Some Animals Are More Equal than Others: Trophic Cascades and Keystone Species (Marine Biology) Short Film

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

False. Some members of a food web can have effects that are disproportionate to their abundance.

Explain the reasoning to:
Why "all members of a food web are equal in abundance and on their relative effects on one another" is false.

Some species, which are not as abundant as some other species, can have large effects on the structure of their ecosystems.

True or False:
Every member of a food web is the prey of another member of the food web.

False. Apex predators are at the top of food webs and don't have predators.

Why are the mussels in Mukkaw Bay able to quickly cover the rockface in Paine's experiment?

Starfish feed on mussels, so when the starfish were removed the mussels no longer had a predator and their populations grew unchecked.

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 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.

I. only

How did the green world hypothesis differ from this "bottom-up" view?

The green world hypothesis
explained that the number of herbivores is also limited by predators from the top down.

Imagine a simple food chain: Grass -> Grasshoppers -> Mice. If snakes that eat mice are added to this ecosystem, how would you redraw the food chain to represent this change?

Grass -> Grasshoppers -> Mice -> Snakes

Grass -> Grasshoppers -> Mice -> Snakes
After the snakes are added, would you expect the amount of grass to increase or decrease? Explain your reasoning.

The snakes will cause a reduction in mouse populations, leading to an increase in grasshoppers that will then eat the grass

In 1997, which species is the apex predator in the food chain?

Killer whales

Draw two diagrams that show the food chains for the control and experimental plots. Include interactions among predatory beetles (if present), ants, caterpillars, and piper plants.

Control Experimental
piper plants piper plants ants caterpillars caterpillars beetles

What statement do Letourneau and Dyer's results support?

Adding beetles reduced ant numbers and triggered a trophic cascade that decreased the mean leaf area left on plants.

Do the results of the Letourneau and Dyer experiment support or refute the green world hypothesis? Explain your answer.

The experiment supports the green world hypothesis because changes in plant leaf area were caused by predators controlling herbivore numbers from the top down. Adding beetles decreased ant predation on caterpillars and led to increased caterpillar herbivory and decreased plant leaf area.