Counting Burrows and a Glimpse into a Hidden World



Authored by Houston Chandler

In the field

Combine a full day of walking in the Georgia sun with ample amounts of sand and sweat and you are halfway to experiencing what a long day sampling for Gopher Tortoise (Gopherus polyphemus) in late summer feels like. Near the end of August, we began surveying suitable habitat patches (sandhills) on the Orianne Indigo Snake Preserve (OISP) for Gopher Tortoises. This year’s surveys are a follow-up to initial surveys that we completed from late 2011 to early 2012. Resurveying the OISP about every five years allows us to track changes in the Gopher Tortoise populations through time and potentially see the benefits of consecutive years of high-quality habitat management and restoration.

Surveying for Gopher Tortoises presents two primary challenges: one common to most wildlife studies and the other somewhat unique to tortoises. First, on most properties or survey sites it is impossible to actually count all of the Gopher Tortoises because of time and other constraints. Thus, a method for accurately estimating the tortoise population is needed. The most widely used method for estimating the size of Gopher Tortoise populations is called line transect distance sampling (LTDS). Other similar distance sampling techniques are widely applied in wildlife biology. To complete LTDS surveys, 1–3 observers walk transects (straight lines) through suitable Gopher Tortoise habitat and count burrows that are encountered along the transect. When using three observers, one person walks on the transect line and navigates, while the other two observers walk to either side, increasing the distance from the line that burrows are detected.

Gopher Camera

After a burrow is detected, usually via the large apron or patch of sand in front of the burrow, the second challenge of working with Gopher Tortoises appears. How do you tell if there is a tortoise in the burrow? Active burrows often have tortoise tracks or appear better maintained than older burrows, but this is not a foolproof way to identify whether or not a tortoise is present. To solve this problem, we use a specially designed burrow camera (a camera on a ~25-foot long cord that is attached to a monitor) to search the burrow all the way to the back. This is generally the toughest part of tortoise surveys as burrows can have patches of deep sand and/or twists and turns that make it difficult to push a camera all the way to the end. Once we have successfully gotten the camera to the end of the burrow (or observed the tortoise), the burrow width is measured because it correlates strongly with Gopher Tortoise size. Gopher Tortoises generally make their burrows wide enough for them to turn completely around. Finally, and most importantly, the distance from each burrow to the transect is recorded either in the field or later using GIS. These distance measures, the occupancy status of each burrow, and other covariates are then combined into statistical models that estimate the Gopher Tortoise population size and density at the study site.

Gopher Burrow

To date, we have completed surveys on approximately one-third of the OISP, encountering just under 300 burrows and over 70 tortoises. Gopher Tortoise burrows provide habitat for over 300 other species, and we often encounter other animals utilizing these important habitats, including many obligate invertebrates (i.e., species that require tortoise burrows to complete their life cycle), snakes, lizards, frogs, and the occasional mammal. Notable commensal species observed so far include several Southern Toads (Anaxyrus terrestris), a Southern Chorus Frog (Pseudacris nigrita), a single snake of unknown species, and an Eastern Cottontail (Sylvilagus floridanus) that gave Ben Stegenga quite the scare by rapidly fleeing a burrow! We plan to finish surveying the other two portions of the OISP over the next couple months before analyzing all of the data and assessing the current status of this important Gopher Tortoise population.