Photo Credits - Cover Page 
Top left: Water hyacinth ( Eichhornia crassipes), W.V. Evans, Bugwood.org 
Top right: Common Reed (. Phragmites australis ), J.M. Randall, TNC, Bugwood.org 
Bottom left: Zebra mussels (Dreissenapolymorpha), R. Westbrooks, USGS, Bugwood.org 
Bottom right: Green Crab ( Carcinus maenes ), J.T. Carlton 
