Headwater refuges: Flow protects Austropotamobius crayfish from Faxonius limosus invasion

This study explores the geospatial relationship between the invasive crayfish species Faxonius limosus and the native Austropotamobius bihariensis and A. torrentium crayfish populations in Eastern Europe, identifying the environmental factors which influence the invasion. We used species distribution modelling based on several climatic, geophysical and water quality variables and crayfish distributional data to predict sectors suitable for each species within the river network. Thus, we identified the sectors potentially connecting invasive and native population clusters and quantified the degree of proximity between competing species. These sectors were then extensively surveyed with trapping and hand searching, doubled by eDNA methods, in order to assess whether any crayfish or the crayfish plague pathogen Aphanomyces astaci are present. The predictive models exhibited excellent performance and successfully distinguished between the analysed crayfish species. The expansion of F. limosus in streams was found to be limited by flash-flood potential, resulting in a range that is constrained to lowland rivers. Field surveys found neither crayfish nor pathogen presence in the connective sectors. Another interesting finding derived from the screening efforts, which are among the most extensive carried out across native, apparently healthy crayfish populations, was the existence of a latent infection with an A. astaci strain identified as A-haplogroup. Our results provide realistic insights for the long-term conservation of native Austropotamobius species, which appear to be naturally protected from F. limosus expansion. Conservation efforts can thus focus on other relevant aspects, such as ark-sites establishment for preventing the spread of more dangerous invasive crayfish species and of virulent crayfish plague pathogen strains, even in locations without direct contact between crayfish hosts.