Impact of the Highly Invasive Signal Crayfish ( <i>Pacifastacus leniusculus</i> ) on Freshwater Macroinvertebrate Communities

ABSTRACT Invasive non‐native species threaten freshwater ecosystems, disrupting the functioning and structure of the food chain and potentially leading to biodiversity loss. As omnivores, crayfish can modify their environment, directly affecting organisms such as macroinvertebrates. Macroinvertebrates, intermediate consumers in food chains, are influenced by both top‐down and bottom‐up forces, playing fundamental functional roles within freshwater ecosystems. The main aim of this study was to assess the effects of the signal crayfish Pacisfatacus leniusculus on the freshwater macroinvertebrate community in a mountainous area with very low human disturbance, while controlling for other measured environmental factors (e.g., T°C, pH, TDS). To this end, 34 sites (18 invaded and 16 non‐invaded) in the Rabaçal and Tuela River basins (Portugal) were sampled. Comparisons were made between basins and invaded and uninvaded sites regarding abundance, biomass, richness and indices of macroinvertebrate community diversity and functionality. A total of 38,529 organisms from 133 freshwater macroinvertebrate taxa were identified. Our results showed a decline in the biomass, richness and diversity of macroinvertebrates with increasing crayfish abundance. Few differences in functional diversity were found, probably due to functional redundancy; however, crayfish did affect certain functional groups: drillers and periphyton‐associated macroinvertebrates exhibited a negative correlation with crayfish abundance, whereas scrapers and gravel‐associated macroinvertebrates demonstrated a positive response. In addition to the abundance of signal crayfish, temperature, total dissolved solids and pH showed some influence on the macroinvertebrate community. Overall, these findings are fundamental for better understanding, predicting and managing crayfish impacts in low disturbance areas of upper catchments. It is important to continue monitoring not only non‐native crayfish populations but also macroinvertebrate (and other) communities, as our findings clearly demonstrate the sensitivity of these organisms to crayfish presence. Future research should investigate the main mechanisms of this change in the macroinvertebrate community.