Since plants can grow by converting the sun’s energy directly into their tissues, they are known as producers in the ecosystem. The plants are used by herbivorous animals as food, which gives them energy. A large part of this energy is used up for day to day functions of these animals such as breathing, digesting food, supporting growth of tissues, maintaining blood flow and body temperature. Energy is also used for activities such as looking for food, finding shelter, breeding and bringing up young ones. The carnivores in turn depend on herbivorous animals on which they feed. Thus the different plant and animal species are linked to one another through food chains. Each food chain has three or four links. However as each plant or animal can be linked to several other plants or animals through many different linkages, these inter-linked chains can be depicted as a complex food web. This is thus called the ‘web of life’ that shows that there are thousands of interrelationships in nature.
The energy in the ecosystem can be depicted in the form of a food pyramid or energy pyramid. The food pyramid has a large base of plants called ‘producers’. The pyramid has a narrower middle section that depicts the number and biomass of herbivorous animals, which are called ‘first order consumers’. The apex depicts the small biomass of carnivorous animals called ‘second order consumers’. Man is one of the animals at the apex of the pyramid. Thus to support mankind, there must be a large base of herbivorous animals and an even greater quantity of plant material. When plants and animals die, this material is returned to the soil after being broken down into simpler substances by decomposers such as insects, worms, bacteria and fungi so that plants can Energy Cycle absorb the nutrients through their roots.
Primary consumers only obtain a fraction of the total solar energy—about 10%—captured by the producers they eat. The other 90% is used by the producer for growth, reproduction, and survival, or it is lost as heat. You can probably see where this is going. Primary consumers are eaten by secondary consumers. An example would be birds that eat bugs that eat leaves. Secondary consumers are eaten by tertiary consumers. Cats that eat birds that eat bugs that eat leaves, for instance.
At each level, called a trophic level, about 90% of the energy is lost. What a shame. So, if a plant captures 1000 calories of solar energy, a bug that eats the plant will only obtain 100 calories of energy. A chicken that eats the bug will only obtain 10 calories, and a human that eats the chicken will only obtain 1 calorie of the original 1000 calories of solar energy captured by the plant. When you think about this way, it would take 100 1000-calorie plants—those would be enormo plants, by the way—to produce a single 100-calorie piece of free-range chicken. You are now recalling all of the plants you have ever forgotten to water in your life and feeling really, really terrible about it, aren't you?
The relationships among producers, primary consumers, secondary consumers, and tertiary consumers are usually drawn as a pyramid, known as an energy pyramid, with producers at the bottom and tertiary consumers at the top. You can see from the example above why producers are at the bottom of this pyramid. It takes a lot of producers for higher-trophic-level consumers, like humans, to obtain the energy they need to grow and reproduce.
This is the answer to the great mystery as to why there are so many plants on Earth. We will even spell it out for you because it is so important to understand: there are so many plants on Earth because energy flow through ecosystems is inefficient. Only 10% of the energy in one trophic level is ever passed to the next.