Intrinsic properties and thermal adaptations enhancing the invasion of marsh frogs, Pelophylax ridibundus s.l.

Human globalization indirectly increases the number of invasive species and contributes to climate change, two major threats to Earth’s biodiversity. Amphibians, which typically have to go through a complex life cycle and for which body temperature is mainly regulated by their thermal environment and behavioural thermoregulation, are one of the most threatened classes of vertebrates. Climate change and invasive species, paradoxically often other amphibians, contribute to native amphibian population declines. Marsh frogs (Pelophylax ridibundus), are one of these invaders. Originating from various geographic areas, including the Balkans, marsh frogs now show a widespread distribution across Western Europe due to massive introductions events and because of their successful invasion. Indeed, some invasive species are able to quickly establish and spread in their introduced environment because of genetic predispositions and adaptive plasticity. Despite previous studies on their establishment and impact on native communities, few studies have attempted to better understand the factors driving the invasion success of marsh frogs, particularly from a global warming perspective, even if some authors have alerted the scientific community on how warmer temperatures may have favoured their dispersal.
The goal of this thesis is to better understand, through behavioural and physio-anatomical measurements, the intrinsic potential of marsh frogs as successful invaders. First, my PhD aims to elucidate which anatomical features favour the physical performance of marsh frogs. More specifically, we investigate whether some traits better predict physical performance than others, making them selective targets that may affect the invasion of marsh frogs. Second, since amphibians are ectotherms and because the current climate is rapidly changing, my PhD aims to assess how thermal variations affects these physical performances. By linking their thermal responses to temperature in their introduced environment we will be able to suggest if predicted climate change would enhance or limit their invasion. Finally, as most amphibians show a complex life cycle we test if these changes in temperature may similarly impact the biological activity of each developmental stage. Even if one stage might benefit from warmer temperatures, this might not be the case of other stages during their development, bringing therefore valuable knowledge for their management.
Our results showed that invasive marsh frogs present a panel of traits and plastic responses promoting their invasion. Anatomical features, such as bigger kidneys, gonads, and glutes muscles were found to be key traits predicting their physical performance and for which selection may lead to physiological changes that may affect each step of their invasion. These physical performances were shown, in adults, to benefit from warmer climate because of their broad thermal tolerances combined with a high thermal preference and traits maximized at temperatures which can be higher than their current environmental air temperature. This may therefore allow marsh frogs to expand their activity period and colonize previously underexploited shaded habitats. We also observed that warmer temperatures affect other stages such as tadpole and climax stages (at the onset of metamorphosis). However, both stages are able to handle temperatures that are greater than the current range of temperatures of their pond. Overall, these results show that alien marsh frogs are able to establish and invade new areas over Western Europe through the association of intrinsic and plastic traits. They also suggest that, in at least the next century, invasive marsh frogs will benefit from global warming because of their wide thermal tolerance and broad thermal physical performance breadth at each life stage.
In conclusion, through these studies, we improved our understanding of an invasive species, its adaptations and its responses to temperature. Overall, our results shed light on anatomical features and on the thermal range that drives the physiology of invasive marsh frogs, revealing selective targets that could enhance their invasion but at the same time providing essential knowledge for their management. We also hope that these data will promote the adoption of integrative research methodologies, this through the investigation of multiple traits and the use of numerous test temperatures to bring a more nuanced understanding and, therefore, enhancing the characterization of species adaptations in light of their inherent complexity.