PrepTest 134, Section 1, Question 8
According to the "bottom-up" theory of how ecosystems(56 more words not shown)
Which one of the following,(9 more words not shown)
- AIn an effort(40 more words not shown)
- BAfter hunting virtually(24 more words not shown)
- CAfter many of(27 more words not shown)
- DAfter a new(40 more words not shown)
- EPlants that are(31 more words not shown)
Reading the argument
Plants feed herbivores, and herbivores feed predators. I can see why the amount of edible plant life would limit how many animals an ecosystem supports.
But why would removing predators have little effect?
Imagine wolves eating deer and deer eating plants. If many wolves disappear, more deer might survive. More deer then eat more plants. A change at the top of the food chain could work its way down through the whole system. Plants could still be essential to the ecosystem while predators also have a substantial influence.
What we’re looking for
That gives us a clear response to the theory's second claim. We don't have to reject the entire idea that food supplies matter. The question asks for evidence against the theory, and evidence that reducing predators dramatically changes the rest of the ecosystem would challenge something the theory expressly predicts.
Working the answers
- A
This fits the bottom-up picture. Releasing more monkeys doesn't solve the shortage of the trees they need for food. If there isn't enough food to support the animals, the population-building effort can fail. That supports the importance of the plant supply rather than showing that predators substantially affect the ecosystem.
- BCorrect
Predators were virtually eliminated, herbivore populations increased more than tenfold, and plants were dramatically reduced. That is the cascade we imagined: fewer animals eating the herbivores, many more herbivores eating plants, and a major change in plant density. The theory predicts little impact from reducing predators, but this example describes an enormous impact extending through the ecosystem.
- C
Here the initial change occurs in the plants. Trees are cleared, the ants that require that forest decline, and the anteaters decline too. That moves from the plant level to the animals depending on it, just as the bottom-up account would lead us to expect.
- D
The herbivores lost one kind of food but could thrive on its replacement. The identity of the fern changed without removing the edible food supply supporting those animals. Stable herbivore numbers in those circumstances don't conflict with the theory's focus on available plants. A plant change doesn't have to mean a shortage of food.
- E
The pigs' food has flourished, and their numbers have remained fairly stable despite hunting. That doesn't show a dramatic effect spreading from fewer predators to herbivores and plants. In fact, a strong food supply could help the pig population replace animals lost to hunting. This can fit the idea that the plant supply helps sustain the population; it does not provide the clear conflict that B does.
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