Predation pressure of invasive marsh frogs: a threat to native amphibians?

Anurans have been introduced in many parts of the world and have often become invasive
over large geographic areas. Although predation is involved in the declines of invaded amphibian
populations, there is a lack of quantitative assessments evaluating the potential risk posed to native
species. This is particularly true for Pelophylax water frogs, which have invaded large parts of
western Europe, but no studies to date have examined their predation on other amphibians in their
invaded range. Predation of native amphibians by marsh frogs (Pelophylax ridibundus) was assessed
by stomach flushing once a month over four months in 21 ponds in southern France. Nine percent of
stomachs contained amphibians. Seasonality was a major determinant of amphibian consumption.
This effect was mediated by body size, with the largest invaders ingesting bigger natives, such as
tree frogs. These results show that invasive marsh frogs represent a threat through their ability to
forage on natives, particularly at the adult stage. The results also indicate that large numbers of
native amphibians are predated. More broadly, the fact that predation was site- and time-specific
highlights the need for repeated samplings across habitats and key periods for a clear understanding
of the impact of invaders.