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Name: Susan
Status: educator
Grade: 4-5
Location: NC
Country: USA
Date: Fall 2009

Question:
We are currently studying ecosystems and talking about food chains and food webs. This is only my second year teaching 5th grade science and last year I did it without a teacher's manual. Anyway, my question is "Are humans always on the top of the food chain?" I told my students that they were because we are not naturally a part of any other animal's food chain. However, I later found a side note in the teacher's manual saying humans aren't always at the top because we can be eaten by tigers or sharks. So, what is the correct information?



Replies:
An interesting question. Most humans now are at the top of their food chains, even to the point in the U.S. that many of our bodies are not even returned to the food chain by decomposition. But certainly the occasional human falls victim to a shark or other large predator, including a rare mountain lion or grizzly bear attack in North America. I would not say that makes us part of a food chain, or more properly "food web," since people are not a regular part of the ecosystem in that way - the contribution of energy to the system is so tiny as to be completely insignificant. One might make the point with sharks or Asian tigers that the rare human meal contributes enough to be called part of a food web. It may be a bit too much for 5th graders, but there is an opportunity in this discussion to talk about the larger role humans play in the Earth's ecosystems, and that not so long ago, in evolutionary terms, humans were very much contributors and not just consumers in food webs.

J. Elliott


I don't think there's a "right" or "wrong" answer -- it depends on how you choose to define the terms. It's true that humans are not typically prey for any animal (we've done a pretty good job of protecting ourselves from the species that try to eat us, and so they've for the most part learned to find other meals). And it's true that a human could be eaten by another animal.

However, I prefer a view of ecosystems that takes into account the full life cycle of all organisms. Not all interactions between animals are predator-prey. For example, a fungus might break down a tree and the nutrients may then be used by another plant. Or a bacterium may live in a person, even feed off the person, yet not kill the person. Humans eventually die and their bodies are broken down by various microorganisms (if the person wasn't eaten by a tiger or shark). Those microorganisms produce nutrients that feed plants, which then grow and are eaten, and the same atoms may end up again in another human. The substance of all life, regardless of organism, has been recycled and reused for billions of years.

Hope this helps,

Burr


Both are correct. If you're eaten by a tiger, then you're clearly in the tiger's food chain, but we're not often eaten by tigers, so it's not very useful to spend your time thinking about this branch of the chain. (Chains have branches?) "Food chain" is not a precisely bounded notion; it's a useful notion in part *because* it's not precisely bounded. It gets some important ideas across in an easily visual way, and it accomplishes this by not sweating the details.

There's plenty of occasions to sweat the details, and it's important to do some of this. But it's also useful to have high-level concepts, like food chain, that leave the details in a softer focus.

Tim Mooney


They forgot polar bears and crippled lions.. Usually, humans are on the top as we are omnivores - we eat anything. We are not the preferred prey of any of any animal except the crippled lion (everything else is too fast for it to catch).

For a fifth grade class, I think your statement that we are at the top of the chain quite adequate. Introducing exceptions will just serve to confuse some students.

Being 5th graders, I'd expect some of them will come up with the exceptions anyway.

Hope this helps.

R. W. "Bob" Avakian
Instructor
B.S. Earth Sciences; M.S. Geophysics
Oklahoma State Univ. Inst. of Technology



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