I'd like to make a pitch for introducing fuzzy logic in trophic indices.
Using fractional levels is ambiguous, as TL=3 could either be primary
carnivores or organisms who eat both plants and primary carnivores. I would
rather replace the latter by 50% TL=1 and 50% TL=3.
For students this is actually easier. If half of what they eat is vegetables
and half is meat, then they are 50% TL=2 and 50% TL=3.
It also makes it easy to get more sophisticated. If 20% of what they eat is
beef or some other herbivore, that is 20% TL=3. But if they are eating 10%
salmon, which is a carnivorous fish, that is 10% TL=4. Supposing that the
rest is veggies and fruits, what is more informative, to say nothing of
easier to calculate?
70% TL=2, 20% TL=3, 10% TL=4 OR
TL=(2*0.7+3*0.2+4*0.1)=2.4?
Then if you get into trophic efficiency, you can calculate the energy
required to produce food broken down by trophic level, but not if all you
know is the average.
Bill Silvert
----- Original Message -----
From: "Hilary Callahan" <[email protected]>
To: <[email protected]>
Sent: Monday, December 22, 2008 2:55 PM
Subject: Re: [ECOLOG-L] Need suggestions for hands-on activity about trophic
efficieny/biomass pyramids
I don't know if this qualifies as hand-on, but in my non-majors college
student course I teach trophic levels quantitatively ... we do this so
that
we can understand the difficulty of figuring out whether trophic levels
are
indeed declining in various large marine ecosystems and globally.
As I introduce the basics of pyramids of #, biomass, and energy, I also
ask
them to fill out a very small questionnaire about their diets. I somewhat
arbitrarily assign a trophic value of 2.0 for strict vegans, 2.2 for
lacto-ovo-vegetarians, 2.4 for omnivores, and 2.6 for students who consume
meat at 2+ meals per day.
I collect and compile these, looking at the class average for trophic
level.
This info becomes valuable the next session, when we get into the issue
that
there are in-between cases (organisms that are not strictly primary
consumer
or secondary consumer), the difficulty of assigning a trophic level to an
entire species given variation among individuals such as across
developmental stages (omnivores as juvenile, vegetarian as adult), and
also
perhaps variation in ecological context (vegetarian on campus but omnivore
at home), etc.
We also talk about how data can be biased or inaccurate. What if people
lied
on the questionnaire? We then get into methods for validating trophic
level
estimates using multiple mechanisms, using analysis of nitrogen stable
isotopes, or of stomach contents.
This may be a little advanced, and not exactly hands-on, but it makes the
topic relevant to their lives and the exercise is adaptable to other
audiences and contexts. I find that almost everyone -- non-majors and
majors
alike -- likes to think about food, including how individual and human
diets
situate our species in the trophic pyramid, and the associated energetics.
