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AP Biology: Unit 8, Energy Flow & Trophic Levels (Drill 34)

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About This Drill

AP Biology: Unit 8, Energy Flow & Trophic Levels (Drill 34) is a practice drill. It contains 5 original questions created by Brian Stewart, a Barron's test prep author with over 20 years of tutoring experience.

Practice quantitative reasoning about energy transfer across trophic levels in this AP Biology drill on Unit 8 ecology. Apply the ecological efficiency formula, interpret energy flow tables, and evaluate claims about ecosystem productivity.

Passage

Ecologists studying a freshwater lake ecosystem measure energy transfer across four trophic levels over one growing season. Energy content is expressed in kilocalories per square meter per year (kcal/m2/yr). Net production represents energy available to the next trophic level after respiration losses.
Trophic LevelOrganismGross Production (kcal/m2/yr)Respiration (kcal/m2/yr)Net Production (kcal/m2/yr)
1Phytoplankton20,0003,00017,000
2Zooplankton3,4001,2002,200
3Planktivorous fish680320360
4Piscivorous fish725022

Questions & Explanations

Question 1. Using the data in the table, what is the approximate ecological efficiency from trophic level 1 to trophic level 2?

  • A) 6%
  • B) 11.8%
  • C) 17% ✓
  • D) 33%

Explanation: Ecological efficiency = (gross production of consumer level / gross production of prey level) x 100. Zooplankton gross production is 3,400 kcal/m2/yr; phytoplankton gross production is 20,000 kcal/m2/yr. 3,400 / 20,000 x 100 = 17%. A (6%) incorrectly uses respiration at level 2 in the numerator: 1,200 / 20,000 x 100 = 6%. B (11.8%) applies a net-to-net calculation: 2,200 / 17,000 x 100 = 12.9%, which is plausible but not the standard ecological efficiency formula. D (33%) inverts the ratio.

Question 2. Which calculation correctly determines the percentage of energy transferred from trophic level 2 to trophic level 4?

  • A) (680 / 3,400) x 100
  • B) (72 / 3,400) x 100 ✓
  • C) (22 / 3,400) x 100
  • D) (72 / 20,000) x 100

Explanation: To find the percentage of energy from trophic level 2 that reaches trophic level 4, divide gross production at level 4 by gross production at level 2: 72 / 3,400 x 100 = 2.1%. A calculates efficiency from level 2 to level 3 only (680 / 3,400), not the full span to level 4. C uses net production at level 4 rather than gross production. D spans all four trophic levels, from level 1 to level 4, rather than from level 2 to level 4.

Question 3. Based on the data, how much energy is lost to respiration at trophic level 3?

  • A) 360 kcal/m2/yr
  • B) 320 kcal/m2/yr ✓
  • C) 680 kcal/m2/yr
  • D) 1,200 kcal/m2/yr

Explanation: The table directly states that respiration at trophic level 3 (planktivorous fish) is 320 kcal/m2/yr. This can be confirmed: net production = gross production - respiration, so 680 - 320 = 360, which matches the net production column. A (360) is the net production of level 3, the energy remaining after respiration, not the respiration value itself. C (680) is gross production at level 3. D (1,200) is respiration at trophic level 2.

Question 4. A researcher proposes that increasing nutrient input to the lake (eutrophication) would increase phytoplankton productivity and therefore proportionally increase energy available at trophic level 4. Which reasoning best evaluates this proposal?

  • A) The proposal is well-supported because energy flows directly from producers to top consumers without loss.
  • B) The proposal is flawed because eutrophication always decreases phytoplankton productivity by increasing water turbidity.
  • C) The proposal is partially supported -- increased phytoplankton productivity would provide more energy at level 1, but energy losses compound across three trophic transfers (0.17 x 0.20 x 0.11 = 0.0037), so only about 0.4% of that increase would reach trophic level 4. ✓
  • D) The proposal is well-supported because ecological efficiency is constant across trophic levels, so any increase at level 1 is passed on equally at each subsequent level without substantial loss of usable chemical energy between organisms, so top consumers benefit nearly as much as producers.

Explanation: Energy losses compound multiplicatively. Using efficiencies from the data -- approximately 17% from level 1 to 2, 20% from level 2 to 3, and 11% from level 3 to 4 -- the cumulative transfer is 0.17 x 0.20 x 0.11 = 0.0037, meaning only about 0.4% of that increase would reach trophic level 4. A is incorrect -- substantial energy is lost at every transfer. B is incorrect -- eutrophication initially stimulates phytoplankton growth; light limitation is a later consequence. D is incorrect -- ecological efficiency varies by level and does not preserve proportional increases.

Question 5. Which trophic level demonstrates the greatest metabolic cost relative to its energy intake?

  • A) Trophic Level 1 (Phytoplankton)
  • B) Trophic Level 2 (Zooplankton)
  • C) Trophic Level 3 (Planktivorous fish)
  • D) Trophic Level 4 (Piscivorous fish) ✓

Explanation: Metabolic cost relative to energy intake = respiration / gross production. Level 1: 3,000 / 20,000 = 15%. Level 2: 1,200 / 3,400 = 35%. Level 3: 320 / 680 = 47%. Level 4: 50 / 72 = 69%. Trophic level 4 has the highest proportion of gross production consumed by respiration. This reflects the general pattern that higher-level consumers, often more active and metabolically demanding, retain proportionally less of their energy intake as net production.