| Summary: | The American signal crayfish, Pacifastacus leniusculus, is an invasive species from North America that has been widely introduced through Europe, where it is a major threat to native European crayfish species and causing increasing concern because of its impact on the wider aquatic ecosystem. Although widely considered invasive, little is known of the rates of invasion and colonisation. The temporal and spatial pattern of spread of signal crayfish populations in two upland rivers in Northern England is described. Range expansion of up to 2.4 km year-1 were recorded from an established population, but rates over an order of magnitude less were recorded in the initial stages of establishment. Range expansion of both populations was strongly biased in a downstream direction, which has implications for directing possible management efforts and determining the timescale of threat which expanding signal crayfish populations pose.
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