AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/9/9-8)
Unit 9 · Topic 9.8
9.8 Invasive Species
Invasive species are non-native species that spread rapidly and harm the ecosystems, economies or health of the places they're introduced to. They usually succeed because they are generalists that reproduce quickly and face few natural predators or competitors in their new home.
Key terms
- invasive species
- non-native species
- generalist
- r-selected species
- zebra mussel
Non-native versus invasive
A non-native species (also called exotic or introduced) is one that people have moved, on purpose or by accident, outside its natural range. Many non-native species are harmless or useful; most food crops in the Americas, such as wheat, are non-native.
A non-native species becomes invasive when it spreads aggressively and causes harm, such as outcompeting native species, preying on them, spreading disease or damaging property and agriculture. Invasive species are one of the leading causes of biodiversity loss, especially on islands. Even a harmful invader can have a side benefit: zebra mussels filter so much plankton that lake water becomes clearer. But invaders usually outcompete native species, and that is what exam questions focus on.
Why some species become invasive
- Generalists: they can eat many foods and live in many habitats, so they can find what they need in a new place (topic 3.1).
- r-selected traits: they mature fast and produce many offspring, so populations grow quickly (topic 3.2).
- Wide tolerance: they can handle a broad range of temperatures and conditions.
- Enemy release: their natural predators, parasites and diseases were left behind, so nothing keeps their numbers in check.
- Native species haven't evolved defenses against them, so prey may not recognize them as a threat.
How they arrive
Ships take on ballast water (water carried for stability) in one port and release it in another, along with any organisms in it. Other routes include the pet and aquarium trade, ornamental plants for gardens, hitchhiking on cargo, firewood, boats and vehicles, and deliberate introductions intended to control pests or prevent erosion that backfire.
Examples to know
| Species | Origin and introduction | Harm |
|---|---|---|
| Zebra mussel | Black and Caspian Sea region; arrived in the Great Lakes in ballast water, found in 1988 | Clogs water intake pipes; filters out plankton that native species need; smothers native mussels |
| Kudzu | Asian vine; widely planted in the US Southeast in the 1930s–40s to control erosion | Grows over and shades out trees and other plants |
| Cane toad | Central and South America; released in Australia in 1935 to eat beetles in sugarcane fields | Toxic skin poisons native predators that eat it; it failed to control the beetles |
| Burmese python | Southeast Asia; released or escaped pets established in Florida's Everglades | Sharp declines, over 90 percent in some studies, in raccoons, opossums and other mammals |
| Emerald ash borer | Asian beetle; arrived in wood packing material | Has killed hundreds of millions of ash trees in North America |
Controlling invasive species
Prevention is cheapest: rules requiring ships to exchange or treat ballast water, inspections of cargo, bans on importing high-risk species, cleaning boats between lakes, and not moving firewood. Once a species arrives, early detection and rapid removal give the best chance of success. Established species may be managed by trapping, hunting, pulling or herbicides, or by biological control, introducing a natural enemy from the species' home range. Biological control must be tested carefully, because the cane toad shows how an introduced control can become an invader itself.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Explaining a population explosion
A non-native snail is released into a lake. Within ten years it is the most common snail in the lake, and two native snail species have nearly disappeared. Using species traits, explain why the non-native snail succeeded.
Show the solutionHide the solution
- Step 1: Likely a generalist: it can eat many types of algae and detritus, so it can use resources the native snails need.
- Step 2: Likely r-selected: it reproduces quickly with many offspring, so its population can grow faster than the natives'.
- Step 3: Enemy release: the predators and parasites that controlled it in its home range aren't in this lake, so nothing keeps its numbers in check.
- Step 4: Result: it outcompetes the native snails for food and space, and their populations decline.
Answer: As a generalist with r-selected traits and no natural predators in the lake, it reproduced rapidly and outcompeted the native snails for food and space.
Common mistakes
- Using 'non-native' and 'invasive' as synonyms. Only non-natives that spread and cause harm are invasive.
- Describing invasive species as K-selected specialists. They are typically r-selected generalists.
- Suggesting biological control as a risk-free fix. Introduced predators can become invasive themselves, as the cane toad did.
On the exam
- Expect to identify traits that make a species likely to become invasive, or to explain how a specific invader harms an ecosystem.
- When asked for a solution, prevention measures (ballast water rules, inspections) and removal programs are specific, accepted answers.
Connected topics
Videos
Check yourself
4 questions on 9.8 Invasive Species. Pick an answer to see if you got it, and why.
Zebra mussels, native to the region around the Black and Caspian Seas, were discovered in the Great Lakes in 1988. They most likely arrived in ballast water released from cargo ships.
Each female can release hundreds of thousands of eggs a year, and the mussels attach to almost any hard surface. Within a few years they covered rocks, native mussel shells and water intake pipes, and the water in many areas became noticeably clearer as the mussels filtered out plankton.
Described case study
Which characteristics of zebra mussels best explain their success as an invasive species?
The clearer water in the Great Lakes most likely had which effect on the native food web?
A state wants to slow the spread of an invasive aquatic plant between lakes. Which action would be most effective at preventing new introductions?
In 1935, cane toads from the Americas were released in Queensland, Australia, to eat beetles that damaged sugarcane. The toads did little to control the beetles, spread across more than a million square kilometers, and poisoned many native predators that tried to eat them. Which lesson does this case most clearly show?
0 of 4 answered