Imagine you’re driving down the road in a car. As you look out the window you notice a stretch of land with plants as far as the eye can see. Grass, flowers, trees, and other plants are visible in every direction. You probably wouldn’t be too shocked or surprised to see this image. But imagine, instead, looking out the window and only seeing animals as far as the eye can see. Snakes, hawks, rabbits, and other animals so thickly packed together that you would have to step over them to take a walk; what would that sound like and smell like?
There’s no place in the world where we can see animals packed together like this. So then, why does our world have so many more plants than animals? One explanation for this that scientists have developed is called the Pyramid of Energy. You know that all living things need energy, from the smallest bacteria to the largest plants or mammals. You may also know that people use food chains and food webs to show how this energy is passed from the sun to producers (plants) and then to consumers (animals, fungi, bacteria, etc.). What you may not know is that every time energy is passed from one living thing to the next, only a small portion of the energy makes it to that next living thing. Only the tiniest amount of the sun’s energy reaches us here on Earth, only a small amount is used by an apple tree, and then we only get a very small amount of energy from the apple. By the time you get to the end of any food chain, most of the energy that was available at the beginning is no longer available. Scientists came up with the Pyramid of Energy to explain how this works.
The Pyramid of Energy is a model that uses a pyramid shape to show that the energy that consumers (like animals) can use decreases as it travels through organisms. Producers are at the bottom of the pyramid because they are able to transform the sun’s energy into a large amount of plant energy through the process of photosynthesis. Producers are the base of energy for most food chains and food webs, like the basement is for a house.
Primary consumers make up the next level because they rely on plants for their energy. This level of the pyramid is smaller than the producer level because most of the energy used at the producer level is used by producers for their life processes (respiration, photosynthesis, reproduction, etc.) and transformed to heat before they are eaten.
Secondary consumers make up the next level because they rely on primary consumers for most of their energy. This level of the pyramid is smaller than the primary consumer level because most of the energy used at the primary consumer level is used for their life processes (respiration, digestion, reproduction, etc.) and transformed to heat before these animals are eaten.
The highest level can be either tertiary consumers or decomposers. They rely on secondary consumers for most of their energy. This level of the pyramid is smaller than the secondary consumer level because most of the energy used at the secondary consumer level is used for their life processes and transformed to heat before these animals are consumed. It’s possible to have additional levels of consumers, but in most food chains energy runs out by the second or third level.
How much energy is lost from one level to the next? Scientists say that on average 90% of the available energy is used for life processes such as respiration, photosynthesis, and reproduction and transformed to heat energy before an organism is consumed. A molecule called ATP is used to store this energy, and when the energy is taken from ATP, some of the energy is lost as heat. This means only about 10% of the original energy is left to feed the next level. This 10% is stored in the tissues (leaves, stem, muscles, organs, fat, etc.) of the organism.
Let’s take an example, using the wetlands. The wetlands is a biome found in very moist areas, usually on the banks of rivers, oceans or lakes. Here producers such as phytoplankton (types of plants) are producing 500,000 calories of energy from sunlight each day. If you gathered all of this phytoplankton into one spot, they might fill a space the size of a room. Since only about 10% of this energy reaches the zooplankton (types of animals), this leaves about 50,000 calories of energy to support the zooplankton each day. If you gathered all of these zooplankton into one spot, they might fill a space the size of a school dumpster. Since only about 10% of this energy reaches the sunfish, this leaves about 5,000 calories of energy to support the sunfish each day. If you gathered all of these sunfish into one spot, they might fill a space the size of a trash can. Since only about 10% of this energy reaches the great blue herons (a type of bird), this leaves about 500 calories of energy to support herons each day. If you gathered all of the herons into one spot, you would have only one! Fortunately, animals don’t just depend on one type of consumer to meet their energy needs. But in essence it takes the energy of a room full of phytoplankton just to support one great blue heron.
This is why we have so much plant life at the bottom of each food chain yet so few consumers at the top of each food chain. So the next time you take a car ride, you’ll know why there’s so many green plants and so few animals. It’s all about energy!