Logical Reasoning

PT149 · S3 · Q8 Over the last thousand years

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Over the last thousand years, plant species native to islands have gone extinct at a much faster rate than have those native to mainland regions.

Curious Fact

How come, over the past 1000 years, plant species native to islands have gone extinct at a much faster rate than those native to mainland regions.

Biologists' Explanation

Mainland plants have evolved defenses against being eaten by large land mammals, whereas island plants have not, since large land mammals don't show up on islands until after an island is colonized by humans.

Evaluate

The two pressure points for any Explain Curious Fact argument are 1. Alternate Explanations for the curious fact 2. the Plausibility of the Author's Explanation

We can strengthen such an argument by ruling out an alternative explanation or by increasing the plausibility of the explanation offered by the author.

On Strengthen questions (especially those whose question stem asks us to strengthen explanation / hypothesis), the correct answer is much more likely to increase plausibility.

The most common form of that type of answer is (i.e. No Cause / No Effect data points).

The author thinks that caused island plants to go extinct more quickly. So a No Cause / No Effect answer could sound like, .

Goal

Let's look for anything that corroborates this idea that islands don't have large land mammals until humans show up. Once humans show up with large land mammals, those land mammals eat the plants into extinction (sooner than the plants can evolve defense mechanisms).

8.

Which one of the following, if true, most strongly supports the biologist's explanation cited above?

  1. Most of the plant species

    Rate vs. Number

    First of all, this is really just reiterating the background fact. We already know that mainland plant species have gone extinct at a slower rate than have island plant species. We don't need to be convinced of that, but that seems to be what this answer is trying to do: "most living plant species are native to mainland!" But we also wouldn't do anything with this answer no matter what because it's talking about raw numbers, whereas the argument cared about rates. When we think about how much land mass on Earth is mainland vs. how much is island, it's probably at least 10 to 1 in favor of mainland. With 10x as much square footage, we would certainly expect there to be 10x as many plants and maybe 10x as many species on mainland. So we don't care about the raw numbers. We care about what percent of those species are going extinct. It's no fair to compare the raw numbers, since mainland will have such a higher total to start with.

    9% picked this

  2. Many plant species that are

    No Impact

    This is saying that "at least 5 plant species that are native to mainland have been well established on islands". Okay, cool? That's a very weak idea and it doesn't help us assess whether the higher extinction rate of island plants is due to large land mammals.

    4% picked this

  3. Commercial development on many islands

    Weakens

    This points to a potential Alternate Explanation: "Plants on islands aren't going extinct faster because they don't have defenses for large land mammals, they're going extinct faster because we keep uprooting their habitats to build our seaside resorts".

    2% picked this

  4. Correct

    The rate of extinction of

    Why this is right

    This has some punch, "tends to increase dramatically" after human colonization. Did the biologists think that human colonization was part of why island plants go extinct more often? Yes! The story the biologists are telling is that humans colonize an island, then they bring their large land mammals over, then those mammals eat up a bunch of plant species to extinction. In a sense, this answer is providing the No Cause / No Effect form we were predicting, because it's saying, "prior to humans arriving (and thus prior to there being any large land mammals on the island), the rate of extinction was way lower". That makes it seem like the arrival of humans is a big causal difference-maker. And since large land mammals don't arrive until humans arrive, we have a three-way correlation with "no humans || no large mammals || much lower extinction rate" vs. "humans || large mammals || higher extinction rate". It could be that humans are causing these island extinctions some way besides bringing over large land mammals, but the answer doesn't need to prove the biologists are right. It just has to add some plausibility to the causal story they're telling, which this does.

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

    83% picked this

  5. Large land mammals tend to

    Weakens

    This hurts the plausibility of the biologists' causal story. They are saying that humans arrive at an island, then large mammals come with them, then the large mammals eat some of these island plants to extinction. But this answer throws a monkeywrench into that story by saying, "Nahh ... those large land mammals wouldn't have been interested in chowing down on island plants. They prefer mainland plants."

    3% picked this

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