Trophic impact of adult invasive marsh frogs on native amphibians and pond ecosystems

Biological invasions are one of the main causes of biodiversity loss, and they impact most ecosystems worldwide. Because of their spatial isolation and their spatially constrained food webs, lentic freshwater ecosystems are particularly threatened by invasive alien species. More particularly, alien predators have some of the largest impacts on aquatic communities, by directly exerting predation pressure on their native prey, or indirectly through trophic cascades. This kind of impacts are well documented for fully aquatic predators such as invasive fish. However, the trophic impacts of invasive semi-terrestrial predators, such as anurans remain poorly studied in pond ecosystems.
Marsh frogs (Pelophylax ridibundus sensu lato) are the most widely introduced anurans in western Europe, where they became invasive. These ranids have impacted native sister species of the same genus through genetic pollution and replacement. However, no study assessed their trophic impact on pond communities, and more particularly, on the threatened group of amphibians.
The present thesis aimed to assess the trophic ecology and the impacts of adult marsh frogs on the recipient pond communities, with an emphasis on native amphibians. To complete this objective, we first assessed the potential threat exerted by invasive marsh frogs to native amphibians, by assessing their predation pressure based on stomach contents analysis. We then modelized the respective habitat preferences of invasive marsh frogs and native amphibians, as well as their environmental niche overlap to assess the risk incurred by native species at the pondscape scale. To evaluate the impact of marsh frogs on the overall pond communities, including macroinvertebrates and amphibians, we modelized their trophic niche and highlighted their complex functional role.Finally, we tested the effect of invasive marsh frog density on the densities on pond consumers and their trophic structure using stable isotopes. We also considered the cover of submerged aquatic vegetation (SAV) to support pond food webs and to buffer the potential impact of marsh frogs.
We conducted our studies in an area historically devoid of Pelophylax frogs and now invaded by alien marsh frogs (Pelophylax ridibundus sensu lato), the Larzac plateau (southern France). To study the trophic interactions and impacts of marsh frogs, we selected 21 ponds invaded at various densities. Environmental niches and habitat preferences of amphibians were assessed considering a larger sample size of 140 ponds.
Our results showed that alien marsh frogs consumed all species of native amphibians of the area, either at the adult or larval stage. Larger frogs consumed larger amphibians such as adult Mediterranean tree frogs (Hyla meridionalis). The predation pressure exerted by invasive marsh frogs reached its maximum during the reproduction period of native species, in spring. Estimations of the number of adult Mediterranean tree frogs and palmate newts (Lissotriton helveticus) consumed by alien marsh frogs suggested strong predation pressure on their respective populations at the pond scale. At the landscape scale, alien marsh frogs colonized most ponds of the area. They had similar but less pronounced habitat preferences than native amphibians. Invasive frogs exhibited very large environmental niche, therefore overlapping most part of the respective niche of native amphibians. Marsh frogs co-occurred with native species in most ponds, therefore presenting frequent risk to native species. More broadly, alien marsh frogs exhibited generalist and opportunistic feeding strategies, and a particularly wide trophic niche. Their diet was dominated by terrestrial prey, but they also consumed most aquatic macroinvertebrates. Pelagic predators were dominant in the aquatic diet (e.g., heteropteran and coleopteran). Marsh frogs impacted the composition of the guild of pond predators. Some taxa such as Dytiscidae showed lower densities and altered isotopic composition in the presence of large densities of marsh frogs. However, our analyses did not reveal any influence of marsh frogs on the trophic structure of the overall aquatic communities (including primary consumers and omnivores). SAV cover had opposite effect than marsh frogs on the densities of impacted consumers and it supported larger trophic diversity and basal resources.
This doctoral thesis provides new insights on the ecological consequences of the ongoing biological invasion by marsh frogs. Despite the persistence of native aquatic communities in presence of alien marsh frogs, we alert on the risk incurred by native amphibians. In addition to the previously demonstrated replacement of native sister species of Pelophylax, our work showed that invasive marsh frogs exert important predation pressure on native amphibian and some keystone macro-invertebrates. As the biological invasions by water frogs remained cryptic and understudied for a long time, our research highlights the trophic and spatial ecology of one of the most widespread species of the palearctic and give insights on the consequences they may have when translocated outside their native range.