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

Sunday: 25 November 2012

Forty Four Box Turtles, 2012, Part 3  -  @ 09:13:06
We'll get to the exciting stuff below, but first:

This was another of the new box turtles things I saw this year, mating. Many years ago I was doing a little weeding, late in the summer, and saw two box turtles chasing each other across the fairy ring. It was delightful, but I didn't have a camera.

But in this case I came upon these two right at the edge of the newly added study area. I decided that's where they rightly belong, so they didn't get included in this year's population estimate (next year they will). If you don't know what I'm talking about, ask - this is part of the mark and recapture thing, so I can't use data from the newly added territory this year.

I'll still add this warning: the following exciting stuff is mostly for my purposes, but you're welcome to follow along! You wouldn't believe how often I go back to the blog to retrieve numbers and thoughts from previous years. This year was an important year in box turtle observations, therefore more numbers and more thoughts. This adds to the previous three posts here, here, and here.

I'm interested in following the size of the population, to determine if our guys are in decline or otherwise. I actually think that my conservation efforts are encouraging a stable population, but it's better to confirm this. Turtles don't stay in one place necessarily, and there are some predators around.

For analyzing numbers, I'm using mark and recapture experiments. You can find more about the simple Lincoln-Peterson model that I'm using here.

I plotted two indicators to determine if numbers of new turtles discovered were plateauing by the end of the season. Once of these, the red one, is a simple count of turtles previously unseen this season (regardless of whether I'd found them in earlier years). The last three months begins to show some leveling off. Even though I found plenty of turtles Aug-Oct, they had already been seen in 2012. I was finding turtles, but they were largely old friends.

The green line (right axis) is the ratio of new turtles to total 2012 turtles. It too shows flatlining, which is not surprising since it's really the same measure as the red plot.

Notice that the red line starts to level off at 25 new turtles. That's not to say that there are only 25 turtles, it's just to say that new ones are becoming less likely to encounter. To get a measure of the full population, you do mark and recapture.

To do a mark and recapture experiment you need two visits separated in time. In the first visit you capture and mark (in my case, photograph) the turtles in the population.

This year I used the 12 unique turtles I found in 2011 as my first visit, and then all the turtles I found this year as my second visit. That's just one way I could have put the data together.

And so I've plotted that calculated population size at each turtle discovery in the plot below. The red line is one estimate based on the simple equation given here, along with the rationalization. The green line is based on a slightly modified form that is supposed to be less biased. The good thing is that it comes with an error estimate. The population size is then 44 +/- 8 turtles in the old study area.

There's one thing puzzling about the plots. We rather quickly achieved an estimate of 40 or so turtles, in early May. But then there was a rapid rise in the estimate of population size, to 55 turtles by the end of June. Then the estimate fell back to around 45 around mid July, which was maintained through the end of the season.

What's that all about? I speculated about this when I noticed it happening, and that the new turtles seemed to be mostly female (4 new females to 1 male, during this period). It may represent an influx of females into nesting areas during nesting season. They arrive, do their thing, and then leave.

Now, these mark and recapture experiments are usually done much more quickly. Net a bunch of fish and mark their fins. Return them to the environment and let them merge back in. Check back in an hour and do the same thing. With turtles we come back a year later. That's just the way it is, with turtles. I've earlier discussed my concerns about whether I'm doing mark and recapture appropriately, but have since found a number of published population estimates done pretty much this way.

So next year I'll have some additional ways I can use the data to calculate a population. I'll have at least two or three possible first visits to choose from - 2011, 2012, and portions therein. I'll also be doing my second visit in the new study area, and will have an estimate for that extra acreage.

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