#14 - Guam has 40 times more spiders than nearby islands

[lewis]

Strengthen-CE. The correct answer choice is (A)

The biologists make a causal conclusion here, arguing that the cause is the loss of bird species and the effect is the increase in the spider population. That's the relationship we're trying to strengthen. The conclusion is based on the premise that the birds prey on spiders and use spiderwebs for their nests. So, basically, with fewer birds, there are fewer predators eating spiders and destroying their homes.

When you have a causal argument in a strengthen question, your prephrase is essentially that you are looking for one of the 5 ways to strengthen a causal conclusion (in this case, that fewer birds are causing more spiders): eliminate an alternate cause; show that when you have the cause, you have the effect; show that when you don't have the cause, you don't have the effect; eliminate the possibility of the reverse cause and effect; or support the data.

Answer choice (A): This is the correct answer. If birds compete with spiders for insect prey, that's another reason why their presence might keep the spider population down--they're competing for the same food source. So not only do birds eat spiders and destroy their homes, they also eat the things spiders would eat. This is a "support the data" method of strengthening a causal conclusion. It supports the stimulus data by providing another way in which birds may directly keep the spider population down, which strengthens the conclusion that fewer birds would allow the spider population to increase.

Answer choice (B): How the biologists estimated the spider population has nothing to do with whether the loss of bird species is responsible for the increase in the spider population.

Answer choice (C): Comparing the number of spiderwebs on Guam to nearby islands it not relevant to our causal relationship about birds and spiders. We know there's an increase in the spider population on Guam. We want to strengthen the idea that the loss of bird species is the cause.

Answer choice (D): The proliferation of a couple of bird species since the arrival of brown tree snakes has nothing to do with whether the loss of bird species is responsible for the increase in the spider population.

Answer choice (E): The eradication of brown tree snakes have nothing to do with our birds and spiders.

Hello, I have a BIG incomprehension here, I've always treated a ''strengthen the conclusion'' as a question asking you to strengthen the conclusion solely, therefore here '' Guam have 40 times more spiders than nearby islands have.''
And a ''strengthen the arguments'' as a question asking you to strengthen the argument, therefore here the causal relationship.

This approach have always worked out for me as I average -2,-3 on LR until this question! According to my technique answer A would be a no brainer for a ''strengthen the argument'' and answer C would be a no brainer for a ''strengthen the conclusion''
and because of this question now I am lost! your explanation seems to be for a strengthen the argument not a strengthen the conclusion according to my approach.

So, what is the true difference between a strengthen the conclusion and a strengthen the argument then..

[Jeremy Press]

Hi lewis!

I'd encourage you to try not to make an overly sharp distinction between strengthening the conclusion and strengthening the argument. By strengthening a conclusion, it's true that you strengthen the argument (in a very direct way, at that!). But it's also true that by strengthening an argument (strengthening the ties between the premises that lead you to the conclusion), you'll strengthen the conclusion as a result. Yes, it's true that on the LSAT we don't normally look to weaken or strengthen arguments by simply undermining or bolstering the factual nature of the premises of those arguments. But there's a fine line there, and you can sometimes miss something if you're only looking at the conclusion without considering the reasons given for it.

The good news on this question is that answer choice C doesn't actually strengthen the conclusion (let alone the argument). I think part of the problem stems from identifying the conclusion simply as the statement that "Guam has 40 times more spiders than nearby islands have." That's actually not the conclusion. As Kelsey notes in her explanation to this question, the conclusion is the cause/effect claim made in the second sentence, that the cause of Guam's relatively greater number of spiders is "the accidental 1940s introduction into Guam of the brown tree snake," which in turn eliminated bird species and caused the explosion in spider population. To target and strengthen that conclusion, we need something that makes it more likely that the loss of bird species caused the spider population explosion (not something else). Answer choice C doesn't make it any more or less likely that the loss of birds was responsible for the spider population change. Notice how answer choice A speaks directly to the cause (bird species loss) that the biologists argue for, by giving us information that suggests loss of birds could benefit the spider population. That's strengthening both the argument and the conclusion!

I hope this helps!

Jeremy

[owen95]

I got this one right because A was the only answer choice that seemed to strengthen at all, but it took me longer than it should have.

This might've been silly, but my hyper-cautious LSAT brain got hooked on the underlying assumption in the biologist's argument that "fewer bird species" = "fewer birds overall." Maybe the bird population became way less diverse in terms of species, but that doesn't necessarily mean that there are fewer birds. Maybe the brown tree snake only eats 10 of the 12 native bird species, and once the other 10 species were gone, the remaining two species thrived. (Something like this isn't all that uncommon when a species goes extinct). So answer choice A seemed to be making the same problematic jump in the stimulus, i.e. that a fewer number of species necessarily means fewer number of individual birds.

So I was looking for an answer that either justified that jump or used the spiderweb thing to build off of. C was tempting... more spiderwebs could be either 1) because there are less individual birds using them for nests, which would seem to strengthen the argument, or 2) just because there are more spiders already. The possibility of 2) though pretty much made C useless.

I know "strengthen" questions generally have a little more wiggle room and we don't need to account for every weakness in the argument, but any advice for not getting hung up on things like this when they're irrelevant? or was there something in the stimulus that should've shown me that there were in fact fewer individual birds?

Thank you!!

[Jeremy Press]

Hi Owen,

My advice would be to look for the answer that is the most directly relevant to the conclusion, even if it has just a bit of ambiguity to it.

Could the test writers have come up with a stronger strengthen answer than these five answers? Sure (one along the lines of what you discussed). Is (A) the best strengthen answer here? Absolutely. First, it's direct. Despite the slight ambiguity, there's still a highly plausible reading of answer choice (A) that works to strengthen. Second, there's no equally plausible "weaken" side of answer choice (A). There's no way I can assume that fewer species will definitely, or even very likely, mean more overall birds (even though I suppose there's a chance of that happening in certain instances).

I would also quibble with you a little about answer choice (C). I don't see any way that answer choice (C), as stated, helps to show that the introduction of the tree snake led to the increase in spiders. It's an answer that seems to be suggesting there are more spiders on Guam than elsewhere (using spiderwebs as a proxy). But why? I have to shrug my shoulders at that question, because there's no way to tell. The answer is just talking about the end point of the causal chain. The conclusion we're trying to strengthen is about what set off the causal chain initially.

I hope this helps!

[nyc431]

Hi there - in the explanation, you say that the snakes have nothing to do with birds and spiders.

This was my logic in selecting E. Can you help me understand why its flawed? Based on the stimulus, I inferred a causal chain. Upon introduction to the island, the snakes ate the birds (eliminated 10/12 species). And then the spider population expanded because the spiders' thrived in their predators' (the birds') absence? So if the snakes are difficult to eradicate, then the spiders continue to thrive?

[Adam Tyson]

When we said that the "eradication" of snakes has nothing to do with the birds and spiders, we meant that what we may or may not be able to do NOW with those snakes isn't relevant to what caused the spiders to proliferate in the first place. Answer E does nothing to address whether the loss of birds led to the relatively large number of spiders. We need an answer that looks to the past to see what happened, rather than looking to the future to see what could happen next.

Focus on what caused this situation to arise, and support the claim that it was the snakes!

[WildMountainElk]

I understand why A is correct, but I had a difficult time eliminating D.
I identified the conclusion as the biologists attributed the increased spider population to the loss of the bird species. Answer D seems to eliminate an alternate explanation, namely that it wasn't a decrease in the shear number of birds that caused the increased spider population, since the remaining 2 bird species have since proliferated. This would give credence to the conclusion that it was the loss of the species that was the cause.

[Adam Tyson]

I think you may be reading that a little backwards, WildMountainElk, because answer D may actually weaken the argument by suggesting that the number of birds may not have diminished as much as the author implied. It's hard to say for sure what impact this answer has, though, because we can't know if these two bird species prey on spiders or use their webs in their nests, but this definitely doesn't help the argument.

To strengthen, we need more evidence that the snakes caused a loss of bird species, and that loss of bird species is the cause of the growing spider population. I see what you mean about eliminating an alternate cause - it's not the number of birds that matters but the number of bird species - but this doesn't connect back to the brown snake being the initial cause of the difference between this island and other islands. We should keep that in mind along with the claim about the birds, which is part of the causal chain of events but not the triggering cause that started the whole ball rolling. Answer A does a good job of connecting us back to that first cause, because it means the snakes eliminated something that competes with spiders for food.