AP Environmental Science

## Learning Outcomes

- Distinguish between **r-selected** and **K-selected** species using life-history traits
- Explain why many invasive species spread successfully in new habitats
- Describe how invasive species can harm native **K-selected** populations through competition, predation, disease, and habitat alteration
- Understand how invasives can reduce the effective **carrying capacity (K)** for native species
- Link **K-selected** traits to slow recovery and greater risk of local extinction
- Apply these ideas to AP-style examples such as zebra mussels, Asian carp, and island predators

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand invasive impacts on **K-selected** species, with a focus on the following syllabus points:

- Know that **K-selected species** generally produce few offspring, mature later, and invest more parental care
- Know that **r-selected species** generally produce many offspring, mature early, and invest little parental care
- Identify why many invasive species are described as **r-selected generalists** with rapid spread
- Explain how invasive species can reduce native populations through competition, predation, disease, and habitat change
- Understand that invasive species can lower the effective **carrying capacity** for native species by reducing food, habitat, or other resources
- Explain why native **K-selected** species often recover more slowly after invasive-caused decline
- Use precise biodiversity language such as **population decline**, **extirpation**, and **reduced biodiversity**
- Apply these ideas to real examples such as **zebra mussels**, **Asian carp**, and invasive predators on islands

## 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 traits make a species **K-selected** rather than **r-selected**?
2. Why are many invasive species able to spread quickly after introduction?
3. How can an invasive species lower the carrying capacity for a native species?
4. Why are **K-selected** native species often less able to recover after population losses?
5. Name one invasive species and describe one specific way it harms native organisms.

## Introduction

In AP Environmental Science, students often learn **r-selected** and **K-selected** species as a comparison table. That is useful, but exam questions often go one step further. They ask you to explain what happens when these life-history strategies meet real environmental problems, especially **invasive species**.

This matters because invasive species are a major driver of biodiversity loss. Many invaders succeed because they reproduce quickly, spread widely, and tolerate varied conditions. Native **K-selected** species often struggle against that pressure because their populations grow slowly and recover slowly. So the key issue is not just what K-selected species are, but why those traits can become a weakness when an invasive species enters the system.

> **Key Term: K-selected species**
> Species that tend to live near carrying capacity, mature later, produce fewer offspring, and invest more energy in each offspring.

### r-selected and K-selected species: what the exam really wants you to compare

The basic contrast starts with population strategy. **r-selected species** are adapted for rapid population increase. They usually mature early, produce many offspring, and provide little parental care. Their populations often show boom-and-bust patterns, especially when conditions are disturbed or new habitat becomes available.

**K-selected species** are different. They are more typical of stable environments where competition for resources is high. They usually mature slowly, produce fewer young, and invest heavily in survival of each offspring. Their populations tend to stay closer to **carrying capacity** and often match Type I or Type II survivorship patterns.

> **Key Term: r-selected species**
> Species that tend to mature early, reproduce rapidly, produce many offspring, and invest little parental care in each one.
>
> **Key Term: carrying capacity (K)**
> The maximum population size an environment can support over time with available resources.

The important exam point is that these are not labels for “good” or “bad” species. They describe different solutions to survival.

Why does this matter for invasive species? Because many successful invasives have traits more like **r-selected** organisms. If a nonnative species arrives in a new area, rapid reproduction gives it a major advantage. Even if many offspring die, enough may survive to establish a breeding population. Early maturity means the species does not need many years before expanding. A broad diet and wide tolerance range make this even easier.

By contrast, **K-selected** natives are often less flexible. If their numbers fall, they cannot replace lost individuals quickly. A population of insects may rebound in one season. A population of sharks, elephants, or long-lived birds cannot.

Students often miss one subtle point: **K-selected does not mean safe**. A species living near carrying capacity in a stable habitat may seem well established, but that does not protect it from sudden disturbance. In fact, if the disturbance is severe and the species reproduces slowly, it may be less able to recover than an r-selected species.

> **Test Tip:** If an FRQ asks why an invasive species spreads quickly, do not stop at “it has no predators.” Add life-history traits such as early maturity, many offspring, and broad tolerance.

### Why invasive species often hit K-selected natives hardest

An **invasive species** is not just any nonnative species. For APES purposes, it is a nonnative species that spreads and causes harm to the environments, economies, or human health.

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

Many invasive species harm natives through four main mechanisms: **competition**, **predation**, **disease**, and **habitat alteration**.

Start with competition. If an invasive species uses the same food, space, or nesting area as a native species, the native population may decline. This is especially serious for a **K-selected** species because each loss matters more. A species that produces few offspring each year cannot afford repeated failures in feeding or reproduction.

Now consider predation. If an invasive predator eats eggs, juveniles, or adults of a native K-selected species, recovery can be extremely slow. This is particularly severe on islands, where native birds or reptiles may not have evolved defenses against mammalian predators such as rats or cats. If a native bird lays only a few eggs per year and invasive predators remove most of them, the population can crash faster than it can replace itself.

Disease is another pathway. Invasive organisms can bring pathogens or parasites into a system. A slowly reproducing native population may then face extra mortality on top of its already low reproductive rate. Even moderate added mortality can push a K-selected species into long-term decline.

Habitat alteration can be just as damaging. Some invasives physically change the environment so native species can no longer use it effectively. This does not require direct predation at all. A plant invader may crowd out native vegetation. An aquatic invader may block sunlight, change nutrient flow, or alter the food web. The native K-selected species may then lose breeding habitat, prey availability, or shelter.

The central APES inference is this: invasive species often increase **density-dependent limiting factors** for native species. They intensify competition, predation, and disease pressure. For **K-selected** natives, that added pressure is hard to absorb.

A useful cause-and-effect chain is:

invasive species introduced -> native resources or habitat reduced -> native survival and reproduction decline -> population recovery slows -> biodiversity decreases -> local extinction risk rises

> **Key Term: extirpation**
> The loss of a species from one local area, even if the species still exists elsewhere.

### Invasives can reduce carrying capacity for native K-selected species

Students are often taught that carrying capacity is fixed, but in real environments it can change. An invasive species can reduce the effective **carrying capacity** for a native species by lowering the quality or amount of resources available.

Suppose a lake once supported a stable native fish population. If an invasive species consumes much of the plankton base, changes water clarity, or disrupts spawning habitat, the lake may now support fewer native fish than before. In other words, the carrying capacity for the native population has fallen.

This is where the link to **logistic growth** becomes useful. In logistic growth, population increase slows as a population approaches carrying capacity:

$$
\frac{dN}{dt} = rN \left(1 - \frac{N}{K}\right)
$$

You do not always need the formula on the exam, but the logic matters. If **K** gets smaller because of an invasive species, then even a previously stable native population may now be above the new carrying capacity. That can produce decline, dieback, or long-term instability.

> **Key Term: logistic growth**
> Population growth that slows as a population approaches carrying capacity, producing an S-shaped pattern.

A simple numerical example helps.

Imagine a native turtle population in a wetland was stable at 500 individuals because the habitat could support about 520. Then an invasive predator begins consuming hatchlings, and an invasive plant reduces nesting habitat. The effective carrying capacity for the turtle population may fall from 520 to 350. A population that was once stable is now too large for the altered habitat and will likely decline.

This matters more for **K-selected** species than for **r-selected** species because K-selected populations recover slowly after overshoot or crash. A turtle population cannot quickly replace losses if females mature late and produce relatively few surviving offspring.

> **Test Tip:** In graph questions, link invasive spread to rapid increase in the invader, but link the native K-selected species to slower recovery and a lower effective carrying capacity.

### Applied examples you can use in an AP answer

**Zebra mussels** are a classic example. They were introduced into the Great Lakes through ballast water from ships. They reproduce rapidly and spread quickly. Zebra mussels filter large quantities of plankton from the water, which reduces food available to native species. They also attach to hard surfaces, including native mussels, harming or immobilizing them. This can reduce food availability and habitat quality for native organisms, lowering carrying capacity for some native populations.

**Asian carp** are another strong example. They compete with native fish for plankton and habitat in freshwater systems. Because they reproduce quickly and can dominate food resources, they may reduce the population size native fish communities can sustain. A native fish species with slower reproduction will have more trouble recovering from this competitive pressure.

**Water hyacinth** shows habitat alteration clearly. It forms dense mats on the water surface. These mats block sunlight from submerged plants, reduce oxygen conditions indirectly, and clog waterways. The effect is not just “more plants.” It is a change in habitat structure that can disadvantage native species that depend on open water or submerged vegetation.

A more targeted AP-style example involves **K-selected native birds on islands**. If invasive rats are introduced, they may eat eggs and chicks. A native bird that lays only a few eggs each season and matures slowly cannot replace those losses quickly. That makes **local extinction** much more likely than for a fast-reproducing species.

You should be careful not to overclaim. Public environmental agencies often describe invasive harms broadly, but they do not always label every harmed native as explicitly K-selected. On the AP exam, that connection is usually your ecological reasoning: slow reproduction + late maturity + few offspring = slow rebound after invasive-caused losses.

### What AP exam answers should sound like

Strong APES answers use **mechanism**, not vague statements. Compare these:

Weak: “Invasive species reduce biodiversity.”

Better: “Invasive zebra mussels reduce biodiversity by filtering plankton needed by native species and by attaching to native mussels, which lowers survival and reduces carrying capacity for native aquatic organisms.”

Weak: “K-selected species are more vulnerable.”

Better: “K-selected species are more vulnerable because they mature later and produce fewer offspring, so after invasive-caused declines their populations recover much more slowly.”

Notice what improved: the answer explains **how** and **why**.

If you get an FRQ asking why invasives can drive K-selected natives toward extirpation, build a short chain:

1. invasive species increases competition, predation, disease, or habitat change
2. native survival or reproduction falls
3. carrying capacity for the native species decreases
4. K-selected traits slow population recovery
5. local extinction becomes more likely

That is the level of reasoning AP readers reward.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- **K-selected species** tend to mature slowly, produce few offspring, and invest high parental care
- **r-selected species** tend to mature early, reproduce rapidly, and produce many offspring
- Many invasive species succeed because they often show **r-selected** and **generalist** traits
- Invasive species can harm natives through **competition**, **predation**, **disease**, and **habitat alteration**
- Native **K-selected** species are often more vulnerable because they recover slowly after losses
- Invasives can increase **density-dependent limiting factors** such as competition and predation
- Invasive species can lower the effective **carrying capacity** for native species by reducing resources or habitat quality
- A lower carrying capacity can push a once-stable native population into decline
- **Zebra mussels** disrupt aquatic food webs by filtering plankton and attaching to native species
- **Asian carp** compete with native fish for food and habitat
- On islands, invasive predators can drive slowly reproducing native birds toward **extirpation**
- Strong AP answers explain the **mechanism** of harm, not just the outcome

## Key Terms and Concepts

- K-selected species
- r-selected species
- carrying capacity (K)
- invasive species
- extirpation
- logistic growth


K-Selected r-Selected Species - Invasive impacts on K-selected