Humans and Food Web/Chain
Date: Fall 2009
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?
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.
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,
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.
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
Being 5th graders, I'd expect some of them will come up with the
Hope this helps.
R. W. "Bob" Avakian
B.S. Earth Sciences; M.S. Geophysics
Oklahoma State Univ. Inst. of Technology
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