What it covers: Biodiversity — what it is, why it's valuable, and how island biogeography, niches, and species interactions shape it.
Exam weight: About 6–8% of the AP Environmental Science exam.
The big question: What factors determine how much biodiversity an ecosystem has, and how do species interact to maintain it?
Big Idea covered: Interactions Between Earth Systems (ERT).
Key topics at a glance
Levels of Biodiversity
Genetic, species, and ecosystem diversity. Higher genetic diversity helps populations adapt; higher species diversity increases ecosystem resilience.
Ecosystem Services
Four types: provisioning (food, water), regulating (climate, flood control), supporting (pollination, nutrient cycling), and cultural (recreation, spiritual value).
Island Biogeography
Species richness increases with island size (more habitat) and decreases with distance from the mainland (lower colonization rate).
Ecological Niches
A species' role and resource use. Competitive exclusion prevents two species from sharing an identical niche; resource partitioning lets similar species coexist.
Species Interactions
Competition (both lose), predation (one eats the other), and symbiosis: mutualism (+/+), commensalism (+/0), parasitism (+/−).
Keystone Species
Disproportionate impact relative to abundance. Example: sea otters control sea urchins, protecting kelp forests.
Natural Disturbances
Fires, floods, and volcanic eruptions reset succession and can increase long-term biodiversity by creating new habitat niches.
Adaptations
Traits shaped by natural selection that improve survival and reproduction in a specific environment — the mechanism behind ecological tolerance and niche specialization.
The key terms you must know
Biodiversity — variety of life at the genetic, species, and ecosystem levels.
Ecosystem services — benefits ecosystems provide to humans: provisioning, regulating, supporting, cultural.
Island biogeography — species richness predicted by island size and distance from the mainland.
Ecological niche — a species' specific role, habitat, and resource use within its environment.
Competitive exclusion — two species can't indefinitely share the exact same limiting resource.
Resource partitioning — competing species divide a shared resource to reduce competition.
Keystone species — disproportionately large effect on community structure relative to abundance.
Mutualism / commensalism / parasitism — the three types of symbiotic species interactions.
Indicator species — a species whose condition reflects overall ecosystem health.
Ecological tolerance — the range of conditions a species can survive within.
Key themes to remember
Biodiversity has measurable, predictable patterns. Island biogeography reduces a complex ecological outcome to two simple variables — size and isolation.
Every species occupies a niche, and niches structure communities. Competition, predation, and symbiosis are the mechanisms by which niches get carved out and defended.
Small species can have outsized effects. Keystone species and indicator species show that abundance alone doesn't predict ecological importance.
Disturbance isn't always destruction. Natural disturbances can increase long-term biodiversity by creating new niches and resetting succession.
Biodiversity loss threatens ecosystem services. This theme connects Unit 2 directly to the human-impact units later in the course.
Common exam traps
Don't confuse species richness with biodiversity. Richness is just the number of species; biodiversity also accounts for genetic and ecosystem-level variety.
Island biogeography has TWO variables, not one. Size AND distance both matter — a question testing only one is testing half the model.
Mutualism, commensalism, and parasitism are easy to mix up. Use the +/+, +/0, +/− shorthand to keep them straight.
A keystone species is not necessarily the most abundant species. The definition is about disproportionate impact, not population size.
Fundamental niche ≠ realized niche. The realized niche is always equal to or smaller than the fundamental niche once competition is factored in.
Natural disturbances aren't automatically bad for biodiversity. Many ecosystems (like fire-adapted forests) depend on periodic disturbance to maintain diversity.