Ecosystem effects of invasive crayfish increase with crayfish density

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