AP Environmental Science The Living World: Biodiversity — Worked Answer Explanations

Unit 2 · 12 questions explained

Below is a complete answer key for our AP Environmental Science The Living World: Biodiversity practice questions. For each question you'll find the correct choice, a full written explanation of how to get there, and — for every wrong answer — a short note on exactly why it's tempting and where it goes wrong. Reading these straight through is one of the fastest ways to find the gaps in a unit before exam day.

Prefer to test yourself first? Take the timed The Living World: Biodiversity practice test and come back here to review, or head back to the The Living World: Biodiversity unit overview.

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Question 1 · Easy
A population of cheetahs shows very little variation in immune system genes, making them highly susceptible to the same diseases. This problem illustrates a loss of:
- A
  Species diversity
  Why not A: Species diversity refers to the number of different species, not variation within one species.
- B
  Ecosystem diversity
  Why not B: Ecosystem diversity refers to variety of habitat types, not genetic variation within a population.
- C
  Genetic diversityCorrect
- D
  Functional diversity
  Why not D: Functional diversity refers to the range of ecological roles, not gene-level variation.
Explanation
Genetic diversity is the variety of alleles within a population. Low genetic diversity (as in cheetahs, which went through a genetic bottleneck) reduces adaptive potential. When all individuals share similar immune genes, a single pathogen can devastate the entire population.

Key takeaway
Genetic diversity within a population buffers against disease and environmental change.
Question 2 · Easy
Ecological succession on a newly formed volcanic island begins with:
- A
  Climax community establishment
  Why not A: A climax community is the stable endpoint of succession, not the starting point.
- B
  Secondary succession, because existing soil is present.
  Why not B: New volcanic rock has no existing soil or seed bank — this is primary, not secondary succession.
- C
  Pioneer species colonizing bare rockCorrect
- D
  A diverse mix of K-selected species competing for resources
  Why not D: K-selected species come late in succession; pioneer species are typically r-selected.
Explanation
Primary succession starts on bare substrate (volcanic rock, glacial till) with no soil or seed bank. Pioneer species like lichens and mosses colonize first, weathering rock and adding organic matter. Over time, soil develops, enabling less-tolerant species to establish, and the community shifts through seral stages toward a climax community.

Key takeaway
Primary succession: bare substrate → pioneer species (lichens, mosses) → soil formation → intermediate species → climax community.
Question 3 · Easy
A forest fire destroys a pine forest. Five years later, the area is dominated by fast-growing shrubs and grasses. After 50 years, pines re-establish dominance. This process is called:
- A
  Primary succession
  Why not A: Primary succession starts on bare substrate with no soil; here, soil and seed bank remain after the fire.
- B
  Secondary successionCorrect
- C
  Ecosystem facilitation
  Why not C: Facilitation is a mechanism within succession, not the name of the overall process.
- D
  Competitive exclusion
  Why not D: Competitive exclusion describes one species outcompeting another, not the community-level recovery trajectory.
Explanation
Secondary succession occurs after a disturbance that leaves soil and seed bank intact (fire, logging, flood). Pioneers (grasses, shrubs) colonize quickly because soil is already present. Over decades, longer-lived species return, eventually restoring the climax community. Secondary succession is faster than primary because soil already exists.

Key takeaway
Secondary succession: soil remains after disturbance → rapid recolonization → original community restored (faster than primary).
Question 4 · Easy
Which biome is characterized by permafrost, low plant diversity dominated by mosses and grasses, and is found at high latitudes?
- A
  Boreal forest (taiga)
  Why not A: Taiga has coniferous trees and no permafrost in most areas; it is south of the tundra.
- B
  Temperate grassland
  Why not B: Temperate grasslands are at mid-latitudes, have no permafrost, and experience warm summers.
- C
  Arctic tundraCorrect
- D
  Savanna
  Why not D: Savannas are tropical/subtropical grasslands with a dry season; no permafrost.
Explanation
Arctic tundra is defined by permafrost (permanently frozen subsoil), a short growing season (<60 days), and low-growing vegetation (mosses, sedges, lichens, dwarf shrubs) because tree roots cannot penetrate the frozen ground. It encircles the Arctic Ocean at high latitudes.

Key takeaway
Tundra = permafrost + no trees + high latitude. Taiga (boreal forest) is just south and has conifers.
Question 5 · Medium
A small island 500 km from the mainland has fewer bird species than an equally sized island located 50 km from the mainland. This pattern is best explained by:
- A
  The distant island has more volcanic activity, reducing habitat quality.
  Why not A: Volcanic activity is not a principle of island biogeography; distance from the source pool is the relevant variable.
- B
  The distant island has a lower immigration rate from the mainland source pool.Correct
- C
  The distant island experiences higher extinction rates due to its isolation.
  Why not C: Isolation does affect extinction, but the primary driver of fewer species on far islands is reduced immigration, not higher extinction.
- D
  The distant island is older and has less primary succession habitat.
  Why not D: Island age is not a core variable in MacArthur and Wilson's equilibrium theory.
Explanation
MacArthur and Wilson's island biogeography theory predicts that species richness on islands is determined by immigration rate (decreases with distance) and extinction rate (decreases with island size). A distant island has a lower immigration rate because dispersal across greater distances is less likely, so fewer species colonize it.

Key takeaway
Island biogeography: species richness increases with island size and decreases with distance from mainland.
Question 6 · Medium
A tropical rainforest is cleared and replaced by a crop monoculture. Which type of biodiversity loss occurs FIRST and MOST directly?
- A
  Loss of genetic diversity within crop varieties
  Why not A: The monoculture actually introduces uniform genetics, but the forest's genetic diversity is what is directly lost.
- B
  Loss of species diversity (species richness)Correct
- C
  Loss of ecosystem diversity
  Why not C: The ecosystem type does change, but the immediate, measurable direct loss is the species that disappear from the area.
- D
  Loss of biome diversity
  Why not D: Biome is a broad classification; the specific and immediate loss measured is at the species level.
Explanation
Replacing a biodiverse rainforest with a monoculture immediately eliminates hundreds or thousands of species (plants, animals, fungi, microbes) that depended on that habitat. This is a direct, measurable loss of species richness. Genetic and ecosystem diversity losses follow, but species loss is the most direct and rapid consequence.

Key takeaway
Habitat destruction causes immediate species richness loss; this is the leading driver of biodiversity decline.
Question 7 · Medium
The tropical rainforest, coral reef, and Mediterranean shrubland are all considered biodiversity hotspots. What TWO criteria define a biodiversity hotspot?
- A
  High species richness AND location in the tropics
  Why not A: Mediterranean shrublands and some temperate hotspots are not in the tropics; location is not a criterion.
- B
  High endemism AND significant habitat loss (≥70%)Correct
- C
  High genetic diversity AND economic value of ecosystem services
  Why not C: Economic value and genetic diversity are not the defining criteria for hotspot designation.
- D
  Large area AND protected status under international law
  Why not D: Hotspots are often small or fragmented, and protection is an outcome sought, not a definition criterion.
Explanation
Conservation International defines a biodiversity hotspot as a region with at least 1,500 endemic vascular plant species (high endemism) and that has lost at least 70% of its original habitat. The combination of irreplaceability (endemism) and threat (habitat loss) prioritizes conservation effort.

Key takeaway
Biodiversity hotspot = high endemism + severe habitat loss (at least 70% already destroyed).
Question 8 · Medium
Tropical rainforests cover ~6% of Earth's land surface but contain an estimated 50–80% of all terrestrial species. The primary reason for this extraordinary species richness is:
- A
  Tropical soils are exceptionally nutrient-rich, supporting more plant species.
  Why not A: Tropical soils are often nutrient-poor (leached by rain); plant richness is driven by climate stability and light, not soil fertility.
- B
  Year-round warmth and rainfall allow high productivity and diverse niches over long evolutionary time.Correct
- C
  Tropical regions have fewer predators, so more species can coexist.
  Why not C: Tropics have diverse predators; reduced predation pressure is not the explanation for high richness.
- D
  Humans have introduced many species to tropical regions for agriculture.
  Why not D: Agricultural introductions reduce native biodiversity; they are not the cause of high native species richness.
Explanation
Tropical rainforests have stable, warm, wet climates year-round, enabling high productivity and complex vertical stratification (multiple canopy layers). Over millions of years, this has allowed speciation into countless ecological niches. Evolutionary time and habitat complexity are the primary drivers.

Key takeaway
Tropical biodiversity: stable climate + high productivity + evolutionary time + vertical habitat complexity = most species-rich biome.
Question 9 · Medium
A conservation biologist studying wolves reintroduced to Yellowstone observed that elk began avoiding stream banks, allowing willows and aspens to recover. This ripple effect through the ecosystem is an example of:
- A
  Bottom-up regulation
  Why not A: Bottom-up regulation is driven by primary producers (plants); here the change was initiated by a top predator.
- B
  Competitive exclusion
  Why not B: Competitive exclusion describes one species eliminating another from a niche, not a predator-prey-habitat cascade.
- C
  Trophic cascade driven by a keystone predatorCorrect
- D
  Island biogeography equilibrium
  Why not D: Island biogeography applies to species richness on islands/habitat fragments, not predator-driven ecosystem changes.
Explanation
Wolves are a keystone species whose reintroduction initiated a trophic cascade: wolves (apex predator) → changed elk behavior (avoided open areas) → reduced elk browsing on willows/aspens → vegetation recovery → river bank stabilization, beaver return, and changed river geomorphology. This is top-down control through a keystone predator.

Key takeaway
Keystone species have disproportionate effects on ecosystem structure; their removal or addition triggers trophic cascades.
Question 10 · Medium
When two different species of Paramecium are grown together in the same culture on a limited food resource, one species always outcompetes and eliminates the other. This outcome is an example of:
- A
  Mutualism
  Why not A: Mutualism benefits both species; here one species is eliminated.
- B
  Parasitism
  Why not B: Parasitism involves one species living on/in another; these are free-living competitors.
- C
  Competitive exclusionCorrect
- D
  Character displacement
  Why not D: Character displacement is when competing species evolve differences to reduce overlap; here they do not coexist long enough for that.
Explanation
Gause's competitive exclusion principle states that two species competing for identical resources cannot coexist indefinitely — one will be more efficient and will drive the other to extinction from that habitat. In natural systems, species coexist by partitioning resources (niche differentiation), which reduces direct competition.

Key takeaway
Competitive exclusion: two species with identical niches cannot coexist — one outcompetes the other.
Question 11 · Hard
According to the species-area relationship, if an island's area is reduced by 90%, approximately what fraction of its species are expected to survive?
- A
  90% of species survive
  Why not A: A 90% area reduction does not preserve 90% of species; the species-area relationship is logarithmic, not linear.
- B
  ~50% of species surviveCorrect
- C
  ~10% of species survive
  Why not C: 10% species retention would follow a 1:1 area-species ratio, not the actual logarithmic relationship.
- D
  All species survive because they adapt to smaller habitat
  Why not D: Adaptation takes many generations; immediate habitat loss causes near-term extinctions.
Explanation
The species-area relationship: S = cA^z, where z ≈ 0.25–0.30. A 90% area reduction leaves 10% of the original area. Predicted species: S_new/S_old = (0.10)^0.25 ≈ 0.56, or roughly 50% of species. The logarithmic relationship means that halving species richness requires reducing habitat by ~90%, which is why habitat fragmentation is so damaging.

Key takeaway
S = cA^z; reducing area by 90% typically eliminates ~50% of species (z ≈ 0.25).
Question 12 · Hard
A landscape ecologist comparing two forest fragments finds that the 100 ha fragment supports more species than the 10 ha fragment. An additional finding is that even within the 100 ha fragment, interior forest species are more abundant than edge-tolerant species. These observations together support the importance of:
- A
  Biotic resistance and invasive species control
  Why not A: Biotic resistance relates to native communities resisting invasions; the observations relate to habitat area and edge effects.
- B
  Habitat area and interior-to-edge ratio in conservation designCorrect
- C
  Primary productivity as the sole driver of biodiversity
  Why not C: Productivity matters but the fragment comparison is about area and edge effects, not productivity differences.
- D
  The founder effect in isolated populations
  Why not D: The founder effect is a genetic phenomenon for newly isolated small populations; it doesn't explain species richness patterns across fragments.
Explanation
Larger fragments support more species (species-area relationship) and have more interior habitat relative to edge. Edge habitat has different microclimates (more wind, light, temperature variation) and supports generalist species, while interior specialists require core habitat. Conservation design favors large, round preserves with connected corridors to maximize interior area and minimize edge effects.

Key takeaway
Larger, rounder habitat fragments maximize interior-to-edge ratio, supporting more interior-specialist species.