As mentioned at the beginning of this section, all organisms on Earth require energy to live. We have also just learned that most plants get their energy from the sun through the process of photosynthesis. Plants that use the photosynthesis process, along with photosynthesizing microbes, are called autotrophs. The word autotroph comes from the Latin word for “self-feeding.” These plants and microbes are called autotrophs because they make their own food energy by converting solar energy into sugar through the metabolic process of photosynthesis. But, what about other forms of life, such as humans, mammals, and insects that cannot metabolize solar energy? These organisms are called heterotrophs (from the Latin word meaning “different-feeding”), because they must consume organic compounds found within the tissues of other organisms in order to supply the food energy they need. 

Looking Ahead


You will learn in the upcoming Energy and Spirituality that many religions of the world hold that human beings possess an inner ‘spiritual energy’.

Numerous forms of both autotrophs and heterotrophs that live within the same ecosystem maintain harmonious existence with each other through a trophic structure. A trophic structure is a pattern of movement of energy and matter between organisms in a specific ecosystem. The complex trophic structure that maintains balance between autotrophs and heterotrophs, known as a food web, is patterned on the feeding relationships between organisms that coexist within an ecosystem. An example of a simple soil food web (Figure 4) demonstrates the interdependent links between organisms. Each organism plays a role within the ecosystem that is required for the vitality of the other organisms in the food web.  

soil food web
Figure 4: A soil ecosystem food web. Arrows depict the flow of energy from one organism to the next through the food web. 1
There are different trophic levels that are established within a food web. The differentiation of these levels is based on groups of organisms that share the same feeding habits and primary means of obtaining energy. For example, photosynthetic plants make up the primary producer trophic level because they are primary to the food web and they produce their own sugars which then fuels the rest of the food web. Primary consumers, or herbivores, make up the second trophic level, because they consume primary producers. Secondary consumers are carnivores or omnivores that consume primary consumers and sometimes plants. Tertiary consumers (also called top carnivores) consume primary and secondary consumers. Most food webs cannot support more than four trophic levels, due to the inefficiency of energy transfer (the loss of usable energy) from one level to the next (i.e. the Second Law of Thermodynamics). If the plants, microorganisms, and animals in a food web are categorized by their trophic levels, we can display and summarize their relationships, trophic level biomass, and energy transfer within a trophic pyramid (Figure 5).
trophic levels
Figure 5: A simple trophic pyramid demonstrates the loss of energy (kilo calories, or biomass) from one tropic level to the next. Depicted as a pyramid, we can see that the primary producers or photosynthesizing autotrophs, make up the base of most food webs. As energy is transferred up the pyramid toward the top carnivores, approximately 90% is lost at each level, and only 10% is converted into biomass of the next higher trophic level. 2

In the trophic pyramid depicted in Figure 5, there are four trophic levels. At the base of the pyramid are the photosynthetic plants or primary producers. Note that the total biomass (or stored energy) of the base trophic level is many times greater than the other trophic levels, with each successive trophic level containing a significantly smaller total biomass. The reduction in biomass occurs because there is a large energy loss associated with each trophic transfer, while only 10% of the energy is transferred from one level to the next and converted to biomass at the next higher trophic level.

For example, when an herbivore (such as a grass-eating insect) consumes 100 kcal (kilo calories) of grass, it cannot convert 100% of those kcal into insect biomass. Instead, some of the energy is used in the insects’ searching to find suitable food, and in eating and digesting the food. Some of the food energy is consumed through basic metabolic processes in the insect, and some is lost through defecation of feces. Therefore, only approximately 10% of the energy consumed will be converted into new growth of insect tissues or reproduction by the insect.

Trophic efficiency is calculated by the percentage of energy that consumers in one trophic level gain and convert into biomass from the total stored energy of the previous trophic level. Because trophic efficiency is so low, there are very few tertiary consumers (as depicted with the hawk in Figure 5).

As explained in the Biodiversity Chapter, the Earth is home to many different biomes, each supporting hundreds of regionally-unique food webs, like the one featured in Figure 6. Over the past 3 billion years, the number of different life forms has proliferated, increasing the Earth’s biodiversity, and also increasing the total biomass of living organisms. Upon death of an organism, some of this biomass becomes buried in geologic deposits. Over the course of several hundreds of millions of years this biomass has been converted into combustible crude oil, coal, natural gas, and heavy oils due to exposure to heat and pressure in the Earth’s crust. These combustible organic deposits are called fossil fuels. Later in this chapter, we will learn about how humans utilize these fossilized organic deposits as a form of fuel in many modern cultures.