Browsing by Author "Kruger, Natasha"

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    Anti-predator strategies of the invasive African clawed frog, Xenopus laevis, to native and invasive predators in western France
    (Regional Euro-Asian Biological Invasions Centre, 2019) Kruger, Natasha; Measey, John; Herrel, Anthony; Secondi, Jean
    ENGLISH ABSTRACT: When species are translocated to a novel environment, individuals become exposed to new predators against which they may not express very efficient defences at least at an initial stage. The strength of anti-predator defence is an important parameter that may determine the ability of local communities to control the expansion of invasive populations. The African clawed frog, Xenopus laevis, is a globally invasive amphibian that has successfully established invasive populations on four continents. In its invasive distribution in western France, X. laevis encounters novel aquatic predators. Some may be related to the predators in the native range but others may belong to different taxonomic groups and not be functionally or ecologically equivalent. We tested whether naïve X. laevis tadpoles from the invasive French population exhibit anti-predator response to local predators, and whether the response depends on the degree of relatedness with predators encountered in the native range of the frog, or whether individuals may express generic neophobia to any cue they are not familiar with. We exposed naïve lab-reared tadpoles to a native non-predatory water snail, Planorbarius corneus, a native predatory beetle, Dytiscus dimidiatus, and an invasive predatory crayfish, Procambarus clarkii. We found that X. laevis tadpoles innately reduce their activity when exposed to beetle and crayfish stimulus cues, but not to snails. Reducing activity can decrease the probability of being detected by predators. This demonstrates that invasive tadpoles respond to known and novel predators regardless of the evolutionary history. Whether the produced response is always effective against a totally novel predator remains to be tested.
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    Do vulnerable life-history stages of Xenopus laevis reduce its invasion potential?
    (Stellenbosch : Stellenbosch University, 2020-12) Kruger, Natasha; Measey, John; Secondi, Jean; Herrel, Anthony; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Introduced populations must overcome several barriers to become invasive in a novel environment where they may experience new ecological conditions. Phenotypic plasticty and local adaptation are two mechanisms that can assist introduced populations to overcome these barriers. The African clawed frog, Xenopus laevis (Daudin, 1802) is native to southern Africa but is invasive on four different continents. This study aimed to understand four major aspects of the African clawed frogs’ invasion in western France. Firstly, I assessed for the level of phenotypic plasticity and local adaptation of X. laevis tadpoles from the two contrasting rainfall regions in the native range, as this could provide information on the level of phenotypic plasticity in the French invasive range. The winter rainfall region is characterised by colder temperature and a winter rainfall pattern whereas the summer rainfall region is characterised by warmer temperatures and a summer rainfall pattern. I performed a reciprocal exchange experiment using outdoor mesocosms. I measured body size, timing of metamorphosis, and survival of tadpoles (NF stage 45 - 66). I found that both phenotypic plasticity and local adaptation determined the tadpole phenotype in the winter and summer rainfall regions. I also identified a survival cost in individuals translocated to the other region. However, the cost was lower for winter rainfall tadpoles. Thus, the interaction between phenotypic plasticity and local adaptation likely explains the persistence of this species in contrasting environments. Secondly, in amphibians, tadpole phenotypic traits can be coupled or decoupled to dispersal traits in adults. Spatial sorting has been observed in adults of X. laevis in western France. Thus, I tested whether spatial sorting altered the morphology and life-history traits of tadpoles due to the coupling of traits between stages. I conducted common garden experiments in laboratory microcosms and outdoor mesocosms. I compared body size, timing of metamorphosis and survival between tadpoles from the core and from the periphery of the invasive range but found no effects of the position in the invasive range on tadpole phenotype. The only difference in both the outdoor and laboratory experiments was the larger body size of post-metamorphic individuals at the periphery. These findings support the hypothesis of decoupling between adult traits and tadpole phenotypic traits. Decoupling can allow each stage of the complex life cycle to respond independently to environmental conditions. Thirdly, I assessed whether the degree of phenotypic plasticity changed between the core and the periphery during the expansion process. I tested this hypothesis for invasive X. laevis in western France. I measured the critical thermal limits and modelling the thermal performance curves of the burst swimming performance (velocity, acceleration, and sinuosity) of core and periphery tadpoles reared at three developmental temperatures (low, intermediate, and high). I found that both the position in the distribution (core/ periphery) and the developmental temperature was significant in determining burst swimming performance. However, no differences were identified between the performance of core and periphery tadpoles reared in the same developmental temperature. Tadpoles reared at the high temperature acclimated their performance to warmer temperatures, however, tadpoles displayed limited acclimation to cooler temperatures. Finally, I tested whether tadpoles identified and responded to novel predators in their invasive range. In western France, X. laevis can encounter novel aquatic predators. Some may be related to predators found in the native range, but others belong to different taxonomic groups. I tested whether naïve tadpoles from the invasive French population exhibited anti- predator response to local predators, and whether the response depended on the degree of relatedness with predators in the native range. I exposed naïve lab-reared tadpoles to a non- predatory water snail, Planorbarius corneus, a native predatory beetle, Dytiscus dimidiatus, and an invasive predatory crayfish, Procambarus clarkii. I found that tadpoles innately reduced their activity when exposed to beetle and crayfish olfactory cues, but not to snail cue. Reducing activity usually decreases the probability of being detected by predators. This demonstrates that invasive tadpoles respond to known and novel predators regardless of the evolutionary history of the prey-predator interaction. This work focused entirely on tadpoles. It demonstrates the high phenotypic plasticity of larval development in the native range, which may provide the necessary variation for the colonisation of new areas, provided no evidence of coupling between stages, which allow each stage to respond its own constraints, and showed the recognition of local predators, which is expected to enhance survival. Overall, this thesis describes mechanisms whereby X. laevis tadpoles can contribute to the invasion success of the species in France, and globally.