Trophic Strategies of a Non-Native and a Native Amphibian Species in Shared Ponds

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 latterPeer reviewe

Morphological Consequences of Developmental Plasticity in Rana temporaria are not Accommodated into Among-Population or Among-Species Variation

Environmental induced developmental plasticity occurs in many organisms and it has been suggested to facilitate biological diversification. Here we use ranid frogs to examine whether morphological changes derived from adaptive developmental acceleration in response to pool drying within a species are mirrored by differences among populations and across species. Accelerated development in larval anurans under pool drying conditions is adaptive and often results in allometric changes in limb length and head shape. We examine the association between developmental rate and morphology within population, among populations in divergent environments, and among species inside the Ranidae frog family, combining experimental approaches with phylogenetic comparative analyses. We found that frogs reared under decreasing water conditions that simulated fast pool drying had a faster development rate compared to tadpoles reared on constant water conditions. This faster developmental rate resulted in different juvenile morphologies between the two pool drying conditions. The association between developmental rate and morphology found as a result of plasticity was not mirrored by differences among populations that differed in development, neither was it mirrored among species that differed in development rate. We conclude that morphological differences among populations and species were not driven by variation in developmental time per se. Instead, selective factors, presumably operating on locomotion and prey choice, seem to have had a stronger evolutionary effect on frog morphology than evolutionary divergences in developmental rate in the ranid populations and species studied.Peer Reviewe

Trophic strategies of a non-native and a native amphibian species in shared ponds.

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

δ¹³C and δ¹⁵N values and standard ellipse areas for the isotopic niches of <i>B</i>. <i>calamita</i> (A) and <i>D</i>. <i>pictus (B)</i> in each pond under non-sharing conditions.

<p>δ¹³C and δ¹⁵N values and standard ellipse areas for the isotopic niches of <i>B</i>. <i>calamita</i> (A) and <i>D</i>. <i>pictus (B)</i> in each pond under non-sharing conditions.</p

Mean and standard deviation of δ¹³C and δ¹⁵N values for <i>B</i>. <i>calamita</i> (filled circles; n = 21), <i>D</i>. <i>pictus</i> (empty circles) and the controlled diet (filled triangles; n = 21).

<p>Mean and standard deviation of δ¹³C and δ¹⁵N values for <i>B</i>. <i>calamita</i> (filled circles; n = 21), <i>D</i>. <i>pictus</i> (empty circles) and the controlled diet (filled triangles; n = 21).</p

Differential trophic traits between invasive and native anuran tadpoles

How trophic resources are managed is a key factor in our understanding of the success of invasive species. In amphibians that usually occupy ephemeral ponds, the capacity to acquire resources and food selection are especially important because as a pond dries, the larval density increases and food resources are limited. Abundant and high-quality food can increase the final size and reduce the duration of development of amphibians. The aim of this work was to assess the trophic traits of tadpoles of the invasive (originally North African) anuran Discoglossus pictus compared to those of native European Epidalea calamita tadpoles under laboratory conditions. Food of two different levels of quality was supplied, and the feeding activity and food preference of the two species were analysed alone and in co-occurrence. D. pictus was capable of modifying its behaviour and food preferences; while E. calamita displayed much milder differences between treatments. Both alone and in co-occurrence with the native species, the invasive tadpoles obtained higher feeding activity values and showed a stronger preference for high-quality food. Additionally, when high densities of the two species shared food resources, the feeding activity results indicated potential displacement of the native tadpoles to lowquality resources. D. pictus thus presents trophic traits that are favourable for invasion and could limit the fitness of E. calamita when resources are limited or there is a risk of pond desiccation

δ¹³C and δ¹⁵N values and standard ellipse areas for <i>B</i>. <i>calamita</i> and <i>D</i>. <i>pictus</i> in the four ponds where the species coexist (A-D).

<p>δ¹³C and δ¹⁵N values and standard ellipse areas for <i>B</i>. <i>calamita</i> and <i>D</i>. <i>pictus</i> in the four ponds where the species coexist (A-D).</p

δ¹⁵N and δ¹³C descriptive statistics for two species in shared and non-shared ponds.

<p>These statistics were evaluated from original isotopic values (without specific diet discrimination correction).</p><p>δ¹⁵N and δ¹³C descriptive statistics for two species in shared and non-shared ponds.</p

Generalized least squares model for effects of the variables species (<i>D</i>. <i>pictus</i> and <i>B</i>. <i>calamita</i>), pond condition (sharing or non-sharing), and the interaction between the two on stable isotope values (δ ¹⁵N and δ ¹³C).

<p>Generalized least squares model for effects of the variables species (<i>D</i>. <i>pictus</i> and <i>B</i>. <i>calamita</i>), pond condition (sharing or non-sharing), and the interaction between the two on stable isotope values (δ ¹⁵N and δ ¹³C).</p