Logical Reasoning

PT16 · S2 · Q21 Several years ago, as a measure

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Several years ago, as a measure to reduce the population of gypsy moths, which depend on oak leaves for food, entomologists introduced into many oak forests a species of fungus that is poisonous to gypsy moth caterpillars.

Conclusion

The decline in the gypsy moth population is attributable to the presence of the poisonous fungus.

Evidence

Several years ago, as a measure to reduce the population of gypsy moths, entomologists introduced a species of fungus that is poisonous to gypsy moth caterpillars into many oak forests.

Since then, the population of both caterpillars and adult moths has significantly declined in those areas.

Evaluate

We have a correlation between the introduction of poisonous fungus and the decline in the gypsy moth population. The entomologists are assuming a causal relationship, suggesting the fungus caused the population decline.

Curious Fact?

What's causing the decline in gypsy moth populations?

Author's Explanation

The poisonous fungus introduced by entomologists is causing the decline.

To strengthen this argument, we would want evidence that specifically ties the decline in moth populations to the fungus, ruling out other potential causes and making the entomologist's explanation more plausible.

Goal

Let's find an answer that strongly supports the idea that the poisonous fungus is the main cause of the decline in gypsy moth populations, rather than some other factor.

21.

Which one of the following, if true, most strongly supports the conclusion drawn by the entomologists?

  1. Correct

    A strain of gypsy moth

    Why this is right

    This gives us a classic "No Cause, No Effect" plausibility strengthener (the most common form of correct answer when we Strengthen an Explain Curious Fact argument). The author thought that the fungus was causing a decrease in gypsy moths. This answer shows that where there is no danger of the fungus (a strain that's unaffected by it), there is no decrease in gypsy moths (in fact, there's an increase!) To put it another way, "You can tell that the fungus is effective, because the only gypsy moths that are doing okay are the ones that have an immunity to the fungus."

    Skill tested: Strengthen · how this choice captures the argument's function is the move to repeat next time.

    71% picked this

  2. The fungus that was introduced

    Unclear Impact

    It doesn't really matter whether the fungus is a surgically specific killer of moth but not other insects, or whether the fungus is just a big ol' bomb that wipes out lots of insects too. The conclusion is just about whether the fungus is killing the moths, which could happen or not happen in either of those scenarios. We don't know whether other insects are / aren't also in decline. So we have no way to judge whether the actual decreases in insects are moth-specific or more generalized. If we knew that few other insect species in the moth's habitat have been experiencing a decrease since the fungus was introduced, then this answer would strengthen somewhat.

    16% picked this

  3. An increase in numbers of

    Opposite (if anything)

    This answer is talking about how changes in predator population seem to influence changes in the moth population. If anything, that's giving us fodder for an alternate explanation of why the moths are in decline (maybe their predators are on the rise!).

    7% picked this

  4. In the past several years,

    Opposite Impact

    This weakens because it suggests another potential cause for the decline: reduced oak populations due to air pollution and acid rain. If there are fewer oak leaves due to these factors, caterpillars might starve, offering another explanation and thus weakening the argument.

    2% picked this

  5. The current decline in the

    Opposite Impact

    This weakens the argument, because it hurts the plausibility that the fungus is the cause of the decrease in moths, because the decrease in moths is occurring to the same degree in places where there isn't fungus. (a classic Effect w/o Cause weakener)

    4% picked this

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