Observations on the intensity and prevalence of Batrachochytridium dendrobatidis in sympatric and allopatric Epidalea calamita (native) and Discoglossus pictus (invasive) populations

We analyse if the presence of the invasive Discoglossus pictus and the native Epidalea calamita in the same geographic area can shape the dynamics of infection by Batrachochytridium dendrobatidis (Bd). Both amphibian species share breeding habitat preferences in the area (i.e. ephemeral and temporary ponds) and are common syntopic competitors. We sampled adults of either species in a total of nine breeding localities in Catalonia (NE Iberian Peninsula): six localities within sympatric geographic areas, two localities outside the current invasion range of D. pictus, and one locality comprising solely D. pictus due to its recent introduction and the ancient extinction of E. calamita. We analysed the prevalence and intensity of infection by Bd in 183 individuals (81 D. pictus and 101 E. calamita). The presence of Bd was detected in eight of the localities, all of which had not been yet confirmed as positive for the fungus. Only in one locality, with presence of D. pictus, Bd presence was not detected. Mean prevalence was slightly but non-significantly higher in E. calamita (46.5%) than in D. pictus (36.6%). Allopatric populations of E. calamita showed significantly lower prevalence of Bd infection than sympatric ones, but similar differences were not found for infection intensity. Likewise, no significant differences were found in infection intensity between allopatric and sympatric D. pictus populations. The presence of abundant Bd outside the invasion range of D. pictus supports the assertion that Bd has not been introduced in the studied area because of D. pictus invasion. However, the presence of the two species in sympatry may enhance the infection rates and therefore the prevalence of Bd in the native species.