Better Alone Than in Bad Company: Trophic Ecology of Co-Occurring Invasive and Native Crayfish.

The North American signal crayfish, , is one of the most successful invasive crayfish species in Europe. Its broad trophic niche and ability to exploit various food sources across different trophic levels, coupled with the spread of lethal crayfish disease, pose significant threats to native crayfish populations. However, documentation of co-occurrence between invasive signal crayfish and native crayfish in invaded freshwater ecosystems remains rare, and research on their coexistence remains limited. In an Italian stream, signal crayfish coexist with native, white-clawed crayfish, . This study investigated the trophic ecology of signal crayfish and white-clawed crayfish at sites where they co-occurred versus those where they occurred alone. We evaluated whether ecological traits, such as trophic niche, the presence of crayfish plague (), and body condition of signal crayfish at the invasion front, facilitated the invasion progress of signal crayfish and replacement of native white-clawed crayfish. The research employed stable isotope analyses of carbon and nitrogen, using SIBER and MixSIAR mixing models, along with stomach content analyses and Fulton and hepatopancreas indices. When the two species occurred alone, they exhibited trophic niche partitioning. When they coexisted, their trophic niches significantly overlapped. Specifically, signal crayfish shifted their trophic niche to that of white-clawed crayfish, changing from a predatory-omnivorous diet to a primary consumer. A greater occurrence of crayfish was found in the stomachs of signal crayfish compared to white-clawed crayfish, indicating higher cannibalistic behaviour, while both species consumed substantial proportions of macroinvertebrates, detritus, and periphyton when co-occurring. In general, signal crayfish exhibited better conditions when co-occurring with native species compared to allopatric individuals, suggesting higher strength in individuals at the invasion front. This study highlights the complex dynamics of invasive and native crayfish interactions, emphasising the greater trophic plasticity and improved biological conditions exhibited by invasive signal crayfish during co-occurrence.