Ecological Pyramids and the Ten-Percent Energy Rule

Ecological pyramids are diagrams that show the numbers, biomass or energy at each trophic level of a food chain, and the ten-percent rule is the approximation that only about 10 percent of the energy at one trophic level is transferred to the next. In brief: there are three types: pyramids of numbers, of biomass and of energy. Energy pyramids are always upright, because energy is lost at each step as heat. Pyramids of numbers and of biomass can be inverted, for example when one large tree supports thousands of insects, or when phytoplankton with a small standing biomass supports a larger biomass of zooplankton.

Pyramids explain why top predators are rare and why food chains rarely have more than four or five links. This guide explains each pyramid type, the reasons for energy loss between levels, how decomposition fits in, the practical consequences for human food supply, works through calculations of energy transfer and compares the three kinds of pyramid.

What are ecological pyramids?

An ecological pyramid has producers at the base and successive levels of consumers above, with the top predators at the apex. The width of each bar is proportional to the quantity measured.

• Pyramid of numbers: the number of individual organisms at each level. It is easy to count, but it ignores size. A grassland pyramid is wide at the base with millions of grass plants, then fewer grasshoppers, fewer frogs and few hawks. A single oak tree supporting many insects, however, gives an inverted pyramid.
• Pyramid of biomass: the dry mass of living material at each level (g/m² or kg/ha). It is more accurate, but represents a single moment, so it can be inverted for aquatic systems in which phytoplankton reproduce so fast that a small standing mass supports a larger mass of zooplankton that lives longer. Seasonal changes also affect it.
• Pyramid of energy: the energy flow through each level per unit area per unit time (kJ/m²/year). It takes into account the rate of production, so it is always upright. It is the best representation of the real relationship.

The three types of pyramid compared

What is the ten-percent rule?

On average, around 10 percent of the energy stored in the biomass of one trophic level is converted to the biomass of the next level. This rule of thumb is attributed to Raymond Lindeman (1942). The actual efficiency, called the trophic transfer efficiency, ranges from about 1 to 20 percent, depending on the ecosystem and the organisms. For example, it is often higher in aquatic systems and lower in terrestrial ones with many woody plants. It is a useful approximation, not a law.

The energy after n transfers from a producer level with energy E is E × 0.1ⁿ. If producers store 100 000 kJ, then herbivores store about 10 000 kJ, primary carnivores 1 000 kJ, secondary carnivores 100 kJ and tertiary carnivores 10 kJ. Beyond four or five levels, the energy left is too small to support a viable population of a large animal, which explains why food chains are short.

Why is energy lost between trophic levels?

• Respiration: the largest loss. Organisms use a great deal of the energy in food to power their life processes, and it is lost as heat. Endotherms such as birds and mammals use more than 90 percent of their intake for respiration, because they maintain body temperature.
• Parts not eaten: bones, roots, shells and fur, which remain uneaten, as well as dead organisms that are not eaten by the next level but by decomposers.
• Egestion (feces): the indigestible portion of food (such as cellulose) passes through the gut without being absorbed.
• Excretion: energy in urine and other wastes.
• Heat loss and movement: energy used in hunting and fleeing is also lost as heat.

According to the second law of thermodynamics, energy transformations are never completely efficient, and some energy is always dispersed as heat. Energy flows through the ecosystem in a single direction and cannot be recycled, in contrast to nutrients.

What role does decomposition play?

Dead organisms, feces and shed parts form detritus. Decomposers (bacteria and fungi) and detritivores (earthworms, woodlice, dung beetles) feed on this material, and release nutrients back to the soil and CO₂ to the air. They get their energy from the dead matter, and in most terrestrial ecosystems the greater part of NPP (often more than 80 percent) goes to the decomposer food web, not to grazers. The rate of decomposition depends on temperature, moisture, oxygen and the chemical composition of the material. Decomposition is fast in warm, moist conditions, and slow in cold or waterlogged places, where peat can accumulate. Decomposers are not shown in the pyramids, but they are essential in nutrient cycling.

Worked example: energy and biomass pyramids

The table gives the energy stored in each trophic level of a meadow in a year.

1. Efficiencies: 800 ÷ 8 000 = 0.10 (10 percent); 100 ÷ 800 = 0.125 (12.5 percent); 9 ÷ 100 = 0.09 (9 percent).
2. Overall efficiency from producers to owls: 9 ÷ 8 000 = 0.001125, or about 0.11 percent.
3. Energy at the base per unit of energy at the top: 8 000 ÷ 9 ≈ 889, so about 889 units of energy are fixed by the grass for every unit stored in the owls.
4. Area needed: suppose that one owl stores 90 000 kJ of energy in its body over a lifetime of growth and breeding. The meadow produces 9 kJ of owl tissue per m² per year, so 90 000 ÷ 9 = 10 000 m² (1 hectare) of meadow would be needed for one year of owl production. A real owl territory is of this order of magnitude or larger, because it must also cover the energy used by the owl for respiration, which is far greater than what it stores.

If the meadow were used to grow crops for people instead of feeding animals for meat, the energy would be used more efficiently. Eating grain directly takes energy from level 1, but feeding the grain to cattle first and eating the beef takes energy from level 3, with a loss of about 90 percent at each step. Around three to ten times as much land is needed per unit of energy for meat as for grain, depending on the animal. This calculation underlies arguments about diets and land use.

What are the implications for food and conservation?

Large predators such as tigers, orcas and eagles need very large ranges, which makes them vulnerable to loss of habitat. They also accumulate toxins that do not break down, because each step concentrates them, which is called biomagnification. Fisheries that target top predators, such as tuna and sharks, remove the species with the lowest productivity, which recover slowly. Aquaculture of herbivorous fish (carp, tilapia) is more energy-efficient than that of carnivorous fish (salmon), which are fed with fishmeal.

How did the pyramid concept develop?

Charles Elton described the pyramid of numbers in his 1927 book Animal Ecology. Raymond Lindeman (1942) put forward the energy-based view of trophic levels and the efficiency of energy transfer between them. Howard T. Odum’s studies of Silver Springs (1957) provided some of the first full energy budgets of an ecosystem. The 10 percent figure was popularized by textbooks, though Lindeman’s own estimates varied between 5 and 25 percent.

Which mistakes are common?

Mistake 1: Saying that 10 percent of the organisms are passed on

The rule concerns energy, not numbers or mass of individuals.

Mistake 2: Saying that energy pyramids can be inverted

They cannot. Numbers and biomass pyramids can be inverted.

Mistake 3: Thinking that the lost energy is destroyed

It is converted to heat and radiated away. Energy is conserved, but it is no longer available as chemical energy.

Mistake 4: Applying the 10 percent figure exactly

It is a guideline. The real value varies from about 1 to 20 percent.