AP Biology Unit 8: Ecology MCQs – 30 Practice Questions with Answers (College Board CED)
Free AP Biology practice for Unit 8: Ecology, covering responses to the environment, energy flow, population ecology, communities, biodiversity, and disruptions to ecosystems, with a short explanation after every question.
Responses to the Environment
Q1. A plant flowers only when nights exceed a critical length in autumn. What is this response called?
- (A) Chemotaxis
- (B) Phototropism, the growth of a shoot toward a directional light source
- (C) Photoperiodism
- (D) Gravitropism
Show Answer
Answer: C – Photoperiodism is a response to day or night length, used by many plants to time flowering.
Q2. A moth migrates thousands of kilometres to a breeding ground each year. Which type of behaviour is this?
- (A) An innate seasonal movement to a new habitat
- (B) A reflex to light only
- (C) A response to population density
- (D) A learned behaviour acquired in one lifetime
Show Answer
Answer: A – Migration is a seasonal, largely innate behaviour in which animals move between habitats, often for breeding or feeding.
Q3. A student designs an experiment on whether seedlings bend toward light. Which control is most important?
- (A) Seedlings with no water
- (B) Seedlings grown with no light, all else identical
- (C) Seedlings grown in brighter light only, with no group kept in darkness for comparison
- (D) Seedlings of a different species
Show Answer
Answer: B – A valid control keeps every variable identical except the one being tested, so a no-light group isolates the effect of directional light.
Q4. Honeybees perform a waggle dance that indicates the direction and distance of a food source. What is the function of this behaviour?
- (A) To communicate the location of food
- (B) To regulate hive temperature
- (C) To attract a mate
- (D) To defend the hive from predators, by alerting guard bees to an intruder
Show Answer
Answer: A – The waggle dance is a form of communication that transfers information about food location to nestmates.
Q5. An animal becomes active at dawn and rests at night, even when kept in constant darkness for several days. What does this suggest?
- (A) The behaviour is learned from other animals
- (B) The rhythm is driven purely by light
- (C) The animal has an internal circadian rhythm
- (D) The animal is responding to temperature only
Show Answer
Answer: C – Persistence of a roughly 24-hour rhythm without external cues indicates an internal circadian clock.
Q6. A lizard basks in the sun in the morning and moves into shade at midday. What is this behaviour regulating?
- (A) Population density
- (B) Gene expression directly
- (C) Blood glucose level
- (D) Body temperature
Show Answer
Answer: D – Behavioural thermoregulation, such as basking and seeking shade, helps ectotherms keep body temperature within a suitable range.
Q7. Two groups of plants are grown under identical conditions except that one group receives a longer night. Only the long-night group flowers. What is the independent variable?
- (A) Whether the plants flower
- (B) Night length
- (C) Plant height
- (D) Soil type
Show Answer
Answer: B – The independent variable is the factor deliberately changed by the experimenter, here night length; flowering is the dependent variable.
Q8. A researcher claims a chemical attracts insects because more insects were caught in traps with the chemical than in traps without it, but the traps were placed in different fields. What is the flaw?
- (A) Insects cannot smell chemicals, so trap catches cannot reflect attraction at all
- (B) Controls are unnecessary
- (C) The sample size is too large
- (D) Trap location is a confounding variable
Show Answer
Answer: D – Because trap location also differed, the result could be due to location rather than the chemical, so the comparison is confounded.
Energy Flow and Population Ecology
Q9. In a food chain, producers capture 10,000 kJ of energy. Applying the typical 10 percent transfer rule, how much energy reaches the tertiary consumer (fourth trophic level)?
- (A) 1,000 kJ
- (B) 100 kJ
- (C) 10 kJ
- (D) 1 kJ
Show Answer
Answer: C – Energy falls roughly tenfold at each transfer: primary consumers gain about 1,000 kJ, secondary about 100 kJ, and tertiary about 10 kJ.
Q10. Why are food chains usually limited to four or five trophic levels?
- (A) Decomposers remove all energy, leaving nothing for any higher trophic level
- (B) Top predators refuse to eat
- (C) Energy is lost at each transfer
- (D) Producers stop growing
Show Answer
Answer: C – Because most energy is lost as heat and waste at each transfer, higher levels receive too little energy to sustain viable populations.
Q11. A pyramid of biomass for a grassland shows a large producer base narrowing toward top predators. What does the shape mainly reflect?
- (A) The number of species at each level
- (B) The total mass of organisms at each trophic level
- (C) The rate of photosynthesis only, measured as energy fixed per unit area per year
- (D) The age of each organism
Show Answer
Answer: B – A biomass pyramid represents the total dry mass at each trophic level, which usually decreases upward as energy is lost.
Q12. A population of bacteria doubles every 30 minutes in unlimited nutrients. Which growth pattern does this show?
- (A) Zero growth
- (B) Declining growth
- (C) Logistic growth, levelling off as the population reaches carrying capacity
- (D) Exponential growth
Show Answer
Answer: D – Unlimited resources allow a constant doubling rate, producing the J-shaped curve of exponential growth.
Q13. A deer population grows rapidly, then levels off near 500 individuals as food becomes scarce. What does 500 most likely represent?
- (A) The birth rate
- (B) The carrying capacity of the habitat
- (C) The death rate
- (D) The minimum viable population, below which extinction becomes almost certain
Show Answer
Answer: B – Carrying capacity is the maximum population size an environment can sustain given its resources.
Q14. A mark-recapture study marks 50 fish, then later catches 40 fish, of which 10 are marked. What is the estimated population size?
- (A) 100
- (B) 200
- (C) 400
- (D) 90
Show Answer
Answer: B – Using the Lincoln-Petersen estimate, N = (50 x 40) / 10 = 200 fish.
Q15. A survivorship curve shows most individuals surviving to old age, with mortality rising late in life. Which type of organism fits this pattern?
- (A) A large mammal with few offspring and parental care
- (B) An insect with no parental care, laying many eggs that mostly die young
- (C) A plant producing wind-dispersed seeds
- (D) An oyster producing millions of eggs
Show Answer
Answer: A – Type I survivorship, with high survival until old age, is typical of large mammals that invest heavily in few offspring.
Q16. A population graph shows an S-shaped curve that flattens over time. Which model fits, and why does growth slow?
- (A) Declining, because the population is going extinct and births stay below deaths
- (B) Exponential, because resources are unlimited
- (C) Logistic, because growth slows near carrying capacity
- (D) Linear, because births equal deaths from the start
Show Answer
Answer: C – The S-shaped logistic curve flattens as competition for limited resources and other density-dependent effects reduce net growth near carrying capacity.
Density, Communities and Biodiversity
Q17. As a mouse population becomes very dense, disease spreads faster and food per mouse falls. Which type of factors are these?
- (A) Random factors unrelated to population size
- (B) Density-independent factors
- (C) Density-dependent factors
- (D) Abiotic factors only
Show Answer
Answer: C – Disease transmission and competition intensify as density rises, so they are density-dependent factors.
Q18. A severe frost kills 80 percent of insects in a field regardless of how crowded they were. What type of factor is the frost?
- (A) Biotic only
- (B) A carrying capacity factor
- (C) Density-dependent
- (D) Density-independent
Show Answer
Answer: D – Weather events affect a similar proportion of individuals whatever the population density, so they are density-independent.
Q19. A sea otter eats sea urchins, which otherwise overgraze kelp forests. When otters are removed, kelp forests collapse. What role do otters play?
- (A) Decomposer
- (B) Keystone species
- (C) Primary producer
- (D) Invasive species
Show Answer
Answer: B – A keystone species has an effect on its community far larger than its abundance would suggest, as otters do by controlling urchins.
Q20. Two species of barnacle compete for space on the same rocks, and one consistently excludes the other from the lower shore. Which principle does this illustrate?
- (A) Mutualism
- (B) Competitive exclusion
- (C) Commensalism
- (D) Succession
Show Answer
Answer: B – Competitive exclusion occurs when two species competing for the same limited resource cannot coexist indefinitely in the same niche.
Q21. A bird builds its nest in a tree, gaining shelter, while the tree is neither helped nor harmed. Which symbiotic relationship is this?
- (A) Predation
- (B) Mutualism
- (C) Parasitism
- (D) Commensalism
Show Answer
Answer: D – Commensalism benefits one partner while the other is unaffected.
Q22. A cuckoo lays eggs in another bird’s nest, and the host raises cuckoo chicks at the expense of its own. Which relationship is this?
- (A) Competition
- (B) Mutualism
- (C) Commensalism
- (D) Brood parasitism
Show Answer
Answer: D – Brood parasitism benefits the parasite while harming the host’s reproductive success, so it is a parasitic relationship.
Q23. After a volcanic eruption leaves bare rock, lichens colonise first, followed by mosses, grasses, and eventually trees. What is this process called?
- (A) Eutrophication
- (B) Secondary succession
- (C) Primary succession
- (D) Climax stabilisation only
Show Answer
Answer: C – Primary succession begins on newly exposed surfaces with no soil, starting with pioneer species such as lichens.
Q24. An abandoned farm field gradually changes from weeds to shrubs to forest over decades. Why is this secondary rather than primary succession?
- (A) Soil is already present
- (B) No animals are involved
- (C) It ends with lichens
- (D) It starts faster
Show Answer
Answer: A – Secondary succession follows a disturbance where soil remains, so recolonisation starts from existing soil rather than bare rock.
Disruptions, Biodiversity and Human Impact
Q25. A lake receives fertiliser runoff rich in nitrates and phosphates. Algae grow rapidly, then die and decompose, and fish die from lack of oxygen. Which process is this?
- (A) Desertification
- (B) Biomagnification
- (C) Eutrophication
- (D) Succession
Show Answer
Answer: C – Eutrophication is nutrient enrichment that drives algal blooms; decomposition of the algae depletes oxygen and can kill aquatic animals.
Q26. A pesticide is found at low concentration in lake water, moderate concentration in small fish, and very high concentration in fish-eating birds. What explains the increase up the food chain?
- (A) Biomagnification up the food chain
- (B) Eutrophication of the lake
- (C) The pesticide evaporates upward
- (D) The birds produce the pesticide
Show Answer
Answer: A – Biomagnification occurs when a persistent substance becomes more concentrated in organisms at higher trophic levels.
Q27. An introduced plant spreads rapidly on an island, outcompeting native plants that have no defences against it. What is the most likely long-term effect on biodiversity?
- (A) Native species decline and biodiversity falls
- (B) No change, because new species always help
- (C) The island gains soil nutrients only
- (D) Biodiversity increases permanently
Show Answer
Answer: A – Invasive species can displace native species, reducing species richness and overall biodiversity.
Q28. A conservation team compares two forests: one has 40 species with even abundances, the other has 40 species but one dominates. Which forest has higher biodiversity in the fuller sense?
- (A) The first, because biodiversity includes both richness and evenness
- (B) The second, because dominance increases stability
- (C) They are equal, because richness alone defines biodiversity
- (D) Neither, because biodiversity counts individuals only
Show Answer
Answer: A – Biodiversity includes species richness and the relative abundance (evenness) of species, so the more even community is more diverse.
Q29. A government protects a wetland because it filters water, stores carbon, and supports fisheries. Which argument is this based on?
- (A) The species-area curve only
- (B) Ecosystem services provided by biodiversity
- (C) The wetland’s biomass pyramid
- (D) Carrying capacity alone
Show Answer
Answer: B – Ecosystem services are the benefits ecosystems provide to people, including water purification, carbon storage, and food provision.
Q30. Global temperatures rise and a mountain species’ suitable habitat shifts upward until no higher ground remains. What is the most likely outcome for that species?
- (A) Its population grows
- (B) It becomes a keystone species
- (C) It adapts instantly
- (D) Its range contracts and extinction risk rises
Show Answer
Answer: D – When climate shifts a habitat beyond available space, the species loses suitable habitat and faces a higher risk of extinction.
Work through any questions you missed again, then revise those areas before moving on to the next unit.