Inside This Unit: The Full Breakdown
The Living World: Biodiversity covers the diversity of life on Earth, including species diversity, genetic diversity, and ecosystem diversity. This unit examines how biodiversity is measured, why it matters, and what threatens it.
Why it matters
Biodiversity is central to environmental science and appears throughout the AP exam. Understanding island biogeography, ecological succession, and threats to biodiversity provides the foundation for evaluating conservation strategies and environmental policies.
Key concepts
- Biodiversity includes species diversity (number and relative abundance of species), genetic diversity (variation within species), and ecosystem diversity (variety of habitats).
- Island biogeography theory: species richness on an island is determined by the balance between immigration (increases with proximity to mainland) and extinction (decreases with island size).
- Ecological succession is the process of community change over time. Primary succession begins on bare substrate; secondary succession follows a disturbance.
- Keystone species have disproportionate effects on ecosystem structure. Indicator species signal environmental health.
Measuring Biodiversity
Biodiversity is measured at multiple levels. Species richness counts the number of different species in an area, while species evenness describes how equally individuals are distributed among species. A community with 10 species each having 10 individuals is more diverse than one with 10 species where one species has 91 individuals. The Shannon diversity index combines richness and evenness into a single value. Genetic diversity — the variety of genes and alleles within a species — is critical for adaptation and resilience. Ecosystem diversity refers to the variety of habitats, communities, and ecological processes in a region. High biodiversity generally indicates a healthy, resilient ecosystem, while low diversity can signal environmental stress or degradation.
Island Biogeography and Succession
The theory of island biogeography, developed by MacArthur and Wilson, explains species richness on islands (and island-like habitats such as habitat fragments). Species richness reaches an equilibrium determined by the balance between immigration of new species (higher on islands closer to the mainland) and extinction of existing species (higher on smaller islands). This theory has important applications for conservation: habitat fragments act like islands, and smaller, more isolated fragments support fewer species. Ecological succession describes how communities change over time. Primary succession occurs on newly exposed surfaces (bare rock, lava flows) where pioneer species like lichens establish first. Secondary succession follows a disturbance (fire, farming) where soil remains intact, allowing faster recovery. Both types progress toward a relatively stable climax community.
Threats to Biodiversity
The main threats to biodiversity are summarized by HIPPCO: Habitat loss (the leading cause of species decline, primarily from agriculture and urbanization), Invasive species (non-native species that outcompete natives, like kudzu or zebra mussels), Population growth (more humans means more resource demand), Pollution (chemicals, plastics, and excess nutrients degrade habitats), Climate change (shifting temperatures and precipitation alter habitats faster than species can adapt), and Overexploitation (overhunting, overfishing, and overharvesting). Species with small populations, limited ranges, or specialized habitat requirements are most vulnerable. Extinction is permanent — once a species is gone, its genetic information and ecological role are lost forever. Conservation efforts focus on protecting habitat, controlling invasive species, and reducing pollution.
AP exam tip
For AP Environmental Science free-response questions about biodiversity loss, always identify the specific threat (habitat loss, invasive species, etc.) and explain the mechanism by which it reduces biodiversity. Generic answers about "pollution" without specifics earn fewer points.
Connections to other units
- Unit 1 (Ecosystems): Biodiversity affects ecosystem stability, productivity, and resilience to disturbance.
- Unit 3 (Populations): Population dynamics of individual species affect overall community biodiversity.
- Unit 5 (Land and Water Use): Land-use decisions directly determine how much habitat is available for biodiversity.
