PT149 S3 Q8 ExplanationOver the last thousand years

Answer choices, explained

1. A

   Rate vs. Number9% picked this

   Most of the plant species in the world that have not yet gone extinct are

   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%
2. B

   No Impact4% picked this

   Many plant species that are not native to islands have become very well established on

   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%
3. C

   Weakens2% picked this

   Commercial development on many islands has resulted in loss of habitat for

   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%
4. D

   Correct83% picked this

   The rate of extinction of native plant species on an island tends to increase dramatically

   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%
5. E

   Weakens3% picked this

   Large land mammals tend to prefer plants from species native to mainland regions over plants from

   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%