9.8 Invasive Species

Invasive species are species that live and sometimes thrive outside their normal habitat, and they become a problem when they threaten native species. They are often generalist, r-selected species that outcompete natives for resources, and humans control them through prevention, removal, chemical treatment, and biological control.

Why This Matters for the AP Environmental Science Exam

This topic sits in Unit 9, Global Change, which carries one of the larger weightings on the exam. You will need to explain the environmental problems invasive species cause and propose realistic strategies to control them. The most useful skill here is going beyond naming a control method and actually explaining how it works and what its advantages, disadvantages, or unintended consequences are. That kind of cause-and-effect reasoning and solution proposal shows up in both multiple-choice questions and free-response prompts where you analyze a scenario and recommend an action.

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Key Takeaways

• An invasive species lives outside its normal habitat and is labeled invasive when it threatens native species. Some non-native species can be beneficial, so the threat to natives is the key part of the definition.
• Invasive species are often generalist, r-selected species, which helps them reproduce fast and outcompete native species for resources.
• Human activities like trade, transportation, and intentional introductions move species into new areas.
• Prevention, such as inspection and quarantine, is the most effective and cost-efficient control strategy.
• Once a species is established, methods include physical removal, chemical control, biological control, habitat modification, and monitoring.
• Biological control carries a risk: the introduced control agent can itself become a new problem.

What Makes a Species Invasive

An invasive species is a non-native species introduced to a new area where it can establish a population and spread, often harming native biodiversity, the economy, or human health. Invasive species can be plants, animals, fungi, or microorganisms.

Two traits explain why so many invasives succeed:

• They are often generalist species, meaning they can use a wide range of food sources and habitats.
• They are often r-selected species, meaning they reproduce quickly and in large numbers.

These traits let invasives outcompete native species for resources like food, space, and nesting sites. Keep in mind that a non-native species is not automatically invasive. It only counts as invasive when it threatens native species.

How They Spread and What They Do

Human activities such as transportation, trade, and the introduction of exotic species move species around the world. Once established in a new environment, invasive species can:

• Outcompete native plants and animals for resources.
• Change ecosystem processes like nutrient cycling, water flow, and fire regimes.
• Harm human activities such as agriculture, forestry, and recreation.
• Increase the risk of spreading new diseases.

These changes can lead to large economic costs and to declines in native populations.

Strategies for Control

Several human interventions can control invasive species:

1. Physical removal: Manually removing individuals or populations through hand-pulling, cutting, digging, and trapping.
2. Chemical control: Using pesticides or herbicides to reduce survival, reproduction, or growth of invasive species.
3. Biological control: Introducing natural predators, pathogens, or competitors to reduce the invasive population.
4. Habitat modification: Changing the environment to favor native species over the invasive one.
5. Monitoring and early detection: Regularly checking for new invasions and acting quickly when one is found.
6. Public education and outreach: Teaching people how to avoid spreading invasive species.

Prevention is considered the most effective and cost-efficient strategy. This includes quarantine regulations, inspections of cargo and vehicles, and rules on importing and trading live organisms.

When an invasive species is already present, early detection and rapid response (EDRR) matters. EDRR means monitoring for new invasions, spotting them early, and acting fast with removal, chemical, or biological methods to limit spread.

Control methods should match the specific species, the ecosystem, and the management goals. An integrated approach that combines several methods is often the most effective.

Examples to Illustrate the Concept

These are illustrative examples, not required AP content. Use them to practice explaining cause and effect and proposing solutions.

Zebra Mussels

Zebra mussels (Dreissena polymorpha) are small freshwater mussels native to the Black and Caspian Sea regions. They were discovered in the Great Lakes in the late 1980s and have spread widely. They reproduce quickly and outcompete native species for plankton. They clog water intake pipes, damage boat motors, and attach to native mussels and suffocate them. Control includes physical removal, chemical control, and prevention measures like inspecting and decontaminating boats before they enter new waters.

Cane Toads

Cane toads (Rhinella marina) are native to Central and South America. They were introduced to Australia in 1935 as a biological control for sugarcane pests, but became invasive themselves. This is a useful example of how biological control can backfire. They outcompete and prey on native animals, and their toxins poison native predators like snakes and marsupials that are not adapted to eat them. Control efforts include fencing, trapping, and research into genetic methods.

Other Examples

• European starling (Sturnus vulgaris): Introduced to the US in the late 1800s, now numbering in the millions and outcompeting native birds for food and nesting sites.
• Argentine ant (Linepithema humile): Forms dense colonies that displace native ants and disrupt ecosystems. Control includes baiting, pesticides, and physical removal.
• Giant hogweed (Heracleum mantegazzianum): Brought in as an ornamental plant, it outcompetes native plants along riverbanks, forests, and roadsides.
• Asian carp (bighead, silver, grass carp): Reproduce rapidly, consume large amounts of plankton, and compete with native fish.
• Burmese python: Likely released by pet owners into the Florida Everglades, where it preys on native wildlife.
• Kudzu (Pueraria montana): Introduced to the US in 1876 as an ornamental plant and for erosion control, now spreads aggressively.
• Japanese knotweed: Brought to Europe and North America as an ornamental, it outcompetes native plants and damages infrastructure.

Across these examples, the common pattern is fast reproduction, broad resource use, and harm to native species and economies. Control usually combines physical removal, herbicides, biological control, and public awareness.

How to Use This on the AP Environmental Science Exam

Free Response

When a prompt asks you to propose a control strategy, do not stop at naming one. Identify the method, then explain how it actually reduces the invasive population, and describe at least one advantage, disadvantage, or unintended consequence. For example, biological control can lower an invasive population, but the introduced agent may become invasive itself.

MCQ

Watch for questions that test the definition. A non-native species is only invasive when it threatens native species. Also connect invasive success to the traits generalist and r-selected, since those terms tie back to Unit 3 population concepts.

Common Trap

Linking invasive species to the wrong cause or to a vague solution. Be specific. Say which resource the invasive outcompetes natives for, or which exact intervention you would use and why.

Common Misconceptions

• Every non-native species is invasive. Not true. A species is only invasive when it threatens native species. Some introduced species are harmless or even beneficial.
• Invasive species are usually specialists. They are more often generalists and r-selected species, which is exactly why they spread and outcompete natives so well.
• Biological control is always safe. Introducing a predator or competitor can create a new invasive problem, as the cane toad example shows.
• Removal alone solves the problem. Without prevention and monitoring, new invasions keep happening. Prevention is the most cost-effective approach.
• A small population means a species is endangered. Small does not automatically mean threatened. What matters is whether the population can adapt, move, or maintain itself under the pressures it faces.

Related AP Environmental Science Guides

• 9.1 Stratospheric Ozone Depletion
• 9.3 The Greenhouse Effect
• Major Environmental Disasters
• Unit 9 Overview: Global Change
• 9.4 Increases in the Greenhouse Gases
• 9.2 Reducing Ozone Depletion