Ecologist: A local population of red foxes in the Oakridge Forest increased by thirty percent over the last three years, coinciding with a severe decline in the population of coyotes, which compete with foxes for small mammal prey. Since the total abundance of small mammals in the forest remained constant throughout this period, the reduction in competition from coyotes must be the primary cause of the fox population increase.
Which one of the following is an assumption on which the ecologist's argument depends?
- AWhenever a forest's coyote population declines, the local red fox population is guaranteed to expand.
- BCoyotes are the only predatory species in Oakridge Forest that compete with red foxes for prey.
- The increase in the red fox population in Oakridge Forest was not primarily the result of an influx of foxes migrating from adjacent areas.Answer
- DA reduction in competitor populations will always cause predator populations to rise in any ecosystem.
- ERed foxes in Oakridge Forest prey on a wider variety of small mammal species than coyotes do.
Answer
The argument depends on the assumption that the increase in the red fox population in Oakridge Forest was not primarily the result of an influx of foxes migrating from adjacent areas.
The author concludes that reduced competition from coyotes caused the increase in the local fox population. This causal claim requires that an alternative explanation—such as foxes moving in from neighboring territories—is not the real reason for the population gain. When negated, if migration was the primary cause, the author's causal conclusion is invalidated.
Step-by-Step Solution
Key Concept
Necessary Assumption (Defender Assumption)