Q19 - The goblin fern

[nbayar1212]

I don't understand why answer choice E is right.

This is what I gather from the stimulus:
1. that the GF required a thick layer of LL and is disappearing.
2. Where there is no GF, LL is very thin and LR is there.
3. LR eats leaf litter.
Conclusion: It's probably responsible for the fern's disappearance.

E says LR doesn't favor places that have thinner leaf litter than what GR require.

If I negate that to say "LR DOES favor places where the LL is thinner than what the goblin fern requires" I don't seem to destroy the conclusion... It would still make me think, yeah, the LR like thinner LL which explains why they are eating it and I could continue to believe that the LR eating the LL and making it thinner is responsible for the GFs disappearance.

Any help?

[Gelato]

Try thinking of this analogy. Dead antelopes are teaming with ants, so ants are probably responsible for the antelopes' death. Ants aren't responsible for killing the antelope, they just prefer rotten meat or a rotten habitat right?
If L.r does prefer those kinds of habitats, then it might just be a coincidence that ferns are disappearing. The earthworm might not have anything to do with them.
That's what helped me understand it .

[Djjustin818]

I still have no idea how E is the answer

[mehreena379]

E rules out an instance where the situation could be reversed.

[johnscottwilsonsr]

UPDATE: I GET IT NOW!

If L.r does prefer [thick leaf layers], then it might just be a coincidence that ferns are disappearing. The earthworm might not have anything to do with them.

Hold on. I too am confused and not sure your analogy works.

A "MIGHT NOT" prefer thick leaf layers, does not destroy a "PROBABLY responsible"

Even if something, MIGHT NOT happen, it cans still be something that PROBABLY happens.

Where am I still off?

No wait, I get it. The negation test does not work here as well. If E is negated, then the argument can no longer come to the conclusion that the earth worm PROBABLY caused the disappearance. SO, if E is negated, the earthworm MIGHT have caused the disappearance, but we cannot conclude it PROBABLY did so.

I bet this will be on PT70!

[valjohnson948]

Here's my take on it...

Since E says the earthworm DOESN'T favor habitats where the leaf litter layer is CONSIDERABLY thinner, the earthworm might favor habitats where the leaf litter layer is thicker (and this is where goblin fern typically thrives...from the first sentence). So, if it prefers a thicker layer, then it is eating the ferns, thus being responsible for the fern's disappearance.

I'm not 100% sure this is a right explanation, so feel free to correct me!

[tommywallach]

Hey Guys,

Thought I'd do this one top to bottom, as there have been so many questions.

Conclusion: Earthworms responsible for fern disappearance

Premise: Fern needs leaf litter, earthworm eats litter, earthworms always around when fern is gone

Assumption: The assumption here is classic reverse causation. What if the low litter level causes the earthworms to show up, rather than the other way around?

(A) This just restates a premise that was already given (though more extremely). Definitely not an assumption.

(B) is irrelevant. We're concerned about a European earthworm anyway.

(C) is irrelevant. We don't care what the litter is made up of.

(D) is too extreme. We don't need there to be NO places for this argument to work.

(E) is correct. It removes the reverse causation, allowing us to make the inference that the earthworm is responsible for the low level of litter.

Hope that helps!

-t

[agutman]

And here's my top-to-bottom! Hope it helps.

PT69, S1, Q19 (Necessary Assumption)

The trigger "˜thus’ helps us locate the conclusion here: L. rubellus is probably responsible for the fern’s disappearance. This is a causation conclusion: L is causing the fern to disappear. What is this conclusion based on? areas where fern recently vanished are teeming with L and have unusually thin leaf litter (and we’re told L eats leaf litter). It certainly seems reasonable to conclude that this is all L’s fault, but we know we must be very careful when making a causation conclusion. The premises offer us a correlation, but that’s never enough to prove causation. Perhaps an unknown factor is causing the fern to vanish, and L didn’t enter those areas until later? This could all just be one big misunderstanding! Let’s diagram the core:

Spots where fern recently vanished have unusually thin leaf litter, which is required by the fern + those spots are teeming with L, which eats leaf litter --> L is probably responsible for the fern’s disappearance

We’re looking for an answer choice that will make a coincidence less likely, and we’ll get rid of answer choices if negating them doesn’t kill the conclusion:

(A) doesn’t mention L so probably wrong; let’s negate it and see what happens: some North American forests with thick leaf litter don’t have any fern. No one ever said fern has to be in every spot that has thick leaf litter! So, even if the opposite of (A) is true, L could still be the reason for the fern’s disappearance. Get rid of this one.

(B) Let’s negate this one too: some earthworms other than L eat leaf litter. Even so, L could still be responsible for the fern’s disappearance. Get rid of this one.

(C) You’d probably expect to find evidence of dead fern in areas where the fern recently vanished. What if the fern’s dead leaves only made up the smaller part of the leaf litter (again, negation)? This changes nothing... L could still be responsible for the fern’s disappearance. Get rid of this one.

(D) Try negation again: what if you could find some spots in North American forests that have both L and fern? Even then, L could still be responsible for the fern’s disappearance _ perhaps the fern will disappear from those spots as well in the near future. Get rid of this one.

(E) Negation has worked really well for us in the first four answer choices, and at this point we’re hoping (E) is right, otherwise we’re in trouble. Let’s try negating this one as well: L favors habitats where the leaf litter layer is considerably thinner than what is required by the fern. Aha! This means that the fern vanished before L arrived! L couldn’t be responsible for the fern’s disappearance!

So (E) is correct.

#officialexplanation

[pewals13]

So basically A and B are correlated with C and the correct answer is ruling out that B does not cause A?

[christine.defenbaugh]

Good question, pewals13!

You've got the essentials! It's a bit more complicated than that if we really capture all the pieces, but the upshot here is that the argument is assuming that the earthworm must be causing the unusually thin leaf litter. (The argument also needs to assume that the thin leaf litter was the thing that killed the goblin fern, but the answers don't target this.)

Any other relationship between the earthworm and this situation would destroy this argument. Classic alternatives include:

1) reverse causation (unusually thin leaf litter attracts the earthworm)
2) third item causes both (aliens nuked the leaf litter AND dropped off a bunch of earthworms)
3) total coincidence (aliens nuked the leaf litter, and pixies dropped off the earthworms)

Any of these situations would mean that the earthworm probably isn't responsible for the goblin fern death. (E) slam dunks reverse causation!

Does that help a bit?

[daijob]

Hi
I just wanted to double check...
So, thin or thick means the density of the lean in the forest right?

[tommywallach]

Indeed!

[donghai819]

I think B strengthens the argument by ruling out an alternative. Is it correct?

[contropositive]

I am still having trouble with B. It seems like it's eliminating alternative possibility but I guess the problem with B is that its talking about worms that are "native to North America" there could be other worms responsible that are not native to North America

[maryadkins]

Good call, contropositive.

To answer both of your questions, the only way to rule out an answer choice to a necessary assumption is by negating it (unless it's already stated in the premises, or it negates a premise, in which case, you can also rule it out because it is not an assumption).

Avi did this above, drawing the inference that earthworms native to North America will be different than the European earthworms. So the way he put it is:

"Let’s negate this one too: some earthworms other than L eat leaf litter. Even so, L could still be responsible for the fern’s disappearance."

This is the way to rule out (B).

[seychelles1718]

I understand that E blocks reverse causation but I don't undrtstand how the negation of E does not allow the argument to hold. Negation of E would be that L. Rubellus favors habitats with thin leaf litter layer.

But just because L.R PREFERS to live in cetain habitats, doesn't mean it ONLY lives in those habitats. Even if it prefers thin layer, it can still live in thick layers and eat leaf litters, thereby causing the disapearance of the fern.

This is why I eliminated E because I though the negation of E is consistent with the argument.

Can someone please help me with my reasoning?

[ohthatpatrick]

To (badly) weaken an argument, you don't need to refute its conclusion. So there will be a lot of correct answers to NA whose negations are still compatible with the conclusion.

f.e.
All boys like chocolate. Thus, Sam likes chocolate.

We must assume "Sam is a boy", even though "Sam is a girl" doesn't preclude her from liking chocolate.

What it does it remove the relevance of the premise. If Sam is a girl, then the 'premise' about "boys liking chocolate" is rendered irrelevant.

When we're dealing with causal arguments, one of the chief objections we can make is offering a DIFFERENT WAY to explain the background evidence.

Negating (E) won't PROVE the author was wrong, but it makes it more likely that the author's time sequence is completely off.

It's not that there was thick leaf litter, the worm arrived, the worm ate the leaf litter until it was a thin layer, and then the goblin fern started disappearing.

It sounds much more like
the leaf litter got thin (for some unknown reason), which both imperiled the goblin fern and attracted the worm.

I agree that saying the worm FAVORS thin litter environments doesn't preclude it from, at least this one time, going to a thick litter environment and chowing down.

But basically you have to adapt to the question. Sometimes the answer choices aren't 100% consistent with how previous versions of this problem / archetype have felt. We ultimately just have to find the credited response.

If you think of NA as "which answer, if negated, most weakens", can you find a better answer here?

[JohnZ880]

This question is a bit difficult because it's not a classic X causes Y question. Rather it's an X causes Y which causes Z.
X = L. rubellus
Y= unusually thin leaf litter
Z= the disappearance of the Goblin fern

The conclusion in the stimulus only gives us the X causes Z part, which obfuscates the fact that X is not directly causing Z, but rather is causing Y which is causing Z. Plugging in the terms, the worm (X) is causing there to be unusually thin leaf litter (Y), which is resulting in the reduction of Goblin Ferns (Z).

This is important because the correct answer plays on the X causes Y part of the equation, rather than on the X causes Z part of the equation. For example, a correct answer could have stated: actually, Goblin Ferns are disappearing because of climate change. This would have just provided another cause instead of X.

However, in this case, the conclusion plays on the X to Y relationship and says actually Y is causing X so we can't say that X is causing Z. Tricky.

[GabbyM826]

OKAY I GOT IT ..

E Matches because it removes the possibility that the worms just showed up because the leaf litter was thin and the ferns had already died. If the worm favors thin leaf litter it would be unlikely that they would have been present while the leaf litter was thick and ultimately unlikely that they would have been present while the ferns were still thriving. The worms would then be unlikely to have caused the disappearance of the ferns because the fern REQUIRES a think layer of leaf litter so if the worm favored thin leaf litter then their presence at the site where the ferns vanished could be a coincidence and removes them as the cause because then they likely appeared after the leaf litter was already too thin to support the ferns anyways.