We have seen how crayfish are important elements in many different kinds of freshwater ecosystem, often acting as keystone species and enhancing biodiversity through controlling the abundance of many other organisms in the community. However, the all-pervading handiwork of humans has threatened their existence in many ways. In this chapter we look at the negative impacts of environmental deterioration and the introduction of alien species and, in the next, at exploitation.

The Millennium Ecosystem Assessment (MEA; proposed in 2000 and initiated in 2001) noted that different environmental pressures were important on each continent. Among regions containing crayfish, water quality was especially poor in Europe (Füreder et al., 2006), whereas on other continents water shortages, wetland conversion, droughts and fire were among the significant pressures affecting crayfish. In Europe, the few crayfish species were formerly distributed more widely, and were found in many lakes, rivers and brooks until the beginning of the twentieth century, but their distribution has shrunk and become fragmented. The threats considered most important to indigenous European crayfish are crayfish plague, the continued spread of non-indigenous signal, red swamp and spiny-cheek crayfish (Holdich, 2003), and the illegal exploitation and trading of crayfish. Threats considered of lower or local importance include habitat management-related parameters such as predators and habitat alterations, and watercourse alteration – damming, abstraction and the drying out of habitats through drainage (Füreder et al., 2006).