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GED Science: Ecosystems and Energy Flow Concepts

Importance ★★★★★ · 20% (estimate)

Overview

This chapter covers how energy and matter move through ecosystems and how populations affect one another. GED questions often show a food web or energy pyramid and ask what happens when one population changes.

Producers such as plants make their own food from sunlight. Consumers eat other organisms, and decomposers such as fungi and bacteria break down dead matter and return nutrients to the soil. In a food chain or food web, arrows point from the food to the organism that eats it, showing the direction energy flows. Only about 10% of the energy at one level passes to the next; the rest is used for life processes or lost as heat. That is why an energy pyramid gets narrower at the top.

Populations are limited by food, water, space, and predators. The largest population an area can support over time is its carrying capacity. Organisms also interact through symbiosis: in mutualism both benefit, in commensalism one benefits and the other is not affected, and in parasitism one benefits and the host is harmed. Invasive species, habitat destruction, and other disruptions can upset these relationships and cause local extinctions.

Key concepts

  • Producer, consumer, decomposer

    Producers make food from sunlight. Consumers eat other organisms. Decomposers break down dead matter and recycle nutrients.

    Memory tipProducers have no arrows coming in from food.

  • Food web arrows

    Each arrow points from the food to the eater, showing the direction energy flows.

    Memory tipFollow the arrow: grass → rabbit means the rabbit eats grass.

  • 10 percent rule

    About 10% of the energy at one level passes to the next level. Example: 10,000 kcal in producers → 1,000 → 100 → 10.

    Memory tipMultiply by 0.1 for each step up.

  • Carrying capacity

    The largest population an environment can support over time. A population that levels off is near its carrying capacity.

    Memory tipLook for where the population levels off.

  • Symbiosis

    Mutualism (both benefit), commensalism (one benefits, other unaffected), parasitism (one benefits, host harmed).

    Memory tipAsk who benefits and who is harmed.

  • Invasive species

    A non-native species that spreads in a new area and harms the ecosystem, often because it has no natural predators there.

    Memory tipNative = always lived there; invasive = brought in and spreading.

See it in a picture

Energy pyramid with four levels. Producers at the bottom have 10,000 kcal, primary consumers 1,000 kcal, secondary consumers 100 kcal, and tertiary consumers at the top 10 kcal.
About 10% of the energy passes up to each next level, so the pyramid narrows toward the top.

Often tested

  • If an animal has only one food source, it is hit hardest when that food declines.
  • Fewer predators usually means more prey, at least for a while.
  • Mark-recapture estimate: tagged in sample ÷ sample size = total tagged ÷ total population.
  • Habitat destruction usually lowers the number of species in an area.

3 sample questions

Verified practice questions from this unit.

Practice questionReviewedLife Science › Ecosystems and Energy Flow★★★★★

Question 1. The food web shows which organisms eat which. Each arrow points from the food to the organism that eats it. Which organism in this food web is a producer?

Food web with seven organisms. Grass is at the bottom. Arrows go from grass to grasshopper, from grass to rabbit, and from grass to mouse. Arrows go from grasshopper to frog, from frog to snake, from mouse to snake, from mouse to hawk, from rabbit to hawk, and from snake to hawk.
  1. ① Hawk
  2. ② Snake
  3. ③ Grasshopper
  4. ④ Grass
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ④ Grass

Key point: Producers make their own food from sunlight

A producer makes its own food by photosynthesis. In a food web, producers have arrows going out to the animals that eat them, but no arrows coming in from food. Grass is the only producer in this web.

Wrong choices

  • Hawk: The hawk eats other animals, so it is a consumer.
  • Snake: The snake eats frogs and mice, so it is a consumer.
  • Grasshopper: The grasshopper eats grass, so it is a consumer.
Practice questionReviewedLife Science › Ecosystems and Energy Flow★★★★★

Question 2. The food web shows which organisms eat which. Each arrow points from the food to the organism that eats it. A disease kills most of the grasshoppers. Which population is most likely to decrease first because of this?

Food web with seven organisms. Grass is at the bottom. Arrows go from grass to grasshopper, from grass to rabbit, and from grass to mouse. Arrows go from grasshopper to frog, from frog to snake, from mouse to snake, from mouse to hawk, from rabbit to hawk, and from snake to hawk.
  1. ① Snake
  2. ② Grass
  3. ③ Frog
  4. ④ Rabbit
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ③ Frog

Key point: A consumer with only one food source is hit hardest

In this food web, the only arrow into the frog comes from the grasshopper. The frog has no other food, so if most grasshoppers die, the frog population is likely to drop first.

Wrong choices

  • Snake: The snake also eats mice, so it still has food. It may be affected later, but the frog loses its only food.
  • Grass: Grasshoppers eat grass, so fewer grasshoppers would most likely mean more grass, not less.
  • Rabbit: Rabbits eat grass, not grasshoppers. With fewer grasshoppers eating grass, rabbits may even have more food.
Practice questionReviewedLife Science › Ecosystems and Energy Flow★★★★★

Question 3. Bees drink nectar from flowers, and while they do, they carry pollen that helps the flowers make seeds. What kind of relationship is this?

  1. ① Parasitism
  2. ② Predation
  3. ③ Commensalism
  4. ④ Mutualism
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ④ Mutualism

Key point: Mutualism: both organisms benefit

Both partners benefit: the bee gets food (nectar), and the flower gets pollinated. A relationship where both sides benefit is called mutualism.

Wrong choices

  • Parasitism: In parasitism, one organism is harmed. Here both the bee and the flower benefit.
  • Predation: In predation, one animal kills and eats another. The bee does not eat the flower.
  • Commensalism: In commensalism, one benefits and the other is not affected. Here both benefit.
Practice 5 questions from this unit →

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