Impacts of an invasive turtle species (Trachemys scripta) on basking behavior of the western pond turtle (Emys marmorata) in an urban waterway

Background/Question/Methods

The semi-aquatic emydid turtle, Emys (formerly Clemmys) marmorata, is the only pond turtle native to the Western United States and, in California, it is designated as a Species of Special Concern by the California Department of Fish & Game. The habitat of the Western pond turtle (E. marmorata) in California has drastically declined, thus urban and man-made environments have become increasingly important refuges for remaining populations. However, these urban habitats are generally more susceptible to invasion by non-native species because of both their simplified communities and their proximity to human activity. This is true in the arboretum waterway at the UC Davis campus, which supports both E. marmorata and a common pet trade species, the Red-eared Slider (Trachemys scripta). Both species commonly bask, likely for thermoregulation, thus inter- and intraspecific competition for basking sites may exist. Behavioral observations of basking turtles were recorded during high activity periods to assess aggressive behavior (an indicator of basking competition) occurring at common basking sites.

Results/Conclusions

Analyses have shown that (1) the native species is significantly more ‘successful’ in aggressive inter-specific interactions, when defending basking areas than the invasive species; (2) no differences were found in the rates of aggressive interactions across inter- and intra-specific pairs; and (3) no difference was found in how often there was a ‘winner’ of an aggression across inter- and intra-specific aggressions.  Although these results indicate Western pond turtles are not being excluded from basking sites, the Red-eared Slider may be more suited to the urban environment and may be outcompeting the native species for other resources. While these results contrast with our initial predictions, it still may be important to evaluate the physiological costs of additional aggressive interactions and the impacts this may have on the survival of individuals. Implications of this study and future related studies will be informative in adapting management of both Emys marmorata, Trachemys scripta, and possibly other pond turtle species in habitats where Red-eared Sliders have become established.