1.10 Energy Flow and the 10% Rule

The first law of thermodynamics says energy can't be created or destroyed, only changed from one form to another. Sunlight becomes chemical energy in sugar, which becomes motion and heat in an animal. The total amount stays the same.

The second law says that every time energy changes form, some of it becomes heat that can't be used for work. So no energy transfer is 100% efficient. In ecosystems, this is why each feeding step loses usable energy.

On average, only about 10% of the energy stored at one trophic level ends up stored at the next level. Where does the other 90% go? Most is used in cellular respiration to keep the organism alive and is released as heat. Some is in parts that aren't eaten or digested, like bones, fur, roots or wood, which go to decomposers instead.

The real percentage varies from about 5% to 20% depending on the organisms, but 10% is the standard rule for the exam. Moving up one level, multiply by 0.1. Moving down one level, divide by 0.1, which means multiplying by 10.

Animals that keep a constant body temperature, like birds and mammals, burn especially large amounts of energy as heat, so less of what they eat is stored as new body mass. Cold-blooded animals, like insects and fish, often pass along a somewhat higher share.

An energy pyramid shows the energy at each trophic level as stacked bars, with producers at the bottom. Because only about 10% passes up, each bar is about one-tenth as wide as the one below. Energy pyramids are always upright.

A biomass pyramid shows the total mass of living material at each level, and a pyramid of numbers shows how many individuals. These are usually upright too, but they can be inverted. For example, one large oak tree can support thousands of caterpillars, so the numbers pyramid has a narrow base. In some oceans, the tiny phytoplankton at any moment weigh less than the zooplankton eating them, because phytoplankton reproduce so fast.

• Food chains are short: after four or five levels, there isn't enough energy left to support another population.
• Top predators are rare and need large territories, which makes them vulnerable to habitat loss.
• Eating plants is more efficient than eating meat: the same cropland feeds far more people when the crops are eaten directly instead of fed to livestock.
• Toxins can build up at high levels: because a predator eats many prey to get enough energy, it can collect the toxins in all of them (biomagnification, Unit 8).