Ecosystem effects of invasive crayfish increase with crayfish density

Abstract

Abstract The nature and extent of effects of increasing densities of non‐native species on stream ecosystems remain poorly understood. Non‐native crayfish are among the most invasive aquatic species and we hypothesised that, in temperate streams, the extent of trophic modification increases with non‐native crayfish density. Instream flow‐through mesocosms in the River Lune, north‐east England, were used over a 47‐day period in summer to measure density effects of invasive signal crayfish Pacifastacus leniusculus on different ecosystem components (benthic fish, macroinvertebrates, algal biomass, and leaf litter breakdown). Effects were measured through three density treatments (low, medium, and high; four, eight, or 12 crayfish with five bullhead Cottus perifretum per 1.5 m2) and two control (crayfish and fish absent; crayfish absent and benthic fish present) groups. Impacts of crayfish on macroinvertebrates (density, taxonomic composition), fish (growth, diet), algal standing stock, and decomposition rates increased with crayfish density. Direct effects of increased crayfish density were more important than indirect trophic cascade effects. Despite similar invertebrate abundance and richness across enclosures before introducing crayfish and bullhead, they differed significantly from controls at the end of the study, with >80% reduction in macroinvertebrate abundance recorded in the high‐density group. Stable isotope (δ15N, δ13C) analysis showed that the trophic niche of bullhead, but not signal crayfish, changed when the species were in sympatry. Bullhead in treatment enclosures occupied a lower trophic position in the food web than those from the control group. Bullhead in the high‐density group lost 4.2% of body mass over the study period, confirming the existence of resource competition. Leaf litter break down was 59.2% faster, and algal biomass was 91.4% lower in the treatment with the highest crayfish density compared to the control without study animals. This study indicates that signal crayfish, even at a low density, can strongly alter multiple ecosystem components in streams, and emphasises the need for minimising the spread of invasive crayfish within and between streams.