Monday, August 10, 2009

The Threatened Gopher Tortoise

Sticking with the herpetological theme of the last couple of narrative posts (Diamondback Rattle Snakes and Spring Peepers), I thought that I’d share a couple snapshots and a short video clip of a gopher tortoise from a couple of weeks back.

This gopher tortoise, Gopherus polyphemus, was photographed in west-central Florida just outside of a planted slash pine plantation.

Most of the pine plantations in Florida have resident gopher tortoises; however these areas of anthropogenic alteration are far from ideal habitats. The plantations aren’t good for the tortoises primarily because of the fact that the plantations were established for commercial use (wood production) and were therefore seeded very densely to maximize the quantity of trees grown. The extreme density of the canopy trees drastically reduces the amount of light reaching the forest floor and affectively minimizes herbaceous growth and groundcover which the tortoises consume.

In addition to silvaculture (and as with the spring peepers), gopher tortoise populations are also heavily impacted by habitat fragmentation, which is one reason why they are listed as a Threatened Species by the U.S. Fish and Wildlife Service.

Here's a quick video of the above tortoise racing to his burrow:

For those that may not be familiar with the gopher tortoise, here’s a pile of info snagged from the Smithsonian Marine Station’s website:

The gopher tortoise, Gopherus polyphemus, is a large terrestrial turtle having forefeet well adapted for burrowing, and elephantine hind feet. The front legs have scales to protect the tortoise while burrowing. Body length averages approximately 25 cm (10 inches), with the shell ranging in height from 15 – 37 cm (6 – 15 inches). Body mass averages approximately 4 kg (9 pounds). Color is a dark brown to gray-black, with a yellow plastron (bottom shell). A gular projection is evident on the anterior plastron where the head projects out from the shell. Sexual dimorphism is evident, with male gopher tortoises having concave plastrons, while those of females are flat. In addition, the gular projection on male plastrons is generally longer than in females (Ernst and Barbour 1972).

Gopher tortoises dig burrows for cover and for nesting. These can be extensive, measuring approximately 4.7 - 11 m (14 – 40 feet) in length (Witz et al. 1991). Burrow depth is heavily dependent on depth of the local water table (Diemer 1986; Burke and Cox 1988).

Trophic Mode:
Gopher tortoises are primarily herbivorous, with the bulk of the diet consisting of low-growing herbs and grasses. Foods most common in the diet are grasses and legume fruits. They are also known to consume pine needles and seeds, oak mast, prickly pear cactus, asters, palm tree fruits, raspberries, black cherry, and gopher apples (Landers et al. 1980; Auffenberg and Franz 1982; Diemer 1986). Gopher tortoises have also been observed to eat mollusk shells and the bones of dead animals, possibly to supplement their diets with additional calcium.

Predators of gopher tortoises include various snakes, fire ants (Solenopsis saevissima), accipiter hawks, buteo hawks, raccoons, opossums, armadillos, skunks, dogs, foxes, feral cats and man all prey on gopher tortoises. Generally, eggs and hatchling tortoises are significantly more at risk for predation than older animals.

Gopher tortoises use a variety of habitats, including beach dunes, scrub, and pine flatwoods. In all habitat types, soils are generally dry, sandy and well-drained. While generally avoiding swampy areas, gopher tortoises in Brevard County, Florida have been observed to inhabit poorly-drained scrub and slash pine flatwoods (Breininger et al., 1991). In this county, higher densities of gopher tortoises were found in poorly-drained sites than in well-drained sites.
Individuals occupy distinct home ranges, with male home ranges typically being larger than those of females. In east-central Florida, home ranges of male tortoises averaged 1.9 ha (4.7 ac), while those of females averaged only 0.65 ha (1.6 ac). A tortoise excavates several burrows for its use within the home range. Burrows typically are dug at a 30 degree angle from the surface. In Florida studies, male tortoises dug between 8 – 35 burrows. Females tended not to use as many burrows as males, averaging between 3 – 17 burrows (Breininger et al., 1988).

Tortoise densities tend to be higher in fire-adapted communities (Auffenberg and Franz 1982; Diemer 1986). In the absence of fire, canopy trees grow large and shade out the herbaceous vegetation that gopher tortoises rely on as their primary food source.

Associated Species:
Gopherus polyphemus is considered a keystone species in that more than 80 different species live in their burrows, or are dependent on their burrows for protection. Some of these species, such as the gopher frog (Rana areolata), the pine snake (Pituophis melanoleucus) the indigo snake (Dymarchon corais), the scrub jay (Aphelocoma coerulescens) and in inland prairies, the burrowing owl (Athene cunicularia floridana) are rare (Burke and Cox 1988; Spillers and Speake 1988; Stout et al. 1988;Witz et al. 1991).

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