Question

1. review: what happens to the amount of kelp in a coastal ecosystem when the number of sea otters decreases?

otters eat sea urchins but have declined due to overhunting and disease
sea urchins eat kelp holdfasts, leading to a reduction in this ecosystem engineer
a) the amount of kelp remains the same.
b) the amount of kelp decreases because the number of urchins grazing on kelp increases.
c) the amount of kelp decreases because the number of killer whales decreases.
d) the amount of kelp increases because the number of urchins grazing on kelp increases.

Explanation:

To solve this, we analyze the food - web relationships:

1. Sea otters eat sea urchins. So, if the number of sea otters decreases, there will be less predation on sea urchins.
2. As a result, the number of sea urchins will increase (since there are fewer otters to eat them).
3. Sea urchins eat kelp holdfasts. With more sea urchins grazing on kelp, the amount of kelp will decrease.

Now let's analyze each option:

• Option A: The amount of kelp will not remain the same because the change in sea otter population affects sea urchin population and thus kelp. So, A is incorrect.
• Option B: When sea otters decrease, sea urchin numbers increase (due to less predation). And since sea urchins eat kelp, more urchins will lead to more kelp being eaten, so kelp amount decreases. This option is consistent with our analysis.
• Option C: The question is about the relationship between sea otters, sea urchins, and kelp. The number of killer whales is not relevant to this specific relationship described in the problem. So, C is incorrect.
• Option D: If the number of urchins grazing on kelp increases, kelp should decrease, not increase. So, D is incorrect.

Answer:

B) The amount of kelp decreases because the number of urchins grazing on kelp increases.