Energy Pyramid
An energy pyramid is a diagram showing how much energy is available at each trophic level, with about 10 percent passing from one level to the next.
Definition
An energy pyramid is a picture that shows how much energy sits at each step of a food chain, with producers at the wide bottom and each level above narrower because most energy is used up before it reaches the next eater, so if grass holds $1{,}000$ units of energy, the rabbits eating it get only about $100$ and the foxes eating the rabbits get only about $10$. More formally, an energy pyramid represents the energy available at each trophic level, with only about $10$ percent of the energy at one level transferred to the next (the ten percent rule) because the rest is used in respiration, lost as heat, or left in uneaten or undigested material, which is why pyramids of energy are always upright and why ecosystems rarely support more than four or five trophic levels. At the thermodynamic level, an energy pyramid measures productivity in units such as kilocalories or joules per square meter per year and is always upright because the second law of thermodynamics converts some usable energy to heat at every transfer; trophic efficiency, the product of consumption, assimilation, and production efficiency, ranges from about $2$ to $40$ percent, with pyramids of biomass and numbers sometimes inverted when producers are small and turn over rapidly, as with ocean phytoplankton, though a pyramid of energy never is, a trophic-dynamic view introduced by Lindeman's 1942 study of Cedar Bog Lake.
Example
Starting with $10{,}000\ \text{kcal}$ in producers, primary consumers store about $1{,}000\ \text{kcal}$, secondary consumers $100$, and tertiary consumers $10$, which is also why eating grain directly delivers roughly $10$ times more food energy per acre than feeding the grain to cattle first. In a temperate lake, primary production of about $1{,}000\ \text{kcal/m}^2/\text{yr}$ might support $150$ at the herbivore level and $30$ at the carnivore level, efficiencies of $15$ and $20$ percent, while endotherms, with production efficiencies of only $1$ to $3$ percent, narrow a food chain faster than one built on insects and fish.
Key Insight
The energy pyramid is a thermodynamic accounting statement explaining why a field holds millions of insects, thousands of mice, and only a handful of hawks, and because energy cannot be recycled the way matter can, an ecosystem's entire consumer biomass is capped by the sunlight its producers capture, a limit no ecological cleverness can escape.