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Biology 20

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Ecosystems and population change

Ten questions of mixed difficulty, covering Ecosystems and change. Print it, or work through it on screen — the answer key starts on its own page.

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Ecosystems and population change

Biology 20 · maddyhelps.com

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  1. Natural selection requires all of the following except

    1. a) individuals changing their own traits during their lifetime in response to need
    2. b) variation among individuals
    3. c) traits being heritable
    4. d) differences in survival or reproduction

  2. Primary succession begins

    1. a) only in the Arctic
    2. b) after a forest fire
    3. c) in an abandoned farm field
    4. d) on bare rock or other ground with no soil

  3. A population growing exponentially differs from one growing logistically in that

    1. a) exponential growth only happens in large populations
    2. b) logistic growth is always faster
    3. c) the exponential curve keeps rising while the logistic one levels off at carrying capacity
    4. d) the exponential curve levels off sooner

  4. A population of 1000 grows to 1200 in one year. Its growth rate for the year is

    1. a) 2%
    2. b) 20%
    3. c) 12%
    4. d) 200%

  5. Of these, the abiotic factor is

    1. a) soil pH
    2. b) a parasite
    3. c) a competing species
    4. d) a predator

  6. In a predator–prey cycle, the predator population peaks

    1. a) shortly before the prey population peaks
    2. b) shortly after the prey population peaks
    3. c) independently of the prey
    4. d) at exactly the same time

  7. Of these, the density-dependent limiting factor is

    1. a) a forest fire
    2. b) disease spreading through a crowded population
    3. c) a drought
    4. d) an early frost

  8. The carrying capacity of an environment is

    1. a) the rate at which a population grows
    2. b) the number of species present
    3. c) the largest population it can sustain indefinitely
    4. d) the largest population ever recorded there

  9. In a population of insects sprayed repeatedly with the same insecticide, resistance becomes common because

    1. a) the insects learned to avoid the spray
    2. b) resistant insects immigrated from elsewhere
    3. c) the insecticide caused the insects to develop resistance
    4. d) a few individuals were already resistant and survived to reproduce

  10. A climax community is one that

    1. a) never changes at all
    2. b) has the fewest species of any stage
    3. c) contains only one species
    4. d) remains relatively stable until a major disturbance

Answer key · Ecosystems and population change

Biology 20 · maddyhelps.com

  1. a) individuals changing their own traits during their lifetime in response to need — Variation, heritability and differential reproduction are the three conditions. Individuals acquiring useful traits by striving is Lamarck's idea, and it is not how selection works — the variation has to already be there.
  2. d) on bare rock or other ground with no soil — Primary succession starts where there is no soil at all, such as new lava or land exposed by a retreating glacier. Secondary succession starts where a community was destroyed but the soil survived.
  3. c) the exponential curve keeps rising while the logistic one levels off at carrying capacity — Exponential growth is what happens when nothing is limiting — a J-shaped curve. Logistic growth is the same curve until resources start to run short, after which it flattens into an S.
  4. b) 20% — Growth rate is the change divided by the starting population: 200/1000 = 0.20, or 20%. Dividing by the final number instead gives about 17%, which is a different and less useful quantity.
  5. a) soil pH — Abiotic factors are the non-living parts of the environment: temperature, water, light, soil chemistry. The other three are all interactions with other organisms.
  6. b) shortly after the prey population peaks — Predators need time to convert abundant prey into offspring, so their peak lags behind. The lag is what makes the two populations oscillate instead of settling at a fixed pair of numbers.
  7. b) disease spreading through a crowded population — Disease spreads faster the more closely packed the population is, so its effect depends on density. A fire or a frost kills much the same proportion whether the population is dense or sparse.
  8. c) the largest population it can sustain indefinitely — Carrying capacity is set by whatever runs short first — food, water, nesting sites. A population can briefly overshoot it, but not for long, and usually at a cost.
  9. d) a few individuals were already resistant and survived to reproduce — The spray does not create resistance; it selects for whatever resistance already existed by chance. This is why partial treatment and repeated use are such effective ways to breed a resistant pest.
  10. d) remains relatively stable until a major disturbance — It is the stage that persists because the species present can reproduce under the conditions they themselves create. Stable does not mean frozen — individuals are still replaced constantly.