## Learning Outcomes

- Understand why island environmentss often produce **endemic** and **specialist** species
- Explain why island species are especially vulnerable to invasive species
- Identify how invasive species reduce island biodiversity through predation, competition, and habitat damage
- Apply island biogeography to both oceanic islands and fragmented habitat islands
- Use named examples such as **Guam**, **Hawaiʻi**, and the **Galápagos**
- Predict exam answers about which islands have the highest species richness

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand island specialists and invasives, with a focus on the following syllabus points:

- Know that island biogeography explains species richness using **immigration** and **extinction**
- Identify that **large, near islands** usually have the greatest species richness
- Explain why island species often evolve into **specialists** with narrow niches
- Understand that many island species are **endemic**, meaning they are found only in one place
- Explain why invasive species often outcompete or prey on island natives
- Identify common mechanisms of invasive impact, including **predation**, **competition**, **habitat destruction**, and **food-web disruption**
- Apply island biogeography to **habitat islands** formed by fragmentation, not just true islands
- Use specific examples such as **rats**, **feral cats**, **goats**, and the **brown tree snake on Guam**

## 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. Why are many island species specialists rather than generalists?
2. What is the difference between a **native** species and an **endemic** species?
3. Why do invasive species often have such strong effects on islands?
4. Which island would usually have the highest species richness: a large near island or a small far island?
5. Name two ways goats or rats can reduce biodiversity on islands.

## Introduction

Island biogeography is usually taught as a theory about **size** and **distance**: larger islands tend to support more species, and islands closer to the mainland tend to receive more immigrants. That is the basic model. But AP Environmental Science often pushes one step further and asks what kinds of species live on islands and why they are vulnerable.

This matters because islands are often rich in unique life, yet they are also highly sensitive to disturbance. Isolation can produce rare, specialized organisms that survive well under stable local conditions. The same isolation can also leave them poorly prepared for new predators, competitors, or habitat changes caused by invasive species.

> **Key Term: island biogeography**
> The study of species richness and diversification in isolated habitats, based mainly on immigration and extinction.

## Introduction to island specialists

### Why islands produce specialist and endemic species

Island species do not arise in the same way as species on large connected landmasses. Isolation reduces gene flow from outside populations. Once a few colonists arrive, they may spread into open niches with little competition. Over time, natural selection can shape them for very specific local conditions.

> **Key Term: endemic species**
> Species that are native to and found only in one specific geographic area.

A species can be **native** without being endemic. A native species occurs naturally in an area, but it may also live elsewhere. An endemic species is restricted to that one place. Students often mix up these terms, and AP questions may use them carefully.

Islands often favor specialization for three linked reasons:

- resources are limited
- habitats may be unusual or isolated
- competition within a small area can favor narrow niche use
  > **Key Term: specialist species**
  > Species with a narrow ecological niche and specific habitat or resource requirements.

A specialist can be very successful in a stable setting. If food type, nesting site, climate, and predator pressure remain fairly constant, specialization is efficient. A bird with a beak shape fitted to one seed type, or a reptile adapted to one vegetation zone, may outcompete broader but less efficient feeders in that setting.

Darwin’s finches help show the logic. Once ancestral finches reached the Galápagos, isolation and different local conditions on different islands led populations along different paths. Over time, traits such as beak form became linked to particular diets. That is not just “difference for its own sake.” It is a response to different food sources and selective pressures.

But specialization creates risk. A narrow niche means fewer backup options. If the preferred food disappears, if nesting habitat changes, or if a new predator appears, the specialist may not shift quickly enough.

### Why specialist island species are so vulnerable

The weakness of many island species can be shown as a chain:

isolation -> endemism and specialization -> small range and small population -> low genetic diversity and fewer defenses -> high extinction risk

That chain is one of the most useful ways to explain island vulnerability in FRQs.

Many island organisms evolved without the predators or competitors common on continents. Ground-nesting birds on isolated islands may have had little evolutionary experience with mammalian predators. A species that never needed to avoid rats, cats, or snakes may lack effective behaviors such as nest guarding, rapid escape, or predator recognition.

Small population size matters too. If a mainland species loses a local population, immigrants from surrounding areas may recolonize it. On a remote island, recolonization is much less likely. That means one bad event can have a lasting effect.

> **Key Term: generalist species**
> Species with a broad niche that can use a wide range of resources and tolerate varied conditions.

Many invasive species behave more like generalists. That gives them an advantage when conditions change or when they are introduced somewhere new.

> **Test Tip:** If an AP question asks why island natives lose to invaders, do not just say “the invasive species is stronger.” Explain niche breadth: island natives are often **specialists**, while invaders are often **generalists**.

## How invasives disrupt island ecologys

### Why islands are invasion-prone

The equilibrium theory of island biogeography says species richness tends toward a balance between immigration and extinction. Native immigration is often low on remote islands. That means island communities are not constantly refreshed by many new native colonists.

When a harmful nonnative species arrives, the balance changes fast. Native species may go extinct more quickly, but replacement by new native immigrants is slow. So biodiversity can fall sharply.

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

Do not say every nonnative species is invasive. A species becomes invasive when it spreads and causes harm.

The damage from invasives is often greater on islands because:

- food webs are simpler
- populations are smaller
- many species have limited ranges
- many island birds, reptiles, and plants lack defenses against new predators or herbivores
- recovery can be slow because recolonization is limited

### The main ways invasive species reduce biodiversity

There are several recurring mechanisms. Students should know all of them.

First, **predation**. Introduced predators often eat eggs, chicks, reptiles, amphibians, or small mammals that evolved with little defense against them. This is common with rats, cats, and snakes.

Second, **competition**. Invaders may use the same food, nesting sites, or space as native species. If the invasive species reproduces faster or tolerates more conditions, it can displace the native one.

Third, **habitat destruction**. Not all major invaders are predators. Herbivores such as goats can strip vegetation, prevent regrowth, expose soil, and increase erosion. That changes the habitat for many other species.

Fourth, **food-web disruption**. If one native species declines, others that rely on it may also decline. The loss of a prey species, pollinator, or nesting plant can ripple through the system.

> **Key Term: species turnover**
> The replacement of some species by others over time, even when total species number stays near equilibrium.

This matters because equilibrium does not mean “nothing changes.” Species richness may stay similar while the identity of the species changes. On invaded islands, though, the total number of species may also fall.

> **Test Tip:** In FRQs, use a cause-and-effect chain. For example: “Rats prey on eggs and chicks -> seabird reproductive success falls -> bird populations decline -> island biodiversity decreases.”

## Case studies and AP applications

### Classic island invasive examples

The most common AP-relevant examples are **rats**, **feral cats**, **goats**, and the **brown tree snake**.

**Rats** are serious threats because they prey on eggs, chicks, seeds, and small animals. On islands, especially where birds nest on the ground or in burrows, rats can reduce breeding success dramatically.

**Feral cats** are efficient predators of birds, reptiles, and small mammals. Their impact is especially severe where native species did not evolve with mammalian predators.

**Goats** are important because students often focus only on predators. Goats can do major damage through overgrazing. They reduce plant cover, prevent forest regeneration, remove shelter and nesting sites, and increase erosion.

The **brown tree snake** on Guam is one of the clearest examples of invasive predation on islands. It was introduced to Guam and caused devastating declines in native birds. The key point is not memorizing every figure. The key point is understanding why the impact was so severe: Guam’s native birds had limited defenses against a new snake predator.

On Hawaiʻi, rats and cats prey on nests of native birds such as seabirds and other island specialists. On the Galápagos, rats and cats threaten eggs and chicks, while goats damage vegetation needed by native animals and plants.

### Habitat islands and fragmentation

AP Environmental Science does not limit island biogeography to actual islands. A forest fragment surrounded by roads, farms, or urban development can function as a habitat island.

This is where Unit 2 links to later biodiversity topics. Habitat fragmentation creates isolated patches, reduces movement between populations, and increases vulnerability of specialist species.

A forest interior species may survive well in a large continuous habitat but decline in a small fragment because:

- the patch supports fewer individuals
- there is less habitat variety
- edge conditions increase
- invasive species may enter more easily

A fragmented habitat patch can therefore show the same basic logic as a remote island: low immigration, small populations, and elevated extinction risk.

> **Key Term: habitat fragmentation**
> The breaking of a large habitat into smaller, isolated patches.

Specialists are usually hit harder than generalists in fragmented habitats because they need narrower conditions and often larger intact areas.

### Predicting species richness on exams

You should be ready for the standard AP prediction rule:

- **large + near** = highest species richness
- **small + far** = lowest species richness

Why?

Near islands have higher **immigration rates** because they are easier for organisms to reach. Large islands have lower **extinction rates** because they support larger populations and more habitats.

You may also see the species-area idea expressed as:

S = cA^z

Where:

- **S** = number of species
- **A** = area
- **c** and **z** = constants

You do not usually need to calculate with it for this topic, but you should know the meaning: larger area generally supports more species.

One subtle point: an island can have low total species richness but still contain many endemics. Remote islands may have fewer species overall, but a high proportion of those species may be unique.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Island biogeography explains species richness mainly through immigration and extinction
- Large islands near the mainland usually have the greatest species richness
- Island isolation often produces endemic and specialist species
- Endemic means found only in one geographic area, while native does not necessarily mean restricted to one place
- Specialist species have narrow niches and are less adaptable to rapid environmental change than generalists
- Island species are often vulnerable because they have small populations, small ranges, and limited defenses against new predators or competitors
- Invasive species are nonnative species that spread and cause harm
- Invasive species reduce island biodiversity through predation, competition, habitat destruction, and food-web disruption
- Rats and feral cats often prey on eggs, chicks, reptiles, and small mammals on islands
- Goats can reduce biodiversity by overgrazing vegetation and damaging habitat
- The brown tree snake on Guam is a classic case of invasive predation causing native bird declines
- Island biogeography also applies to habitat islands created by fragmentation

## Key Terms and Concepts

- island biogeography
- endemic species
- specialist species
- generalist species
- invasive species
- species turnover
- habitat fragmentation


Island Biogeography - Island specialists and invasives