A species can change others it never eats
- The idea
- Predict
- See it
- Why
- Check
The idea
“If two species aren't directly linked in a food web, a change in one won't affect the other.”
It's easy to think an animal only affects what it eats and what eats it. Its effect actually keeps going, through every species in between.
The short version
The mistake: If two species aren't directly linked in a food web, a change in one won't affect the other.
What's actually true: A change in one species passes through the species in between, so it can affect ones it never eats or touches.
- Trace a change one link at a time. Fewer otters means more urchins survive, and more urchins means less kelp.
- Each step down a simple food chain flips the direction. A predator lowers its prey, which raises what that prey eats. This is called a trophic cascade.
- Kelp also shelters many fish, so losing otters can mean fewer fish. A species with this much effect for its numbers is a keystone species.
On the exam: Follow the change one link at a time, and flip its direction at each step down the chain.