Trophic Cascades

1. True/False. All members of a food web are equal in abundance and in their relative effects on one another.

2. Explain the reasoning or evidence you used to answer Question 1.

3. True/False. Every member of a food web is the prey of another member of the food web. _________

4. Explain the reasoning or evidence you used to answer Question 3.

5. Which statement below explains why the mussels in Mukkaw Bay were able to quickly cover the rockface in
Paine’s experiment?

a. The starfish took up most of the room on the rocks, and when the starfish were removed, the mussels occupied the empty spaces.
b. Paine added more mussels to the rocks, causing the starfish to move to other habitats.
c. The starfish were competing directly with the mussels for food, and removing the starfish allowed the mussels more access to the food.
d. Starfish feed on mussels, so when the starfish were removed the mussels no longer had a predator and their populations grew unchecked.

6. In the film, Paine recalls that a year after the starfish had been removed, the number of species decreased from 15 to eight, after three years the number went down to seven, and after another seven years it was almost all only mussels. In the control plots the number and diversity of species was basically unchanged.

Which statement(s) best explain(s) these results?
I. Keystone species are critical to the diversity and stability of an ecosystem.
II. When a predator is removed, the prey of that predator always increases and species not eaten by the predator always decrease.
III. The disappearance of producers from an ecosystem can cause the number of predators to increase.
a. I only
b. I and II only
c. II and III only
d. I, II, and II

7. Before the 1960s, most ecologists thought that the number of producers in an ecosystem was the only variable that limits the number of herbivores. The idea was that every level was regulated by the amount of food from the trophic level below it.

a. How did the green world hypothesis differ from this “bottom–up” view?
b. Imagine a simple food chain: Grass –> Grasshoppers –> Mice. If snakes that eat mice are added to the ecosystem, how would you redraw the food chain to represent this change?
c. After the snakes are added, would you expect the amount of grass to increase or decrease? Explain your reasoning.