The Living World: Biodiversity

What this unit covers

The topics below follow the published Env. Science course framework for Unit 2. This unit is worth 6–8% of the exam, so budget your time against that rather than against how long the unit takes to teach.

Lessons in this unit

• Biodiversity & Ecosystem Services13 min · 3 objectivesDistinguish the three levels of biodiversity — genetic, species, and habitat · Classify the four categories of ecosystem services with real examples · Calculate and interpret a Simpson’s Diversity Index for a community
• Island Biogeography12 min · 3 objectivesExplain how island size and distance from the mainland shape species richness · Apply the theory of island biogeography to habitat fragments on land · Predict extinction and immigration rates for islands of differing size and isolation
• Ecological Tolerance & Disturbance12 min · 3 objectivesInterpret a tolerance curve and identify the optimal and stress ranges · Distinguish a generalist from a specialist and predict which tolerates change better · Compare periodic, episodic, and random natural disruptions to ecosystems
• Adaptations & Ecological Succession13 min · 3 objectivesExplain how natural selection produces adaptations over generations · Distinguish primary from secondary succession by their starting conditions · Describe the role of pioneer species and predict changes in diversity through succession

Formulas in Unit 2

Every term in Unit 2

All 16 terms we publish for The Living World: Biodiversity, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

Levels of biodiversity

Genetic diversity within a species, species diversity within an ecosystem, and ecosystem diversity across a region.

Why genetic diversity matters

A genetically uniform population has no variants able to survive a new disease or climate, so it can be wiped out by a single pressure.

Ecosystem services from biodiversity

More diverse systems resist disturbance and recover faster, because functional redundancy means losing one species need not end a process.

Island biogeography

Species richness rises with island size and falls with distance from the mainland — the model behind minimum reserve size and wildlife corridors.

Ecological tolerance

The range of conditions a species can survive. Narrow tolerance makes a species a good indicator of environmental change.

Indicator species

A species whose presence or health signals ecosystem conditions — amphibians for water quality, lichens for air quality.

Keystone species

A species whose effect on community structure far exceeds its abundance. Removing sea otters lets urchins destroy kelp forests.

Ecological succession

Primary succession begins on bare rock with no soil and starts with lichens; secondary follows a disturbance that left soil intact and is much faster.

Pioneer species

First colonizers of a disturbed area — lichens and mosses in primary succession — which build soil for later species.

Climax community

The relatively stable end stage of succession, though modern ecology treats it as dynamic rather than fixed.

Adaptations to disturbance

Some species require it: serotinous pine cones open only in fire, and some grasslands need periodic burning to exclude woody species.

Ecosystem resilience vs resistance

Resistance is the ability to withstand disturbance without changing; resilience is the ability to recover after changing.

Simpson's diversity index

D = 1 − Σ(n/N)². Higher values mean greater diversity, accounting for both richness and evenness.

Habitat fragmentation

Splitting habitat into patches increases edge effects, isolates populations and reduces gene flow — often more damaging than the total area lost.

Wildlife corridors

Strips of habitat connecting fragments, allowing migration and gene flow and reducing inbreeding in small isolated populations.

Edge effects

Conditions at a habitat boundary — more light, wind and predators — differ from the interior, so a fragmented habitat has less usable core area.

What examiners penalize here

• When a Simpson’s Index problem appears, the formula is provided on the AP equation sheet — your job is to plug in correctly. The classic error is forgetting the "−1": use n(n − 1) and N(N − 1), not n² and N². Always carry your work; the calculation earns points even if the final decimal is slightly off.
• AP questions often show a graph with immigration falling and extinction rising as species number increases; the equilibrium is where the two curves cross. Moving the curves — a nearer island lifts the immigration curve, a larger island lowers the extinction curve — shifts that crossing point to more species.
• Expect a scenario asking which species is "most vulnerable" to an environmental change. The answer is almost always the **specialist** with the narrow tolerance range or the single food source. Generalists are the survivors; specialists are the ones that end up on the endangered list.
• Know the three adaptation types by their keywords: **structural** = a body part, **physiological** = an internal process, **behavioral** = an action. And remember succession’s trend: diversity and biomass generally *increase* from pioneer stage toward the climax community.

Practice Env. Science

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

Questions about this unit

All 9 units of AP Environmental Science

1. Unit 1 · The Living World: Ecosystems
2. Unit 2 · The Living World: Biodiversity
3. Unit 3 · Populations
4. Unit 4 · Earth Systems & Resources
5. Unit 5 · Land & Water Use
6. Unit 6 · Energy Resources & Consumption
7. Unit 7 · Atmospheric Pollution
8. Unit 8 · Aquatic & Terrestrial Pollution
9. Unit 9 · Global Change