Range expansion of an invasive goby in the River Tiber basin (Central Italy): effects on the abundance and the shelter occupancy of a congeneric native goby

Padogobius nigricans and Padogobius bonelli are two gobies native to Italy and characterized by an allopatric distribution: P. nigricans is endemic to the Tyrrhenian catchment of central Italy, while P. bonelli is endemic to the Adriatic catchment of Northern Italy. In the past two decades P. bonelli has successfully established in central Italy outside of its original area of distribution, coming into contact with populations of P. nigricans. The superiority of P. bonelli in competition for breeding sites is well documented by laboratory studies; however, little evidence from the field has been provided. In this study, the occurrence and abundance of the two gobies were investigated in the River Tiber basin (central Italy) to assess the impacts of P. bonelli expansion on P. nigricans populations. Moreover, to investigate nest interference by P. bonelli towards P. nigricans, shelter occupancy of the two Italian gobies was investigated in the field with respect to individual density. Sampling was conducted by electrofishing in 77 sites throughout the upper River Tiber basin and nest occupation was analyzed in 8 river stretches. Padogobius bonelli has further expanded its distribution in recent years, following both natural and human-mediated dispersal patterns. It is numerically dominant over P. nigricans and fewer P. nigricans juveniles were found in the invaded than the uninvaded areas. Depending on adult density, P. bonelli can occupy up to 97% of nest sites and can seize a significant percentage of shelters even at low adult abundance. The expansion of P. bonelli into newly colonized areas in recent years has increased concern for the conservation of P. nigricans. In the invaded areas, the survival of P. nigricans populations is mostly through migration of individuals from the uninvaded areas located upstream of weirs, allowing isolated populations of this vulnerable and endangered species to persist.