In 1996–2025 English river samples, native invertebrate richness fell by 2.9 families after the first signal crayfish record at 161 sites, more than after first records of any of 309 native species

Question: do native river invertebrate communities change after a non-native invertebrate is first recorded at a monitoring site, and does the change start before or after the record? Data: Environment Agency (England) Biosys open data, 87,736 standard spring and autumn kick samples analysed to species level for many groups, from 12,524 river sites in 551 catchments, 1990-2025. Method: for each of eight non-native taxa, we did a stacked event study of the first record at a site. Controls were sites in the same catchment without a record. The model has site and catchment-year-season fixed effects. The outcome was the number of native family units, without the families that contain the non-native taxa. We applied the same method to the first records of 309 native species as a benchmark for sampling artifacts. Results: after the first signal crayfish (Pacifastacus leniusculus) record at 161 sites, native richness was 2.88 families lower per sample (SE 0.40; about 12%). This was lower than the estimate for each of the 309 native species (mean +0.03, SD 0.45, minimum -1.39). It held in 13 robustness checks (-2.30 to -4.64). The loss was in snails, leeches, cased caddis flies and beetles, not in mayflies, true flies or stoneflies, and WHPT ASPT did not decrease. The decline started 4-6 years before the first record, with a constant slope of -0.34 families per year and no clear step at the record, and the pre-record decline was in the same prey groups. This agrees with crayfish presence some years before kick samples record them, so before-after comparisons that start at the first record can underestimate the effect. Crangonyx (550 sites) gave -0.56 (SE 0.22), but not for records before 2005. Dikerogammarus (62 sites) gave -1.24 (SE 0.70), but the estimate fell to -0.18 when time effects were split by river size. Dreissena, Gammarus tigrinus, Physella, Potamopyrgus and Chelicorophium were within the native range. Limitations: observational design; a first record is not the arrival, and taxa were found in only 11-57% of later samples; site-specific trends not shared within a catchment are not controlled; the crayfish result agrees with earlier before-after studies of the same data source (Mathers et al. 2016, 2020). New here are the eight-taxon comparison, the native benchmark and the timing profile.

invasive species · signal crayfish · Pacifastacus leniusculus · Dikerogammarus · Crangonyx · macroinvertebrates · river monitoring · event study · difference-in-differences · biomonitoring · WHPT · Environment Agency · England · drivers or passengers · detection lag