CONCEPTS 3-4A AND 3-4B 63
Figure 3-14 Greatly simplified food web
in the Antarctic. Many more participants in
the web, including an array of decomposer
and detritus feeder organisms, are not
depicted here. Question: Can you imag-
ine a food web of which you are a part?
Try drawing a simple diagram of it.SquidFishKrill Herbivorous
zooplanktonPhytoplanktonCarnivorousCarnivorous
planktonplanktonCarnivorous
planktońHumansBlue whaleKiller
whaleElephant
sealCrabeater
sealLeopard
seal
Emperor
penguinAdelie
penguinPetrelSperm whaleHeatHeat HeatHeatHeatDecomposersTertiary
consumers
(human)
Primary
consumers
(zooplankton)
Producers
(phytoplankton)
10100Secondary
consumers
(perch)
Usable energy available
at each trophic level
(in kilocalories)10,0001,000Figure 3-15 Generalized pyramid of energy flow
showing the decrease in usable chemical energy
available at each succeeding trophic level in a
food chain or web. In nature, ecological efficiency
varies from 2% to 40%, with 10% efficiency
being common. This model assumes a 10%
ecological efficiency (90% loss of usable energy
to the environment, in the form of low-quality
heat) with each transfer from one trophic level
to another. Question: Why is a vegetarian diet
more energy efficient than a meat-based diet?Energy flow pyramids explain why the earth can
support more people if they eat at lower trophic lev-
els by consuming grains, vegetables, and fruits directly,
rather than passing such crops through another trophic
level and eating grain eaters or herbivores such as cattle.
About two-thirds of the world’s people survive primar-
ily by eating wheat, rice, and corn at the first trophic
level, mostly because they cannot afford meat.