Here is a link to a very interesting article highlighting reasearch that has been carried out by the University of Leeds on the affect of the invasive American Signal Crayfish on native White Clawed Crayfish in England and the freshwater ecosystems as a whole: http://www.sciencedaily.com/releases/2012/03/120315095519.htm
"Their study, published in the online journal PLoS One, compares how quickly the two different species deal with food. The American signal crayfish ate up to 83 per cent more food per day than did their native cousins. The research also
showed that white-clawed crayfish are much more choosy about what they eat, preferring particular types of prey, while the signals eat equal amounts of all prey. The white-clawed crayfish are also affected by a common parasite, porcelain disease, which affects their ability to catch prey, leading affected crayfish to eat 30% less. The American signal crayfish, on the other hand, seem unaffected by the parasite."
Here is the link to the published research paper: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0032229