Energy in Ecosystems — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. In Grade 5 science, what is a keystone species?
A) A species whose removal greatly changes the ecosystem
B) Any large animal in the ecosystem
C) A plant that grows the fastest
D) An animal that lives only in zoos
A keystone species has effects on its ecosystem that are much larger than its numbers would suggest.
2. What happened to elk numbers in Yellowstone after wolves were removed?
A) Elk numbers grew very high
B) Elk numbers stayed exactly the same
C) Elk disappeared completely
D) Elk turned into deer
With no wolves to hunt them, elk populations grew and overgrazed many Yellowstone plants.
3. Why did willow and aspen trees decline after wolves were removed?
A) Elk ate young trees before they could grow
B) The trees got too much rain
C) Beavers ate every single one
D) The trees moved to other parks
Overgrazing by elk kept young willow and aspen from growing into mature trees in Grade 5 ecosystems.
4. What did bringing wolves back to Yellowstone help restore?
A) Biodiversity and healthy riverbanks
B) More highways through the park
C) Year-round snow on the ground
D) Bigger parking lots near rivers
Returning wolves lowered elk numbers, letting plants regrow and supporting beavers, birds, and rivers.
Part B: Fill in the Blank
Write the correct answer on each line.
1. A species that has very large effects on its ecosystem is called a keystone species.
Keystone species hold ecosystems together, just as a keystone holds an arch in place.
2. When wolves were removed from Yellowstone, elk populations increased too much.
Elk grew unchecked without wolf predators, harming the Grade 5 ecosystem balance.
3. Healthy riverbanks returned partly because beavers had more willow to eat.
When elk grazing dropped, willow trees recovered and supported beavers and other species.
4. The variety of life that returned to Yellowstone is called biodiversity.
Biodiversity grew as more plants, birds, and mammals reappeared after wolf reintroduction.
5. When predators and prey numbers stay roughly steady, ecosystems reach equilibrium.
Equilibrium is the balanced state that keystone species help maintain in Grade 5 ecosystems.