Differences in the functional responses of four invasive and one native crayfish species suggest invader-specific ecological impacts.
Invasive non-native species represent a leading threat to global freshwater biodiversity and non-native crayfish species frequently cause extensive ecological damage. However, the extent to which their impact: (1) depends on invader identity and (2) differs from the natural state with native crayfish remains unclear. Comparison of the functional responses of invasive and native species represents a promising approach in this regard. Here, we explored whether four invasive crayfish species (calico crayfish Faxonius immunis, spiny-cheek crayfish Faxonius limosus, signal crayfish Pacifastacus leniusculus, and marbled crayfish Procambarus virginalis) in European freshwaters and the most widespread native species (noble crayfish Astacus astacus) overlap in function in their potential effects on key resources of benthic food webs. First, the impact on gammarids and zebra mussels was assessed by means of comparative functional response analysis using the functional response ratio as impact metric; second, the consumption of macrophytes (Chara sp.) and detritus (leaf litter) was quantified and compared using feeding experiments. Both invader- and resource-specific effects were observed. Invasive calico crayfish and signal crayfish exhibited the strongest per capita effects on gammarids and zebra mussels, respectively, with functional response ratios being 2-fold higher than those of native noble crayfish. Marbled crayfish showed an intermediate effect on both prey species, whereas spiny-cheek crayfish had lower impacts than noble crayfish. In the feeding experiment, calico crayfish consumed the highest amount of detritus, while the consumption of macrophytes did not differ among the five crayfish species. Our work demonstrates as-yet unrecognised differences in functional responses among the four North American crayfish invaders and the European noble crayfish. The lack of congruence across the observed impacts suggests a mostly species-specific pattern and stresses the importance of species and resource identity when considering the ecological impact of crayfish. An initial assessment of invader-specific potential impacts positions calico crayfish and signal crayfish among the most impactful invaders.