An invasive species may be better than none: invasive signal and native noble crayfish have similar community effects

1. Human activities have resulted in the decline of native crayfish and promoted the spread of invasive crayfish species in European fresh waters, threatening ecosystem structure and functioning. 2. We compared effects of native noble crayfish (Astacus astacus), invasive signal crayfish (Pacifastacus leniusculus) and of the absence of crayfish on leaf litter breakdown and invertebrate prey density and biomass in stream ecosystems. 3. In microcosm experiments, invertebrate shredder density was significantly reduced by crayfish presence but similarly affected by the two crayfish species. While crayfish did not directly influence leaf litter breakdown, their presence reduced litter decay rates through predation on invertebrate shredders (i.e. trophic cascade). These effects were again similar between native and invasive crayfish but differed among shredder prey species. 4. Field experiments confirmed that leaf litter breakdown was significantly reduced by crayfish. Although total invertebrate density was not affected by crayfish predation, species-specific effects were observed (e.g. Limnephilidae [Trichoptera] densities were reduced in the presence of crayfish) but did not differ between native and invasive species. 5. Our results show that crayfish can have strong, but species-specific, effects on invertebrate prey and indirect effects on leaf litter breakdown. Contrary to previous findings and popular belief, results also indicate that native noble crayfish and invasive signal crayfish have similar effects on stream communities. Hence, from a management perspective, it may be preferable to have invasive crayfish than no crayfish when impairment results in unsuitable habitats for native species.