This lesson opens the Population Biology domain, which is roughly one-third of the CLEP Biology exam. Expect short, direct "Which of the following..." stems, five answer choices (A–E), and occasional data-table item sets. Focus on being able to apply definitions to unfamiliar examples, not just recite them.
A population is all the individuals of one species living in a defined area and able to interbreed. Three measurable traits describe it:
Population growth rate depends on births, deaths, immigration, and emigration.
[GRAPH: Two curves on axes of population size (N, y-axis) vs. time (x-axis). The exponential curve rises as a steepening J. The logistic curve rises, then bends and flattens into a plateau at a dashed horizontal line labeled K (carrying capacity).]
What limits a population near K?
Two ends of a life-history spectrum:
| Trait | r-selected | K-selected |
|---|---|---|
| Body size | Small | Large |
| Number of offspring | Many | Few |
| Parental care | Little/none | Extensive |
| Maturation | Fast | Slow |
| Lifespan | Short | Long |
| Typical habitat | Unstable, unpredictable | Stable, near carrying capacity |
| Examples | Insects, weeds, bacteria | Elephants, whales, humans, oaks |
r-selected species maximize reproductive rate to exploit open habitat quickly; K-selected species invest in survival and competitiveness in crowded, stable conditions.
A community is all the interacting populations in an area. Key interactions (using +/−/0 for the effect on each partner):
The gradual, somewhat predictable change in community composition over time.
Q1 — D. Correct: 60 mice per hectare is density (count per area); tight grouping around brush piles is dispersion (a clumped spatial pattern). The order asked is "respectively," so density then dispersion. Distractors: (A) reverses roles and adds age structure, which was not measured. (B) names age structure, but no age classes were counted. (C) reverses the order. (E) carrying capacity is a limit, not a described trait here. Fix rule: A raw count per area = density; the pattern of spacing = dispersion.
Q2 — C. Correct: Unlimited resources plus a constant doubling rate with no slowdown is the definition of exponential (J-curve) growth. Distractors: (A) logistic growth would slow near K, but nothing here slows. (B) a density-dependent brake would flatten growth; none is present. (D) zero growth means no change, contradicting doubling. (E) resource partitioning is a competition concept, irrelevant to one culture. Fix rule: No slowdown + unlimited resources = exponential; a slowdown toward a ceiling = logistic.
Q3 — B. Correct: The disease spreads more easily as density rises, so its impact depends on population density — a density-dependent factor. Distractors: (A) and (E) call it density-independent, but its effect clearly scales with crowding. (C) pioneer species relate to succession. (D) commensalism is a symbiosis, not a disease dynamic. Fix rule: If the effect gets stronger as the population gets denser, it's density-dependent.
Q4 — D. Correct: Killing the same percentage regardless of population size is the signature of a density-independent factor, typical of weather like frost. Distractors: (A) and (B) invoke competition/predation, which are density-dependent mechanisms not described. (C) carrying capacity arises from density-dependent limits. (E) resource partitioning concerns competition, not frost. Fix rule: Same fraction killed at any density = density-independent (usually abiotic/weather).
Q5 — D. Correct: Fast maturation, huge seed output, no parental care, and colonizing disturbed ground are classic r-selected traits. Distractors: (A) K-selected species are the opposite — few, well-tended offspring, stable habitats. (B) climax dominants are typically K-selected. (C) mutualism is not implied. (E) commensalism is a symbiosis, not a life-history strategy. Fix rule: Many cheap offspring + disturbed/unstable habitat = r-selected.
Q6 — B. Correct: The barnacle benefits (transport and food access) while the whale is unaffected — a (+/0) relationship, which is commensalism. Distractors: (A) mutualism requires both to benefit; the whale gains nothing. (C) parasitism requires host harm; none occurs. (D) predation involves consuming the partner. (E) competition harms both. Fix rule: One benefits, the other is unaffected (+/0) = commensalism.
Q7 — A. Correct: By feeding in different parts of the same trees, the warblers divide the resource and coexist — this is resource partitioning. Distractors: (B) competitive exclusion would eliminate one species; both persist here. (C) and (D) concern succession and abiotic regulation, not feeding zones. (E) commensalism is a symbiosis, not a partitioned niche. Fix rule: Competitors that split a resource by space/time/method are partitioning to coexist.
Q8 — C. Correct: Each thrives alone but one is driven to zero when both use the identical resource — the classic demonstration of competitive exclusion. Distractors: (A) mutualism would help both. (B) logistic growth describes one population leveling off, not one eliminating another. (D) succession is community change over time, not this experiment. (E) density-independent limitation ignores the competition shown. Fix rule: Two species + one identical limiting resource → one excludes the other = competitive exclusion.
Q9 — C. Correct: Colonization of bare rock with no soil, starting with lichen pioneers, is primary succession. Distractors: (A) secondary succession begins where soil already exists. (B) a climax community is the later stable stage, not the pioneer stage. (D) resource partitioning is a competition concept. (E) logistic growth describes population size, not community change. Fix rule: No soil at the start = primary succession.
Q10 — E. Correct: Plot 2 retains soil, roots, and seeds, so its secondary succession proceeds faster than Plot 1's primary succession on soilless rock. Distractors: (A) absent pioneers slow recovery, not speed it. (B) the two plots begin at different stages (soil vs. no soil). (C) the scenario states soil survived, so it was not sterilized. (D) both plots can undergo succession. Fix rule: Intact soil/seeds = secondary succession = faster recovery.
Q11 — A. Correct: In the logistic model dN/dt = rN[(K−N)/K], as N approaches K the term (K−N)/K approaches zero, so the growth rate approaches zero and the curve flattens. Distractors: (B) unlimited increase describes exponential, not logistic, growth. (C) growth slows toward zero but does not turn permanently negative in the basic model. (D) the rate is not constant; it declines near K. (E) fixed-interval doubling describes exponential growth. Fix rule: Logistic growth rate → 0 as N → K (the population plateaus).
Q12 — E. Correct: A broad pre-reproductive base predicts future growth (Population X), while roughly equal age classes indicate a stable population (Population Y). Distractors: (A) reverses the interpretation of Y. (B) neither diagram indicates decline (which needs a narrow base/top-heavy shape). (C) a growing base does not signal reaching carrying capacity. (D) reverses both readings. Fix rule: Wide young base = growth; even age classes = stable; top-heavy = decline.
Q1 — D. Correct: 60 mice per hectare is density (count per area); tight grouping around brush piles is dispersion (a clumped spatial pattern). The order asked is "respectively," so density then dispersion. Distractors: (A) reverses roles and adds age structure, which was not measured. (B) names age structure, but no age classes were counted. (C) reverses the order. (E) carrying capacity is a limit, not a described trait here. Fix rule: A raw count per area = density; the pattern of spacing = dispersion.
Q2 — C. Correct: Unlimited resources plus a constant doubling rate with no slowdown is the definition of exponential (J-curve) growth. Distractors: (A) logistic growth would slow near K, but nothing here slows. (B) a density-dependent brake would flatten growth; none is present. (D) zero growth means no change, contradicting doubling. (E) resource partitioning is a competition concept, irrelevant to one culture. Fix rule: No slowdown + unlimited resources = exponential; a slowdown toward a ceiling = logistic.
Q3 — B. Correct: The disease spreads more easily as density rises, so its impact depends on population density — a density-dependent factor. Distractors: (A) and (E) call it density-independent, but its effect clearly scales with crowding. (C) pioneer species relate to succession. (D) commensalism is a symbiosis, not a disease dynamic. Fix rule: If the effect gets stronger as the population gets denser, it's density-dependent.
Q4 — D. Correct: Killing the same percentage regardless of population size is the signature of a density-independent factor, typical of weather like frost. Distractors: (A) and (B) invoke competition/predation, which are density-dependent mechanisms not described. (C) carrying capacity arises from density-dependent limits. (E) resource partitioning concerns competition, not frost. Fix rule: Same fraction killed at any density = density-independent (usually abiotic/weather).
Q5 — D. Correct: Fast maturation, huge seed output, no parental care, and colonizing disturbed ground are classic r-selected traits. Distractors: (A) K-selected species are the opposite — few, well-tended offspring, stable habitats. (B) climax dominants are typically K-selected. (C) mutualism is not implied. (E) commensalism is a symbiosis, not a life-history strategy. Fix rule: Many cheap offspring + disturbed/unstable habitat = r-selected.
Q6 — B. Correct: The barnacle benefits (transport and food access) while the whale is unaffected — a (+/0) relationship, which is commensalism. Distractors: (A) mutualism requires both to benefit; the whale gains nothing. (C) parasitism requires host harm; none occurs. (D) predation involves consuming the partner. (E) competition harms both. Fix rule: One benefits, the other is unaffected (+/0) = commensalism.
Q7 — A. Correct: By feeding in different parts of the same trees, the warblers divide the resource and coexist — this is resource partitioning. Distractors: (B) competitive exclusion would eliminate one species; both persist here. (C) and (D) concern succession and abiotic regulation, not feeding zones. (E) commensalism is a symbiosis, not a partitioned niche. Fix rule: Competitors that split a resource by space/time/method are partitioning to coexist.
Q8 — C. Correct: Each thrives alone but one is driven to zero when both use the identical resource — the classic demonstration of competitive exclusion. Distractors: (A) mutualism would help both. (B) logistic growth describes one population leveling off, not one eliminating another. (D) succession is community change over time, not this experiment. (E) density-independent limitation ignores the competition shown. Fix rule: Two species + one identical limiting resource → one excludes the other = competitive exclusion.
Q9 — C. Correct: Colonization of bare rock with no soil, starting with lichen pioneers, is primary succession. Distractors: (A) secondary succession begins where soil already exists. (B) a climax community is the later stable stage, not the pioneer stage. (D) resource partitioning is a competition concept. (E) logistic growth describes population size, not community change. Fix rule: No soil at the start = primary succession.
Q10 — E. Correct: Plot 2 retains soil, roots, and seeds, so its secondary succession proceeds faster than Plot 1's primary succession on soilless rock. Distractors: (A) absent pioneers slow recovery, not speed it. (B) the two plots begin at different stages (soil vs. no soil). (C) the scenario states soil survived, so it was not sterilized. (D) both plots can undergo succession. Fix rule: Intact soil/seeds = secondary succession = faster recovery.
Q11 — A. Correct: In the logistic model dN/dt = rN[(K−N)/K], as N approaches K the term (K−N)/K approaches zero, so the growth rate approaches zero and the curve flattens. Distractors: (B) unlimited increase describes exponential, not logistic, growth. (C) growth slows toward zero but does not turn permanently negative in the basic model. (D) the rate is not constant; it declines near K. (E) fixed-interval doubling describes exponential growth. Fix rule: Logistic growth rate → 0 as N → K (the population plateaus).
Q12 — E. Correct: A broad pre-reproductive base predicts future growth (Population X), while roughly equal age classes indicate a stable population (Population Y). Distractors: (A) reverses the interpretation of Y. (B) neither diagram indicates decline (which needs a narrow base/top-heavy shape). (C) a growing base does not signal reaching carrying capacity. (D) reverses both readings. Fix rule: Wide young base = growth; even age classes = stable; top-heavy = decline.