Writing in the July 2013 issue of the Chelonian Research Foundation’s Chelonian Conservation Biology, Yale University masters student Max Lambert and colleagues recently took a look at basking site usage in the native western pond turtle (Emys [=Actinemys] marmorata) and introduced red-eared sliders (Trachemys scripta) in an urbanized waterway to determine whether native turtles might be excluded from optimal basking sites from either human disturbance and/or competition with introduced turtles.
Over the course of a season, the researchers identified 24 basking sites in the UC Davis Arboretum along the historical north fork channel of Putah Creek since converted to a landscaped waterway through the campus, where an equal number of native and introduced turtles have been recorded. All turtles were identified during opportunistic surveys timed during periods of peak basking activity, and the characteristics of each basking site were measured by variables such as water and air temperature, water depth, distance to the path, substrate, shading, and frequency of human disturbance.
Overall, the researchers found that the basking site characteristics that were most positively correlated with western pond turtle abundance were steeper midslopes, steeper slopes at the water’s edge, shallower water depths near the water’s edge, less disturbance, and concrete substrates, which could be correlated to facilitating quick escape, a behavior common amongst western pond turtles which flush easily. Nevertheless, the researchers found that between the two species, the native western pond turtle was less selective than the introduced red-eared slider, which tended toward shallow slopes, deep water, steel mesh substrates, and high disturbance. This observation suggests several, not necessarily mutually exclusive, scenarios: that sliders are less sensitive to human activity than western pond turtles; that sliders are exhibiting a behavioral change in this modified habitat in response to fewer predators that gives them a competitive advantage; or that each species exhibits an innate preference toward different basking site structural characteristics.
Although the researchers recommend further research to further understand the interactions between these two species, their findings nevertheless demonstrate that the two turtles are using basking habitat differently. Management practices should focus on providing basking sites that offer protection from human activities by introducing protected logs or platforms in the middle of waterways, providing aquatic, terrestrial, and/or vegetative buffers, and constructing visual barriers around basking sites to augment habitat that favors the native western pond turtle. Counterintuitively, augmenting habitat that favors introduced turtles might also benefit native turtles by drawing sliders away from optimal habitat for native species, and minimizing competition by increasing the overall availability of basking sites in an area.
Full Citation: Lambert, Max R., Sharell N. Nielsen, Amber N. Wright, Robert C. Thomson, and H. Bradley Shaffer. 2013. Habitat Features Determine the Basking Distribution of Introduced Red-Eared Sliders and Native Western Pond Turtles. Chelonian Conservation and Biology: 12(1): 192-199.