GCSE ecology/biology

[Rat]

So, someone I know was marking GCSE papers from this year's exams. GCSEs are the exams that (more or less) every 16-year-old sits. The paper in question was a foundation-level science paper. Now, I wasn't allowed to look at the papers that had been handed in, because the marker obviously isn't allowed to show them around, but I did look at the paper, and I found what I thought to be, at least, a confusing question. It goes something like this (from memory):

This is part of a food chain:

A ban on cod fishing has been proposed. Some scientists say that this will decrease the numbers of sand eels, while others claim that the numbers of sand eels will increase. Name a) a reason why sand eel numbers might decrease, and b) a reason why they may increase.

Obviously the first part is easy enough, but the second part I found confounding. I could not come up with a reason that was fully coherent, and that I could expect a GCSE student (this is the foundation paper) to know.

I have looked at the guidance paper for markers, telling them what to give marks for, and therefore effectively what the right answers are, and I can't say I agree with them.

Can anyone come up with anything decent and convincing?

Cheers,
Rat.

 

[Lamuella]

of the top of my long-time-since-GCSEs head:

a) a ban on cod fishing increases the number of cod in the seas. Cod's main surce of food per the food chain is sand eels. More cod mean more cod eating sand eels.
b) A ban on cod fishing increases the number of cod in the seas. Minkes eat cod, sand eels, and herring. If minkes have more cod to eat, they may eat less sand eels.

note that my answer to B is very suspect indeed. I don't trust it and don't think it works. Other answers to (a) include that if people are eating less cod, they may compensate by eating more sand eels.

 

[Hellbound]

Well, I answered the wrong question initially. But I'd agree with Lumuella's second reason.

If the Cod overpopulate, they become a much easier prey for the Minkes. Minke predation on Cod will increase, especailly as the number of Cod grows, and predation on Sand Eels will decrease.

I also don't think it's a likely possibility, but it is a possibility, which is all they are asking for. I think the idea behind this question is to force people to look at multiple sides of an issue (a test of critical thinking) as well as understanding a bit of how inter-related and complex food webs can be.

 

[Rat]

Well, what you have for B is more or less what the marking guidance says, but I'm really not happy with it. The fact remains that although the minke are eating more cod instead of sand eels (and nothing in the question implies that they prefer them), there are still more cod, which are eating the sand eels that the minke aren't. Wouldn't that more or less balance it out?

Cheers,
Rat.

 

[Hellbound]

Well, what you have for B is more or less what the marking guidance says, but I'm really not happy with it. The fact remains that although the minke are eating more cod instead of sand eels (and nothing in the question implies that they prefer them), there are still more cod, which are eating the sand eels that the minke aren't. Wouldn't that more or less balance it out?

Cheers,
Rat.

In theory. BUt if the main reason Minke aren't eating more Cod now is because humans get to them first, then it is possible that the increase in Cod from lack of human predation will be balanced by the increase in Minke predation, and thus Sand Eels don't have any increased Cod predation (as well as having a reduced Minke predation).

 

[Rat]

I suppose. Bear in mind though, that as far as I'm aware, nobody got the right answer, and a large number of people said that there would be more sand eels because they would have more cod to eat. That's the sort of level we're looking at. At the very least, I think the examiners were aiming a little high with this question.

Cheers,
Rat.

 

[blutoski]

I suppose. Bear in mind though, that as far as I'm aware, nobody got the right answer, and a large number of people said that there would be more sand eels because they would have more cod to eat. That's the sort of level we're looking at. At the very least, I think the examiners were aiming a little high with this question.

Cheers,
Rat.

I agree. To complicate matters, when a predator has several prey to choose from, they usually prioritize and only switch to secondary prey when they exhaust the primary. It's called "prey switching."

If the Minke eat Cod, Herring, and Sand Eel, and they prefer Sand Eel to Cod, the Cod abundance will be irrelevant until the Sand Eel are extinct.

 

[Dilb]

Sand eels are eaten by cod, cod eaten by humans. Humans eat less cod, more cod are available to be eaten by minke, which then eat less sand eels. Assumes that the cod population doesn't eat more sand eel as a result of not being fished. Easy enough to guess if you redraw the arrow from cod->human to cod->minke, I'd think.

If, as is probably the case, cod populations increase, sand eel populations would go down, even if minke eat cod instead of sand eel, due to the energy lost from going sand eel to cod to minke, rather than sand eel to cod. Probably more complicated than I'd expect from 16 year olds.

More likely though, fishermen can't catch cod and so catch sand eel and herring, which due to the decrease in herring could increase the sand eel population.