Within the state, these efforts are paying off. The spread of Eurasian watermilfoil, for example, was rising rapidly before new laws and public information efforts slowed the increase. In 1999 there was no documented spread of flowering rush, spiny waterfleas, ruffe, or round gobies within Minnesota. And the program's partnership with counties to use imported beetles to eat purple loosestrife is showing results. New threats from outside the state are what has the exotics program staff most concerned.
Most states have exotics species laws far more lax than Minnesota's.
They strain virtually everything from the water, including game fish larvae. Rendall says that in some Mississippi backwaters, Asian carp now make up more than 90 percent of the biomass. Bighead carp feed on plankton, competing with paddlefish and other native fish species for food. Conover worries that the species robs food from native fish, including game species such as walleye, catfish, and northern pike. Elsewhere along the river, black carp pose a huge threat to native mussel populations.