AP Environmental Science

## Learning Outcomes

- Define an **invasive species** in AP Environmental Science terms
- Distinguish between **native**, **nonnative**, **introduced**, and **invasive** species
- Explain why not all introduced species are invasive
- Identify the main kinds of harm invasive species can cause
- Apply the definition to examples and exam-style questions
- Connect invasive species to biodiversity loss and habitat change

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand invasive species definition, with a focus on the following syllabus points:

- Know that an invasive species is a **nonnative species that causes harm**
- Identify the distinction between **introduced/nonnative** species and **invasive** species
- Explain that harm may be **ecological**, **economic**, or related to **human health**
- Understand how invasive species can outcompete native species for food, space, water, or light
- Recognise other mechanisms of harm, including predation, disease spread, and habitat alteration
- Understand that invasive species are a major cause of biodiversity decline and are part of **HIPPCO**
- Identify common traits that often make invasive species successful, such as rapid reproduction and broad tolerance
- Apply the definition carefully to examples such as zebra mussels, water hyacinth, and European green crabs

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What makes a species invasive rather than simply nonnative?
2. Why are not all introduced species considered invasive?
3. Name three different kinds of harm an invasive species may cause.
4. How can an invasive species reduce biodiversity in an habitat?
5. What is the difference between a native species and an introduced species?

## Introduction

In AP Environmental Science, invasive species matter because they are one of the main human causes of biodiversity loss. They appear in Unit 9 because global change is not only about climate and oceans. It is also about how human movement, trade, transport, and land use shift species into places where they did not evolve.

Students often lose marks here because they define invasive species too broadly. A species is not invasive just because it comes from somewhere else. The exam expects a more exact definition: the species must be **nonnative** and it must **cause harm**. That harm can affect the environments, economies, or human health.

> **Key Term: invasive species**
> A nonnative species that establishes and spreads in a new area and causes environmental, economic, or human-health harm.

### The core definition: nonnative plus harm

The most exam-safe definition is simple: an invasive species is a **nonnative organism that establishes, spreads, and causes harm**. That wording matters because each part does a job.

First, **nonnative** means the species is outside its natural range. It did not originate in that environment. In APES questions, you may also see the terms **introduced species**, **alien species**, **exotic species**, or **non-indigenous species**. These all point to the same broad idea: a species has been moved beyond the area where it naturally occurs.

Second, the species usually has to **establish** itself. A few seeds or animals arriving in a new place do not automatically make a species invasive. If they cannot survive and reproduce, they are not really changing the environment.

Third, the species typically **spreads**. That spread is one reason invasive species are such a serious problem. A single introduction event may become a regional issue if the species reproduces quickly and faces few checks.

Fourth, and most importantly, the species must **cause harm**. This is the part students most often leave out. Without harm, the species is merely nonnative or introduced, not invasive.

> **Key Term: nonnative species**
> A species living outside its natural or historical range, usually because it was moved by human activity.
>
> **Key Term: introduced species**
> A species brought into an area where it does not naturally occur.

Notice what these definitions show: nonnative and introduced do not automatically mean invasive. The invasive label depends on impact.

A good one-sentence exam answer would be: an invasive species is a nonnative species whose introduction and spread cause ecological, economic, or human-health harm.

> **Test Tip:** If a question asks you to define invasive species, include both ideas: **outside its natural range** and **causes harm**. If you miss the harm part, your definition is incomplete.

### Why not all nonnative species are invasive

This is the distinction APES examiners want you to make clearly. Many species are transported by humans, intentionally or accidentally, but only some become invasive.

Some introduced species fail immediately. They may arrive in a new habitat but find unsuitable temperature, rainfall, predators, disease, or lack of food. In that case, they do not establish a population. They are introduced, but not invasive.

Other introduced species may survive without causing major disruption. They exist in the new environment, but they do not outcompete native species, alter food webs, or create measurable damage. Again, these are nonnative, but not invasive.

A species becomes invasive only when its presence changes the system in harmful ways. This is why rapid spread alone is not enough. A fast-spreading nonnative plant that causes no significant ecological or economic damage would not meet the full APES definition.

This distinction matters because the vocabulary can seem interchangeable in everyday speech, but on the exam it is not. The term **invasive** is narrower and more precise.

Think of the categories like this:

- **Native species**: evolved in that habitat and occur there naturally
- **Nonnative or introduced species**: moved there from elsewhere
- **Invasive species**: a nonnative species that spreads and causes harm
  > **Key Term: native species**
  > A species that occurs naturally in an area and is part of that environment without human introduction.

This also explains why AP questions may describe a species being transported by ships, trade, tourism, landscaping, or agriculture and then ask whether it is invasive. You cannot answer from transport alone. You must ask: did it establish, spread, and cause harm?

A common trap is to define invasive species as “species from another area.” That definition is too vague and would include harmless introduced species.

### What counts as harm?

The harmful impact of invasive species can be grouped into three big categories: **environmental harm**, **economic harm**, and **human-health harm**. APES most often stresses environmental harm, but the other two also count.

Environmental harm includes direct damage to native species or ecological systems function. For example, invasive species may outcompete native species for food, water, light, or space. They may prey on native organisms, spread disease, hybridise with native species, or alter habitats.

Economic harm includes damage to agriculture, fisheries, forestry, tourism, or infrastructure. Zebra mussels are a classic example because they clog water intake pipes, attach to boats and docks, and create high management costs. Water hyacinth can block waterways and choke irrigation systems. These effects are not just biological; they also cost money.

Human-health harm may happen if an invasive species spreads pathogens, worsens allergies, increases disease vectors, or changes environmental conditions in ways that harm people. This is less commonly the focus in introductory examples, but it is still part of the accepted definition.

The key idea is that harm goes beyond “I do not like this species.” It must be real, measurable damage to natural systemss, the economy, or health.

Here are the main mechanisms of environmental harm that APES students should know:

- **Competition**: invasive species outcompete native species for resources
- **Predation**: invasive predators consume native prey that may lack defences
- **Disease transmission**: invasive organisms bring pathogens or parasites
- **Hybridisation**: breeding with native species can weaken local gene pools
- **Habitat alteration**: changing light levels, soil conditions, fire patterns, or hydrology
- **Food web disruption**: changing predator-prey relationships and energy flow
- **Biogeochemical change**: altering nutrient cycling or other system processes
  > **Key Term: biodiversity loss**
  > A reduction in the variety of life in an area, including fewer species, less genetic variation, or less habitat variety.

In practice, these forms of harm are often linked in a chain. For example, an invasive aquatic plant may grow densely, block sunlight, kill submerged plants, reduce oxygen, and then change fish populations. That is why invasive species are treated as a major biodiversity threat rather than just a minor inconvenience.

> **Test Tip:** If a question asks how invasive species affect native communitiess, do not stop at “they harm biodiversity.” Add a mechanism such as **outcompeting natives**, **predation**, or **habitat alteration**.

### Why invasive species are often successful

The APES definition does not require you to list traits, but knowing them helps explain why some introduced species become invasive while many do not.

The textbook traits linked to invasive success include:

- broad diet
- high dispersal rate
- high reproductive rate
- short generation time
- high tolerance for varied environmental conditions
- close association with humans
- ability to colonise disturbed areas
- sometimes clonal or vegetative reproduction

These traits overlap strongly with **generalist** and often **r-selected** characteristics. A generalist can use many resources and cope with changing conditions. An r-selected species reproduces quickly, produces many offspring, and can establish a foothold fast.

This matters because many native species most harmed by invasives are more specialised. Specialist native species are adapted to narrow conditions or a limited food source. They may do well in stable environments, but they often struggle when a flexible new competitor arrives.

The same logic explains why islands and isolated habitats are especially vulnerable. Species in those places often evolved with fewer predators or competitors, so a newly introduced generalist can gain an immediate advantage.

The human role also matters. Invasive species often move with global trade, ship ballast water, vehicles, farming, landscaping, pets, and tourism. Human disturbance can create empty or damaged habitats that invasive pioneer species exploit quickly.

### Applying the definition to examples

Examples help, but on the exam they should illustrate the definition, not replace it. Saying “zebra mussels are invasive” is not enough unless you explain why.

**Zebra mussels** are a strong example because they were introduced from Eurasia into the Great Lakes through ballast water. They are nonnative, spread rapidly, outcompete native species, and clog intake pipes and infrastructure. That means they cause both ecological and economic harm.

**Water hyacinth** is another strong example. It is an introduced aquatic plant that forms dense mats on water surfaces. Those mats reduce light penetration, crowd out native aquatic plants, and clog waterways and intake pipes. Again, it is not invasive merely because it came from elsewhere; it is invasive because it causes harm.

**European green crabs** fit the definition because they are nonnative in places such as the San Francisco Bay area, where they compete with native species and consume native shellfish, threatening fisheries.

Notice how each example can be analysed using the same structure:

1. It is outside its natural range
2. It established and spread
3. It caused ecological or economic damage

That structure is much safer than relying on memorised names alone.

### How invasive species connect to Unit 9 global change

Invasive species belong in Unit 9 because global change includes human movement of organisms around the planet and the resulting effect on biodiversity. The topic connects directly to **HIPPCO**, where the “I” stands for invasive species.

It also overlaps with climate change. Warming temperatures can make new areas suitable for species that previously could not survive there. That does not change the definition of invasive species, but it can increase the risk of invasions.

The topic also links to environmental systems change more broadly. Invasive species can alter food webs, change nutrient cycles, and reduce the resilience of ecological communitiess. In some cases, they push already vulnerable native species toward threatened or endangered status.

For APES, this means you should not memorise the definition in isolation. You should understand why the definition matters: invasive species are a human-driven cause of biodiversity decline and ecological balance disruption.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- An invasive species is a **nonnative species that establishes, spreads, and causes harm**
- The harm can be environmental, economic, or related to human health
- **Nonnative** and **invasive** are not the same thing
- Not all introduced species are invasive
- The key distinction is the **harm criterion**
- Invasive species often harm native habitatss by competition, predation, disease spread, or habitat alteration
- They can disrupt food webs and ecological processes
- Common successful traits include rapid reproduction, broad diet, and wide environmental tolerance
- Many invasive species are generalists and often show r-selected features
- Zebra mussels, water hyacinth, and European green crabs are examples that fit the full definition
- Invasive species are part of **HIPPCO** and are a major cause of biodiversity loss
- On the exam, a full definition must include both **nonnative status** and **harm**

## Key Terms and Concepts

- invasive species
- nonnative species
- introduced species
- native species
- biodiversity loss


Invasive Species - Invasive species definition