AP Biology Unit 8: Ecology practice questions

Question 1

A plant is placed on its side in a dark room. Its shoot grows upward and its root grows downward — with no light cue. What mechanism drives this?

Accepted Answer

Gravitropism (geotropism) — the plant is responding to gravity, not light. Shoots grow against it (negative), roots grow with it (positive).

Answer

Phototropism: light from an unseen source directs stem growth upward

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Thigmotropism: physical contact with the soil surface redirects root growth

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Chemotropism: chemical gradients in the soil pull the root downward

Question 2

A moth flies toward a flame even though it gets burned. A moth larva stops moving when touched. Both are behavioral responses — but they're different types. What distinguishes taxis from kinesis?

Accepted Answer

Taxis is directed movement toward or away from a stimulus (the moth flying toward light). Kinesis is a change in movement rate or turning frequency in response to a stimulus — not directed toward or away from it (the larva increasing random movement when disturbed).

Answer

Taxis involves learned behavior; kinesis is purely instinctual

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Kinesis is directed movement; taxis is undirected change in movement rate

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Both are identical — taxis and kinesis are synonymous terms for stimulus-response movement

Question 3

A deer herd's feeding behavior shifts from daytime to nighttime grazing after wolves are introduced to the area. No wolves actually attack the deer during this period. What's driving the behavioral change?

Accepted Answer

The deer respond to an external cue — wolf scent or sound signals danger, shifting grazing. It's a behavioral response to a threat cue, not an actual attack.

Answer

Natural selection rapidly evolved nocturnal feeding instinct in the herd within one generation

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The wolves altered light availability by hunting in daylight, forcing deer into darkness

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Reduced food competition at night drove the shift, unrelated to wolf presence

Question 4

Your friend says energy cycles through an ecosystem the same way carbon does — in and out, used and reused indefinitely. Where is your friend wrong?

Accepted Answer

Energy flows one way through ecosystems and does not cycle the way matter does. It enters as sunlight, moves up trophic levels, and is lost as heat — carbon cycles, energy doesn't.

Answer

Your friend is correct — energy cycles between producers and decomposers indefinitely

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Energy cycles only at the producer level; it flows at higher trophic levels

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Both energy and matter flow in one direction and neither cycles through ecosystems

Question 5

Carbon cycles through ecosystems, but energy flows through them. One returns to its starting point; the other doesn't. What makes matter different from energy in how ecosystems handle them?

Accepted Answer

Matter is conserved and cycles between organisms and reservoirs; energy isn't and is lost as heat. Atoms get reused; heat energy can't be recaptured.

Answer

Energy cycles through decomposers, which convert heat back into chemical energy for producers

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Matter flows in one direction too — from producers to consumers without returning to abiotic reservoirs

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Both energy and matter cycle, but energy's cycle is much slower than matter's

Question 6

A forest is clearcut and the land is paved. Trace what happens to the local carbon cycle at the ecosystem level.

Accepted Answer

Photosynthesis stops (no producers), so atmospheric CO2 is no longer fixed into organic molecules. Decomposition of remaining organic matter continues briefly, releasing stored carbon. Net result: carbon moves from biotic storage to the atmosphere without being recaptured, disrupting the cycle.

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Carbon cycling accelerates because decomposers work faster without competition from plants

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The carbon cycle is unaffected because carbon in soil reservoirs remains intact

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Combustion of paving materials replaces photosynthesis as the main carbon-fixing process

Question 7

A farmer applies a broad-spectrum antibiotic to soil to kill a bacterial pathogen. Crop yields drop sharply the following season despite adequate rainfall and sunlight. What process did the antibiotic disrupt?

Accepted Answer

Nitrogen fixation — soil bacteria convert N2 into ammonia and nitrates that plants absorb. Killing them breaks the nitrogen cycle, starving plants of nitrogen.

Answer

The antibiotic disrupted the phosphorus cycle by killing phosphate-solubilizing fungi

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Denitrification accelerated, converting soil nitrates back to N2 faster than plants could absorb them

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Decomposition stopped, so organic matter accumulated and blocked root water uptake

Question 8

Transpiration and evaporation both move water into the atmosphere — so a plant and a puddle do the same thing for the water cycle. Is that right?

Accepted Answer

Not exactly — both make water vapor, but transpiration is biologically regulated, not passive. Plants control stomata; a puddle just evaporates.

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Evaporation adds water vapor; transpiration adds oxygen — they contribute to different cycles

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Transpiration occurs only at night when stomata open; evaporation occurs only during daylight

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They are identical processes — the term used depends only on whether a living organism is nearby

AP Biology — Unit 8: Ecology

What AP Biology Unit 8 covers

AP Biology Unit 8 practice questions

Key terms in Unit 8

Drill all of Unit 8 with spaced repetition