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One of the critical factors for understanding the establishment, success and potential impact on native species of an introduced species is a thorough knowledge of how these species manage trophic resources. Two main trophic strategies for resource acquisition have been described: competition and opportunism. In the present study our objective was to identify the main trophic strategies of the non-native amphibian Discoglossus pictus and its potential trophic impact on the native amphibian Bufo calamita. We determine whether D. pictus exploits similar trophic resources to those exploited by the native B. calamita (competition hypothesis) or alternative resources (opportunistic hypothesis). To this end, we analyzed the stable isotope values of nitrogen and carbon in larvae of both species, in natural ponds and in controlled laboratory conditions. The similarity of the delta 15N and delta 13C values in the two species coupled with isotopic signal variation according to pond conditions and niche partitioning when they co-occurred indicated dietary competition. Additionally, the non-native species was located at higher levels of trophic niches than the native species and B. calamita suffered an increase in its standard ellipse area when it shared ponds with D. pictus. These results suggest niche displacement of B. calamita to non-preferred resources and greater competitive capacity of D. pictus in field conditions. Moreover, D. pictus showed a broader niche than the native species in all conditions, indicating increased capacity to exploit the diversity of resources; this may indirectly favor its invasiveness. Despite the limitations of this study (derived from potential variability in pond isotopic signals), the results support previous experimental studies. All the studies indicate that D. pictus competes with B. calamita for trophic resources with potential negative effects on the fitness of the latter.

Temporal or evolutionary changes in the effects of invasive competitors on native species have not been studied in great depth. In this study, we explored possible modifications in the trophic shifts of native and invasive tadpoles in a set of ephemeral ponds with different amounts of time elapsed since the invasion, i.e., with different degrees of naiveté of the native species to its invasive competitor. Using stable isotopes analysis, we found that the native (Epidalea calamita) and invasive (Discoglossus pictus) species always segregated in their trophic position within ponds. Furthermore, the isotopic signature of the tadpoles was affected by the composition and diversity of the surrounding vegetal and animal communities. The amount of time elapsed since the invasion did not influence the magnitude of the trophic differences between the species, but it did affect the nature of this segregation. Segregation at the trophic level occurred most frequently during the first stages of invasion, with the invasive species occupying higher trophic levels. However, segregation was progressively attained through the consumption of different items within the same trophic level when the amount of time since invasion increased. Thus, our results point that the native species no longer uses a lower trophic level after several generations of coexistence. In contrast, changes in the trophic niche width of either species during the invasion process were largely undetected.

One of the critical factors for understanding the establishment, success and potential impact on native species of an introduced species is a thorough knowledge of how these species manage trophic resources. Two main trophic strategies for resource acquisition have been described: competition and opportunism. In the present study our objective was to identify the main trophic strategies of the non-native amphibian Discoglossus pictus and its potential trophic impact on the native amphibian Bufo calamita.We determine whether D. pictus exploits similar trophic resources to those exploited by the native B. calamita (competition hypothesis) or alternative resources (opportunistic hypothesis). To this end, we analyzed the stable isotope values of nitrogen and carbon in larvae of both species, in natural ponds and in controlled laboratory conditions. The similarity of the δ15N and δ13C values in the two species coupled with isotopic signal variation according to pond conditions and niche partitioning when they co-occurred indicated dietary competition. Additionally, the non-native species was located at higher levels of trophic niches than the native species and B. calamita suffered an increase in its standard ellipse area when it shared ponds with D. pictus. These results suggest niche displacement of B. calamita to non-preferred resources and greater competitive capacity of D. pictus in field conditions. Moreover, D. pictus showed a broader niche than the native species in all conditions, indicating increased capacity to exploit the diversity of resources; this may indirectly favor its invasiveness. Despite the limitations of this study (derived from potential variability in pond isotopic signals), the results support previous experimental studies. All the studies indicate that D. pictus competes with B. calamita for trophic resources with potential negative effects on the fitness of the latter.

Food availability and pond desiccation are two of the most studied factors that condition amphibian metamorphosis. It is well known that, when food is abundant, organisms undergo metamorphosis early and when they are relatively large. The capability of anurans to accelerate their developmental rate in response to desiccation is also common knowledge. These two variables must act together in nature, since we know that, as a pond dries, the per capita resources decrease. We conduct an experiment to evaluate the effects of desiccation and food availability separately and in combination in tadpoles of the painted frog (Discoglossus pictus). We demonstrate that food deprivation leads to slow growth rates, which delay metamorphosis and produce smaller size and weight. The capability to accelerate metamorphosis when facing a drying pond is also confirmed, but, nevertheless, with factor interaction (when the pool is drying and resources are scarce) the capacity to respond to desiccation is lost. In addition, slow drying rates are shown to be stressful situations, but not enough to provoke a shortening of the larval period; in fact, the larval period becomes longer. We also demonstrate that the interaction of these factors changes the allometric relationship of different parts of the hind limb, which has implications for the biomechanics of jumping. Due to low mortality rates and an adequate response to both environmental factors, we expect D. pictus to have a great invasive potential in its new Mediterranean distribution area, where lots of temporary and ephemeral ponds are present.

Biological invasions are regarded as a form of global change and potential cause of biodiversity loss. Xenopus laevis is an anuran amphibian native to sub-Saharan Africa with strong invasive capacity, especially in geographic regions with a Mediterranean climate. In spite of the worldwide diffusion of X. laevis , the effective impact on local ecosystems and native amphibian populations is poorly quantified. A large population of X. laevis occurs in Sicily and our main aim of this work was to assess the consequences of introduction of this alien species on local amphibian populations. In this study we compare the occurrence of reproduction of native amphibians in ponds with and without X. laevis , and before and after the alien colonization. The results of our study shows that, when X. laevis establishes a conspicuous population in a pond system, the populations of Discoglossus pictus , Hyla intermedia and Pelophylax synklepton esculentus show clear signs of distress and the occurrence of reproduction of these native amphibians collapses. In contrast, the populations of Bufo bufo do not appear to be affected by the alien species. Since the Sicilian population of X. laevis shows a strong dispersal capacity, proportionate and quick interventions become necessary to bound the detriment to the Sicilian amphibians populations.

Environmental contamination, especially due to the increasing use of pesticides, is suggested to be one out of six main reasons for the global amphibian decline. Adverse effects of glyphosate-based herbicides on amphibians have been already discussed in several studies with different conclusions, especially regarding sublethal effects at environmentally relevant concentrations. Therefore, we studied the acute toxic effects (mortality, growth, and morphological changes) of the commonly used glyphosate-based herbicide formulation Roundup® UltraMax on early aquatic developmental stages of two anuran species with different larval types (obligate vs. facultative filtrating suspension feeders), the African clawed frog ( Xenopus laevis ) and the Mediterranean painted frog ( Discoglossus pictus ). While X. laevis is an established anuran model organism in amphibian toxicological studies, we aim to establish D. pictus as another model for species with facultative filtrating larvae. A special focus of the present study lies on malformations in X. laevis embryos, which were investigated using histological preparations. In general, embryos and larvae of X. laevis reacted more sensitive concerning lethal effects compared to early developmental stages of D. pictus . It was suggested, that especially the different morphology of their filter apparatus and the higher volume of water pumped through the buccopharynx of X. laevis larvae lead to higher exposure to the formulation. The test substance induced similar lethal effects in D. pictus larvae as it does in the teleost standard test organism used in pesticide approval, the rainbow trout ( Oncorhynchus mykiss ), whereas embryos of both species are apparently more tolerant and, conversely, X. laevis larvae about two times more sensitive. In both species, early larvae always reacted significantly more sensitive than embryos. Exposure to the test substance increased malformation rates in embryos of both species in a concentration-dependent manner, but not at environmentally relevant concentrations. However, the assumed field safety, based on calculated surface water concentrations of the active ingredient (glyphosate), should be validated with realistic field data and buffer strips have to be urgently regarded to any aquatic amphibian habitat.

Although the purely ecological impacts of biological invasions have been well studied, a less thorough effort has been made in terms of their evolutionary ecology. Previous studies show that anti-predator phenotypic plasticity may be one of the major ecological forces driving survival and rapid evolution of prey facing new predators. In turn, this means that biological invasions embody a perfect case for studying the tradeoffs and evolution of phenotypic plasticity per se. Here, we studied the plastic responses of native (Pelodytes punctatus) and invasive (Discogbssus pictus) anurans facing a native (dragonfly Anax sp.) and two invasive (fish Gambusia holbrooki and crayfish Procambarus clarkii) predators. Marked responses were reported against the native predator from both the native and the invasive anuran, but they both responded mildly to the exotic predators as well. Native P. punctatus displayed a morphological reaction to invasive P. clarkii after scarcely 30 years of coexistence with this predatory crayfish and responded behaviorally to the invasive fish G. holbrooki. Invasive D. pictus reacted behaviorally to all predators, but unexpectedly only reacted morphologically to native Anax sp. All these results support high prey-predator specificity in these reactions and an evolutionary dissociation between behavioral and morphological plasticity in anurans. Each species displayed a particular set of tradeoffs between plastic responses and their costs, which is probably due to differences in ecological niche and evolutionary history, but interestingly we usually detected unexpected patterns in combinations using introduced predators. This suggests that perhaps singular plastic shifts usually occur when tadpoles face recently introduced species. Given the speed in which these evolutionary changes become noticeable and their potential in avoiding prédation risk, this study supports that phenotypic plasticity might play an important role in population dynamics during biological invasions.

We examined niche occupancy of Discoglossus pictus , an anuran recently established in Europe, comparing the niches of native (North Africa) and alien populations (south-western Europe) at two spatial scales to determine whether adaptive divergence had occurred between these two populations. Additionally, we determine whether the alien species showed a wider larvae niche and higher phenotypic variability compared with co-occurring anurans. We characterized the breeding habitats and the climatic space occupied by native and alien groups of populations of D. pictus and examined morphological traits of D. pictus and sympatric anuran larvae. Our results revealed no divergence in breeding habitat use between native and alien populations. A shift was observed between the realized niches occupied by the native and alien populations, but this shift might only reflect cryptic niche conservatism. The range of reproductive habitats selected by D. pictus was not wider than those of most native species. In the invaded range, D. pictus showed morphological overlap with some native species and broader phenotypic variability, but the adaptive advantages of this latter attribute were uncertain. Our results suggest that the invasive capacity of this species depends on favourable abiotic conditions rather than on its adaptive advantages over native anurans.

The painted frog, Discoglossus pictus , was introduced into the Iberian Peninsula 100 years ago and its distribution has steadily increased since then. We studied the effects of this non-native amphibian species on the native ones in the northeastern area of the peninsula. We compared amphibian assemblages in regions with and without D. pictus to estimate niche overlap between species. Additionally, we carried out a laboratory evaluation of the effects of competition between the non-native and the two native species with which it overlaps most commonly: Bufo calamita and Pelodytes punctatus . The presence of D. pictus larvae reduced the survival, body mass and activity of B. calamita , and increased time to metamorphosis. Furthermore, D. pictus showed the highest consumption rate while P. punctatus showed the lowest. One possible consequence of these competitive interactions is an alteration of species co-occurrence patterns in the recipient community on a regional scale. In the non-invaded area, the checkerboard score (C-score) of co-occurrence indicated that the community was structured, whereas the standardized C-score in the invaded area indicated that the community did not differ significantly from having a random structure. These results suggest that competition between native and non-native species can cause recipient communities to become less structured.

The introduction of exotic species is a major cause of ecological disturbance and has recently been shown to promote the decline of some amphibian populations. In Western Europe, several amphibian species have been introduced in recent decades, among them Discoglossus pictus , impact of which in native amphibian communities is still unknown. We studied the potential impact of D. pictus by analysing the degree of niche overlap, assuming the possible existence of competitive interactions with native species. We have studied the structure of the anuran assemblage at local level, defining the morphospace occupied by native species and the habitat occupied by the different ecomorphs. The analysis of distance matrices suggested that there was a covariation between morphological characters and habitat selection. We determined the position of D. pictus within the morphospace of the native anuran community, patterns of co-occurrence among alien, and native species and habitat selection. The potential effect of climate on local assemblages had been controlled based on data obtained from climate models. These analysis showed that D. pictus was clustered with the benthic guild, selected positively small ephemeral ponds and was a thermophilic species. Based on these results, a greater degree of niche overlap was expected with Bufo calamita and Pelodytes punctatus. The definition of morphological groups can be useful to understand the invaded assemblage structure and the potential effect of an alien frog on native communities.