15.6–15.8: Energy and chemicals flow within ecosystems.

A Parson’s chameleon (Calumma parsonii) eating a grasshopper in Madagascar.

All life on earth is made possible because energy flows perpetually from the sun to the earth. Looking at an ecosystem such as a desert savanna, for example—trying to understand how all the species, from grasses and trees to birds and mammals and worms, interact with one another, and what role each plays within the system—can seem overwhelming. But if we focus on just one aspect of the ecosystem—the pathways energy takes as it flows through the system—a simple and logical order becomes clear.

The sun is where our pathway of energy flow begins. Most of the energy is absorbed or reflected by the earth’s atmosphere or surface, but about 1% of it is intercepted and converted to chemical energy through photosynthesis. That intercepted energy is then transformed again and again by living organisms, making about four stops as it passes through an ecosystem. Let’s examine what happens at each of the stops, known as trophic levels (FIGURE 15-12).

Figure 15.12: Follow the fuel.

First stop: producers. When it comes to energy flow, all the species in an ecosystem can be placed in one of two groups: producers and consumers. Plants (along with some algae and bacteria) are the producers. They convert the sun’s light energy into chemical energy through photosynthesis, as discussed in Chapter 4. We use another word to describe that chemical energy: food. The amount of organic material produced in a biome is called its primary productivity level.

Second stop: primary consumers—the herbivores. Cattle grazing in a field, gazelles browsing on herbs, insects devouring the leaves of a crop plant—these are the primary consumers in an ecosystem, the animals that eat plants. Plant material such as cellulose can be difficult to digest. Consequently, most herbivores, the animals that eat plants, need a little help in digesting their food. Primary consumers, from termites to cattle, often have bacteria living in their digestive system. These microorganisms benefit the organism in which they live by breaking down the cellulose, enabling the herbivore to harness the energy held in the chemical bonds of the plants’ cell walls.

Third stop: secondary consumers—the carnivores. The energy that the herbivore harnesses fuels its growth, reproduction, and movement, but that energy doesn’t remain in the herbivore forever. Carnivores, such as cats, spiders, and frogs, are animals that feed on herbivores. They are also known as secondary consumers. As they eat their prey, some of the energy stored in the chemical bonds of carbohydrate, protein, and lipid molecules is again captured and harnessed for their own movement, reproduction, and growth.

Fourth stop: tertiary consumers—the “top” carnivores. In some ecosystems, energy makes yet another stop: the tertiary consumers, or “top carnivores.” These are the “animals that eat the animals that eat the animals that eat the plants.” They are several steps removed from the initial capture of solar energy by a plant, but the general process is the same. A top carnivore, such as a tiger, eagle, or great white shark, consumes other carnivores, breaking down their tissues and releasing energy stored in the chemical bonds of the cells. As in each of the previous steps, the top carnivores harness this energy for their own physiological needs.

This path from producers to tertiary consumers is called a food chain. We see later in this chapter why a food chain almost never extends to a fifth stop.

The food chain pathway from photosynthetic producers through the various levels of animals is a slight oversimplification. In actuality, food chains are better thought of as food webs, because many organisms are omnivores and can occupy more than one position in the chain (see Figure 15-12). When you eat a simple meal of chicken and vegetables, after all, you’re simultaneously a carnivore and a herbivore. On average, about 30% to 35% of the human diet comes from animal products and the remaining 70% to 65% from plant products. Many other animals, from bears to cockroaches, also have diets that involve harvesting energy from multiple stops in the food chain.

In every ecosystem, as energy is transformed through the steps of a food chain, organic material accumulates in the form of animal waste and dead plant and animal matter. Decomposers, usually bacteria or fungi, and detritivores, including scavengers such as vultures, worms, and a variety of arthropods, break down the organic material, harvesting energy still stored in the chemical bonds (FIGURE 15-13). Decomposers are distinguished from detritivores because the decomposers are able to break down a much larger range of organic molecules. But both groups release many important chemical components from the organic material, which can eventually be recycled and used by plants and other producers.

Figure 15.13: Nothing is wasted.

Energy flows from one stop to the next in a food chain, but not in the way that a baton is passed by runners in a relay race. The difference is that at every step in the food chain, much of the usable energy is lost as heat. An animal that eats five pounds of plant material doesn’t convert that into five new pounds of body weight. Not by a long shot. In the next section, we’ll see how this inefficiency of energy transfers ensures that most food chains are very short.