AP Biology: Ecology — Practice Questions & Answers
Ecology examines how organisms interact with each other and their environment across scales from individuals to the biosphere. This unit covers behavioral and physiological responses to the environment, energy flow through trophic levels, population growth models, community interactions, biodiversity, and large-scale disruptions such as invasive species and climate change.
951 practice questions available for this unit — here are 10 with full answers and explanations.
A plant shoot bends toward a light source coming from one side of a window. Which term best describes this directional growth response?
Thigmotropism
Phototropism
Gravitropism
Photoperiodism
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WhyPhototropism is the directional growth of a plant in response to light. Growth toward the light is positive phototropism, driven by the redistribution of auxin to the shaded side, which causes faster elongation there.
2Multiple choice · Easy
Which of the following is an example of innate (instinctive) behavior rather than learned behavior?
A rat navigating a maze faster after several trials
A dog sitting in response to a trained command
A spider spinning a complete web the first time without practice
A bird singing the local dialect after hearing adults
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WhyA spider spinning a web correctly the first time, without prior practice or observation, is innate behavior encoded by genes. Learned behaviors, by contrast, develop through experience and are modified over time.
3Multiple choice · Medium
In a grassland food chain, plants capture 20,000 kcal of energy. Applying the 10% rule, approximately how much energy is available to secondary consumers?
2,000 kcal
200 kcal
20 kcal
18,000 kcal
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WhyEach trophic transfer passes roughly 10% of energy upward. Primary consumers receive about 10% of 20,000 = 2,000 kcal. Secondary consumers receive about 10% of 2,000 = 200 kcal. The rest is lost mainly as heat from cellular respiration.
4Multiple choice · Medium
Why do most food chains rarely contain more than four or five trophic levels?
Predators at high levels reproduce too slowly to form populations
Decomposers consume all the energy before it reaches top consumers
So little energy remains after repeated 10% transfers that higher levels cannot be supported
Top predators are always larger and require less total energy
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WhyBecause only about 10% of energy is transferred between trophic levels, the energy available diminishes sharply at each step. After several transfers, too little energy remains to support another level of consumers.
5Multiple choice · Medium
A population of rabbits grows according to the logistic model. As the population size approaches the carrying capacity (K), what happens to the population growth rate?
It increases without limit
It remains constant at its maximum
It slows and approaches zero
It becomes negative and the population goes extinct
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WhyIn logistic growth, the per-capita resources decline as N approaches K, so the growth rate slows and approaches zero. Growth is fastest near K/2, where the absolute number of individuals added per unit time is greatest.
6Multiple choice · Easy
Which set of traits is characteristic of an r-selected species?
Few offspring, high parental care, long lifespan
Many offspring, little parental care, rapid maturation
Large body size, late reproduction, stable populations near K
Whyr-selected species typically have many offspring, little or no parental care, small body size, rapid maturation, and short lifespans. They are favored in unstable or unpredictable environments where rapid reproduction is advantageous.
7Multiple choice · Hard
A bacterial population doubles every 30 minutes under ideal conditions, showing J-shaped growth. Starting with 100 cells, how many cells are present after 3 hours, and what does this curve indicate about resources?
6,400 cells; resources are unlimited
600 cells; resources are limiting
3,200 cells; the population has reached carrying capacity
1,600 cells; density-dependent factors are limiting growth
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WhyIn 3 hours there are 6 doublings (180/30). The population is 100 x 2^6 = 100 x 64 = 6,400 cells. A J-shaped (exponential) curve indicates that resources are effectively unlimited, so growth is not yet constrained by carrying capacity.
8Multiple choice · Hard
The logistic growth equation is dN/dt = rN((K - N)/K). A population has r = 0.5 per year, K = 1,000, and a current size N = 400. What is the instantaneous growth rate dN/dt?
200 individuals per year
120 individuals per year
300 individuals per year
80 individuals per year
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WhySubstitute: dN/dt = 0.5 x 400 x ((1000 - 400)/1000) = 0.5 x 400 x (600/1000) = 0.5 x 400 x 0.6 = 120 individuals per year. The (K-N)/K term reduces growth as N rises toward K.
9Multiple choice · Hard
Which scenario best illustrates a density-dependent factor regulating a population?
A sudden freeze kills a large fraction of an insect population
A volcanic eruption destroys a forest habitat
A contagious disease spreads faster as the population becomes crowded
A flash flood washes away a riverbank colony
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WhyDensity-dependent factors intensify as population density rises. The spread of a contagious disease accelerates when individuals are crowded, increasing transmission. Abiotic events like floods or freezes act independently of density.
10Multiple choice · Medium
Two species of paramecium are grown together; one species drives the other to local extinction because both require the same limiting resource. This outcome best demonstrates which principle?
Mutualism
Resource partitioning
Competitive exclusion
Commensalism
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WhyThe competitive exclusion principle states that two species competing for the identical limiting resource in the same niche cannot coexist indefinitely; the superior competitor eliminates the other unless niches diverge.
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Ecology: The scientific study of how organisms interact with one another and with the physical and chemical features of their surroundings.
Biosphere: The global sum of all regions of Earth where life exists, spanning the lowest parts of the atmosphere, the oceans, and the upper crust.
Biome: A large geographic region defined by a characteristic climate and the communities of plants and animals adapted to it, such as tundra, desert, or tropical rainforest.
Ecosystem: A functional unit consisting of all the organisms in an area together with the nonliving environment they exchange energy and matter with.
Community: All the populations of different species that live together and interact within a particular area.
Population: A group of individuals of a single species occupying the same area at the same time and capable of interbreeding.
Species: A group of organisms whose members can interbreed and produce fertile offspring under natural conditions.
Habitat: The specific place or type of environment in which an organism naturally lives and obtains the resources it needs.