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

The North American signal crayfish, Pacifastacus leniusculus, 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 lethalcrayfish disease, pose significant threats to native crayfish populations. However, documentation of co-occurrence betweeninvasive signal crayfish and native crayfish in invaded freshwater ecosystems remains rare, and research on their coexistenceremains limited. In an Italian stream, signal crayfish coexist with native, white-clawed crayfish, Austropotamobius pallipes.This study investigated the trophic ecology of signal crayfish and white- clawed crayfish at sites where they co-occurred versusthose where they occurred alone. We evaluated whether ecological traits, such as trophic niche, the presence of crayfish plague(Aphanomyces astaci), and body condition of signal crayfish at the invasion front, facilitated the invasion progress of signalcrayfish 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. Whenthe two species occurred alone, they exhibited trophic niche partitioning. When they coexisted, their trophic niches significantlyoverlapped. 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 comparedto white-clawed crayfish, indicating higher cannibalistic behaviour, while both species consumed substantial proportions ofmacroinvertebrates, detritus, and periphyton when co-occurring. In general, signal crayfish exhibited better conditions whenco-occurring with native species compared to allopatric individuals, suggesting higher strength in individuals at the invasionfront. This study highlights the complex dynamics of invasive and native crayfish interactions, emphasising the greater trophicplasticity and improved biological conditions exhibited by invasive signal crayfish during co-occurrence.