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Invasive species

What causes a species to be considered invasive. How invasive species can damage ecological communities.

Key points

  • Invasive species are species that have been introduced into areas outside their native range and can cause—or have caused—harm in their new area.1
  • Invasive species may outcompete native species for resources or habitat, altering community structure and potentially leading to extinctions.
  • Asian carp illustrate the potential impact of invasive species. Introduced into the United States by humans, these fish species have colonized waterways and may threaten native fish populations, and fisheries, in the Great Lakes.


Take a look at the photo below. Just another pretty morning drive in the hills of Tennessee! But wait a minute ... those trees ... they're covered with something. Look closer, and you'll see that almost the entire landscape is covered with a thick, green blanket. This blanket is made up of an invasive plant called kudzu.
Image credit: Kudzu by Katie Ashdown, CC BY 2.0
Kudzu is one dramatic example of what can happen when a species gets introduced into a new ecosystem where it has abundant resources and few predators. The kudzu plant was introduced to the United States from Asia in the late 1800s as an ornamental plant, and it and was planted widely in the South in the early 1900s to reduce soil erosion. What the people who planted this vine did not know was that it would rapidly take over the landscape, growing as much as a foot a day and enshrouding ground, shrubs, trees, and even houses and old cars in a suffocating girdle of vines.2
Invasive species like kudzu are a vivid—and scary!—example of how ecological changes, including those caused by humans, can alter communities and ecosystems. In this article, we'll look in more detail at what an invasive species is and how invasive species can disrupt ecosystems—often reducing the numbers of native species and altering the overall structure of the community.

What is an invasive species?

An invasive species is a species that has been introduced to an area outside of its native range and has the potential to cause harm—or has already caused harm—in its new location.1 Many invasive species are found in the United States, and a few examples are shown in the pictures below. Whether you're enjoying a forest hike, taking a summer boat trip, or just walking down a city street, chances are that you've encountered an invasive species.
Image modified from Community ecology: Figure 16 by OpenStax College, Biology, CC BY 4.0; modified from original work by—from left to right, starting at top left—Liz West; M. McCormick, NOAA; E. Dronkert; Dan Davison; USDA; Don DeBold

Case study: Asian carp

Let's take a look at what's arguably the kudzu of the aquatic world: the Asian carp. Since they were introduced to the United States in the 1970s, Asian carp have rocketed in numbers thanks to their vast appetite and speedy reproduction, now forming up to 95% of the biomass in some Mississippi and Illinois rivers. Not only that, these fish have led to an international lawsuit about waterway access between the United States and Canada! This is a dramatic example of what can happen when an invasive species gets a foothold in a new place.
Where did this story begin? Asian carp—which are not a single species, but a group of related species—were introduced to the United States in the 1970s.2 They were imported largely by fisheries and sewage treatment plants, which used the carp's filter feeding abilities to rid ponds of excess plankton. However, some fish escaped. By the 1980s, these fish had colonized waterways of the Mississippi River basin, including the Illinois and Missouri Rivers.
Because they are big eaters and fast reproducers, Asian carp can often outcompete native fish species with whom they share habitats and food sources. Black carp eat mussels and snails, limiting their availability for native fish and damaging shellfish populations. Another Asian carp species, the silver carp, eats plankton, a key food for many native fish species in their larval and juvenile stages.2
Although Asian carp can be eaten, the fish are bony and are generally not a desired food in most parts of the United States. Also, if you go out for a fishing trip, be careful: you could get smacked by an Asian carp! The fish, frightened by the sound of approaching motorboats, often thrust themselves into the air and may land in the boat or directly hit the boaters, see jumping carp below.
Image credit: Asian carp by Steve Hillebrand, USFWS, CC BY 2.0
The Great Lakes and their prized salmon and lake trout fisheries are also threatened by the Asian carp. These invasive fish have already colonized rivers and canals that lead into Lake Michigan, including the major supply waterway linking the Great Lakes to the Mississippi River.
To keep the carp from leaving this canal, electric barriers have been used to discourage migration. However, the threat is serious enough that several states and Canada have sued to have the Chicago channel permanently cut off from Lake Michigan. We don't yet know whether the Asian carp will prove to be mostly a nuisance—like invasive species such as the zebra mussel—or whether it will ultimately destroy the largest freshwater fishery in the world.
The story of the Asian carp shows how population and community ecology, fisheries management, and politics can intersect and how ecology can have very real importance for the human food supply and the US economy. On a more personal note, it also shows how ecology can matter for folks enjoying a day on the river ... who might happen to get smacked with a flying carp!

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