Topic 1.10 Notes – Energy Flow and the 10% Rule

What the 10% Rule Means

In a food chain, energy flows sun → producers → consumers → heat. One move from one trophic level to the next is one energy transfer.

The 10% rule says that only about 10% of the energy at one trophic level is passed to the next level as biomass. Biomass means the stored chemical energy in living tissue. That stored energy is what the next consumer can actually use.

• About 90% is not transferred to the next trophic level.
• That does not mean energy disappears.
• It means that energy is no longer available to the next step in that food chain.

A common mix-up is at the producer level. The energy at the base of an energy pyramid is usually chemical energy stored by photosynthesis, not all sunlight hitting the ecosystem.

There are two different efficiencies here:

• Photosynthesis efficiency: producers capture only about 1% of incoming solar energy
• Trophic transfer efficiency: about 10% passes from one trophic level to the next

Energy also differs from matter. Matter cycles through ecosystems, but energy does not. Ecosystems need a constant outside input, usually sunlight.

Why Energy Decreases at Higher Trophic Levels

Less energy is available higher in the food chain because transfer is never complete.

• Some organisms or body parts are never eaten
• Some eaten material is not digested or assimilated
• Some energy leaves as feces and other wastes
• Much assimilated energy is used for respiration, movement, maintenance, growth, repair, and reproduction
• Some organisms die without being eaten by the next consumer

The energy pyramid below ties those losses together. Most of the energy captured by producers never becomes biomass at higher consumer levels, and some is released as heat at each transfer.

Energy pyramid

Uneaten biomass and wastes can still enter detrital pathways through decomposers and detritivores. So when APES says energy is “lost,” it means lost to the next trophic level in that pathway, not destroyed.

This connects to thermodynamics:

• First law of thermodynamics: energy is conserved. It changes form instead of being created or destroyed.
• Second law of thermodynamics: every energy transfer increases entropy, so some usable energy becomes low-quality heat. No transfer is 100% efficient.

That second law is the reason less usable energy is left to build biomass at each higher trophic level.

How to Calculate Energy Transfer

The basic equations are:

$$E_{\text{higher}} = E_{\text{lower}}(0.10)$$

$$E_n = E_0(0.10)^n$$

Here, $n$ is the number of transfers, not the number of trophic levels named.

Moving through trophic levels

• Going up one level → multiply by 0.10 or divide by 10
• Going down one level → divide by 0.10 or multiply by 10

A classic trap is this: a tertiary consumer is the fourth trophic level, but it is only three transfers above producers.

Energy not transferred

$$E_{\text{not transferred}} = E_{\text{lower}} - E_{\text{higher}}$$

Under the 10% rule, that is usually 90% of the lower level.

Measured transfer efficiency

If real data is given, use it:

$$\text{transfer efficiency} = \frac{E_{\text{higher}}}{E_{\text{lower}}} \times 100$$

If the answer comes out to 15%, keep 15%. Do not force it to 10%.

Units

Common units are J, kJ, cal, kcal, often per area per time, like kJ/m²/yr. Multiplying by 0.10 does not change units. A percentage has no unit.

What Energy Pyramids Show and Why They Matter

An energy pyramid shows available energy at each trophic level. Producers form the widest base, then primary, secondary, and tertiary consumers get narrower.

Energy pyramid and trophic levels

In this example, the side label for decomposers is a reminder that decomposers act on material from every trophic level. Energy pyramids are always upright because each transfer leaves less usable energy. Pyramid width shows amount of energy, not body size of individual organisms.

This explains a lot:

• Food chains are usually only 4 or 5 trophic levels
• Higher trophic levels usually support fewer organisms
• Top predators need large feeding territories
• Eating lower on the food chain supports more consumer biomass

Compact example:

• eelgrass → grazing invertebrates → small fish → larger predatory fish

Each arrow is one transfer, so if eelgrass has 10,000 kJ, the larger predatory fish would have $10{,}000(0.10)^3 = 10$ kJ.