Energy flow and the ecological pyramid

Follow the energy rather than the matter, and a brutal rule appears: at each tier, most of it vanishes.

The 10 % rule

   producers        10,000 units of energy
        |  ~ 10 %
        v
   herbivores        1,000
        |  ~ 10 %
        v
   carnivores I        100
        |  ~ 10 %
        v
   carnivores II        10

Where do the lost 90 % go?

- a large share is never EATEN (roots, wood, uneaten carcasses)
- a share is not DIGESTED and leaves as droppings
- most of the rest is burned by RESPIRATION, dissipated as heat

The actual percentage varies from 5 to 20 % depending on the habitat, but the order of magnitude holds: about 90 % is lost at each level.

The ecological pyramid

              ▲          carnivores II     10
            ▲▲▲▲         carnivores I     100
        ▲▲▲▲▲▲▲▲▲▲       herbivores     1,000
   ▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲    producers     10,000

Two direct consequences:

Chains are short. Beyond four or five links, not enough energy remains to feed another level. This is a thermodynamic constraint, not a biological coincidence.

Top predators are rare. A huge production base is needed to sustain a few individuals at the top. It is also why they are the first to go when an ecosystem degrades.

The application to human food

   1 hectare of cereals eaten DIRECTLY
        ->  feeds about 10 times more people than
   1 hectare of cereals fed to livestock, then eaten as meat

Adding a trophic level costs about 90 % of the energy. This purely energetic calculation underlies most analyses of farmland use — independently of any moral or cultural consideration.

Bioaccumulation: what concentrates instead

Energy dilutes as you climb the pyramid. Some substances do exactly the opposite.

   water              0.000003 ppm  of DDT
   phytoplankton      0.0004 ppm          ×130
   small fish         0.5 ppm             ×1000
   large fish         2 ppm               ×4
   fish-eating birds  25 ppm              ×12

   overall factor: about 10 million

Fat-soluble pollutants — DDT, mercury, PCBs — are neither excreted nor broken down: they accumulate in fat. A predator ingests everything its prey accumulated over their lifetimes.

This mechanism nearly wiped out the bald eagle in the United States — DDT thinned eggshells — and explains the advice limiting consumption of large predatory fish, tuna or swordfish, because of mercury.

Summary

• About 90 % of the energy is lost at each trophic level.
• Losses: uneaten parts, undigested parts, and above all respiration.
• Hence the ecological pyramid, and chains of 4 or 5 links at most.
• Top predators are necessarily rare.
• Eating a plant directly is ~10 times more efficient than going through an animal.
• Conversely, fat-soluble pollutants concentrate up the chain.