15.7: Energy pyramids reveal the inefficiency of food chains.

Look out of the nearest window. What organisms can you see? Almost without fail, you will see green plant life. Maybe some trees, possibly bushes and grasses as well. You’ll have to look longer and harder to see any animals, but you’ll probably see a few, most likely small animals and various insects that eat plants. On the other hand, you might stare out of the window all day and not see any animals (other than some fellow humans) that eat other animals. Why? And why are big, fierce animals so rare? Also, why are there so many more plants than animals?

The answers to these questions are closely related to our observation that an animal consuming five pounds of plant material does not gain five pounds in body weight from its meal. The actual amount of growth such a meal can support is far, far less—about 10%—and this is fairly consistent across all levels of the food chain. So the herbivore consuming five pounds of plant material is likely to gain only about half a pound in new growth, while the remaining 90% of the meal is either expended in cellular respiration or lost as feces. Similarly, a carnivore eating the herbivore converts only about 10% of the mass it consumes into its own body mass. Again, 90% is lost to metabolism and feces. Additionally, non-predatory deaths reduce the transfer of energy from one trophic level to the next. And the same inefficiency holds for a top carnivore as well. Let’s explore how this 10% rule limits the length of food chains and is responsible for the rarity of big, fierce animals outside your window and across the world

Biomass is the total weight of living or non-living organic material in a given volume, such as a single organism, or, on a larger scale, the weight of all plant and animal matter in an ecosystem. Given the 10% efficiency with which herbivores convert plant biomass into their own biomass, how much plant biomass is necessary to produce a single 1,200-pound (500 kg) cow? On average, that cow would need to eat about 12,000 pounds (5,000 kg) of grain in order to grow to weigh 1,200 pounds. But that 1,200-pound cow, when eaten by a carnivore, could only add about 120 pounds of biomass to the carnivore, and only 12 pounds to a top carnivore. That’s a huge amount of plant biomass required to generate a tiny amount of our top carnivore, which explains why big, fierce animals are so rare (and why vegetarianism is more energetically efficient than meat-eating). Multiply that 5,000 kg of grain by several hundred—or more appropriately, thousands—and you can see that millions of kilograms of grain are required to support only a few top carnivores.

How much plant biomass would be required to support an even higher link on the food chain? Ten times as much—so much that there might not be enough land in the ecosystem to produce enough plant material. And even if there were, the area required would be so large that the “top, top carnivores” might be so spread out and so busy trying to eat enough that they’d be unlikely to encounter each other in order to mate. Hence, the 10% rule limits the length of food chains.

We can illustrate the path of energy through the organisms of an ecosystem with an energy pyramid, in which each layer of the pyramid represents the biomass of a trophic level. In FIGURE 15-14, we can see that for terrestrial ecosystems, the biomass (in kilograms per square meter) found in photosynthetic organisms, at the base of the pyramid, is reduced significantly at each step, given the incomplete utilization by organisms higher up the food chain. FIGURE 15-15 illustrates the huge variation in primary productivity across a variety of ecosystems. It is highest in tropical rain forests, marshes, and algal beds, and lowest in deserts, tundra, and the open ocean. In each case, the shapes of the energy and biomass pyramids are similar. With a smaller base, though, the ability of an ecosystem to support higher levels in the food chain is reduced. One dramatic exception is seen in some aquatic ecosystems where the producers are plankton. Because plankton have such short life spans and rapid reproduction rates, a relatively small biomass can support a large biomass of consumers, giving rise to an inverted pyramid (see the bottom pyramid in Figure 15-15). However, if you quantified the amount of energy available to consumers (rather than measuring biomass), the pyramid would resemble those seen in terrestrial ecosystems.