Native Plants, Habitat Restoration, and Other Science Snippets from Athens, Georgia

Wednesday: 18 December 2013

Box Turtle Overview  -  @ 11:10:25
Winter is the time for analyzing box turtle data that I took from March until early November. For those who are just joining us, I've been increasingly involved over the last eight years in documenting box turtle encounters and movements on our property here in the northeast Georgia piedmont. For the first five or six years those observations were casual and no particular effort was made to look for turtles. I did take identification photos, though. Then in mid 2011 I began excursions specifically to look for active box turtles. I increased these in 2012, and broadened the study site to include the newly purchased west twenty acres, as well as more of the east portion of the property. In 2013 I further increased the mileage covered, and began to measure dimensions and weights of the turtles I found. You can get more description and rationalization here, and here. I used mark and recapture model to estimate the population size here, at the end of 2012.


So to kick off the data analysis season, here's a representation 1(via Google Earth with a USGS topo overlay) of the overall property, encompassing 60 acres. You can see a larger image (<200 Kb)by clicking on the smaller image below.

The black lines mark the extent of the full 60 acres. There is a quiet road that ends in a cul de sac at the northeast corner. The eastern boundary also marks the transition from young oak-hickory forest to neighboring pasture. The north to west blue line is Goulding Creek, and the south to north thinner blue line is the feeder creek, SBS Creek, that runs through the property. Home is where the house is. I've marked the observed position of several frequently seen box turtles here.



Let's look more closely. Below is portion of the above map including the most frequently walked and best examined part of the property. There are fifteen turtles I've seen more than twice. Here I've included the positions of only 2six often observed turtles (that 1 acre legend may be more like 2-3 acres).



I was interested in estimating the territories and overlaps for each turtle.

For each I've drawn a polygon by connecting the observation sites. A better way would be to calculate a centroid point, and then draw a circle around it with a radius the length from centroid to whichever observation point was at greatest distance. Maybe I'll do that, however see the notes below3.

Sylvia, in red at the northwest end, occupies the smallest territory, probably just an acre or two. I found her first in 2006, and have observed her 17 times since. This year I found her 11 times. She seems to be an older turtle, with her smoothed carapace and blurred markings. I don't go across the creek on a regular basis, but it's possible that she does. Goulding Creek is certainly traversible by box turtles, though it may represent a soft barrier.

Ernest (yellow), who is actually a female, has a territory just a bit bigger than Sylvia's, by this measure. I first found Ernest in 2008, and saw her four times this year. She's a small turtle. At 375 grams, she's 70 grams under the female average. Ernest tends to keep close to lower SBS Creek, but in 2010 the neighbors to our north photographed her close to their house. This year I found her mating with Reuben.

I've seen Reuben, who is in blue, eleven times since I saw him first in 2012. Five of those times were this year. He ranges along SBS Creek similarly to Ernest, but I've found him uphill to the northeast as well, several times.

Megan, in green, has a territory approximately the size of Reuben's, but the two don't overlap. I've never found Megan anywhere except for the south-southeast part of the property. I've found her frequently along upper SBS Creek, and she makes excursions of considerable distance uphill and into the pine woods east. I've seen Megan eight times since 2008, four of those times this year. She's very distinctive, as the above link shows. At 520 grams, she's as much above the average female 445 grams as Ernest is below it.

I've only seen Austin, in white, four times, beginning in 2012. But this year my sole observation had him located far from the other three sightings. In fact, he was mating with Torri, on Nov 2. His territory does not seem to be much larger than the above two. At 460 grams, he's considerably larger than the male average of 402 g.

We'll wind this up with Torri, in purple. She is the largest turtle I've found, at 530 grams. I first found her at the end of turtle season in 2011; since then I've seen her covering great distances. A veritable chelonian Empress, Her territory is much larger than that of any of the others, and overlaps them all. She ranges from upper to lower SBS Creek, and commands the slopes almost to our house.

Earlier in the fall, I documented encounters with timber rattlesnakes. Snakes generally are more active and certainly faster than box turtles, and they have larger ranges as well. The ever helpful Forest Service Fire Effects Information website has an extensive entry on timber rattlers. It tells us that generally timber rattlers will range 0.5-1.0 miles from a central location (often the den). Females have smaller ranges than males, who in search of mates may travel 5 miles from home.

Just for fun I drew two circles around the site of rattler observation this past fall. The smaller one is 0.5 miles diameter (130 acres), and the larger one is 1 mile diameter (500 acres). For comparison, the range for Sylvia, our first box turtle above, closeby is marked as a red circle.



Notes:

1 Anyone who has done hiking will recognize the USGS quad topo maps in the first two figures above. That's the apparent origin of this mapping background. They're crude, old, and use a nauseating shade of green, but they allow for a much larger zoom than Google Earth's default backgrounds. GE gives excellent zoom for satellite views, but it's hard to detect topographic features that way. GE's terrain background, as seen in the third figure above, is great, but you can only use it down to a zoom level that's way below what I want. FYI, in the USGS topo maps the numbered contour at the level of Goulding Creek is 600 feet elevation, and the increments look to be 20 feet per contour.

2 There are nine other turtles I've seen more than twice that I might have included. There are an additional fifteen turtles that I have seen just twice.

3 A circle with a radius the largest distance from the centroid is probably misleading. There exist hard and soft barriers to turtle movements. A soft barrier might be Goulding Creek. Yes, the turtle can cross it, but is perhaps inhibited to some extent. Similarly for steep slopes within a turtle's apparent territory: turtles can certainly climb steep slopes but may opt for easier routes. There aren't many non fatal hard boundaries around here - those that bring a turtle's inclination or crossing success to zero. A large lake might be one, but I don't have a large lake. There is a long deep (10 feet) gully that a turtle would have trouble with, but I've found Reuben on both sides of it. I assume he walked around it.


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