What is an Invasive Species?


Wisconsin Statute defines invasive species as “nonindigenous species whose introduction causes or is likely to cause economic or environmental harm or harm to human health.”  These species can be aquatic or terrestrial weeds, insect pests, nuisance animals, or disease-causing organisms. They can occur in all types of habitats and affect urban and rural areas throughout the state.

DCIST often focuses on invasive plants.  An invasive plant is a non-indigenous species or strain that becomes established in a natural plant community and replaces native vegetation.  When these species are introduced into a new area, they leave behind the predators and pathogens that controlled their populations in their original environment.  Without natural checks-and-balances, these invaders have greater success reproducing, surviving, and replacing our native flora.  Their aggressive growth is often aided by the production of compounds and chemicals that can inhibit the growth of other plants around them.

The majority of invasive species have origins in other countries, such as Europe or Asia.  It should be noted that not all non-native plants act aggressively—instead they are deemed invasive when their introduction causes, or is likely to cause, adverse impacts to the economy, environment, and human welfare.  On a broad scale, invasive species decrease the ecological,  aesthetic, recreational and social values of natural environments.  These decreased values may impact the economy of a local area or region.  On a more refined scale, the invasion of aggressive, non-native plants is a serious threat because of the following:

  • Changes in nutrients and water levels that may result from monotypic vegetation, such as dense-stands of Phragmites, and lower the aesthetic values of coastal and natural areas.
  • Dense, invasive plant growth within navigable waterways impedes fish migration and recreational opportunities.  In addition, fish habitat and food sources necessary for other native wildlife may be degraded when invasive species displace native plants.  A more diverse mix of native species rarely causes such problems.  
  • Invasive plants shade out wildflowers and prevent tree seedlings from emerging to regenerate the forest.  This results in a decrease in spring wildflower tourism and a loss in forest and timber.
  • Non-native plants outcompete native plants for resources, which can lower the species diversity and limit crucial habitat for rare and endangered species.  Door County has at least 105 state endangered and threatened plant and animal species.  Of those species listed under the federal Endangered Species Act, 42% of them have invasive species implicated in their decline.  
  • Costs to eradicate or control invasive plants in agricultural and public lands eventually appear in higher food prices, higher taxes, and other indirect costs.  Recent number show that more than $137 BILLION is spent on invasive species each year in the United States alone.

Humans play multiple roles in the invasion and spread of non-native plants and animals.  For starters, we are the primary means of introduction.  We have long used plants from all over the world for landscaping and decoration and have brought new trees to our homes for shade.  We have unintentionally moved plants and animals in ballast water of ships and continue to spread them on our trailers, boats, and vehicles.  Other means of introduction include landscape restoration, biological pest control, sport, the pet industry, and food processing.  Once introduced, their spread has been aided by the alteration and restriction of natural processes (e.g. fire regimes), making conditions more favorable for non-native species.