When One Predator Disappears: Trophic Cascades

Science · Ocean Life · ages 12-14

Name ______________________ Date ____________

What do sea otters eat that helps protect kelp forests?
- Kelp
- Urchins
- Fish eggs
About what percentage of energy moves from one trophic level to the next in a food chain?
- 10%
- 1%
- 50%
A trophic cascade means that removing one predator can change an entire ecosystem.
Circle one: True False
Boats used to catch huge tuna and sharks. Now many boats catch small fish like sardines and anchovies instead. What is this pattern called?
- Fishing down the food web
- Ocean rewilding
- Energy transfer up the chain
Sea otters were hunted almost to extinction for their fur. What happened next in the kelp forest?
- Kelp grew faster because otters were gone
- Nothing changed because otters don't affect kelp
- Urchins boomed and ate through the kelp
A kelp forest has 1000 units of energy stored in kelp. Using the 10 percent rule, about how many units of energy reach the sea urchins that eat the kelp?
Answer: ______________
A classmate says fisheries just need to protect the one species with the most value, and the rest of the ecosystem will be fine. What is the problem with this idea?
- It is correct because each species lives completely separately
- It is correct because energy transfer is always 100% efficient
- It ignores cascades when key species are removed
Scientists reintroduce sharks to a reef and whale populations recover nearby. What is the best reason this could help the ecosystem?
- Sharks and whales have no effect since they are just two species
- They cycle nutrients and keep prey numbers in balance
- More predators always means less food for everyone

Answer key

For grown-ups. Fold this page away before handing over the rest.

When One Predator Disappears: Trophic Cascades W1-mt_NPTjGJIyb3-s1

Urchins · Otters eat sea urchins, and urchins are the ones that eat kelp, so otters help keep kelp forests alive.
10% · Only about 10 percent of the energy at one level passes up to the next. The rest is used for living and moving.
True · That is exactly what a trophic cascade is: one change ripples through many species.
Fishing down the food web · Fishing down the food web happens when the biggest predator fish are caught out, so fishing moves to smaller species.
Urchins boomed and ate through the kelp · With no otters to eat them, sea urchins multiplied and destroyed the kelp forest, leaving bare rock called an urchin barren.
100 · 10 percent of 1000 is 100, so about 100 units of energy pass up to the sea urchins.
It ignores cascades when key species are removed · Ecosystems are connected through food webs, so protecting only one valuable species can still let cascades collapse the rest of the system. That is why ecosystem-based management looks at the whole web.
They cycle nutrients and keep prey numbers in balance · Predators like sharks keep prey populations in check, and whales carry nutrients from deep water to the surface, feeding tiny ocean plants that support the whole food web.