Food Webs and Energy — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. Why does an energy pyramid get narrower at the top?
A) There are more predators than prey
B) Energy is gained at each level
C) Energy is lost as heat at each level so less is available
D) Top predators create their own energy
About 90% of energy is lost as heat at each trophic level. Only ~10% transfers up, so each level supports fewer organisms.
2. In a pond food web, algae are eaten by snails, and snails are eaten by fish. What role do the snails play?
A) producers
B) decomposers
C) primary consumers
D) top predators
Snails eat algae (the producer), making snails primary consumers (first-level consumers).
3. If the frog population in a food web suddenly drops, what is the most likely effect?
A) Insect populations increase because fewer frogs eat them
B) Snake populations increase because they have more food
C) Plant populations decrease because frogs no longer eat them
D) Decomposer populations immediately disappear
Frogs eat insects. With fewer frogs, less predation pressure means insect populations can grow.
4. Which organism would be found at the first trophic level?
A) hawk
B) grasshopper
C) grass
D) frog
The first trophic level consists of producers. Grass photosynthesizes to make its own food.
Part B: Fill in the Blank
Write the correct answer on each line.
1. The original source of energy for almost all food webs on Earth is the sun.
Solar energy drives photosynthesis in producers, which forms the energy base for virtually all ecosystems.
2. A tertiary consumer eats secondary consumers.
Tertiary consumers occupy the fourth trophic level and eat secondary consumers.
3. Organisms that eat dead organic matter are called decomposers.
Decomposers (fungi, bacteria, some worms) break down dead organisms, recycling their matter back into the ecosystem.
4. In an energy pyramid, the level with the least energy is the top.
Because energy is lost at each trophic level, the apex (top predators) receives the least total energy.
5. Photosynthesis converts light energy into chemical energy stored in food.
Photosynthesis stores solar energy as chemical energy in glucose molecules, making it available to consumers.