AP Biology

## Learning Outcomes

- Define biodiversity and distinguish species diversity, genetic diversity, and ecosystem diversity.
- Explain how species richness and species evenness affect measurements of biodiversity.
- Use Simpson’s Diversity Index to compare biodiversity between communities.
- Describe why high biodiversity can help ecosystems withstand environmental disruptions.
- Explain how keystone species, invasive species, habitat destruction, and climate events can alter biodiversity.
- Connect changes in biodiversity to natural selection and evolution in populations.

## AP Biology Syllabus

In AP Biology Unit 8, biodiversity is tested as part of ecology, community interactions, and ecosystem stability. You should be ready to interpret data, calculate or compare diversity values, and explain how changes in one part of an ecosystem can affect populations, communities, and evolutionary outcomes.

- Biodiversity exists at multiple levels: genetic, species, and ecosystem diversity.
- Species diversity depends on both the number of species and the relative abundance of each species.
- Simpson’s Diversity Index is used to estimate and compare biodiversity.
- More diverse ecosystems are often better able to withstand environmental change.
- Keystone species can have effects that are much larger than their abundance suggests.
- Invasive species can reduce native biodiversity by outcompeting, consuming, or poisoning native species.
- Human activities, geological events, and severe weather can disrupt ecosystems and alter biodiversity.
- Environmental change can change which adaptations are favored by natural selection.

## 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 is the difference between species richness and species evenness?
2. Why can a population with low genetic diversity be more vulnerable to environmental change?
3. How does Simpson’s Diversity Index use both the number of species and their relative abundance?
4. Why did reintroducing wolves to Yellowstone affect plant biodiversity?
5. How can an invasive species reduce biodiversity in a community?

## Introduction

Biodiversity refers to the variety of life in a biological system. In AP Biology, you should think about biodiversity at three main levels: the variety of species in a community, the variety of alleles and traits within populations, and the variety of ecosystems in a region. These levels are linked. For example, if a wetland is drained, the ecosystem diversity of a region decreases, species that depend on wetland habitat may decline, and the remaining populations may lose genetic variation.

Biodiversity is not just a list of organisms. A forest containing ten tree species in similar numbers is usually more diverse than a forest where one tree species makes up nearly all individuals and the other nine species are rare. Because AP Biology often asks you to connect data to biological claims, you need to describe biodiversity using clear evidence: species counts, relative abundances, graphs, or diversity index calculations.

> **Key Term: biodiversity**
> The variety of life in a biological system, including variation at the genetic, species, and ecosystem levels.

### Three Levels of Biodiversity

### Species Diversity, Genetic Diversity, and Ecosystem Diversity

The three main levels of biodiversity are species diversity, genetic diversity, and ecosystem diversity. They are often tested together because a change at one level can affect the others.

> **Key Term: species diversity**
> The variety of species in a community, including both the number of species present and their relative abundances.

Species diversity includes two related ideas: how many species are present and how evenly individuals are distributed among those species. A coral reef with many fish, coral, algae, mollusk, and crustacean species has high species diversity. A crop field planted with only one corn variety has much lower species diversity, even if it contains many individual corn plants.

Species diversity affects ecological interactions. More species can mean more feeding relationships, more pollinators, more decomposers, and more possible routes for energy and matter to move through a food web. However, AP Biology questions usually avoid simple claims such as “more species is always better.” Instead, you may be asked to explain how biodiversity affects resistance to disturbance, population stability, or community structure.

> **Key Term: genetic diversity**
> The variety of alleles and genetic traits within a population or species.

Genetic diversity is variation within a population’s gene pool. A population with many different alleles may contain individuals with different tolerances to temperature, disease, drought, toxins, or food availability. If an environment changes, some individuals may already have traits that improve survival and reproduction under the new conditions. Those individuals can pass their alleles to offspring, allowing the population to evolve.

Low genetic diversity can increase risk. If most individuals are genetically similar, a single disease, environmental stress, or predator pressure may affect nearly all of them in the same way. Small populations can lose genetic diversity through genetic drift, founder effects, bottlenecks, and inbreeding. Although those mechanisms are often taught in the evolution unit, they connect directly to ecology because population size and habitat disruption influence genetic variation.

> **Key Term: ecosystem diversity**
> The variety of ecosystems, habitats, communities, and ecological processes in a region.

Ecosystem diversity refers to the range of habitats and ecological systems in an area. A region containing forests, grasslands, wetlands, rivers, and coastal habitats has greater ecosystem diversity than a region dominated by one habitat type. High ecosystem diversity can support many species because different organisms require different abiotic and biotic conditions. For example, amphibians may need ponds or wetlands for reproduction, while certain birds may depend on mature forest for nesting.

Ecosystem diversity also affects recovery from disturbance. If one habitat is damaged, species may persist in nearby habitats or recolonize from less affected areas. Habitat fragmentation, urban development, pollution, and conversion of natural habitats to agriculture often reduce ecosystem diversity.

> **Test Tip:**
> If an AP question asks about “biodiversity,” do not automatically discuss only the number of species. Check whether the question is asking about species diversity, genetic diversity, ecosystem diversity, or a combination.

### Measuring Species Diversity with Richness, Evenness, and Simpson's Index

### Species Richness and Species Evenness

Species diversity has two main components: richness and evenness.

> **Key Term: species richness**
> The number of different species present in a community.

A pond with 12 species has greater species richness than a pond with 5 species. Richness is simple to count, but it does not show whether one species dominates the community.

> **Key Term: species evenness**
> How similar the abundances of different species are in a community.

Imagine two communities, each with four species and 100 total organisms:

- Community A: 25, 25, 25, 25 individuals
- Community B: 97, 1, 1, 1 individuals

Both communities have the same species richness because each has four species. However, Community A has much higher evenness. In Community B, one species dominates, so the community is less diverse overall.

This distinction matters on exams. If a graph shows that the number of species stayed the same but one species became much more abundant, biodiversity may have decreased because evenness decreased.

### Simpson’s Diversity Index

AP Biology commonly uses Simpson’s Diversity Index to measure biodiversity. The equation is:

\text{Simpson's Diversity Index} = 1 - \sum \left(\frac{n}{N}\right)^2
where:

- $$n$$ = the number of individuals of one particular species
- $$N$$ = the total number of individuals of all species in the community
- $$\sum$$ = add the values for all species

> **Key Term: Simpson's Diversity Index**
> A biodiversity measure that uses the relative abundance of each species in a community; higher values indicate greater diversity.

The index increases when species richness is higher and when individuals are more evenly distributed among species. Values closer to 1 usually indicate higher diversity. Values closer to 0 indicate lower diversity, often because one species dominates.

Consider two simplified forest samples.

Forest A has 100 organisms:

- Species 1: 25
- Species 2: 25
- Species 3: 25
- Species 4: 25

For each species, $$n/N = 25/100 = 0.25$$. Then:

$$
1 - [(0.25)^2 + (0.25)^2 + (0.25)^2 + (0.25)^2]
1 - [0.0625 + 0.0625 + 0.0625 + 0.0625] = 1 - 0.25 = 0.75
$$

Forest B also has 100 organisms:

- Species 1: 70
- Species 2: 10
- Species 3: 10
- Species 4: 10

The calculation is:

$$
1 - [(0.70)^2 + (0.10)^2 + (0.10)^2 + (0.10)^2]
1 - [0.49 + 0.01 + 0.01 + 0.01] = 1 - 0.52 = 0.48
$$

Forest A has the higher Simpson’s Diversity Index because individuals are evenly distributed among the four species. Forest B has the same richness but lower evenness, so its diversity value is lower.

> **Exam Warning:**
> Do not rank biodiversity by total organism number alone. A community with 1,000 organisms from one species can have lower species diversity than a community with 100 organisms spread evenly across many species.

For AP Biology free-response questions, show enough work to prove that you used the formula correctly. You may receive credit for setting up the calculation, finding the index, and explaining what the value means in biological terms. A final number without interpretation is often not enough.

> **Test Tip:**
> When comparing two diversity index values, write a sentence such as: “The index increased from 0.62 to 0.78, so biodiversity increased because the species were more evenly represented and/or more species were present.”

### Why Diversity Changes Ecosystem Stability

### Biodiversity and Resistance to Disturbance

Greater biodiversity can help ecosystems withstand disruptions. A disturbance is any event that changes an ecosystem’s structure or function, such as drought, fire, disease, pollution, volcanic eruption, hurricane, or the arrival of an invasive species. Diverse ecosystems often contain species with overlapping ecological roles. If one species declines, another species may partly fill a similar role, reducing the effect on the whole system.

For example, a grassland with many plant species may be more likely to continue primary production during a drought than a grassland dominated by a single plant species. Some plants may be drought tolerant, while others may grow better in wetter years. This variation can stabilize total productivity across changing conditions.

Genetic diversity also supports stability. In a plant population with high genetic variation, some individuals may carry alleles that improve survival during drought or disease outbreaks. If the environment changes, those individuals may reproduce more successfully. In a genetically uniform population, most individuals may be vulnerable to the same stress.

Be careful with wording. High biodiversity does not make an ecosystem immune to change. Severe disturbances can reduce even highly diverse systems. The AP Biology claim is usually that greater biodiversity can increase the ability of ecosystems to resist or recover from environmental change.

### Keystone Species and Trophic Cascades

Some species affect biodiversity far more than their population size would suggest.

> **Key Term: keystone species**
> A species that has a disproportionately large effect on an ecosystem relative to its abundance.

Wolves in Yellowstone National Park are a classic example. When wolves were hunted to very low numbers, deer and elk populations increased. Heavy grazing reduced plant biodiversity, which then affected other animal species that depended on those plants for food or habitat. When wolves were reintroduced, they limited deer and elk populations and changed grazing patterns. Plant species and other animal populations increased, raising biodiversity across parts of the ecosystem.

This is linked to the idea of a trophic cascade. A change at one trophic level can affect many other trophic levels. Another example involves sea otters, sea urchins, and kelp forests. Sea otters eat sea urchins. When sea otter numbers decline, sea urchin populations can increase. More sea urchins consume more kelp, reducing kelp forests and the habitat available for many marine organisms. A change in predator abundance can therefore change primary producer abundance and habitat structure.

In exam answers, name the interaction and trace the cause-and-effect chain. For instance: fewer predators → more herbivores → more plant consumption → less plant biomass and habitat → lower diversity of dependent species.

> **Exam Warning:**
> A keystone species is not necessarily the most abundant species. Its key feature is its large effect on community structure compared with its numbers.

### Disturbance, Human Impact, and Evolutionary Responses

### Invasive Species and Loss of Native Biodiversity

> **Key Term: invasive species**
> A nonnative species that spreads in a new habitat and harms native species, community structure, or ecosystem function.

Invasive species can decrease biodiversity in several ways. They may outcompete native species for food, water, light, nesting sites, or space. They may prey on native species that lack defenses. They may introduce disease or alter nutrient cycling. They may also lack natural predators in the new environment, allowing their populations to grow rapidly.

Cane toads in Australia are a well-known example. They were introduced in the early twentieth century to control cane beetles in sugarcane fields. Instead, cane toads spread widely. They have few natural predators in Australia, compete with native species for resources, and produce toxins that can kill animals that attempt to eat them. Their introduction has contributed to declines in many native species.

On the AP exam, avoid saying only that an invasive species “takes over.” Explain the mechanism. A stronger answer says: “The invasive species lacks predators, so its population increases. It consumes resources used by native species, causing native populations to decline. As the invasive species makes up a larger fraction of the community, species evenness and biodiversity decrease.”

### Habitat Destruction, Pollution, and Disease

Human activities can reduce biodiversity by changing or removing habitats. Building cities, roads, farms, and dams can destroy habitats directly or fragment them into smaller pieces. Smaller habitat patches often support fewer species and smaller populations. Smaller populations are more likely to lose genetic diversity through drift and may be more vulnerable to inbreeding.

Pollution can make habitats less suitable for many species. Chemical runoff can affect water quality, reduce oxygen availability, or introduce toxins. If sensitive species die or fail to reproduce, species richness may decline. If tolerant species increase and dominate, species evenness may decline.

Human movement into previously undisturbed habitats can also change disease dynamics. Humans may introduce new pathogens to wildlife, domestic species may transmit disease to wild populations, or humans may come into contact with diseases that were previously rare in human populations. Disease can reduce population size and genetic diversity, especially if mortality is high.

Climate-related changes and severe weather events can also affect biodiversity. Hurricanes can remove vegetation and alter habitats. Prolonged drought can change plant communities and reduce water availability. Volcanic eruptions can destroy existing communities but may also create new surfaces for ecological succession. AP questions may ask you to predict whether diversity increases or decreases after a disturbance; use the data provided rather than assuming one direction every time.

### Environmental Change and Natural Selection

Disruptions can alter which traits are favored. Mutations create new alleles randomly, but natural selection is not random: individuals with traits that improve survival and reproduction in a specific environment leave more offspring. When the environment changes, the traits that provide an advantage may also change.

For example, during a drought, plants with deeper roots or better water conservation may have higher survival. In an area with increased pollution, individuals with alleles that improve toxin tolerance may reproduce more successfully. In a habitat where a new predator appears, prey with better camouflage, faster escape responses, or defensive behaviors may be favored.

Rapid environmental change can increase the strength of selection. However, evolution requires heritable variation. If a population has very low genetic diversity, it may not contain alleles that help individuals survive the new conditions. In that case, the population may decline or go extinct rather than adapt.

This is a common AP Biology connection: biodiversity supports the potential for future evolution. Genetic diversity provides raw material for natural selection, species diversity can stabilize community interactions, and ecosystem diversity provides many habitats and ecological roles.

> **Test Tip:**
> When explaining evolution after an environmental change, include all three parts: variation already exists or arises by mutation, the environment selects among phenotypes, and individuals with advantageous heritable traits leave more offspring.

## Key Point Checklist

This article has covered the following key knowledge points:

- Biodiversity includes genetic diversity, species diversity, and ecosystem diversity.
- Species diversity depends on both species richness and species evenness.
- Species richness is the number of species present in a community.
- Species evenness describes how similar the population sizes of different species are.
- Simpson’s Diversity Index is calculated using relative abundance values for each species.
- A higher Simpson’s Diversity Index usually indicates greater biodiversity.
- Communities with the same richness can have different diversity values if their evenness differs.
- Greater biodiversity can help ecosystems withstand or recover from environmental disruptions.
- Keystone species have large effects on ecosystems despite not necessarily being abundant.
- Invasive species can reduce native biodiversity by competing, predating, spreading disease, or lacking predators.
- Human impacts such as habitat destruction, pollution, and disease introduction can decrease biodiversity.
- Environmental change can alter selection pressures, causing populations with heritable variation to evolve.

## Key Terms and Concepts

- biodiversity
- species diversity
- genetic diversity
- ecosystem diversity
- species richness
- species evenness
- Simpson's Diversity Index
- keystone species
- invasive species
- trophic cascade
- habitat destruction
- disturbance
- natural selection
- species evenness
- species richness


Biodiversity - Species, genetic, ecosystem diversity