AP Environmental Science, AP exams, 6–8%

Unit 2: The Living World: Biodiversity

Unit 2 asks why biodiversity matters and what maintains it. Ecosystem services, island biogeography, ecological tolerance, and succession give a framework for predicting how a community responds to disturbance. Adaptation ties the unit back to evolutionary reasoning.

Position: 2 of 9 units
Published weighting: 6–8% in the official framework
Topics covered: 6 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Environmental Science. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Introduction to biodiversity: Genetic, species, and ecosystem diversity, and the link between diversity and resilience.
Ecosystem services: Provisioning, regulating, supporting, and cultural services, with concrete examples of each.
Island biogeography: How island size and distance from the mainland shape species richness, and why habitat fragments behave like islands.
Ecological tolerance: Ranges of tolerance and optimal ranges, and how a change in conditions shifts species distributions.
Natural disruptions to ecosystems: Periodic, episodic, and random disturbances such as fire, storms, and volcanic activity.
Ecological succession: Primary and secondary succession, pioneer and keystone species, and the path toward a climax community.

Study tips

How to practice Biodiversity.

Section-specific guidance that follows from how this material is usually tested.

Practice naming a specific ecosystem service for a named ecosystem; generic answers score poorly on free-response questions.
Contrast primary and secondary succession using one worked example each, including what starts the sequence.
Connect island biogeography to habitat fragmentation now; the exam usually tests it in the applied form.

Where this fits

Unit 2: The Living World: Biodiversity is 2 of 9 units in the AP Environmental Science guide, and it carries a published weighting of 6–8% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Environmental Science guide Official exam information

Sample Biodiversity questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
What type of ecological succession begins on bare rock or a fresh lava flow where no soil is present?
1. APrimary succession
2. BSecondary succession
3. CEutrophication
4. DFragmentation
Show answer
Answer: A. Primary succession
Primary succession starts where there is no soil, so pioneer species such as lichens and mosses must first break down rock and build soil, which makes the process very slow. Secondary succession is the tempting distractor, but it follows a disturbance such as a fire or abandoned farmland where soil and a seed bank remain. That head start is why secondary succession reaches a mature community much faster.
Key insight: The presence or absence of soil is what separates secondary from primary succession.
Question 2Foundational
What are the three levels at which biodiversity is measured?
1. AGenetic, population, and biome diversity
2. BSpecies, habitat, and climate diversity
3. CGenetic, species, and ecosystem diversity
4. DGenetic, trophic, and watershed diversity
Show answer
Answer: C. Genetic, species, and ecosystem diversity
Biodiversity is described at the level of alleles within a population, of different species within a community, and of different ecosystems across a landscape. Genetic diversity matters most for a population's ability to adapt, which is why small isolated populations are vulnerable even when species counts look healthy. The other options mix in terms such as biome and watershed that describe places rather than levels of diversity.
Key insight: Losing genetic diversity can doom a species long before the species count in an ecosystem changes.
Question 3Intermediate
According to island biogeography theory, which island is expected to support the greatest number of species?
1. AA large island far from the mainland
2. BA large island close to the mainland
3. CA small island close to the mainland
4. DA small island far from the mainland
Show answer
Answer: B. A large island close to the mainland
Larger islands hold more habitat and larger populations, which lowers extinction rates, and islands nearer the mainland receive more colonists, which raises immigration rates. Species richness settles at the balance of those two rates, so large and close maximizes it. The same reasoning applies to habitat fragments surrounded by developed land, which is why conservation planners favor large, connected reserves.
Key insight: Size controls extinction and distance controls immigration; richness is where those two rates balance.
Question 4Intermediate
A coastal wetland filters pollutants out of water and absorbs storm surge that would otherwise flood nearby land. Which category of ecosystem service does this describe?
1. AProvisioning services
2. BCultural services
3. CSupporting services
4. DRegulating services
Show answer
Answer: D. Regulating services
Regulating services are the benefits people get when an ecosystem moderates a natural process, such as water purification, flood control, or climate regulation. Provisioning services are the tempting distractor, but those are tangible products taken from the ecosystem such as fish, timber, or fresh water. Cultural services cover recreation and aesthetic value, and supporting services are the underlying processes such as nutrient cycling.
Key insight: Provisioning gives you a product; regulating keeps a process in check.
Question 5Advanced
A fish species has an optimal temperature range of 12 to 18 degrees Celsius and a full tolerance range of 8 to 24 degrees Celsius. What is the most likely outcome if its stream warms to a steady 22 degrees Celsius?
1. AThe population survives but grows and reproduces less successfully
2. BThe population is completely unaffected because 22 degrees is within its tolerance range
3. CThe entire population dies within hours of the change
4. DThe population's optimal range shifts upward within a single generation
Show answer
Answer: A. The population survives but grows and reproduces less successfully
At 22 degrees the fish are inside the tolerance range but outside the optimal range, a zone of physiological stress where individuals survive yet spend more energy on maintenance and less on growth and reproduction. Saying the population is unaffected is the tempting answer, but only the optimal range supports full performance. Tolerance ranges are set by physiology and cannot shift in one generation, although natural selection can shift them over many.
Key insight: Between the optimal range and the limits of tolerance, organisms persist but perform poorly, so populations decline quietly.

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Unit 2: The Living World: Biodiversity