Invasive species are plants, animals and pathogens that are "out of place." A species is regarded as invasive if it has been introduced by human action to a location, area or region where it did not previously occur naturally (i.e., is not native), becomes capable of establishing a breeding population in the new location without further intervention by humans and spreads widely throughout the new location.
One of the reasons that invasive species are able to succeed is that they often leave their predators and competitors behind in their native ecosystems. Without these natural checks and balances they are able to reproduce rapidly and out-compete native species.
Invasive species can alter ecological relationships among native species and can affect ecosystem function, economic value of ecosystems and human health. More than 180 species of plants, plankton, fish and pathogens have been introduced into Lake Michigan and Lake Superior. Most of these species have arrived in ballast water of sea-going ships. Others have been introduced by recreational boaters and water gardeners or through escape from aquaculture facilities.
Some native species can become a nuisance when environmental conditions change. Cladophora is a good example of a nuisance species. This native green algae is part of a healthy lake ecosystem. But, in Lake Michigan, bright green Cladophora blankets the lake bottom. Large amounts of Cladophora wash onto shore and form stinky rotting mats. The recent overgrowth of this algae is caused by clearer water and local nutrient availability created by invasive mussel species (zebra mussel and quagga mussel). The presence of these non-native mussels has upset the balance of Lake Michigan’s ecosystem, giving Cladophora a competitive advantage.
The following resources contain more information about invasive and nuisance species in Wisconsin's Great Lakes.