Monday, August 9, 2010

Sliders? In My Neighborhood?

Me with non-native Pond Slider (Trachemys scripta).
What exactly is Trachemys scripta? Most sources treat it as a 'megaspecies' composed of 15 subspecies ranging from Virginia to New Mexico to Colombia (e.g. Turtles of the World), however this notion is an unfortunate holdover from The Age of Lumping. Seidel (2002) argues that differences in breeding behavior, broadly non-overlapping distributions, and distinct morphology* imply many of the 'subspecies' are in fact full species; a phylogeny determined from 52 morphological traits placed United States Sliders (scripta, troostii, elegans) in a distinct clade, causing Seidel to restrict Trachemys scripta to the three subspecies and moving all others to new or previously existing species. The molecular phylogeny in Stephens and Wiens (2004) also supports a scripta/troostii/elegans clade and shows the traditional "Trachemys scripta" to be strongly polyphyletic; curiously "Trachemys" itself was shown to be paraphyletic, but this was not directly commented on. Bickham et al. (2007) used the taxonomy proposed by Seidel (2002), making it about as 'canonical' as a proposed taxonomy can get, so from here on out I will use Trachemys scripta in the strictest sense.

* According to Turtles of the World, some 'subspecies' barely reach an SCL of 20 cm (troostii, taylori, cataspila) while grayi reaches a colossal 60 cm (2 feet), and presumably weighs over 30 times as much. If the traditional Trachemys scripta species concept was valid, it would have to take the cake for size variation within a species.

T. scripta is the only Trachemys species with black markings on the plastron (Seidel 2002).
Trachemys scripta naturally ranges across a considerable portion of the United States east of the Continental Divide*, but thanks to human introductions, breeding colonies are now established on every continent except Antarctica, as well as several major island groups (Ernst and Lovich 2009). In addition to being the world's most widely distributed turtle species, Pond Sliders are also one of the most genetically variable vertebrates (Scribner et al. 1995). To demonstrate, here are three possibly related individuals:

The middle individual is the same one I'm holding at the top, approx SCL of 20 cm (8 inches).
The above turtles are residents of Wood Pond in Barrington, Rhode Island, a tiny body of water (0.16 hectares/0.4 acres) designed to collect highway runoff - note the unnatural sheen at the water's surface. Despite these factors, and undoubtedly thanks to people constantly throwing in bread, I've counted at least a dozen Sliders, a handful of Painted Turtles (Chrysemys picta picta), hundreds of Goldfish (Carassius auratus auratus), and a couple dozen Mallards (Anas platyrhynchos). Feeding waterfowl is a bad idea, so I decided to attract the turtles with a few pellets of old cat food. Sliders were present in the pond 7 years ago, and judging by how small some of them are, it seems possible this is a reproducing population. The turtle I am holding at the top of the page was stuck in a construction ditch and may have been on its way to deposit eggs. The presence of Painted Turtles raises the possibility that the Sliders can colonize local freshwater bodies of water - however the highway and brackish waterways will (hopefully) hinder their movements*. Still, if anyone wants some pet sliders, these are already fairly tame and easy to catch.

* They are occasionally present in brackish waters, but this has only been observed in Georgia and South Carolina (Ernst and Lovich 2009)

Introduced Trachemys scripta have had negative impacts on turtle populations in Europe and Western North America due to direct competition, including competition for basking sites (Ernst and Lovich 2009 - citing Cadi and Joly 2003, 2004, Spinks et al. 2003), however it is not clear what sort of impact they would have if present in areas with a multitude of other turtle species. Wild T. scripta prefers shallow freshwater habitats with plentiful basking sites and aquatic vegetation such as lakes, swamps, sloughs, and slow moving rivers where they feed on assorted plant and animal matter, shifting towards the former with maturity (Ernst and Lovich 2009, also citing various). While I though a density of about 30/hectare looked like a lot in Wood Pond, only one population density study cited by Ernst and Lovich (2009) was similar (28/hectare in a small pond) and several were much larger, the largest being 983/hectare! T. scripta also take up a large percentage of the total number of turtles in many areas, the studies cited in Ernst and Lovich (2009) generally observed over 50% T. scripta, with a low of 0.06% and a high of 87%. Despite the propensity towards invasiveness, anthropogenic influences (habitat destruction, pollution, roadways, over-collection) are causing decreases in wild populations (Ernst and Lovich 2009). I don't think there's a species of turtle out there that can't be described by the previous sentence.


Bickham, J.W., Iverson, J.B., Parham, J.F., Philippen, H-D, Rhodin, A.G.J., Shaffer, H.B., Spinks, P.Q., van Dijk, P.P. (2007). An Annotated List of Modern Turtle Terminal Taxa with Comments on Areas of Taxonomic Instability and Recent Change. Chelonian Research Monographs 4, 173-199. Available.

Ernst, C.H., and Lovich, J.E. (2009). Turtles of the United States and Canada. John Hopkins University Press: Baltimore, Maryland. ISBN 13:978-0-8018-9121-2

Ernst, C.H., Altenburg, R.G.M., and Barbour, R.W. Turtles of the World. World Biodiversity Database.

Scribner, K.T., Morreale, S., Smith, M.H., Gibbons, J.W. (1995). Factors contributing to temporal and age-specific genetic variation in the freshwater turtle Trachemys scripta. Copeia. 1995, 970-977.

Seidel, M.E. (2002). Taxonomic Observations on Extant Species and Subspecies of Slider Turtles, Genus TrachemysJournal of Herpetology 36(2), 285-292. Available.

Stephens, P.R., and Wiens, J.J. (2004). Convergence, Divergence, and Homogenization in the Ecological Structure of Emydid Turtle Communities: The Effects of Phylogeny and Dispersal. The American Naturalist 164(2), 244-254. Available.

Secondary References:

Cadi, A. and Joly, P. (2004). Impact of the introduction of the red-eared slider (Trachemys scripta elegans) on survival rates of the European pond turtle (Emys orbicularis). Biodiversity and Conservation 13, 2511-2518.

Cadi, A., and Joly, P. (2003). Competition for basking places between the endangered European pond turtle (Emys orbicularis galloitalica) and the introduced red-eared slider (Trachemys scripta elegans). Canadian Journal of Zoology 81(1), 392-398.

Spinks, P.Q., Pauly, G.B., Crayon, J.J., Shaffer, H.B. (2003). Survival of the western pond turtle (Emys marmorata) in an urban California environment. Biological Conservation 113, 257,267