#8 - Over the last thousand years, plant species native to

[Administrator]

Please post your questions below! Thank you!

[mepstlsat24]

Hi why is C wrong? Wouldnt commercial development lead to more humans which lead to more mammals which eat the plants? I had a hard time distinguishing between C and D.

Thanks!

[James Finch]

Hi Mepst,

The conclusion that needs to be strengthened in this question is that large land mammals are causing the extinction of native plants on islands. The premise is that human colonization is what leads to the presence of these large land mammals. So we need to strengthen the link between human colonization, introduction of large land mammals, and extinction of native plant species, or

human colonization introduction of large land mammals extinction of native plant species

Answer choice (C) is positing an alternate cause, commercial development, which if true would weaken the conclusion. Human colonization is not synonymous with commercial development, and does not imply introduction of large land mammals.

Answer choice (D) ignores the large land mammals and simply creates a linkage between human colonization and extinction of native plant species. While this does leave open the possibility that some other byproduct of human colonization is causing the extinctions, it does provide more evidence for the causal chain we see in the stimulus, making it a correct Strengthen answer choice.

Hope this helps!

[Michaeltinti22]

Just a clarification question, isn't the conditional chain in the stimulus populations of animals established the island has been colonized by humans, since until works like unless?

[Ben DiFabbio]

Just a clarification question, isn't the conditional chain in the stimulus populations of animals established the island has been colonized by humans, since until works like unless?

Hi Michael,

You're exactly right.

"Populations of large land mammals are not established on islands until after the island is colonized by humans."

This last sentence in the stimulus works conditionally like an "unless" statement. If populations of large land mammals were established, then we definitely know that the island has been colonized by humans.

[Iam181]

Just a clarification question, isn't the conditional chain in the stimulus populations of animals established the island has been colonized by humans, since until works like unless?

Hi Michael,

You're exactly right.

"Populations of large land mammals are not established on islands until after the island is colonized by humans."

This last sentence in the stimulus works conditionally like an "unless" statement. If populations of large land mammals were established, then we definitely know that the island has been colonized by humans.

But in answer D we only know that the necessary "the human colonization" is met. How does show large mammals are stablished. It just make it more possible that they were established?

I'm confused how by meeting the necessary condition here in this problem it led us to believe the sufficient condition has also been met.

[KelseyWoods]

Hi Iam181!

I think there's some confusion over James's initial diagram for this question. Yes, we normally reserve arrows for conditional diagramming. But in this case, James was not providing a conditional reasoning diagram, he was making a causal diagram. So first you have human colonization, then populations of large land mammals become established, then plant species become extinct.

It's true that "until" is a conditional indicator word and we can diagram that last sentence in a conditional way (as Michael and Ben discussed). But this question is more easily thought of as a causal reasoning question because the biologist's explanation (large land mammals causing the extinction of island plant species) is causal, rather than conditional, in nature. So you need the humans first, they cause the populations of large land mammals to become established, and that causes the plant extinctions.

You're correct that human colonization doesn't guarantee that large land mammals become established because it is a necessary condition, but remember we're only trying to strengthen this explanation, not prove it 100%. And satisfying that necessary condition, or the first cause in our causal relationship chain, does strengthen the idea that it's the large land mammals causing the extinction. As James says, it still leaves open the possibility that it's some other by-product of human colonization that is responsible for the extinctions. But again, that's ok because we're only trying to help the argument, not definitively prove it.

Hope this helps!

Best,
Kelsey

[nosracgus]

Hi! Why is A incorrect?

My reasoning is that 1. D strengthens much more, and 2. A is basically a restatement of the first sentence (and therefore would be a better MBT answer than a Strengthen answer).

Am I on the right track?

Thanks!
C

[Jeremy Press]

Hi C,

Answer choice A doesn't actually repeat anything that's stated in the stimulus, including the first sentence. The first sentence speaks to the rate of extinction (native island species have a faster rate of extinction), whereas answer choice A speaks to the composition of non-extinct plants (more than half native to mainland regions). Just because the rate of island extinctions has sped up doesn't necessarily mean there are now more mainland than island species. However, the reason answer choice A is incorrect is that it doesn't shed any light on why the island species are going extinct at a faster rate. That's the issue we're being asked to strengthen (the "biologist's explanation," i.e. the reason they give for the faster rate). Since answer choice A doesn't shed any light on the biologist's explanation, it does not strengthen it. So it's incorrect for that reason alone.

I hope this helps!

[Emilymarie]

I'm confused by the "rate of extinction tends to increase dramatically" part of Answer Choice D. I thought the stimulus was saying that native plant species have a higher rate of extinction BECAUSE they haven't gotten to build up defenses to large land mammals. Once humans colonize the island and large land mammals arrive, wouldn't the rate of extinction decrease? Or is the assumption here that the rate of extinction temporarily goes up as the plants get used to the large land mammals eating them, to which the plants develop defenses over time and the rate of extinction goes down? Let me know if my understanding is off here! I thought this was an opposite answer because of this rate of extinction piece.