Alteration of Species Composition and Population Trends of Freshwater Turtles Following Invasion of Competitive and Predatory Alien Animals in Southern Kyoto, Japan

In Japan, countrywide spread of the red-eared slider turtle (RES; Trachemys scripta elegans) and the raccoon (Procyon lotor), both of which are invasive alien animals, has been raising concerns that they may interfere with the growth and survival of indigenous turtles, thereby altering the turtle fauna in freshwater ecosystems. Using a trap-capture method, we conducted a two-year (2015–2016) field study to reveal transition of turtle demographics in irrigation ponds located in the southern part of Kyoto Prefecture, Japan. In a previous study conducted in 1999, when raccoons had not yet invaded in the adjacent areas, these ponds were abundant with the Japanese pond turtle (JP; Mauremys japonica, endemic to Japan) followed by Reeves' pond turtle (RP; Mauremys reevesii) and almost devoid of RES. In the present study, a total of 536 turtles were recorded and RES completely replaced JP as the dominant species. Age structure was skewed towards older age classes in the JPs but towards younger age classes in the RESs, suggesting unsuccessful recruitment of young JPs as opposed to high reproductive potentials of RESs. The sex ratio was even in the JPs and RPs but significantly biased toward female in the RESs, suggesting high fertility of the RESs. Furthermore, limb loss individuals strongly suggesting predation attacks by feral raccoons were most frequently found in the JPs followed by the RPs, but were not found in the RESs. This vulnerability of the JPs might confer relative advantages to the survival of the sympatric RESs, resulting in further growth of the RES population and an accelerating progression toward local extinction of the JPs, as symbolized by their aging population structure.