What you'll learn
Every ecosystem runs on energy that starts as sunlight. In plain words: energy passes from plants to the animals that eat them, and the number of living things goes up and down with the food supply.
This lesson follows that energy as it moves from plants to animals. It also explains why populations rise, fall, and level off. You will learn:
- how to read food chains and food webs and name each organism's role (producer, consumer, decomposer);
- what trophic levels are and why an energy pyramid shrinks by about 90% at each step;
- how carrying capacity and limiting factors control population size;
- how predator–prey, competition, and symbiosis (mutualism, commensalism, parasitism) shape communities.
How it shows up on the GED: expect a food web diagram, an energy pyramid, a population graph, or a short experiment. Then questions ask you to:
- name each organism's role;
- predict what happens if one species disappears;
- apply the 10% rule;
- explain a population trend.
These are among the most common life-science stimuli, and they reward careful reading of arrows and axes.
Words to know
- ecosystem — all the living things in an area, plus their nonliving surroundings. Example: a pond with its fish, plants, water, and sunlight.
- producer (autotroph) — makes its own food, usually by photosynthesis (grass, algae).
- consumer (heterotroph) — gets energy by eating other organisms (rabbit, hawk).
- herbivore / carnivore / omnivore — eats plants / eats animals / eats both (deer / wolf / bear).
- decomposer — breaks down dead material and wastes, returning nutrients to the soil (bacteria, fungi).
- food chain — one path of energy: grass → rabbit → fox.
- food web — many connected food chains in one ecosystem.
- trophic level — an organism's feeding position (producer = level 1, primary consumer = level 2, and so on).
- energy pyramid — a diagram showing energy available at each trophic level.
- population — all members of one species in an area (all the rabbits on an island).
- community — all the populations of different species in an area.
- carrying capacity — the largest population an environment can support over time.
- limiting factor — anything that keeps a population from growing larger (food, water, space, predators, disease).
- symbiosis — a close, long-term relationship between two species.
- mutualism / commensalism / parasitism — both benefit / one benefits, the other is unaffected / one benefits, the other is harmed.
- competition — two organisms need the same limited resource (two plant species competing for sunlight).
Food chains, food webs, and roles
Arrows show the direction energy flows: from the food to the eater. "Grass → rabbit" means the rabbit eats the grass. This is the single most important rule for reading GED food webs.
| Trophic level | Role | Examples |
|---|---|---|
| 1 | Producer | grass, trees, algae, phytoplankton |
| 2 | Primary consumer (herbivore) | grasshopper, rabbit, zooplankton |
| 3 | Secondary consumer | frog, songbird, herring |
| 4 | Tertiary consumer | snake, hawk, seal |
In a food web, one organism can sit at more than one level. A hawk that eats a mouse (level 2) is acting as a secondary consumer. The same hawk eating a snake that ate a frog is acting at a higher level.
Decomposers (bacteria, fungi, worms) are nature's clean-up crew. They feed on dead organisms at every level. They return nutrients such as nitrogen and phosphorus to the soil. Then producers reuse them. They are often left off the diagram but are essential.
Changes ripple through a web, like a stone dropped in a pond. If a species is removed:
- its prey usually increase (fewer are eaten);
- its predators lose a food source and may decline or switch to other prey;
- organisms two steps away can be affected in the opposite direction. Example: remove snakes → frogs increase → grasshoppers decrease. An invasive species (a newcomer with no natural predators) can disrupt a web by out-competing native species.
Energy pyramids and the 10% rule
In plain words: at each step up the food chain, most of the energy gets used up. Only a small part is passed on.
Think of a paycheck. Most of it goes to rent and bills. Only a little is left to pass on. Energy enters an ecosystem as sunlight. Producers capture it. It passes to consumers when they eat. At each step, only about 10% of the energy becomes part of the next level. What happens to the other ~90%?
- It is used for life processes (cellular respiration, movement, staying warm) and lost as heat.
- Or it stays in parts that are not eaten or digested (bones, fur, wood).
Example: if grass captures 10,000 kcal, grasshoppers get about 1,000 kcal, frogs about 100 kcal, and snakes about 10 kcal.
Consequences the GED likes to test:
- Food chains are short (usually 4–5 levels) because little energy reaches the top.
- There are far fewer top predators than producers (fewer hawks than mice, fewer mice than grass plants).
- Eating lower on the food chain wastes less energy. Grain can feed more people directly than by feeding that grain to cattle.
Energy flows; matter cycles. Energy moves one way and leaves as heat, so ecosystems need a constant supply of sunlight. Matter (carbon, nitrogen, water) is recycled again and again by producers, consumers, and decomposers.
Each level holds about one-tenth of the energy of the level below it.
Populations: growth, carrying capacity, and predator–prey cycles
In plain words: a population grows until it runs short of food, water or space, then it levels off.
Think of a new restaurant in town. At first, customers pour in. Then the tables fill up, and the number of diners each night levels off. With plenty of resources, a population grows faster and faster (exponential growth, a J-shaped curve). As resources run short, growth slows and the population levels off near the carrying capacity (an S-shaped, or logistic, curve).
Limiting factors set the carrying capacity:
- Density-dependent factors grow stronger as a population gets crowded: food and water shortages, competition, predation, disease.
- Density-independent factors affect a population regardless of its size: floods, fires, droughts, harsh winters.
Example: a pond that can support about 500 frogs will not hold 2,000 for long, even if many eggs hatch. More insects for food could raise the carrying capacity; a drought could lower it.
Predator–prey cycles:
- Prey increase.
- Predators have more food and increase.
- Predators eat more prey, so prey decrease.
- Predators run short of food and decrease.
- Prey recover, and the cycle repeats. On a graph, the predator peaks come after the prey peaks. This delay is a time lag. Predators are also usually far fewer than prey. Population graphs often use two vertical axes, so always check which axis belongs to which line.
Relationships between species
| Relationship | Species 1 | Species 2 | Example |
|---|---|---|---|
| Mutualism | + helped | + helped | bees get nectar; flowers get pollinated |
| Commensalism | + helped | 0 unaffected | barnacles riding on a whale |
| Parasitism | + helped | − harmed | tapeworm in a dog's intestine; tick on a deer |
| Predation | + helped | − killed and eaten | owl eats mouse |
| Competition | − | − | two bird species eating the same seeds |
Mutualism, commensalism, and parasitism are the three types of symbiosis. A parasite usually lives on or in a host for a long time without killing it quickly. A predator kills its prey.
Competition lowers the success of both species. Sometimes two species need exactly the same limited resource. Then one often wins. The other declines or disappears from that area — like two coffee shops on one small street. Example: in the lab, two protist species are fed the same bacteria. Both grow less well together than alone, and one may die out.
The 10% rule as a formula
E_{\text{next level}} \approx 0.10 \times E_{\text{level below}}For n steps up from the producers:
E_n \approx E_{\text{producers}} \times (0.1)^nExample: producers 50,000 kcal, three steps up: 50{,}000 \times 0.1^3 = 50 kcal.
Percent of energy transferred, when the GED gives real numbers:
\text{transfer efficiency} = \frac{E_{\text{upper level}}}{E_{\text{lower level}}} \times 100\%The Science test has no formula sheet, but the passage usually states the 10% rule.
Example 1: reading a food web
Use the meadow web: grass → grasshopper, mouse, rabbit; grasshopper → frog, songbird; frog → snake; mouse → snake, hawk; rabbit → hawk; songbird → hawk; snake → hawk.
Question: Which organism is a secondary consumer only?
A. Grass B. Songbird C. Hawk D. Rabbit
Think-aloud:
- Grass is the producer (A is out). Rabbit eats grass, a primary consumer (D is out).
- Songbird eats only grasshoppers (primary consumers), so it is always a secondary consumer.
- The hawk eats mice, which makes it secondary. But it also eats snakes and songbirds, which makes it tertiary or higher. So it is not "secondary only."
Answer: B.
Example 2: applying the 10% rule backwards
Question: In a lake, the secondary consumers (perch) receive about 400 kcal/m²/yr. About how much energy did the producers capture?
Think-aloud:
- Secondary consumers are two steps above producers.
- Going down one level means dividing by 0.1 (multiplying by 10).
- Primary consumers: 400 \times 10 = 4{,}000. Producers: 4{,}000 \times 10 = 40{,}000.
- Check: 40{,}000 \times 0.1 \times 0.1 = 400 ✓.
Answer: about 40,000 kcal/m²/yr.
Example 3: a carrying capacity graph
Data: deer on a reserve: year 0, 50; year 2, 140; year 4, 260; year 6, 300; year 8, 305; year 10, 298.
Question: What is the approximate carrying capacity, and what would most likely happen if wolves were reintroduced?
Think-aloud:
- Growth slows after year 4 and levels off around 300, so the carrying capacity is about 300 deer.
- Wolves are predators, a limiting factor. More predation would lower the deer population below 300.
- Over time, fewer deer could also mean more young trees and shrubs, a ripple effect through the food web.
Answer: about 300 deer; the deer population would most likely decrease.
Example 4: naming a relationship
Passage: "Cattle egrets follow grazing cattle. As the cattle walk, they stir up insects, which the egrets catch. The cattle do not appear to gain or lose anything."
Question: Which type of relationship is this?
Think-aloud:
- Egret: gets food → +.
- Cattle: "do not appear to gain or lose anything" → 0.
- A +/0 relationship is commensalism. It is not mutualism (cattle do not benefit) and not parasitism (cattle are not harmed).
Answer: commensalism. Note: if the passage had said the egrets also ate ticks off the cattle, the answer would change to mutualism. Always use the passage's details.
Common traps
- Backwards arrows. Wrong: "grass → rabbit means grass eats rabbits." Right: arrows point from food to eater (direction of energy flow).
- Energy is destroyed. Wrong: "90% of the energy disappears." Right: it is used and lost as heat. Energy changes form but is never destroyed.
- Recycling energy. Wrong: "Decomposers return energy to plants." Right: decomposers return nutrients (matter); energy must come from new sunlight.
- Carrying capacity = birth rate. Wrong: "More babies raise the carrying capacity." Right: carrying capacity is set by resources and limiting factors.
- Ignoring the second axis. Wrong: "There are more foxes than rabbits." Right: check which axis each line uses.
- Predation vs. parasitism. A predator kills and eats its prey; a parasite feeds on a living host over time.
GED tactics for ecosystems
- Trace with your finger (or cursor). Follow arrows one step at a time; list what each organism eats and what eats it.
- For "what if" questions, start at the changed species and move one arrow at a time, predicting increase or decrease.
- For pyramids, confirm the 10% pattern with two levels you know before calculating the unknown one.
- For population graphs, read the legend and axes first, find peaks and when they occur, and look for a time lag.
- For tables with many columns, point at the exact column the question names.
- Use +/0/− notation for symbiosis and competition questions.
- Avoid absolute claims (always, will definitely) and predictions beyond the data.
Check yourself
A Meadow Food Web
A food web shows feeding relationships in an ecosystem. Each arrow points from an organism to the organism that eats it, showing the direction in which energy flows. The food web below shows some of the organisms in a meadow. Grass is the only producer shown. Decomposers, such as bacteria and fungi, are present in the meadow but are not shown.
Which organism in the food web is a primary consumer?
Hint
A primary consumer eats producers. Which organism has an arrow coming to it from the grass?
Step-by-step solution
Evidence: the food web shows arrows from grass to the grasshopper, the mouse, and the rabbit. The passage explains that "Each arrow points from an organism to the organism that eats it."
Step by step:
- Grass makes its own food by photosynthesis, so it is the producer.
- A primary consumer eats the producer. The mouse eats grass (arrow from grass → mouse).
- The frog eats grasshoppers (a primary consumer), so the frog is a secondary consumer.
- The hawk eats mice, rabbits, songbirds, and snakes, so it is a higher-level consumer, not primary.
The answer is D. Mouse.
Tip: follow the arrows. An arrow means "is eaten by," so count how many steps an organism is from the producer.
Check yourself
A Meadow Food Web
A food web shows feeding relationships in an ecosystem. Each arrow points from an organism to the organism that eats it, showing the direction in which energy flows. The food web below shows some of the organisms in a meadow. Grass is the only producer shown. Decomposers, such as bacteria and fungi, are present in the meadow but are not shown.
Suppose a disease kills most of the snakes in the meadow. Which change is most likely to happen soon after?
Hint
Find what the snake eats. Animals that are no longer being eaten tend to increase in number.
Step-by-step solution
Evidence: in the food web, arrows go from the frog → snake and the mouse → snake, and from the snake → hawk.
Step by step:
- Snakes eat frogs and mice. With fewer snakes, fewer frogs are eaten.
- So the frog population is likely to increase. (Mice would likely increase too, but that is not offered as a choice.)
- Check the others: B is backwards (mice lose a predator, so they increase, not decrease). C is also backwards: more frogs eat more grasshoppers, so grasshoppers would likely decrease. D is wrong: hawks lose one food source (snakes), so hawks would not be expected to increase.
The answer is A.
Tip: for "what if a species disappears" questions, look one arrow away in both directions: its prey usually increase; its predators lose food.
Summary
- Arrows in food chains and webs point from food to eater; producers are level 1.
- Only about 10% of energy passes to the next trophic level. The rest is used and lost as heat, so pyramids narrow.
- Energy flows one way; matter (carbon, nitrogen) is recycled, with decomposers returning nutrients.
- Populations level off at the carrying capacity set by limiting factors such as food, space, predators, and disease.
- Predator peaks lag behind prey peaks; competition lowers both species.
- Mutualism (+/+), commensalism (+/0), parasitism (+/−): judge each partner from the passage's details.