The water shrew is Britain's largest shrew and one of very few venomous mammals, found along clean unpolluted streams and rivers where it preys on aquatic invertebrates and small amphibians.
Water shrews are acutely sensitive to water pollution and loss of riparian vegetation. Agricultural runoff, sewage discharge, and pesticide contamination reduce aquatic invertebrate populations that form their prey base. Removal of bankside vegetation exposes them to predation and reduces microhabitat complexity. Their exceptionally high metabolic rate means even brief periods of food scarcity are life-threatening. Climate change may alter invertebrate availability timing, creating mismatches with shrew breeding cycles.