9.8 - Invasive Species

Learning Objectives

The main objective is to explain the environmental problems associated with invasive species. The associated skill is proposing a solution to an environmental problem, specifically invasive species.

I. Invasive Species Basics

Definition: An invasive species is a non-native species that’s typically introduced to an ecosystem through human activity.

Non-native isn't the same as invasive: a species is invasive only when it spreads and threatens native species, ecosystems, or the economy. Many non-native species live outside their normal range without causing harm, and some are helpful: honeybees, brought to North America by European colonists in the 1600s, pollinate crops, and most US crops (like wheat and soybeans) came from other continents.
Thriving outside the home range: without the predators, parasites, and diseases that kept them in check back home, invasive species can reach much higher numbers in a new place.

Human Activity Examples: Moving boats between different lakes or rivers can transport species like the zebra mussel that attach to the hull. The exotic animal trade means pets like the Burmese python get released into the wild. Large cargo ships can carry species like the zebra mussel in their ballast water, which then gets discharged at the destination.

Characteristics of Success:

Lack of Natural Predators: Invasive species typically thrive because there’s no natural predator in the new ecosystem to control their population.

High Competitiveness: They’re often aggressive feeders and can outcompete native organisms for space, soil nutrients, or water.

High Adaptability (Generalists): They’re usually very adaptable, able to tolerate new precipitation or temperature conditions and adapt to new food sources. Generalists are more likely to thrive than specialists.

Rapid Reproduction (r-Selected): They often fall into the r-selected category, meaning they’re rapid reproducers with a higher biotic potential. That lets them reproduce fast in a new ecosystem and quickly take over.

II. Specific Examples and Environmental Problems

Invasive Species	Origin / Introduction	Problems Caused
Zebra Mussel	Native to the Black and Caspian Seas. Introduced to the U.S. and Canada via ballast water in the St. Lawrence Seaway.	Aggressive filter feeders that outcompete native organisms for small particulate matter, zooplankton, and algae. They reproduce rapidly, up to a million eggs a year, and can clog intake pipes for industrial or agricultural purposes, which causes economic damage.
Kudzu Vine	Native to Japan and China. Brought to the U.S. in 1876 for the Philadelphia Centennial Exposition. In the 1930s and 1940s the government paid Southern farmers to plant it to control erosion on worn-out farmland.	Grows extremely fast, estimated up to a foot a day. It outcompetes native plants for sunlight and water by growing right over them and shading them out. It also has no native herbivore pressure in the southeastern U.S.
Asian Carp	Introduced initially to control algae populations in aquatic farms, then escaped into the Mississippi River.	Aggressive feeding fish that outcompete native fish species for food and space. That decreases fishery value, both commercial and recreational. They can also injure boaters or damage boats by jumping out of the water.
Emerald Ash Borer (EAB)	Discovered in Michigan in 2002. Believed to have been introduced via wood packaging materials shipped on planes or boats.	The larvae bore into the bark and eat the phloem (which carries sugars) and the outer wood, cutting off the flow of nutrients and water, which kills the tree rapidly. EAB’s range is expanding with global warming, since areas that used to freeze earlier and stay frozen longer now thaw out earlier, extending their season.
Burmese Python	Introduced to Florida through the exotic animal trade, as released pets.	Aggressive feeding habits. They eat almost any mammal, including deer, cats, and even alligators. In Everglades National Park, road surveys from 2003 to 2011 found about 99% fewer raccoons and opossums than in the 1990s, and rabbits and foxes weren't seen at all (Dorcas and others, 2012).

III. Economic Costs and Control Methods

Economic Impact: In 2005, a study concluded that the United States loses about $120 billion annually in economic activity or costs tied to invasives. Costs come from lost agricultural productivity, declining tourism income, lost fishery productivity, the cost to remove and control the species, and property value decline, like when trees get killed.

Control Methods:

Laws and Prevention: Create laws to prevent transport, like cracking down on firewood transport to stop the spread of the emerald ash borer.

Example: in 2012 the US banned importing Burmese pythons under the Lacey Act.

Removing Hosts: For the emerald ash borer, this means cutting down ash trees, especially dead ones, to thin the population and reduce the chance of the invasive spreading.

Inspection and Removal: Carefully inspecting boats and removing “aquatic hitchhikers” like the zebra mussel. It also includes inspecting ships to make sure ballast water is dumped before reaching port.

Since 2017, an international treaty (the Ballast Water Management Convention) requires most ships to treat or exchange their ballast water so they don't carry species into new ports.

Biological Control: Introducing natural predators, part of Integrated Pest Management (IPM). One example is the use of three species of Chinese wasp in the Midwest that paralyze or lay eggs in the larvae of the emerald ash borer.

Risk (unintended consequences): a control species can become invasive itself. In 1935, cane toads were released in Queensland, Australia, to eat beetles in sugarcane fields. They didn't control the beetles, spread across northern Australia, and poison native predators like quolls and snakes that try to eat them. That's why biological control species are now tested for years to make sure they only attack the target.

Physical Removal: Good old-fashioned physical removal, like pulling out garlic mustard, running aggressive hunting campaigns for pythons, or spraying and scraping zebra mussels off boats and intake valves.

Big picture

Invasive species thrive because of their r-selected, generalist traits and the absence of natural predators in their new home. The resulting economic and ecological harm, like the destruction of ash trees by the emerald ash borer and the clogging of water systems by zebra mussels, requires control methods ranging from physical removal all the way to introducing natural predators through biological control.

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