Competition, Niches and What Allows Coexistence · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Why rivals can share one world

Science · Ecology & Conservation · ages 20-22
Name ______________________   Date ____________

  1. Warblers A and B each catch ten insects. A takes eight from upper branches and two from lower branches. B takes three upper and seven lower. What resource is partitioned?

    • Insect food divided by feeding height within the tree
    • Insect food divided by feeding time within the tree
    • Branch space divided by nesting height within the tree

  2. In the standard two-species Lotka-Volterra competition model, which condition supports stable coexistence?

    • One species grows from rarity against an established rival
    • Both species grow from rarity against an established rival
    • Both species decline from rarity against an established rival

  3. Both species persist alone in a constant, well-mixed single-nutrient system with no other limits, enemies or immigration. A has R* = 3 mg/L; B has R* = 6 mg/L. What equilibrium outcome is predicted?

    • B excludes A by lowering the nutrient to its own R* of 6 mg/L
    • A excludes B by lowering the nutrient to its own R* of 3 mg/L
    • Both persist with the nutrient halfway between their two R* values

  4. Matched plots are randomly assigned to competitor removal or sham handling. Habitat stays alike. The remaining species expands its diet only after removal. What does this support?

    • The species avoided those foods because of its inherited feeding preference
    • The competitor's presence had restricted the species' use of those foods
    • Handling disturbed the plots and changed the species' use of those foods

  5. Two rodent species prefer different seed sizes. No data on seed scarcity or population trends are available. Which conclusion is supported?

    • Diet differences suggest partitioning; competition and persistence remain untested
    • Diet differences establish coexistence; seed limitation remains untested
    • Diet differences establish competition; long-term persistence remains untested

  6. A predator reduces a dominant competitor. The shared resource increases, and a weaker competitor then persists. Which indirect mechanism fits these observations?

    • The predator provides food directly to the weaker species after each hunt
    • The predator lowers the weaker species' demand for the shared resource
    • Predation reduces resource use by the dominant rival, leaving food available

  7. In the standard two-species competition model, each species declines when rare against its established rival. This proves the same species wins from every starting mixture.

    Circle one:   True   False

  8. A plant grows better in nitrogen-poor soil when associated with nitrogen-fixing microbes. Which mechanism can explain the advantage?

    • Microbes convert nitrogen from the plant into gas that escapes to the atmosphere
    • Microbes convert carbon from their plant partner into nitrogen that the plant can use
    • Microbes convert atmospheric nitrogen into forms that their plant partner can use
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Why rivals can share one world W1-mt_VS9Yud3rCQ-s1

  1. Insect food divided by feeding height within the tree · A feeds mainly in upper branches and B mainly in lower branches. Their insect use differs by height, while still overlapping.
  2. Both species grow from rarity against an established rival · Each species must increase at low density while its rival is at its single-species equilibrium. Both invasion tests are needed.
  3. A excludes B by lowering the nutrient to its own R* of 3 mg/L · A can replace its losses at 3 mg/L and reduce the nutrient below B's 6 mg/L threshold. Under the stated model, A excludes B.
  4. The competitor's presence had restricted the species' use of those foods · Expansion only after removal supports release from competition. Other changes caused by removal should still be checked before assigning the entire effect to one mechanism.
  5. Diet differences suggest partitioning; competition and persistence remain untested · Different seed-size preferences suggest resource partitioning. They do not show whether seeds limit the rodents or whether both populations persist.
  6. Predation reduces resource use by the dominant rival, leaving food available · The predator suppresses the dominant competitor. More shared resources then become available to the weaker species, an indirect benefit.
  7. False · When each species declines against an established rival, either may win depending on starting numbers. This is a priority effect.
  8. Microbes convert atmospheric nitrogen into forms that their plant partner can use · Nitrogen fixation makes atmospheric nitrogen available in usable forms. This can ease nitrogen limitation without removing other growth limits.
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