Lesson 01: Ecosystems & Energy Flow

Warm-Up

A field of grass soaks up sunlight all summer. A grasshopper eats the grass, a shrew eats the grasshopper, a hawk eats the shrew. Here's the puzzle that runs through all of environmental science: why can't that field support very many hawks? Why is the world full of grass and grasshoppers but you have to search for a top predator? The answer is not about space or meanness — it's about energy, and how much of it is lost at every hand-off in a food chain. Master that single idea and you've unlocked a huge fraction of Unit 1. Grab a pencil; this lesson is where APES becomes a numbers game.

Core Concept

Energy enters ecosystems from the sun

Almost every ecosystem on Earth runs on solar energy. Producers (autotrophs) — plants, algae, cyanobacteria — capture sunlight through photosynthesis and convert it into chemical energy stored in glucose:

6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

A small number of ecosystems (deep-sea hydrothermal vents) instead run on chemosynthesis, where bacteria oxidize inorganic molecules like hydrogen sulfide. But for the exam, assume solar energy and photosynthesis unless a vent is explicitly named.

Trophic levels

Energy passes from organism to organism in a food chain, and each feeding position is a trophic level:

• Producers (1st trophic level) — make their own food
• Primary consumers (2nd) — herbivores that eat producers
• Secondary consumers (3rd) — carnivores that eat herbivores
• Tertiary consumers (4th) — carnivores that eat other carnivores
• Decomposers (bacteria, fungi) and detritivores (earthworms, vultures) — break down dead material and recycle nutrients back to producers

Real ecosystems are not simple chains but food webs — interconnected chains where most organisms eat, and are eaten by, several species.

[DIAGRAM: Food web. Grass → grasshopper and rabbit. Grasshopper → shrew and sparrow. Rabbit → hawk and fox. Shrew → hawk. Sparrow → hawk. Fox and hawk at top. Arrows point in the direction energy flows (from eaten to eater). Decomposer box (fungi/bacteria) receives arrows from every organism.]

The 10% rule

Here is the master concept: only about 10% of the energy at one trophic level is passed up to the next. The other ~90% is lost, mostly as heat from cellular respiration, plus energy spent moving, energy in undigested waste, and energy in parts not eaten.

[DIAGRAM: Energy pyramid, 4 levels. Base = producers 10,000 kcal/m²/yr; primary consumers 1,000; secondary consumers 100; tertiary consumers 10. "×10% transfer" arrow between each level. Width of each bar proportional to energy.]

This is why pyramids narrow at the top, and it answers the warm-up: there simply isn't enough energy left at the top to support many large predators. It's also why food chains rarely exceed 4–5 links — beyond that, there's almost nothing left to eat.

The 10% figure is an average; real ecological efficiency ranges roughly 5–20%. On the exam, use 10% unless told otherwise.

Productivity: GPP and NPP

Primary productivity is the rate at which producers capture and store energy.

• Gross primary productivity (GPP) = the total rate of photosynthesis — all the energy producers capture.
• Net primary productivity (NPP) = the energy left for growth and for consumers after the producers use some for their own respiration:

NPP = GPP − respiration (R)

NPP is what matters ecologically, because it's the energy actually available to the rest of the food web. Units are usually energy per area per time (kcal/m²/yr) or biomass per area per time (g/m²/yr).

The most productive ecosystems per unit area are tropical rainforests, estuaries, swamps, and coral reefs; the least productive are deserts, open ocean, and tundra. (The open ocean has low productivity per square meter but, because it's so vast, contributes the largest total NPP on Earth — a classic trap.)

Why this matters

Every energy question on the exam — pyramids, biomagnification of toxins up food chains, why eating lower on the food chain feeds more people — traces back to the 10% rule and the GPP/NPP distinction. Lock these in now.

Worked Examples

Example 1 (easy): Applying the 10% rule up a chain

Producers in a meadow store 45,000 kcal/m²/yr. Using the 10% rule, how much energy reaches the secondary consumers?

Strategy: Multiply by 10% (×0.10) for each level climbed. Producers → primary → secondary is two steps.

Solution:

45,000 kcal × 0.10 = 4,500 kcal   (primary consumers)
4,500 kcal × 0.10  = 450 kcal      (secondary consumers)

Answer: 450 kcal/m²/yr.

Interpretation: Two trophic steps means two factors of 10 — energy dropped by 100×.

Example 2 (medium): Calculating NPP

In a forest, gross primary productivity is 12,000 kcal/m²/yr and plant respiration consumes 6,500 kcal/m²/yr. Find NPP.

Strategy: NPP = GPP − R.

Solution:

NPP = 12,000 − 6,500 = 5,500 kcal/m²/yr

Answer: 5,500 kcal/m²/yr is available to consumers and for plant growth.

Interpretation: Less than half the captured energy is left after the plants "pay their own bills." Respiration is a real energy sink, not a rounding error.

Example 3 (AP-style): Working backward

A population of hawks requires 30 kcal/m²/yr and sits at the 4th trophic level. Assuming the 10% rule, how much energy must the producers capture (NPP)?

Strategy: Going down the pyramid, divide by 0.10 (multiply by 10) per level. From 4th to 1st is three steps.

Solution:

Tertiary (hawks): 30 kcal
Secondary: 30 ÷ 0.10 = 300 kcal
Primary:   300 ÷ 0.10 = 3,000 kcal
Producers: 3,000 ÷ 0.10 = 30,000 kcal

Answer: 30,000 kcal/m²/yr.

Interpretation: Supporting a top predator takes a thousand times more energy at the base — the ecological reason apex predators need huge territories.

Example 4 (AP-style): Efficiency that isn't 10%

A lab finds only 8% of energy transfers from producers (5,000 kcal) to primary consumers. How much reaches primary consumers, and how does this compare to the 10% assumption?

Solution:

5,000 × 0.08 = 400 kcal   (at 8%)
5,000 × 0.10 = 500 kcal   (at 10%)

Answer: 400 kcal, which is 100 kcal less than the 10% estimate would predict.

Interpretation: Read the problem — if it gives an efficiency, use it. The 10% rule is only the default.

Common Mistakes

• Reversing the arrows. Food-web arrows point from the organism eaten to the organism that eats it — the direction energy flows. Students draw predators pointing at prey. Wrong.
• Confusing GPP and NPP. GPP is total capture; NPP is what's left after respiration. Consumers eat NPP, not GPP. On an FRQ, "energy available to herbivores" = NPP.
• Multiplying when you should divide (and vice versa). Going up the pyramid, multiply by 0.10. Going down (working backward from a predator), multiply by 10. Sketch the pyramid every time.
• Calling decomposers a trophic level that "starts over." Decomposers recycle nutrients (matter), but energy still leaves the system as heat. Matter cycles; energy flows one way and exits.
• Assuming open ocean is unproductive. Low productivity per m², but the largest total NPP because of its size.

Practice Problems

11. (FRQ-style) A grassland stores 50,000 kcal/m²/yr as NPP. Using the 10% rule, calculate the energy available to (a) primary consumers and (b) tertiary consumers. Show your work.

12. (Data) A study reports GPP = 20,000 and NPP = 7,000 kcal/m²/yr. (a) Calculate respiration. (b) Explain what a low NPP-to-GPP ratio suggests about the plants' energy use.

FRQ Practice — Designing an Investigation (10 pts)

Ecologists want to test the hypothesis that fertilizer runoff increases the net primary productivity of pond algae.

(a) Identify the independent and dependent variables. (2 pts) (b) Describe a controlled experimental design to test the hypothesis, including a control group. (3 pts) (c) Identify one variable that must be held constant and explain why. (2 pts) (d) The algae's GPP is measured at 3,000 mg O₂/L/day and respiration at 1,200 mg O₂/L/day. Calculate NPP. (1 pt) (e) Describe one way fertilizer runoff reaching a natural pond could ultimately harm the ecosystem despite raising productivity. (2 pts)