How biodiversity is measured and why it matters. Ecosystem services, island biogeography, ecological niches, and the interspecific relationships — competition, predation, symbiosis — that structure every community. You'll also meet keystone species and the natural and human disruptions that threaten biodiversity.
Seven topics from the College Board CED, in order.
Topic 2.1
Introduction to Biodiversity
Genetic, species, and ecosystem diversity — the three levels of biodiversity and why they matter for ecosystem resilience.
Topic 2.2
Ecosystem Services
The benefits humans receive from healthy ecosystems: regulating, supporting, provisioning, and cultural services.
Topic 2.3
Island Biogeography
How island size and distance from the mainland determine species richness, colonization, and extinction rates.
Topic 2.4
Ecological Tolerance
The range of conditions a species can survive in, and the zone of physiological stress at the edges of that range.
Topic 2.5
Natural Disruptions to Ecosystems
Fires, floods, volcanic eruptions, and other natural disturbances that reset or reshape ecological communities.
Topic 2.6
Adaptations
How natural selection produces traits suited to an organism's environment, increasing survival and reproduction.
Topic 2.7
Ecological Succession
How communities change over time — primary vs. secondary succession, pioneer species, and the path toward a climax community.
About Unit 2
Unit 2 zooms in from the energy and nutrient flows of Unit 1 to focus on biodiversity itself — what it is, how it's measured, and why it's valuable. You'll learn how island biogeography predicts species richness from just two variables (island size and isolation), how every species occupies a specific niche, and how interactions like predation, competition, and symbiosis hold communities together.
This unit is roughly 6–8% of the AP Environmental Science exam and takes about 11–12 class periods. The concepts here — especially ecosystem services, ecological tolerance, and succession — reappear constantly in later units on land use, pollution, and global change, since biodiversity loss is one of the central themes of the course.
The College Board ties Unit 2 to a single Big Idea:
Big Idea: ERT
Interactions Between Earth Systems — biodiversity, ecological tolerance, disturbance, and succession