Definition
An ecological pyramid is a diagram that stacks up, from the bottom, the number of organisms, the biomass (weight), or the energy at each trophic level (each position in who-eats-whom) of a food chain. Because the energy passed along drops sharply at each step up, it usually narrows into a pyramid shape.
Why does it get narrower toward the top?
No matter how much food is on a buffet table, we use most of it up just eating, digesting, and breathing. Living things do the same with the energy they take in: they burn almost all of it just to stay alive. Of the energy a plant makes from sunlight, only about 10 percent or so actually reaches the next level, the grasshopper.
When a grasshopper eats grass, the energy goes into keeping its body warm, jumping, and breathing, and some leaves as waste. Only a tiny share gets stored as flesh and muscle that a frog can eat.
Each step up, from frog to the snake that eats it, and then to the eagle that eats the snake, the energy shrinks to roughly one tenth. Depending on the ecosystem and the organism, the share can range from a few percent to about 20 percent. That is why feeding the top predator takes a tremendously large layer of producers holding up the bottom.
The three faces of the pyramid
Ecological pyramids come in three main kinds, depending on what you measure. The "pyramid of numbers" counts living individuals. The "pyramid of biomass" measures the total weight of living things. The "pyramid of energy" measures energy.
Usually all three narrow toward the top: millions of blades of grass, tens of thousands of grasshoppers, thousands of frogs, and one hawk. But in a forest where thousands of caterpillars live on a single giant oak, the pyramid of numbers can turn into an upside-down jar shape.
This happens because living things come in all sizes. The surface pictures, numbers and weight, can flip from one ecosystem to another. But when you measure the energy flowing through, every ecosystem keeps a perfect pyramid shape, without exception.
A closer look: does the pyramid flip upside down in the ocean?
In the open ocean, you can see something that looks very strange. Observers find a biomass pyramid that is upside down: the animals that eat phytoplankton, the ocean's primary producers, such as zooplankton and small fish, weigh more than the phytoplankton itself.
How can the eaters outweigh the food and the ecosystem still hold up? The secret is growth speed, or turnover. Phytoplankton are tiny, but they multiply incredibly fast, so whatever gets eaten is replaced by new individuals within days.
So if you snap a photo at one moment and weigh what you see, the producers look small. But add up the energy they make over a whole year, and phytoplankton still come out far ahead. The rule that holds up the base of an ecosystem works in the ocean too, without the slightest error.
Common misconceptions
An ecosystem's pyramid is always a right-side-up triangle, no matter what.
In a forest where one big tree hosts many insects, or in ocean ecosystems, the pyramid of numbers or biomass can flip upside down. The pyramid of energy, however, never flips in any case.
Where you meet it
On the African savanna, there are tens of thousands of zebras and wildebeests, but you can count the lions on your fingers.
In a forest, a single giant oak can feed hundreds of caterpillars and birds, which shows how the pyramid of numbers can look.
In one sentence
An ecological pyramid is the balanced structure of an ecosystem that narrows naturally because only about 10% of the energy passes up to each higher level.