Biological and trophic consequences of genetic introgression between endemic and invasive Barbus fishes.

Genetic introgression with native species is recognized as a detrimental impact resulting from biological invasions involving taxonomically similar invaders. Whilst the underlying genetic mechanisms are increasingly understood, the ecological consequences of introgression are relatively less studied, despite their utility for increasing knowledge on how invasion impacts can manifest. Here, the ecological consequences of genetic introgression from an invasive congener were tested using the endemic barbel populations of central Italy, where the invader was the European barbel Barbus barbus. Four populations of native Barbus species (B. plebejus and B. tyberinus) were studied: two purebred and two completely introgressed with alien B. barbus. Across the four populations, differences in their biological traits (growth, body condition and population demographic structure) and trophic ecology (gut content analysis and stable isotope analysis) were tested. While all populations had similar body condition and were dominated by fish up to 2 years of age, the introgressed fish had substantially greater lengths at the same age, with maximum lengths 410-460 mm in hybrids versus 340-360 mm in native purebred barbel. The population characterized by the highest number of introgressed B. barbus alleles (81 %) had the largest trophic niche and a substantially lower trophic position than the other populations through its exploitation of a wider range of resources (e.g. small fishes and plants). These results attest that the genetic introgression of an invasive congener with native species can result in substantial ecological consequences, including the potential for cascading effects. Supplementary Information: The online version contains supplementary material available at 10.1007/s10530-021-02577-6

Food web properties vary with climate and land use in South African streams

1. Land use intensification and climate change are two prominent drivers of variation in biological communities. However, we know very little about how these two potential environmental stressors interact. Here, we use a stable isotope approach to quantify how animal communities respond to urban and agriculture land use, and to latitudinal variation in climate (rainfall and temperature), in 29 streams across South Africa. 2. Community structure was shaped by both land use and climatic factors. The taxonomic diversity of invertebrates was best explained by an independent negative effect of urbanisation, while abundance declined in summer. However, we could not use our variables to predict fish diversity (suggesting that other factors may be more important). 3. Both trophic functional diversity (quantifed using isotopic richness ) and food chain length declined with increasing temperature. Functional redundancy (quantifed using isotopic uniqueness ) in the invertebrate community was high in wet areas, and a synergistic interaction with urbanisation caused the lowest values in dry urban regions. There was an additive effect of agriculture and rainfall on abundance‐weighted vertebrate functional diversity (quantified using isotopic dispersion ), with the former causing a decline in dispersion, with this partially compensated for by high rainfall. 4. In most cases, we found that a single dominant driver (either climate or land use) explained variation between streams. We only found two incidences of combined effects improving the model, one of which was amplified (i.e., the drivers combined to cause an effect larger than the sum of their independent effects), indicating that management should first focus on mitigating the dominant stressor in stream ecosystems for successful restoration efforts. 5. Overall, our study indicates subtle food web responses to multiple drivers of change, only identified by using functional isotope metrics – these are a useful tool for a whole‐systems biology understanding of global change

Variability of isotopic partitioning between sympatric brown trout ( Salmo trutta ) and European grayling ( Thymallus thymallus ) in the Loue River (France)

International audienceTrophic partitioning, defining how individuals or populations differ in their resource use, is expected to promote coexistence of sympatric species by reducing resource overlap. We used stable isotope measurements (delta C-13 and delta N-15) to address niche characteristics (delta C-13 and delta N-15 ranges, isotope convex hull and standard ellipse areas, mean nearest neighbour distances and trophic positions) and trophic partitioning of sympatric brown trout (Salmo trutta, n = 110) and European grayling (Thymallus thymallus, n = 63) in two sampling locations of a French river. Aquatic resources predominantly fuelled both species, yet both terrestrial resources (TER) and trophic positions (TP) were higher for trout (similar to 36 +/- 13% TER, 3.6 +/- 0.7 TP) than grayling (similar to 26 +/- 9% TER, 2.7 +/- 0.6 TP) supporting difference in their trophic niches. Isotope analyses showed that trout had a larger isotope niche than grayling suggesting more opportunistic trophic behaviour. Their isotopic overlapping was higher at the upstream site (isotopic nestedness = 0.8 +/- 0.1) than that at the downstream site (isotopic nestedness = 0.4 +/- 0.2). Euclidian distances of stable isotopes and TP for the two species increased with salmonid size, while aquatic resource use decreased with salmonid size. These results demonstrate an increase in isotopic niche partitioning and change in trophic attributes with ontogeny. Our study showed that despite relying on similar resources, these two sympatric salmonids exhibited clear trophic differences that were amplified with ontogeny. The consideration of fish ontogenetic dietary shift would hence be a determinant driver of the trophic niche partitioning for these sympatric salmonids