Why are there so many mice in a field but only a few owls? Why do wolves need to roam over hundreds of square kilometres? And why do food chains almost never have more than four or five links? The answers all come from one idea: energy flows through an ecosystem in one direction, and a lot of it is used up at every step. Every living thing needs energy to grow, move, keep warm and repair itself. In this lesson you will follow that energy from the Sun, through plants, to the animals that eat them, and see why there is always much less energy at the top of a food chain than at the bottom.
Producers, consumers and decomposers
- Producers (also called autotrophs, "self-feeders") make their own food. Most do it by photosynthesis, capturing light energy to make sugar. Plants, algae and some bacteria are producers. They are the entry point for energy in almost every ecosystem.
- Consumers (heterotrophs, "other-feeders") get energy by eating other organisms. Herbivores eat plants (a deer browsing on cedar), carnivores eat animals (a lynx eating a hare), and omnivores eat both (a black bear eating berries and fish).
- Decomposers, mainly bacteria and fungi, break down dead organisms and wastes, using the energy left in them and releasing nutrients back into the soil and water. Without them, dead material would pile up and nutrients would never be reused.
- Scavengers and detritivores, such as turkey vultures, earthworms and millipedes, eat dead material and help decomposers by breaking it into smaller pieces.
Food chains and trophic levels
A food chain shows one path that energy takes through an ecosystem, for example: algae → zooplankton → yellow perch → northern pike. Each arrow points from the food to the eater, because that is the direction energy flows. Read the arrow as "is eaten by" or "gives energy to." Each step in a food chain is a trophic level (trophic means "feeding"). Producers are the 1st trophic level, primary consumers (herbivores) the 2nd, secondary consumers (animals that eat herbivores) the 3rd, and tertiary consumers the 4th. Some food chains have a 5th level, but very few go longer, for a reason you will see below.
Real ecosystems are messier than one chain. A perch eats zooplankton, but it also eats mayfly larvae, and it is eaten by pike, loons and people. A food web shows many food chains linked together. Food webs make ecosystems more stable: if one food source becomes scarce, many consumers can switch to another. An ecosystem with a more complex food web usually recovers better from a disturbance than one with a simple web.
Why energy shrinks at each level: the 10% rule
When a hare eats grass, most of the energy in the grass does not end up stored in the hare's body. Some grass is not eaten at all (roots, tough stems). Some is eaten but not digested and leaves as waste. And most of what is digested is used by the hare for its own life processes: running from predators, keeping its body warm, breathing, and repairing cells. This energy is released by cellular respiration and ends up as heat that escapes into the environment. Only the energy stored in the hare's new body tissue is available to the lynx that eats it. On average, only about 10% of the energy at one trophic level is passed on to the next. This is called the 10% rule. It is only a rough guide: real transfers usually range from about 5% to 20%, depending on the organisms.
An energy pyramid shows the energy available at each trophic level as a stacked bar, with producers at the base. It always gets narrower toward the top, because energy is lost at every step. This explains several patterns in nature. Top predators like wolves and eagles are rare and need large territories, because there is so little energy available at their level. Food chains rarely have more than four or five levels, because after that there is too little energy left to support another population. And the same farmland can feed more people if they eat plants directly than if the plants are first fed to animals, because each step up the pyramid wastes about 90% of the energy.