# Biodiversity

> AP Biology · AP Biology 2020 CED
> Source: https://www.owlsprep.com/study/ap-biology-u8-biodiversity/

This sub-topic explains how we define and measure biodiversity, the link between biodiversity and ecosystem function, and how disturbances shape biodiversity levels. You will learn to compare biodiversity between different ecological communities.

**Prerequisites:** [Community structure and interspecific interactions](https://www.owlsprep.com/study/ap-biology-u8-community-interactions/)

## Learning objectives

- Distinguish between species richness and species evenness as components of biodiversity
- Explain the relationship between biodiversity and ecosystem function
- Describe how disturbance impacts biodiversity per the intermediate disturbance hypothesis
- Relate biodiversity to ecosystem resilience and stability

## Defining and Measuring Biodiversity

Biodiversity describes the variety of life across all levels of biological organization, from genetic variation within populations to ecosystem diversity across entire regions. For AP Biology exam questions, you will most often be asked to analyze species diversity within local ecological communities.

**Species Richness** — The count of how many different species are present in a given community. It does not account for how many individuals of each species exist.

*Example:* A 1-acre prairie with 38 different plant species has higher species richness than a 1-acre prairie with 19 plant species.

**Species Evenness** — The relative abundance of individuals across all species in a community. It measures how evenly distributed individuals are between different species.

*Example:* Two communities with the same number of species can have very different levels of evenness.

**Worked example:** Compare the overall biodiversity of two wildflower meadows. Meadow 1 has 4 species, with 25 individuals of each species. Meadow 2 has 4 species, with 91 individuals of Species A, and 3 individuals each of Species B, C, and D. Which meadow has higher biodiversity?

1. Step 1: Compare species richness. Both meadows have 4 species, so richness is equal.
2. Step 2: Compare species evenness. Meadow 1 has an equal number of individuals across all four species, so it has maximum evenness. Meadow 2 is extremely uneven, with one species making up over 90% of the total population.
3. Step 3: Evaluate overall biodiversity. Since richness is equal but evenness is much higher in Meadow 1, Meadow 1 has higher overall biodiversity.

> **tip**
>
> AP exam questions almost always test the distinction between richness and evenness. Never assume higher richness equals higher biodiversity without checking evenness.

## Biodiversity and Ecosystem Function

Biodiversity is strongly linked to key ecosystem services and functions, including primary productivity, nutrient cycling, resistance to invasive species, and resilience after disturbance. Higher biodiversity allows more effective niche partitioning, increasing the overall efficiency of resource use by the entire community.

**Ecological Resilience** — The ability of an ecosystem to recover from disturbance and return to its original, stable state after disruption.

*Example:* A diverse coral reef can regrow after a mild hurricane faster than a low-diversity reef.

Keystone species have a disproportionately large impact on overall biodiversity relative to their population size or biomass. Removing a keystone species almost always leads to a dramatic drop in community-wide biodiversity and often total community collapse.

**Worked example:** In a coastal kelp forest ecosystem, sea otters control populations of sea urchins, which graze on kelp. Kelp provides habitat and food for dozens of fish, invertebrate, and marine mammal species. When sea otters were hunted to near extinction, urchin populations exploded, kelp forests were completely destroyed, and 80% of all other species in the ecosystem were lost. Identify the keystone species and explain why biodiversity dropped.

1. Step 1: Identify the species with a disproportionate impact on ecosystem structure. Sea otters control the entire community through their top-down regulation of urchin populations, even though they make up a small fraction of the total biomass.
2. Step 2: Confirm the definition of a keystone species applies. Removing otters caused an 80% drop in overall biodiversity, which matches the definition of a keystone species.
3. Step 3: Explain the mechanism of biodiversity loss. Otter removal released urchins from predation, leading to overgrazing of kelp, the foundation species that provides habitat for most other species in the ecosystem. Without kelp, most dependent species could not survive.

> **exam_tip**
>
> When explaining why higher biodiversity increases ecosystem function, always mention functional redundancy: multiple species filling similar roles means if one is lost, another can compensate.

## Disturbance and Biodiversity

Ecological disturbances are any events that remove organisms or alter resource availability in an ecosystem. Disturbances can be natural (wildfires, hurricanes, droughts) or human-caused (deforestation, pollution, urban development). The intermediate disturbance hypothesis describes how disturbance intensity and frequency impacts overall biodiversity.

**Intermediate Disturbance Hypothesis** — Theory predicting that maximum species diversity occurs when disturbance is moderate: neither too frequent/intense nor too rare/weak.

At very high disturbance levels, only the fastest-growing, fastest-colonizing species can survive, so diversity is low. At very low disturbance levels, competitively dominant species exclude weaker competitors, so diversity is also low. Moderate disturbance removes enough dominant species to allow weaker competitors to persist, leading to maximum diversity.

**Worked example:** A land manager wants to maximize native plant biodiversity in a regional forest. Should they implement a policy of no wildfires, annual low-intensity wildfires, or moderate-intensity wildfires every 10-20 years? Explain your answer.

1. Step 1: Apply the intermediate disturbance hypothesis, which predicts maximum biodiversity at moderate disturbance.
2. Step 2: Evaluate the two extreme options. A policy of no wildfires (very low disturbance) allows competitively dominant tree species to outcompete understory wildflowers and shrubs, reducing overall diversity. Annual wildfires (very high disturbance) only allow fast-colonizing invasive species to survive, reducing diversity.
3. Step 3: Conclude that moderate, occasional wildfires are the best option. They remove enough dominant species to allow less competitive native species to persist, but do not eliminate most species, leading to maximum overall biodiversity.

## Common pitfalls

- **Wrong:** Assuming higher species richness always equals higher biodiversity
  - Why it fails: Biodiversity depends on both richness and evenness. A very uneven community with high richness can have lower overall biodiversity than a less rich but more even community.
  - Correct: Always consider both richness (number of species) and evenness (relative abundance) when comparing biodiversity between two communities.
- **Wrong:** Confusing ecological resilience with disturbance resistance
  - Why it fails: The two terms have distinct meanings that are often swapped on multiple choice questions.
  - Correct: Remember: Resistance = ability to stay unchanged during disturbance; Resilience = ability to recover after disturbance.
- **Wrong:** Assuming all disturbances reduce biodiversity
  - Why it fails: Many students assume all human and natural disturbance is harmful, but moderate natural disturbance often increases biodiversity.
  - Correct: Apply the intermediate disturbance hypothesis: only extreme (very high or very low) disturbance reduces biodiversity.
- **Wrong:** Thinking keystone species are always top predators
  - Why it fails: While many keystone species are top predators, any organism can be keystone if it has a disproportionate impact on biodiversity.
  - Correct: Identify keystone species by their impact on biodiversity, not by their trophic role. Keystone species can be pollinators, ecosystem engineers like beavers, or even plants.

## Cheatsheet

| Concept | Definition | Key AP Exam Point |
| --- | --- | --- |
| Species Richness | Number of different species in a community | Only counts species, not abundance |
| Species Evenness | Relative abundance of individuals across species | Required to calculate overall biodiversity |
| Ecological Resilience | Ability to recover after disturbance | Higher biodiversity increases resilience |
| Keystone Species | Species with disproportionate impact on biodiversity | Loss causes massive biodiversity decline |
| Intermediate Disturbance Hypothesis | Max diversity at moderate disturbance | Low/high disturbance both reduce diversity |

## What's next

Understanding biodiversity is the foundation for studying conservation biology and human impacts on ecosystems, the remaining topics in AP Biology Unit 8. Biodiversity loss driven by human activity is one of the most frequently tested long free-response question topics on the AP Biology exam, and builds directly on the core concepts you learned here about the link between biodiversity and ecosystem function. You will apply your knowledge of richness and evenness to analyze how habitat fragmentation, invasive species, and climate change reduce biodiversity, and evaluate strategies for conserving threatened ecosystems. This topic also connects to earlier units on evolution, where you learned that genetic diversity (a core component of biodiversity) drives the ability of populations to adapt to changing environmental conditions.

- [Population Genetics and Genetic Diversity](https://www.owlsprep.com/study/ap-biology-u7-population-genetics/)
- [Disruptions to Ecosystems](https://www.owlsprep.com/study/ap-biology-u8-disruptions-to-ecosystems/)

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