ANTI-PREDATOR RESPONSES TOWARDS AN INVASIVE CRAYFISH IN AMPHIBIAN LARVAE
Creators: name: MELOTTO, ANDREA
Publisher: name: Universit� degli Studi di Milano
Year: 2020

description: Invasive species pose a severe threat to native ecosystems and represent the second cause of biodiversity loss on global scale after habitat destruction. In particular, invasive predators are major drivers of rapid population declines and local extinctions in native prey. During biotic invasions, native prey become abruptly exposed to novel predators with which they share no history of coevolution. Thus, the lack of common evolutionary history often hampers prey effective response to the novel predation pressures; native prey can both fail to recognise predators as a threat and exhibit anti-predator strategies that are inadequate. Nonetheless, mechanisms such as rapid adaptation and phenotypic plasticity can allow prey to cope with new selective pressures and drive evolutionary changes that can help native species withstanding invasive ones. Due to their relative ecological isolation, freshwater ecosystems are particularly sensitive to invasive species impacts and likewise are most of the organisms exploiting them like amphibians; aquatic and semi-aquatic amphibians are effective indicators of freshwater habitats conservation with an excellent potential for studying the responses towards invasive species. Indeed, generally, amphibians show high level of developmental and behavioural plasticity and have relatively short life cycles, which could allow the detection of important adaptive patterns even few years after the spreading of invasive organisms. This thesis investigated how amphibians can respond to a novel predation pressure, by assessing their modulation of anti-predator responses on multiple traits (i.e. behavioural morphological, life-history). In particular, I aimed to shed light on (i) the role of phenotypic plasticity in mediating the expression of anti-predator responses towards invasive predators; (ii) how novel predation pressures can interact with extant selective forces and foster rapid adaptation in native prey; and (iii) which evolutionary mechanisms are involved in allowing predator recognition by na�ve prey. To this extent, I evaluated the expression of anti-predator responses in amphibian larvae towards the American red swamp crayfish (Procambarus clarkii). This widespread crayfish is listed among the 100 worst invasive alien species, and is a voracious predator of amphibian eggs and larvae, often associated to strong declines of amphibian populations outside its native range. In brief, the research consisted in two main experimental studies, the first of which focused on developmental shifts of a single anuran species, while the second one assessed behavioural responses to the invasive crayfish across the amphibian community of Northern Italy. In the first study, I exposed tadpoles of an endemic frog (Rana latastei) from recently invaded (10- 15 years) and uninvaded populations to the non-lethal presence of the invasive crayfish during their ontogenesis. Tadpoles from invaded populations showed rapid adaptation in life-history traits (reduced development time), and this caused the disappearance of pre-existing adaptive divergence between R. latastei populations exploiting environments with different climatic regimes. However, even if early metamorphosis in invaded populations probably has great advantages as it reduces exposure to crayfish predation, this shift can produce potential carry-over effects on post-metamorphic traits. Indeed, the observed development acceleration was not without a cost, as faster-developing froglets were smaller and displayed poorer jumping performances. Besides, experimental exposure to the invasive crayfish revealed tadpoles were also able to modulate both their development time and morphological traits through phenotypic plasticity. An acceleration in development time was observed even in exposed tadpoles. Moreover, tadpoles reared in presence of P. clarkii showed shift in body shape together with an increase in tail muscle size, which is a trait associated to faster swim and can increase escape from predators. By contrast, tadpole behaviour was not influenced nor by origin (invaded or not invaded populations) neither by crayfish exposure. The second study was performed on several species composing Northern Italy amphibian community (five urodele and eight anuran species); for each species I assessed how different stimuli mediated novel predator recognition and elicited the expression of anti-predator responses in na�ve species. To this extent, I evaluated multiple larval behavioural traits after brief non-lethal exposure to crayfish-released cues (four treatments: visual cues, chemical cues, contemporary exposure to both cues and control). Moreover, I tested if these responses were influenced by the coevolutionary history some of these species shared with a similar native predator, the European crayfish (Austropotamobius italicus). I showed that all species altered their behaviour when exposed to the invasive crayfish while the modality and intensity of response was highly heterogeneous. However, almost all behavioural responses were driven by visual cues, while chemical cues elicited feeble and contrasting outcomes. Finally, I found no support for a coevolutionary history hypothesis between native amphibians and native crayfish, as responses to the invasive predator were not affected by species coexistence with native crayfish. Instead, behavioural responses observed in na�ve species was likely elicited by recognition of general predator traits (e.g. an approaching large shape). The broad implication of my thesis is that amphibian facing invasive predators can both exhibit rapid adaptation to the novel selective pressures and modulate their developmental traits through phenotypic plasticity. Moreover, the expression of these responses is context-dependent and can highly vary in relation to the experienced conditions and across species. For instance, varying typology of risk exposure can produce marked difference in anti-predator response (e.g. activation of short-term behavioural response vs long-term morphological responses). Future studies assessing responses to invasive predators should evaluate multiple traits and carefully consider risk exposure conditions when planning experiments. Finally, the expression of anti-predatory responses of native amphibians, and particularly their effectiveness towards invasive predators, need further extensive investigation, and future conservation plans should take into account both species trends and their responsiveness to global change stressors.