Answer Key: Energy Flow in Ecosystems

Full credit for any reasonable notice + wonder. Ideal: Notice — "there are way more plants than hawks." Wonder — "why can't the land support as many hawks as grasshoppers?" Accept any on-topic wonder. Science note (not required from students): so little energy reaches the top trophic level (the 10% rule) that only a few top predators can be supported.

Accept any reasonable prior idea. Target ideas: the energy originally comes from the Sun (captured by producers in photosynthesis); as it moves up, most of it is lost (as heat / used for life processes), so less is available at each higher level. Do not penalize incomplete prior knowledge here — this is a warm-up.

Most energy: producers (grass), 10,000 kcal — the bottom level. Least energy: tertiary consumers (hawk), 10 kcal — the top level. Why it narrows: only about 10% of the energy at each trophic level is passed to the next; the rest (~90%) is lost as heat, so less energy is available at each step upward.

a) Grass — trophic level 1, producer (autotroph).
b) Grasshopper — trophic level 2, primary consumer (herbivore).
c) Hawk — trophic level 4, tertiary consumer (top carnivore).
d) Soil fungi — decomposer (breaks down dead matter; acts on every level, so no single trophic-level number is required — accept "decomposer, not a numbered feeding level").

5a. 10,000 kcal × 0.10 = 1,000 kcal passed to the primary consumers.
5b. 1,000 kcal × 0.10 = 100 kcal passed to the secondary consumers.
5c. Yes — the calculated values (1,000 and 100 kcal) match Table 1. This tells students that this ecosystem follows the ~10% rule closely: each level holds about one tenth of the level below it. (Accept any answer noting the match and that ~90% is lost at each step.)

Formula: percent transfer = (energy in higher level ÷ energy in lower level) × 100.
= (10 kcal ÷ 100 kcal) × 100 = 0.10 × 100 = 10%. This is consistent with the 10% rule. (Full credit for correct setup and 10%; accept "about 10%.")

a) O — a value read directly (10,000 kcal).
b) I — a reasoned conclusion ("can only be a few hawks because…").
c) O — a value read directly (10 kcal).
d) I — a conclusion reasoned from the pattern (energy lost as heat is not shown as a single measured value; it is inferred from the drop between levels).
Observation = measured/read directly; inference = a conclusion reasoned from evidence.

Full credit if the student ties the wide-bottom, narrow-top shape to energy loss between trophic levels via the 10% rule. Model: "The pyramid is wide at the bottom and narrow at the top because each trophic level only passes about 10% of its energy to the next (the 10% rule). Since ~90% is lost as heat at each step, far less energy is available higher up, so the top levels are small." Accept equivalent wording.

(a) Grass: would likely increase / grow more at first, because far fewer grasshoppers are eating it.
(b) Hawks: would decline — with fewer grasshoppers, the shrews (secondary consumers) lose their food and decrease, so the energy reaching the hawks drops and their numbers fall.
Why: energy flows upward through the food chain; removing the primary consumers cuts the pathway that carries energy from producers to the higher levels. Defensible alternate: a student may note shrews could switch to other prey if available; accept if reasoned. Direction (grass up, hawks down) plus an energy-flow justification earns full credit.

Model answer. Claim: Top-level consumers such as hawks are limited in number because only a tiny fraction of the ecosystem's energy reaches the top trophic level. Evidence: (1) In Table 1, producers have 10,000 kcal but the hawk level has only 10 kcal — 1,000 times less. (2) In Figure 1, each band holds about one tenth of the level below it (10,000 → 1,000 → 100 → 10 kcal). Reasoning: By the 10% rule, only ~10% of the energy at each trophic level passes to the next; the other ~90% is lost as heat. After three transfers only about 0.1% of the producers' energy remains, so there is enough energy to support only a few top consumers.
Defensible alternates: A student may cite the percent-transfer calculation from Q6 (10%) or the shape of the pyramid as evidence. Accept any claim supported by two correct numeric pieces of evidence and sound 10%-rule reasoning.

Articulate: accept any accurate plain-language statement (e.g., "only about a tenth of the energy at one level gets to the next; the rest is lost as heat").
Connect: the cited pair must show a 10× drop, e.g., producers 10,000 → primary consumers 1,000 kcal, or 100 → 10 kcal.
Extend: any correct non-grassland food chain with ≥3 labeled trophic levels (e.g., algae → minnow → bass → osprey; or leaves → caterpillar → songbird → hawk). More than one answer is defensible.

Reward a coherent four-level pyramid with a real producer at the base, plausible organisms at each level, and energy values that drop by ~10× per step (any reasonable producer starting value). The "what if the top disappeared" sentence should note the level below would increase and/or the chain above collapses.

Starting producers (phytoplankton) = 50,000 kcal. Apply ×0.10 each step:

• Phytoplankton (producer): 50,000 kcal
• Zooplankton (primary consumer): 50,000 × 0.10 = 5,000 kcal
• Small fish (secondary consumer): 5,000 × 0.10 = 500 kcal
• Tuna (tertiary consumer): 500 × 0.10 = 50 kcal
• Shark (quaternary consumer): 50 × 0.10 = 5 kcal

Percent of original energy reaching the shark = 5 ÷ 50,000 × 100 = 0.01%. Why sharks need a huge area: only one ten-thousandth of the producers' energy survives four transfers, so an enormous amount of phytoplankton (and ocean area) is needed to supply enough energy for even a small number of large top predators. (Full credit for correct chained multiplications and 0.01%.)