Food Webs and Energy — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. In an ocean food web, phytoplankton are eaten by zooplankton, which are eaten by small fish. What trophic level are the small fish?
A) first trophic level
B) second trophic level
C) third trophic level
D) fourth trophic level
Phytoplankton are the first trophic level (producers), zooplankton are the second (primary consumers), so small fish eating zooplankton sit at the third trophic level as secondary consumers.
2. A forest food web has oak trees, deer, wolves, and mushrooms. Which organism recycles nutrients back to the soil?
A) oak tree
B) deer
C) wolf
D) mushroom
Mushrooms are decomposers that break down dead leaves, wood, and animal remains into nutrients. Those nutrients return to the soil where oak trees and other producers can absorb them again.
3. Why do food webs usually have fewer top predators than herbivores?
A) Top predators are larger and need more space
B) Only about ten percent of energy passes to each next level
C) Herbivores reproduce more slowly than predators
D) Predators eat only plants so they need less energy
Only about 10% of energy passes from one trophic level to the next, with the rest lost as heat. By the time energy reaches top predators, so little remains that the ecosystem can only support a small number of them.
4. A farmer removes all the hawks from a field ecosystem. What is the most likely short-term result?
A) Mouse populations will decrease
B) Grass populations will increase right away
C) Mouse populations will increase because fewer predators eat them
D) Snake populations will disappear
Hawks are key predators that keep mouse numbers in check. Without hawks hunting them, mice face less predation pressure and their population would quickly grow.
Part B: Fill in the Blank
Write the correct answer on each line.
1. Plants convert sunlight into chemical energy through photosynthesis.
During photosynthesis, plants store the sun's light energy as chemical energy inside sugars and starches. This chemical energy is what animals later use when they eat the plants.
2. In a food web diagram, arrows show which direction energy moves.
Arrows in a food web diagram point from the organism being eaten to the one eating it. They trace the path that energy follows as it moves through the ecosystem.
3. A keystone species is one that many other organisms in a food web depend on.
A keystone species plays such a critical role that removing it would cause major changes throughout the food web. Many other organisms depend on it for food, shelter, or balance.
4. Organisms that eat at more than one trophic level, such as bears, are called omnivores.
Bears eat berries (producers) and fish (consumers), which means they feed at multiple trophic levels. This ability to eat both plants and animals is what defines an omnivore.
5. The ten percent rule means only a small fraction of energy moves to the next level.
The ten percent rule means that at each trophic level, roughly 90% of energy is used up or lost as heat. Only about 10% gets passed on to the next level.