This lesson sits in the Population Biology domain (about one-third of the exam). The CLEP exam frequently attaches 2–3 questions to a shared data table or pyramid diagram and asks you to interpret the numbers. Practice reading tables carefully — the arithmetic is simple, but you must know which row or level to use.
A food chain is a linear sequence of who-eats-whom; a food web is the interconnected set of many overlapping chains in a community. Each feeding step is a trophic level:
Energy enters as sunlight, flows one way through the levels, and is ultimately lost as heat. Energy flows; matter cycles. This is the single most testable contrast in the domain.
At each transfer between trophic levels, most energy is lost — used for the organism's own respiration and metabolism, lost as heat, or never eaten/assimilated. On average, only about 10% of the energy at one level is stored as biomass available to the next level.
Consequences: - Food chains are short (usually 3–5 links) because energy runs out. - Top predators are relatively rare — there is little energy left to support them. - Eating lower on a food chain feeds more people per unit of producer energy.
Worked example (10% rule): If producers fix 10,000 kcal, primary consumers capture ≈ 1,000 kcal, secondary consumers ≈ 100 kcal, and tertiary consumers ≈ 10 kcal.
[GRAPH: A five-block vertical energy pyramid. Bottom (producers) is widest at 10,000 kcal; each block above is one-tenth the size — primary consumers 1,000, secondary 100, tertiary 10 — narrowing sharply toward the top.]
Three ways to display trophic structure:
Because energy is always lost upward, the pyramid of energy can never be inverted — a key exam point.
The CLEP exam presents a shared table, then asks 2–3 questions about it. Example energy-budget table for one trophic level:
| Energy term | Value (kcal/m²/yr) |
|---|---|
| Energy ingested | 1,000 |
| Energy lost in feces (not assimilated) | 300 |
| Energy assimilated | 700 |
| Energy lost to respiration | 500 |
| Energy stored as new biomass (production) | 200 |
Key relationships to know: - Assimilated = Ingested − Feces (700 = 1,000 − 300) - Production = Assimilated − Respiration (200 = 700 − 500) - Only the production (new biomass, 200) is available to the next trophic level — not the whole ingested amount.
Matter cycles between organisms and the physical environment. Know the reservoir and the biological step for each.
Q1 — C. Correct: Energy enters as sunlight, moves one way through trophic levels, and exits as heat, while matter (nutrients) is recycled — "energy flows, matter cycles." Distractors: (A) energy is not recycled. (B) reverses the two. (D) matter is not lost one-way; it cycles. (E) reverses which one is lost as heat. Fix rule: Memorize "energy flows (one way, lost as heat); matter cycles."
Q2 — B. Correct: Secondary consumers are two transfers above producers: 8,000 × 0.10 = 800 (primary consumers), then 800 × 0.10 = 80 kcal/m²/yr. Distractors: (A) is the producer value with no transfer. (C) stops one level early (primary consumers). (D) applies the rule three times, one too many. (E) halves the value, ignoring the 10% rule. Fix rule: Count the number of arrows above producers and multiply by 0.10 that many times.
Q3 — E. Correct: Because ~90% of energy is lost at each step, the pyramid of energy always shrinks upward and can never be inverted. Distractors: (A) numbers pyramids can invert (one tree, many insects). (B) biomass is greatest at the top only in rare inverted cases; the reasoning is wrong. (C) numbers pyramids are not guaranteed upright. (D) decomposers do not add mass upward through trophic levels. Fix rule: Only the energy pyramid is guaranteed upright.
Questions 4–5 refer to the following pyramid-of-biomass data for a lake ecosystem (standing dry biomass measured at one time), listed from lowest to highest trophic level.
| Trophic level | Standing biomass (g/m²) |
|---|---|
| Phytoplankton (producers) | 4 |
| Zooplankton (primary consumers) | 21 |
| Small fish (secondary consumers) | 11 |
| Large fish (tertiary consumers) | 1 |
Q4 — B. Correct: Tiny phytoplankton reproduce and are eaten so fast that their standing (instantaneous) biomass stays low even though they supply enough energy over time to support a larger consumer mass — the classic inverted-biomass explanation. Distractors: (A) zooplankton are consumers, not photosynthetic. (C) energy cannot be created. (D) the 10% rule still governs energy flow; only standing biomass is inverted. (E) large fish are top consumers, not producers. Fix rule: Inverted biomass = fast-turnover producers with low standing stock, not a violation of energy flow.
Q5 — C. Correct: Reading the table, zooplankton hold 21 g/m², the largest of the four values. Distractors: (A) phytoplankton are only 4. (B) large fish are only 1. (D) small fish are 11. (E) the values differ, so they are not equal. Fix rule: For a "greatest/least" table question, scan the column and pick the extreme value directly.
Q6 — B. Correct: Bacteria carry out fixation, nitrification, and denitrification (with archaea contributing to some steps). Distractors: (A) plants assimilate nitrogen but do not fix or nitrify it. (C) insects play no direct role in these conversions. (D) fungi drive decomposition/ammonification, not these three steps alone. (E) algae are producers, not the nitrogen-transforming agents here. Fix rule: The nitrogen cycle's chemical conversions are bacteria-driven.
Questions 7–8 refer to the following energy-budget table for one trophic level (all values in kcal/m²/yr).
| Energy term | Value |
|---|---|
| Energy ingested | 2,000 |
| Energy lost in feces | 600 |
| Energy lost to respiration | 1,100 |
| Energy stored as new biomass (production) | ? |
Q7 — A. Correct: Assimilated = 2,000 − 600 = 1,400; Production = Assimilated − Respiration = 1,400 − 1,100 = 300 kcal/m²/yr. Distractors: (B) 900 subtracts feces from respiration incorrectly. (C) 1,400 is the assimilated amount, not production. (D) 200 comes from a different table. (E) 1,100 is the respiration value itself. Fix rule: Production = Ingested − Feces − Respiration.
Q8 — D. Correct: Only the energy stored as new biomass (production) can be eaten by the next level; the rest is lost to feces, respiration, and heat. Distractors: (A) ingested energy includes losses not passed on. (B) and (C) are losses, not transferred energy. (E) sums two losses, which are unavailable to consumers. Fix rule: Only production (new biomass) moves up a trophic level.
Q9 — E. Correct: The phosphorus cycle has no meaningful gaseous phase; phosphate moves through rock, soil, water, and organisms. Distractors: (A) carbon has a major atmospheric CO₂ phase. (B) nitrogen's largest reservoir is atmospheric N₂. (C) the water cycle is dominated by its atmospheric (vapor) phase. (D) oxygen has a large atmospheric phase. Fix rule: Phosphorus = the "no gas phase" cycle (rock/soil/water).
Q10 — A. Correct: NPP = GPP − producer respiration = 20,000 − 12,000 = 8,000 kcal/m²/yr. Distractors: (B) adds instead of subtracts. (C) is respiration alone. (D) is GPP, ignoring respiration. (E) is an arithmetic error. Fix rule: NPP = GPP − respiration; NPP is what consumers can use.
Q11 — D. Correct: Combustion of fossil fuels releases stored carbon as CO₂, directly adding to the atmospheric carbon reservoir. Distractors: (A) burning fuel does not remove soil nitrate. (B) phosphorus has no gaseous phase, so burning does not create one. (C) transpiration is a plant process, not stopped by combustion. (E) denitrification is a bacterial nitrogen step, unrelated to fuel burning. Fix rule: Burning carbon-based fuels loads CO₂ into the carbon cycle.
Q12 — C. Correct: The ~90% energy loss at each transfer leaves too little energy to sustain many levels, capping most chains at three to five links. Distractors: (A) predator "refusal" is not a biological cause. (B) decomposers act at all levels, not as a cutoff. (D) matter continues to cycle regardless of chain length. (E) the number of producer-supported species is not the limiting reason. Fix rule: Short food chains are a direct consequence of the 10% energy-transfer rule.
Q1 — C. Correct: Energy enters as sunlight, moves one way through trophic levels, and exits as heat, while matter (nutrients) is recycled — "energy flows, matter cycles." Distractors: (A) energy is not recycled. (B) reverses the two. (D) matter is not lost one-way; it cycles. (E) reverses which one is lost as heat. Fix rule: Memorize "energy flows (one way, lost as heat); matter cycles."
Q2 — B. Correct: Secondary consumers are two transfers above producers: 8,000 × 0.10 = 800 (primary consumers), then 800 × 0.10 = 80 kcal/m²/yr. Distractors: (A) is the producer value with no transfer. (C) stops one level early (primary consumers). (D) applies the rule three times, one too many. (E) halves the value, ignoring the 10% rule. Fix rule: Count the number of arrows above producers and multiply by 0.10 that many times.
Q3 — E. Correct: Because ~90% of energy is lost at each step, the pyramid of energy always shrinks upward and can never be inverted. Distractors: (A) numbers pyramids can invert (one tree, many insects). (B) biomass is greatest at the top only in rare inverted cases; the reasoning is wrong. (C) numbers pyramids are not guaranteed upright. (D) decomposers do not add mass upward through trophic levels. Fix rule: Only the energy pyramid is guaranteed upright.
Q4 — B. Correct: Tiny phytoplankton reproduce and are eaten so fast that their standing (instantaneous) biomass stays low even though they supply enough energy over time to support a larger consumer mass — the classic inverted-biomass explanation. Distractors: (A) zooplankton are consumers, not photosynthetic. (C) energy cannot be created. (D) the 10% rule still governs energy flow; only standing biomass is inverted. (E) large fish are top consumers, not producers. Fix rule: Inverted biomass = fast-turnover producers with low standing stock, not a violation of energy flow.
Q5 — C. Correct: Reading the table, zooplankton hold 21 g/m², the largest of the four values. Distractors: (A) phytoplankton are only 4. (B) large fish are only 1. (D) small fish are 11. (E) the values differ, so they are not equal. Fix rule: For a "greatest/least" table question, scan the column and pick the extreme value directly.
Q6 — B. Correct: Bacteria carry out fixation, nitrification, and denitrification (with archaea contributing to some steps). Distractors: (A) plants assimilate nitrogen but do not fix or nitrify it. (C) insects play no direct role in these conversions. (D) fungi drive decomposition/ammonification, not these three steps alone. (E) algae are producers, not the nitrogen-transforming agents here. Fix rule: The nitrogen cycle's chemical conversions are bacteria-driven.
Q7 — A. Correct: Assimilated = 2,000 − 600 = 1,400; Production = Assimilated − Respiration = 1,400 − 1,100 = 300 kcal/m²/yr. Distractors: (B) 900 subtracts feces from respiration incorrectly. (C) 1,400 is the assimilated amount, not production. (D) 200 comes from a different table. (E) 1,100 is the respiration value itself. Fix rule: Production = Ingested − Feces − Respiration.
Q8 — D. Correct: Only the energy stored as new biomass (production) can be eaten by the next level; the rest is lost to feces, respiration, and heat. Distractors: (A) ingested energy includes losses not passed on. (B) and (C) are losses, not transferred energy. (E) sums two losses, which are unavailable to consumers. Fix rule: Only production (new biomass) moves up a trophic level.
Q9 — E. Correct: The phosphorus cycle has no meaningful gaseous phase; phosphate moves through rock, soil, water, and organisms. Distractors: (A) carbon has a major atmospheric CO₂ phase. (B) nitrogen's largest reservoir is atmospheric N₂. (C) the water cycle is dominated by its atmospheric (vapor) phase. (D) oxygen has a large atmospheric phase. Fix rule: Phosphorus = the "no gas phase" cycle (rock/soil/water).
Q10 — A. Correct: NPP = GPP − producer respiration = 20,000 − 12,000 = 8,000 kcal/m²/yr. Distractors: (B) adds instead of subtracts. (C) is respiration alone. (D) is GPP, ignoring respiration. (E) is an arithmetic error. Fix rule: NPP = GPP − respiration; NPP is what consumers can use.
Q11 — D. Correct: Combustion of fossil fuels releases stored carbon as CO₂, directly adding to the atmospheric carbon reservoir. Distractors: (A) burning fuel does not remove soil nitrate. (B) phosphorus has no gaseous phase, so burning does not create one. (C) transpiration is a plant process, not stopped by combustion. (E) denitrification is a bacterial nitrogen step, unrelated to fuel burning. Fix rule: Burning carbon-based fuels loads CO₂ into the carbon cycle.
Q12 — C. Correct: The ~90% energy loss at each transfer leaves too little energy to sustain many levels, capping most chains at three to five links. Distractors: (A) predator "refusal" is not a biological cause. (B) decomposers act at all levels, not as a cutoff. (D) matter continues to cycle regardless of chain length. (E) the number of producer-supported species is not the limiting reason. Fix rule: Short food chains are a direct consequence of the 10% energy-transfer rule.