Crayfish plague, carried by invasive signal crayfish, has devastated white-clawed crayfish populations across England and Wales, causing the near-extinction of a native species.
Crayfish plague causes acute welfare harm to white-clawed crayfish populations - the disease kills rapidly after exposure, affecting entire populations simultaneously. Individual suffering during the dying process from this fungal infection, while uncertain in its subjective character, represents harm at population scale. The welfare significance is compounded by the ecological role these animals play: white-clawed crayfish are keystone invertebrates whose decline affects the entire food web of affected river systems. Signal crayfish introduced irresponsibly for commercial purposes now sustain a disease reservoir that may never be eliminated from affected catchments.