Zebra mussels - are a serious introduced species in this country. They first appeared in the Great Lakes in the 1980's and have spread throughout many American waterways. They are incredibly prolific as adults may produce hundreds of thousands of eggs per year. Because of this, they can kill off native species of mussels, and impact other species due to the consumption of food sources. They are having a devastating effect on some ecosystems because they are so numerous and out compete other species. Also, because of their numbers, they can cover the underwater section of any water-based structure, including boats and docks, and even entire streambeds. They clog underwater pipes used for water delivery to industrial plants, and used to provide water for municipalities. It's estimated that the zebra mussel problem has cost the U.S. billions of dollars.
European starlings - are an introduced bird species that tend to flock in large numbers and cause considerable damage to crops. For many of us, they are a nuisance bird as well as they can appear in the thousands, make a tremendous amount of noise, and damage trees from the sheer weight of their numbers. The damage they cause to agriculture is considerably more serious though. They will eat a variety of fruit and vegetable crops. And will eat livestock feed - and in the process, contaminate much of what they don't eat with their droppings. In addition to this, they have been known to introduce insect parasites into poultry houses (like bird mites) and can pass some diseases on to humans and livestock. Control efforts for starlings consists of closing off areas and structures to make them starling-proof, and to limit their access to livestock feed.
Asian carp - a large fish species originally imported to keep catfish farms clean, they are a threat to the ecosystems of the waters they enter. They got out of these farms when floods allowed them access to the Mississippi river in the 1990's. These huge fish can reach 4 feet long and weigh over 100 pounds. The also can eat a tremendous amount of food, which deprives many native species of enough to survive on. Because of this and because of the rate at which they reproduce, the existing food chain of any waterway they enter can be severely disrupted. They also damage habitat, which adversely affects native fish species. These fish are primarily filter feeders and eat plankton. The young of other fish, such as many sport fish species, eat plankton as well, and so can be severely affected by this.
Flathead catfish - are a species native to many waters in this country, but is considered an invasive species in other parts. It's a danger because it is a predator to other fish species. It eats only live fish and it has a voracious appetite. The introduction of this fish into new waters can seriously impact native fish. Where introduced, native species have usually declined in numbers. Usually, flatheads are introduced as sport fish because they grow Quite large (some in excess of 100 pounds), and can provide good eating. But it's estimated that they need to eat 10 pounds of live fish - and they only eat live fish - in order to gain 1 pound; so their impact on other fish populations is considerable. Fishermen are encouraged to keep, and not release, any flathead catfish that they catch in order to help reduce their numbers. Currently this is the primary means of control.
Unwanted, non-native species are a problem wherever they are introduced. We can all help prevent the spread of these un-welcomed species, and should do so whenever we can.