Between 1980 and 2000 the sea otter population of the Aleutian Islands

Although (B) is irrelevant. The use of meaning their in the sentence below is totally ambiguous. I'm not sure whether their refer to otters or orcas

Orcas will eat otters when seals, their normal prey, are unavailable,. Anyone wants to explain?

It is nowhere implied in option C that the "surviving otters" survived from the orcans. They could have survived from anything. It could have been the orcans or climate change or lack of food sources etc.The argument is concerned with the decline in population of the otters. From that standpoint option C can be interpreted in different ways. If it was actually the orcans then it strengthens. If it was climate change or lack of food sources then it weakens.

I feel the option is ambiguous. I arrived at the option through process of elimination. Can anyone clarify this?

(C) tells us that some otters were still surviving as of the year 2000, while some were not.

In addition, it gives us some information about those surviving otters: most of them live in bays that are inaccessible to orcas.

This certainly doesn't prove that the orcas killed off the otters living in more accessible areas -- but it does support that conclusion. Otters in orca-infested waters died off, while otters in orca-free waters survived.

Our job is just to strengthen the argument, not to prove that it is 100% ironclad. (C) does exactly that, and none of the other answer choices come close, so (C) is the correct answer.

I hope that helps!

However, B states that Seals do not eat sea otters and the premise states that Oscars eat otters when seals are unavailable. In addition, Seal population declined dramatically. Therefore, taking both together, one can conclude that seals do not eat sea otters contribute to the fact that Oscars eat most of the otters, leading to the decline of the otter population.

Could GMATNINJA please advise where do I go wrong on the above reasoning? Thanks.

Here's the relevant information from the passage:

• Between 1980 and 2000 the sea otter population of the Aleutian Islands declined precipitously.
• the Aleutian Islands seal population declined dramatically in the 1980s.
• Orcas will eat otters when seals, their normal prey, are unavailable
• Therefore, orcas were most likely the immediate cause of the otter population decline.

What does (B) add to this chain of logic?

On the surface, this looks promising! If seals do not eat sea otters, then perhaps it strengthens the argument that the orcas are to blame.

However, this doesn't hold up to closer analysis of the information in the passage. The seal population "declined dramatically in the 1980s." We are concerned about the decline of the sea otter population "between 1980 and 2000."

If the seals caused the sea otter decline, then it makes no sense that the otters continued to dwindle for at least a decade after the seal population collapsed. In fact, if the seals were the issue, we would expect the sea otter population to increase if the seal population decreased. That's not what happened, so just from the information in the passage we can be confident that the seals are not the problem.

Receiving further confirmation of this in (B) doesn't really strengthen the author's argument. (C), on the other hand, provides additional information that points to orcas as the immediate cause of the otter population decline. (B) is out and (C) is the correct answer.

I hope that helps!

The correct answer is option (C).

Let us understand why.

Understanding the passage:

1. Between 1980 and 2000, the population of Sea Otters in the Aleutian Islands has declined sharply
2. Probable reasons for this:
a) Disease of malnutrition - not observed
b) Eaten by Predators - observed
(i) Seals (primary predators)
(ii) Orcas
3. Orcas will eat otters when Seals are not not available
4. In Aleutian Islands, the Seal population has decreased dramatically in the 1980s.
5. Inference: Seals were not available in the time period between 1980 to 2000 (the time period in question)
6. Conclusion: Orcas were the reason behind the decline in Otter population.

Question: Find the strengthener for the conclusion above.

Thought Process:
What would be an effective strengthener here?
1. Any statement that provides us some sort of proof that a majority of the otters were eaten by the Orcas
2. Any statement that provides us some sort if proof that the few remaining otters only survived because they could not be attacked by Orcas (i.e. safe from Orcas)

Let us analyse the option choices.

(A) The population of sea urchins, the main food of sea otters, has increased since the sea otter population declined.

Irrelevant to our conclusion. This only provides another data point to tell us that the sea otter population declined. This is a known premise in the passage, and has no impact on the conclusion. Hence not a strengthener.

(B) Seals do not eat sea otters, nor do they compete with sea otters for food.

This is an interesting choice. From the passage, the seal is a known predator, which eats sea otters. So, there is some ambiguity here. The inference from this statement is only that seals were not the major cause of declining otter population (neither do they eat otters, nor do they compete with Otters for food, depriving otters of their food).
However, we already know seals were essentially unavailable in this time period. This statement does not provide any point to strengthen the conclusion that the Urcas were the reason behind the declining Otter population. It only strengthens the belief that seals were not the reason behind the decline in Otter population.
Hence, irrelevant.

(C) Most of the surviving sea otters live in a bay that is inaccessible to orcas.

This is in line with out strengthener 2. Most (>50%) of the surviving sea otters live in a bay inaccessible to Orcas. This means these otters were not exposed to Orcas (i.e. they were safe from Orcas). So, the most probable reason these otters survived is because they were safe from Orcas (not exposed to them). This strengthens our belief that Orcas are the main reason behind the overall decline in Otter population.

(D) The population of orcas in the Aleutian Islands has declined since the 1980s.

This only tells us that the Orca population has also come down. But, it does not in anyway have any bearing on whether the Orcas were the main reason behind decline in Otter population. Even if the Orcas population declined (we do not know if this decline is gradual or exponential), the existing Orcas could still be eating enough Otters to create the steep decline in Otter population observed. But not necessarily. The impact of this statement is not clear. Hence not a strengthener.

(E) An increase in commercial fishing near the Aleutian Islands in the 1980s caused a slight decline in the population of the fish that seals use for food.

At best, this tells us that seal population probably declined due to this reason. but we already know this. This has no bearing on the conclusion. Hence, irrelevant.

Cheers!

Can Option E as well strenghthen the argument?
If the food of seal is declined, then the population of seal may also decline. Because the population of seal declined, so Orcas now had to move towards eating Otters. And in this way conclusion that Orcas were the cause of Otters decline is strenghtend.

The thing to notice here is that the passage has ALREADY STATED the the seal population has declined. So, this choice makes no difference.

Responding to a pm:



If we know that seals do not eat otters, it tells us that there is no connection between seals and otters. The fact that A, B or C do not eat otters, doesn't strengthen that orcas are eating otters. The conclusion doesn't say that only orcas can be responsible. It says that orcas are most likely the cause. We need to strengthen that orcas are most likely the cause so we need to strengthen the link between orcas and otters.

VeritasKarishma

if in option E instaed of slight decline if steep decline mention can we infer from that due shortage of seals food -->seals population declined -->so orcas food declines --> when orcas food(seal) declines thy eat otters

We are already given that seal population has declined sharply. Even then this option would add nothing new.

Few evident assumptions are:
- Orcas was available in the 1980s and the population of Orcas was enough between 1980 and 2000 to cause the decline of the otter population
- The seal population never increased post the decline in the 1980s causing the otter population to decline more than the decline caused by Orcas in the 1980s

Let's see if any answer choice strengthens by stating these assumptions:

(A) The population of sea urchins, the main food of sea otters, has increased since the sea otter population declined - Sea urchins, thus have one less predator. This only strengthens the point that, 'sea otter population declined' by pointing out to the evidence that the population of sea urchins increased. But it doesn't strengthen the point that, 'orcas were most likely the immediate cause of the otter population decline'. Reject

(B) Seals do not eat sea otters, nor do they compete with sea otters for food. - Out of context!

(C) Most of the surviving sea otters live in a bay that is inaccessible to orcas. - What about the sea otters that declined between 1980 and 2000? At first this seems out of context, but if most of the surviving sea otters live in a bay that is inaccessible to orcas, then the those sea otters which were eaten might be eaten by Orcas since the sea otters were accessible to them.

(D) The population of orcas in the Aleutian Islands has declined since the 1980s. - A single orcas could eat many otters. Thus a decline in population of Orcas doesn't guarantee that it would not impact the population of otters. Possibly a weakner! Reject

(E) An increase in commercial fishing near the Aleutian Islands in the 1980s caused a slight decline in the population of the fish that seals use for food - Doesn't affect the conclusion that 'orcas were most likely the immediate cause of the otter population decline'

No answer choice stated the assumption pointed out earlier. But option C pointed out a scenario, which if true, could strengthen the point that Orcas were responsible for the decline of sea otter population between 1980 and 2000.

Some kudos or a thorough analysis of my post. Either can help me reach 700!

Between 1980 and 2000 the sea otter population of the Aleutian Islands declined precipitously. There were no signs of disease or malnutrition, so there was probably an increase in the number of otters being eaten by predators. Orcas will eat otters when seals, their normal prey, are unavailable, and the Aleutian Islands seal population declined dramatically in the 1980s. Therefore, orcas were most likely the immediate cause of the otter population decline.

Causality: Orcas --> sea otter declined

Which of the following, if true, most strengthens the argument‘?

To strengthen the argument, we must strengthen the causality.

(A) The population of sea urchins, the main food of sea otters, has increased since the sea otter population declined. It offers an alternative way to explain the decline of sea otter --> weaken

(B) Seals do not eat sea otters, nor do they compete with sea otters for food. it didn't offer any explanation for the reduction of sea otter population

(C) Most of the surviving sea otters live in a bay that is inaccessible to orcas. Yes. The structure is: no Orcas, sea otter survives. No cause, no consequence --> Strengthen

(D) The population of orcas in the Aleutian Islands has declined since the 1980s. If orcas population declined, then they may need to consume less food. So the population of seal reduction didn't cause the orcas to search for alternative source of food, such as, in this case, sea otter

(E) An increase in commercial fishing near the Aleutian Islands in the 1980s caused a slight decline in the population of the fish that seals use for food. It offers an alternative way to explain the decline of sea otter --> weaken

Hi VeritasKarishma - had option B said

-- Seals do eat sea otters and they DO compete with sea otters for food

Is this tweak in option B an example of a "Strengthener" ?

Per my understanding this tweak in option B would Strengthen and below is my thought process

Reasoning

- Seals NOW eat sea otters
- Seal population declining
- Inference : sea otters should be increasing

But given sea otters are declining (rather than increasing) per the premise , this shows there is more of a chance (just a slight more chance) that most likely some other animal is the cause for the reduction in sea otters

This would give a little bit more support towards the conclusion that perhaps Orca's killed off Sea otters (not a 100 % strengthener but perhaps a bit more strengthener)

Thoughts ?

Hi VeritasKarishma,

How is (B), out of scope? Does it not eliminate other cause and effect to the argument?

Thanks

Think of the food chain: Orcas on top. They eat Seals. When seals are not available, they eat sea otters.

The argument tells us that seal population has declined. From this, we are concluding that orcas are eating otters. Now, what seals are eating is kind of pointless, right? Anyway, their population is declining. If we were worrying about increasing population of another species, we would have been interested in what seals are eating. But what we are interested in is the decreasing population of another species. This species acts as an alternate to seals as prey. Hence, the relation we are focusing on is orcas to seals/otters.

I wouldn't think so. Even if seals were eating otters, this would just tell us that the population of otters is less affected by seals now. But it would not strengthen that orcas are the ones reducing otter population. We need to find the reason for reduction in otter population.