer and Hermann 1997 for information on burrow size and longevity at sites in Georgia and Alabama). Burrows have been found to be significantly shorter in clayey soils than sandy soils which may be a result of respiratory limitations (Ultsch and Anderson 1986); oxygen decrements and carbon dioxide increments were greatest in clayey soils and were positively correlated with burrow length. The high humidity associated with the burrow may offer the tortoise protection from desiccation (Auffenberg and Weaver 1969, Means 1982). At the mouth of each burrow is a mound of subsoil excavated by the burrow resident. Kaczor and Hartnett (1990) found that these soil mounds undergo microsuccession and contribute toward increased plant species diversity in the surrounding habitat.
In northern Florida, Diemer (1992c) found that, on average, adult male tortoises use 5.5 burrows and adult female tortoises use 2.7 burrows per activity season (April-December). In Georgia, tortoises were reported to use 7 and 4 burrows for males and females, respectively (McRae et al. 1981b). Average number of burrows used annually by juvenile tortoises was 1.1 by 0-1- year-olds, 2.2 by 2-year-olds, and 1.7 by 4-5-year-olds in a southern Georgia population (McRae et al. 1981b) and 4.4 by 1-4-year-olds in a central Florida population (Wilson et al. 1994). At the Kennedy Space Center in Florida, Smith et al. (1997) documented the use of individual burrows by several tortoises at different times and occupation of individual burrows by two tortoises at the same time. Suggested reasons for differences in burrow use between populations include differences in ground cover, soil composition, temperature extremes at different latitudes, and number of disturbances to burrows.
Although juvenile tortoises use several burrows they spend most of their time in a primary burrow. Annual use of the primary burrow for juvenile tortoises in a central Florida population was 75% of the use of all burrows (Wilson et al. 1994). The data for estimated use of the primary burrow for adult gopher tortoises are not available. Hatchlings dig burrows (Epperson and Heise 2003) and, in their first year, use multiple burrows (Butler et al. 1995). Several studies have noted that gopher tortoises sometimes use shallow depressions, possibly as resting sites when traveling far from their burrows (Fucigna and Nickerson 1989, Godley 1989, Stout et al. 1989, Diemer 1992c), and windrows, possibly for protection from cattle and machinery (Diemer 1992c). On pine plantations in Alabama, most burrows of juveniles were associated with stumps, fallen logs or tree limbs, or shrub stems, which could interfere with excavation attempts by predators (Aresco 1999). Ashton and Ashton (2001) documented the use of apparently abandoned burrows by juveniles in Citrus County, Florida.
In Northern Florida, Diemer (1992b) studied tortoise populations for several years. She found that the number of burrows showing signs of recent activity increased in April, peaked in July, and remained high through October. The burrow surveys showed a continuous cycle of burrow creation and abandonment. The ratio of captured tortoises to burrows (active and inactive) varied among sites and years; the ratio of burrows to tortoises ranged from 0.45-0.69. Percentages of adult individuals in the three populations studied ranged from 40-62%.
Gopher tortoises desiccate more rapidly when deprived of a burrow than any other member of the genus Gopherus (Auffenberg and Weaver 1969). They may withstand relatively high body temperatures (Bogert and Cowles 1947) but froth at the mouth and breathe rapidly when heat stressed. Critical thermal maximum is reported as 43.9 C (Hutchinson et al. 1966).
Many vertebrate and invertebrate species have been recorded from gopher tortoise burrows (Young and Goff 1939, Brode 1959, Hansen 1963, Franz 1986, Jackson and Milstrey 1989, Lips 1991, Witz et al. 1991), including protected species such as the eastern indigo snake (Drymarchon corais COUPERI) and the gopher frog (Rana capito) (Auffenberg 1969, 1978; Diemer 1986). Some burrow associates prefer active burrows over inactive or abandoned ones (Lips 1991); these can be distinguished by characteristics of the burrow entrance (Auffenberg and Franz 1982, Cox et al. 1987). Eisenberg (1983) found that 73.7% of gopher frogs censused were found in active tortoise burrows. Witz and coworkers (1991) excavated 1019 burrows and found that of the vertebrate symbionts captured only lizards were found significantly more often in active burrows than in either inactive or abandoned burrows. Fecal material and other organic debris in the enlarged area at the bottom of the burrow serves as an important food source for some burrow associates (Milstrey 1986).
Often occurs in more or less isolated colonies of up to about 57 individuals; population density within a colony was estimated at 3-27/ha in Florida (Auffenberg and Franz 1982). Folk (1993) estimated density at about 1-2/ha, 3-5/ha, and 16-25/ha on 3 TNC preserves in Florida. At the Kennedy Space Center, Florida, fall densities ranged from a mean of 2.7/ha in disturbed habitat to 0.0/ha in saw palmetto habitat; spring densities ranged from a mean of 2.5/ha in saw palmetto habitat to 0.7/ha in oak-palmetto habitat (Breininger et al. 1994).
A comprehensive study of about 50 populations of gopher tortoises in Florida (McCoy and Mushinsky 1988) found several trends. Gopher tortoise populations residing on sites that had experienced severe area reduction (greater than 25% reduction over the past 20 years), or occurred on sites with greater than 50% tree canopy, or occurred on sites of small size (< 2 ha), tended to have truncated demographic profiles. A truncated profile suggests little recruitment of individuals into the population and abandonment of the site by larger, mature individuals. In contrast, tortoise populations on sites with no or limited area reduction, or sites with less than 50% tree canopy, or relatively large sites (> 2 ha) tended to have a high proportion of large, mature individuals and evidence of recruitment of young into the population (McCoy and Mushinsky 1988).
Comparisons of tortoise populations on true islands with populations on the mainland suggested that tortoises do respond to relatively small, isolated habitats (Mushinsky and McCoy 1994). Both island and mainland tortoise populations show a positive relationship between the number of active and inactive burrows and the area of habitat. Density of burrows, however, decreased as area increased on the mainland, but density of burrows was not related to area on the islands. Also, on the mainland, the ratio of inactive to active burrows (a measure of the tendency of individuals to construct new burrows) increased with area of habitat, and burrow density increased with increasing herbaceous vegetation, but neither of these relations could be demonstrated on islands. Collectively, these findings suggest that tortoises have a greater selection of habitats on the mainland than on islands. Tortoises on islands are confined and may be forced to live in less than ideal conditions. The implications of these findings are profound for tortoises living in small, fragmented "habitat islands" on the mainland. In time, perhaps a few decades, as the quality of their habitat island is degraded, mature adults may be forced to abandon a site in search of better habitat quality. Such individuals, which may be forced to abandon isolated patches of habitat in areas surrounded by human dwellings seem doomed to perish. From a practical perspective, prior to this study (Mushinsky and McCoy 1994), observation of large numbers of active and inactive gopher tortoise burrows in a confined area likely would have been viewed as indicators of a "healthy" population; however, these findings suggest just the opposite. Rather than a signal of a healthy population, large numbers of active and inactive gopher tortoise burrows, relative to the actual number of tortoises, may signal a stressed population (see also Stewart et al. 1993).