Abstract
It has been hypothesised that freshwater pollution may favour the establishment of tolerant invaders at the expense of less-tolerant residents. To test this, this study capitalises on the aftermath of a farm pollution incident, a slurry discharge to a small British stream, which eliminated the resident macroinvertebrate assemblage. Stream transplant bioassay experiments at the discharge site revealed a higher pollution tolerance of the invasive amphipod Crangonyx pseudogracilis and the isopod Asellus aquaticus than that of the native amphipod Gammarus duebeni celticus and the invasive Gammarus pulex. A. aquaticus recolonised the stream up to the discharge point within days, followed by C. pseudogracilis, then G. pulex, with G. duebeni celticus undetected after a month. Values of a macroinvertebrate-derived biotic index of organic water quality, the Average Score Per Taxon (ASPT), gradually increased up to the discharge point in the 30 days following the incident. In a larger-scale survey (72 river and 12 pond sites), inclusion/exclusion of Gammarus spp. had no effect on ASPT values, while inclusion of C. pseudogracilis increased values, despite an assumed equivalence of these amphipods in the index. Pollution could facilitate the establishment of tolerant invaders, and this has implications for biomonitoring.
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