#18 - A stingray without parasites is healthier than it

[moshei24]

I am a little bit confused as to why answer A is better than answer D. For both choices you need to add an extra Assumption for it to reconcile the discrepancy. Then again, I see that D has an extra issue that it's assuming that the parasites are very simple organisms. Is that why that answer is more wrong or less reconciles the discrepancy then A does?

Four answer choice a, we need the assumption that the reason the stingrays are not there is because they died out because of the lack of that environmentally vulnerable organisms. The parasites not being there doesn't have to be because the environmentally vulnerable organisms are not there. That could also not be there for many other reasons. Can you please explain to me why the answer is A? I think I understand it pretty well, but if you explain it's me I will most definitely have a better understanding of why A is the best answer and reconciles the discrepancy.

Thank you very much.

[Adam Tyson]

Answer D really doesn't help us any, moshei. Think of that conditional relationship: IF only a few simple organisms can live there, THEN that environement can be considered stressed. In our situation, we have stingrays living in the environment, without any parasites. Has our sufficient condition been met? I don't know, but I suspect not - I don't thnk stingrays are simple, and the answer tells me nothing about what else might be living there when the stingrays are parasite-free. Does the necessary condition have to happen? Maybe, maybe not - it can happen whether the sufficient condition is met or not, right? It's all a bit of a mess, really.

More importantly, how does it resolve the paradox here? What does it tell us about how a more-healthy stingray (without parasites) might indicate a problem in the environment? That's the paradox, right? In a healthy environment you would expect healthy animals, but here we have a healthy animal indicating an unhealthy environment.

Answer A helps by showing us a situation in which the lack of parasites could indicate a problem with the environment. If they aren't there, it may be because their other hosts, the vulnerable shrimp and oysters, aren't there to support them through their life cycle, and when there are no shrimp and oysters you have a sign of a problem in the environment. We don't have to make any assumptions here - all we need to resolve a paradox is an answer that might explain the situation for us. If A is true, it might explain why a lack of parasites is a bad sign for the environment, even though it might be good for the stingrays.

I hope that helped clear it up.

Adam

[bk1111]

Answer A helps by showing us a situation in which the lack of parasites could indicate a problem with the environment. If they aren't there, it may be because their other hosts, the vulnerable shrimp and oysters, aren't there to support them through their life cycle, and when there are no shrimp and oysters you have a sign of a problem in the environment. We don't have to make any assumptions here - all we need to resolve a paradox is an answer that might explain the situation for us. If A is true, it might explain why a lack of parasites is a bad sign for the environment, even though it might be good for the stingrays.

I hope that helped clear it up.

Adam

I don't understand how this resolves the paradox. Is (A) implying that when parasites are not using stingrays are their hosts, they are using oysters/shrimp, which are already environmentally vulnerable? Additionally, what does pollution have to do with anything?

[Jonathan Evans]

Hi, BK111,

Good question. Let's identify the two sides of the issue here:

1. Absence of parasites indicates healthier stingray conditions.
2. Absence of parasites indicates stingray habitat under environmental stress.

How could it be possible that absence of parasites could both (1) indicate healthier stingrays but also (2) stingray habitat under environmental stress?

Answer choice (A) gives you a situation that would account for this apparent discrepancy. On the one hand, no parasites, great! On the other hand, the parasites are gone because their intermediate, pre-stingray hosts were likely absent, victims of pollution or other environmental stress.

In other words, you have to identify explicitly the two sides of the paradox. Once you identified these parts, then ask what is incongruous between them. The credited response will address both sides, in this case, how the absence of parasites could simultaneously usually be a good thing for stingrays while also perhaps an indicator of other environmental problems.

Does this explanation help?

[WarnerHuntingtonIII]

I understand why A is correct. I do not see the problem with D. I would like someone to explain why my rationale, described below, is incorrect:

"An ecosystem can be considered stressed if only a few species of very simple organisms can live there."

Ok, so a lack of parasites, very simple organisms (this feels like a rewording element and not a distractor element), indicates that at least one kind of very simple organism might not be able to live in the stingray's current environment. This could indicate that only a few very simple organisms can live there, and thus, the healthy stingray's ecosystem is under stress."

[Adam Tyson]

You may be making some unwarranted assumptions there, Warner.

Are parasites simple, or did you just assume that they are?

The paradox arises when there are stingrays present without parasites. Are stingrays simple? Did you assume something about them that wasn't supported by the text?

Does the lack of parasites mean that only a few species of simple organisms can live there, or did you just assume that was the case? Maybe billions of species of simple organisms can live there, even though parasites are not found in stingrays?

Answer D requires so many unwarranted assumptions before it can be any help at all, and even if we make all of them - that parasites and stingrays are simple and that not much else can live in their environment if it is stressed - how would this resolve the paradox that the stingrays without parasites are still considered healthier than if they had some? If they are living in a stressed environment, like one that's polluted, wouldn't that harm their health in way? I still find myself scratching my head no matter how much help I offer to answer D, and that's why it's not a good resolution.

Put another way, answer D might explain why, in a stressed environment, stingrays lack parasites. Maybe it's because the parasites can't survive in that environment. But this does nothing, and I mean absolutely nothing at all, to explain why the stingrays are healthier under those circumstances. To get there you still have to make another assumption, that the health-related advantages of being parasite-free outweigh the disadvantages of living in a polluted environment.