Functional and temporal facets of predation by marsh frogs across the aquatic-terrestrial ecotone of ponds and implications in the context of biological invasions

1. Freshwater vertebrate predators can exert trophic control over aquatic and littoral communities. Among these predators, post-metamorphic anurans exhibit a biphasic trophic spectrum by foraging in both terrestrial and aquatic habitats. Many studies have described their diet through the classical taxonomic classification of prey. However, these singular diet habits imply a complex, time-dependent, realised trophic niche in which predation pressure occurs over many consumers that fill diverse functional roles throughout the aquatic and terrestrial interface of ponds. Among anurans, marsh frogs (Pelophylax ridibundus) have been introduced outside their range in many countries and are now invading nationwide areas, particularly in western Europe. Focusing on their foraging specificities will therefore further the understanding of the trophic role of these alien taxa in pond environments that are highly colonised. 2. We collected stomach contents from 761 marsh frogs from introduced populations in 19 ponds in southern France once a month over 4 months of their active period in the spring. The populations of marsh frogs were studied in a geographic area that was devoid of native water frogs and their origin tracks back as far as southeastern Europe (i.e., more than 1,000 km from the studied sites, as evidenced previously by genetic analyses). 3. Marsh frogs exhibited generalist and opportunistic feeding strategies. The trophic niche was strongly asymmetrical and broader in the terrestrial environment than in the aquatic environment. However, predation occurred in communities of large freshwater macroinvertebrates and amphibians. Whereas the composition of the terrestrial diet exhibited strong seasonal variations, predation pressure was continuously exerted on the same aquatic organisms over time. Primary consumers and consumers at higher trophic levels frequenting aquatic benthic, vegetated, pelagic, and surface microhabitats were preyed upon, underlying the multidimensional extent of the predation spectrum. 4. The diversified feeding strategies of alien marsh frogs highlight the extent of potential ecological control by predation on pond communities. Because of their wide trophic niche, they exert predation pressure on most pond organisms, triggering possible top-down control of the overall aquatic communities.