ECOLOGICAL IMPACTS OF THE INVASIVE AND CRYPTIC BARBU BARBUS (L.) (PISCES:CYPRINIDAE)

Invasive species are among the principal causes of community and ecosystem integrity loss worldwide and freshwaters fishes are among the most threatened and introduced species. The invasive riverine fish Barbus barbus was used in this thesis as a model to study the ecological consequences deriving by two key mechanisms: interspecific trophic interactions and introgressive hybridisation. B. barbus is a large bodied cyprinid native to central Europe that has been introduced outside its native range in western England and Italy. The consequences of interspecific competition with functionally analogous fishes were tested in a serious of experimental conditions at different scales (from tank aquaria to mesocosms) with impacts measured on trophic niches and fish growth rates. Trophic ecology of B. barbus was also investigated in 11 wild populations of the UK also in relation to the use of angler’s baits (pelletized meal) that can act as trophic subsidies and facilitate B. barbus integration into the invaded communities. Introgressive hybridization consequences on functional traits (i.e. trophic ecology, morphology and life traits) was instead tested in wild Italian populations where B. barbus readily hybridize with native co-generic analogous B. plebejus and B. tyberinus. Finally, a further aspect that was considered in this study was the cryptic diversity of Barbus fluvio-lacustrine species in Italy that can lead to an underestimation of the extinction risk faced by barbels also in relation to B. barbus invasion. The experimental approaches demonstrated that competitive interaction among B. barbus and other analogous cyprinids (i.e. Leuciscus idus and Squalius cephalus) can result in suppressed growth rate but trophic niche segregation and constriction (i.e. diet diversification and specialisation) allow fish to co-occur and avoid outcompetition. Compared to intra-specific competition, the effects on fish growth rate were similar (i.e. reduced in both cases), but contrastingly, intraspecific competition produces an increase in niche size (i.e. generalization of diets). This provided experimental evidence for the niche variation hypothesis and explains the strong niche partitioning observed in previous studies on invasive B. barbus populations in English rivers. Moreover, although B. barbus appeared as a weaker competitor than the invasive L. idus, its introduction can result in isotopic niche reorganizations that can scale out to other community members with this requiring further elucidations. In agreement with previous studies, we found that some adult individuals in 11 UK wild B. barbus populations specialized their diets on allochthonous anglers’ baits as shown by their carbon isotope ratio (δ13C) strongly differentiated from that of freshwater macroinvertebrates. However, this varied considerably over space also according to angling pressure and it is unlikely that it helped to ease the interspecific competition of the barbel with native species that is instead more likely to be driven by niche variation processes. Introgressive hybridization with Italian native barbel populations resulted in hybrid populations, with mitochondrial DNA skewed toward B. barbus genotype and only 23% to 4% purebred native genotypes remaining in nuclear DNA. Significant alterations in morphology, enhanced growth rate, different diet and trophic position were detected in one hybrid population highlighting as introgressive hybridization is not only eroding the genetic integrity of native barbel species, but it has the potential to alter the functional role of barbel with consequent impacts that may influence also non-barbel members of the receiving community. Conversely, the detection of hybrid vigour underlined the adaptive role of introgression with hybrids that may be able to persist in areas where native barbel are disfavoured thus raising contrasting conservation perspectives. Purebred native species are likely to be confined to locations where barriers prevent B. barbus expansion and therefore there is a need to reconcile conservation needs to restore fluvial connectivity with the important role of isolated river stretches in offering refuge to native species. Geometric morphometrics and molecular analyses revealed the presence of two previously undetected barbel lineages in southern Italian basins for which a new description (B. samniticus sp. nov.) and a re-establishment (B. fucini Costa 1853) are proposed. Evolutionary history of these lineages may reveal some new insights into the evolution of the southern Italian basins and are therefore of great conservation interest. However, like B. plebejus and B. tyberinus species, the southern Italian lineages are already threatened especially by fish translocations and B. barbus and other exotic species invasions and they urgently require adequate protection. In conclusion, this thesis enhanced our understanding of the complex mechanisms governing the ecological and evolutionary consequences associated with biological invasions and brought new insights into Barbus genus diversity in Italy with important conservation implications