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

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

Análisis multi-escalar de la capacidad invasora de Discoglossus pictus auritus (Amphibia, anura)

[spa] Las Especies Exóticas Invasoras siguen tres fases: introducción, establecimiento y expansión, siendo pocas las que consiguen llegar a la última fase con éxito. Son muchos los factores que explican su éxito en el área invadida y que conforman su capacidad invasora, variando los mismos entre especies y escalas de estudio. A pesar de ser los anfibios uno de los grupos más amenazados, pocos trabajos han estudiado los factores que favorecen la invasión de sus especies, y sus efecto negativos sobre las especies nativas. En esta tesis doctoral se ha desarrollado un estudio multi­ escalar de los principales factores que conforman la capacidad invasora del sapillo pintojo mediterráneo, Discoglossus pictus auritus, especie introducida en Banyuls sur Mer (Francia) a finales del siglo XIX. En la actualidad cuenta con poblaciones abundantes en expansión tanto en Francia como en España. Trabajos previos han indicado su potencial efecto sobre la comunidad nativa y la necesidad de su estudio en profundidad. Los factores que actuan a macro-escala son ampliamente utilizados en el estudio de las invasiónes biologicas y se consideran los mas efectivos a la hora de predecir el exito de estas especies. En este trabajo se analizaron cuatro factores clave: el alcance potencial de expansión (spatial spread), la capacidad de expansión (dispersal ability), la plasticidad del nicho realizado (realized niche plasticity) y la progresión de su expansión en un futuro (increasing/decreasing expansión at the future). Se observo que las condiciónes encontradas por D. pictus en el area de invasión son favorables a su expansión, al ser similares a las del area de origen (niche conservatism) y que las translocaciónes registradas dentro de esta area, podrian acelerar el proceso. Su capacidad de expansión presenta un patron tipico: una fase lag inicial, seguida de un aumento exponencial. A pesar de que los anfibios presentan menores tasas de expansión que otros grupos, se espera una progresión y aumento de su invasión en un futuro y por tanto es recomendable su seguimiento. A meso-escala las comunidades de anfibios se distribuyen en un gradiente de sistemas acuaticos con diferente hidroperiodo, quedando las especies organizadas en función de su capacidad a hacer frente a las diferentes amenazas: la desecación en los habitats efimeros, y la depredación en los ambientes acuaticos mas estables. En una serie de experimentos se analizo la respuesta de D. pictus a la desecaci6n bajo diferentes condiciónes de disponibilidad de recursos, asi como su reacci6n a diferentes depredadores del area de introducci6n y su comparaci6n con la de un anuro nativo. Los resultados obtenidos demuestran que D. pictus responde de forma eficaz a la desecaci6n, una respuesta sin embargo dependiente de la disponibilidad de los recursos. Por otra parte, present6 plasticidad frente a los diferentes depredadores, a pesar de ser menor que la de la especie nativa. Cualidades en su conjunto, que permiten entender la preferencia de D. pictus por los sistemas acuaticos efimeros con poca abundancia de depredadores, donde cabe esperar un elevado exito reproductivo. El ultimo capitulo analiz6 la gesti6n de los recursos tr6ficos realizada por D. pictus y su potencial interacci6n con la especie nativa con la que presenta mayor solapamiento espacial y temporal (Buja calamita), tanto en campo como en laboratorio. Los resultados obtenidos indicaron que D. pictus presenta una efectiva estrategia tr6fica que optimiza su crecimiento y desarrollo en los ambientes efimeros, asi como un desplazamiento de la especie nativa hacia recursos de menor calidad pudiendo afectar la capacidad de B. calamita para responder efectivamente al riesgo de desecaci6n de dichos sistemas acuaticos.[eng] Invasive Alien Species follow three phases: introduction, establishment and expansion. Few of the introduced species, reach the expansion phase successfully. Many traits are associated to invasiveness of a species and explain their invasion success. These traits vary between species and spatial scales. Despite being one of the most threatened groups, few studies have examined the factors that favour the invasion of amphibian species and its potential impact on native communities. The objective of this thesis was the analysis of the invasiveness of the painted frog (Discoglossus pictus auritus) at macro-, meso- and micro-scale. The species was introduced at final of XIX century in Banyuls sur Mer (France) and currently is expanding both in France and in Spain with abundant populations. Previous studies indicate its potential negative effect on the native community and the importance of increase the knowledge about the painted frog as an invasive species. While macro-scale provide useful predictive information of invasion success, meso-scale defines how species insert in new communities organization and micro-scale which particular traits favor its success. Our results at macro-scale showed that D. pictus introduction accomplished niche conservatism during its introduction. The existence of numerous suitable areas to colonize and the good dispersal abilities to expand throughout more unsuitable areas, predicts a range expansion in the future, mostly to the north because of climatic change. In the invaded area the species occupies mainly ephemeral ponds where presumably would obtain high reproductive success. D. pictus present good abilities to overcome desiccation events, but its plasticity is restricted to resources availability. It also displays phenotypic plasticity facing new predators but milder than the observed in native species. This reduced response could limit reproductive success of D. pictus in ponds with larger duration where predation and competition pressure increase. By last, the painted frog showed good trophic traits that are favourable for invasion of ephemeral ponds and could limit the fitness of a native species (B. calamita) when resources are scarced or there is a risk of pond desiccation. The integrative view derived of the three scales study let us to a better understanding of species invasion

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

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

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