Cross-ecosystem effects of light pollution and invasive signal crayfish on riparian spiders

Ecosystems face multiple abiotic and biotic stressors that interact and extend beyond ecosystem boundaries. Both artificial light at night (ALAN) and invasive species are major threats to freshwater biodiversity in Europe. We used a riparian stream mesocosm facility, with 16 replicated units each containing an artificial flume and adjacent terrestrial habitat, to investigate the effects of ALAN, the invasive signal crayfish Pacifastacus leniusculus and their interaction. We specifically addressed their impact on aquatic insect emergence and the potential bottom-up control of riparian spiders. As expected, crayfish reduced insect emergence by 35 % after one-week, while ALAN had no significant effect. Moreover, spider numbers, particularly of riparian Tetragnathidae Pachygnatha degeeri and Tetragnatha extensa , correlated positively with insect emergence, indicating indirect negative effects of the crayfish. During the first week of our experiment, spider numbers increased by 22 % in pitfall traps exposed to ALAN, but decreased by 25 % in suction samples. This difference is likely driven by differences in species composition between the two sampling methods. All in all, the direction and strength of crayfish and ALAN effects were taxon- and time-dependent. This study provides evidence that the impact of signal crayfish can extend beyond aquatic systems and suggests that the effects of crayfish invasion and ALAN are largely independent.