biomass biomass pyramid energy pyramid
primary productivity pyramid of numbers
All organisms require energy for growth, reproduction, and, in some species, movement. But there is a limited amount of energy available in an ecosystem—an "energy budget" that is divided among the different trophic levels. This energy budget influences the types and numbers of organisms in the ecosystem.
Productivity of Ecosystems
What determines an ecosystem's energy budget? For most ecosystems, the answer begins with the amount of sunlight that enters the ecosystem. Much of the sunlight that bombards Earth every day bounces back into space or is absorbed by the atmosphere. Of the light energy that reaches plants and other producers, only a tiny fraction—about 1 percent—is captured by photosynthesis. The producers convert this light energy to the chemical energy stored in organic compounds. Even such a small percentage of the sun's total energy output is enough to enable Earth's producers to manufacture billions of kilograms of organic material, or biomass, each year.
The rate at which producers in an ecosystem build biomass is called primary productivity. The level of primary productivity in an ecosystem sets the energy budget. In other words, primary productivity determines the maximum amount of energy available to all the higher trophic levels in an ecosystem.
An ecosystem's primary productivity is the rate at which the producers build biomass. Factors such as rainfall and
temperature influence productivity. Ecological Pyramids
Only 15 percent of the energy stored in this leaf will be converted to new caterpillar biomass through growth of new tissue. (The caterpillar converts slightly more energy to biomass than the average 10 percent.)
To depict information about energy, biomass, and numbers of organisms at different trophic levels, ecologists use three types of diagrams: energy pyramids, biomass pyramids, and pyramids of numbers. In each case, the foundation of the pyramid is the producer level. The primary consumers form the next block, and so on.
This generalized energy pyramid indicates that only 10 percent of the energy available at a trophic level is typically converted to new biomass in the next trophic level.
Notice in Figure 36-7 that the amount of energy available to the top-level consumer is tiny compared to that available to primary consumers. For this reason, it takes a lot of vegetation to support higher trophic levels. This explains why most food chains are limited to three or four levels; there is simply not enough energy at the top of an energy pyramid to support another trophic level. For instance, lions and killer whales have no natural predators; the energy stored in populations of these top-level consumers is not enough to feed yet another trophic level.
Biomass Pyramids A biomass pyramid represents the actual biomass (dry mass of all organisms) in each trophic level in an ecosystem. Most biomass pyramids narrow sharply from the producer level at the base to the top-level consumers at the peak (Figure 36-8). There are some exceptions, however. In certain aquatic ecosystems, the zooplankton (primary consumers) consume the phytoplankton
Biomass pyramids and pyramids of numbers are two other ways of modeling
information about an ecosystem. A biomass pyramid (left) represents the dry mass of all organisms at each trophic level in an ecosystem. A pyramid of numbers (right) depicts the number of organisms at each trophic level.
Pyramids of Numbers A pyramid of numbers depicts the number of individual organisms in each trophic level of an ecosystem. These pyramids are also organized like energy pyramids, with producers found at the foundation and higher trophic levels on each step above them. In most cases, the foundation is again the widest section, indicating that there are more individual producers than there are primary consumers, and so on (Figure 36-8). This pyramid emphasizes how few top-level consumers an
ecosystem can support. Exceptions to the usual shape of a number pyramid occur when small organisms eat larger ones. For example, a single tree (producer) may be the sole food source for hundreds of insects (primary consumers).
Concept Check 36.2
1. What does primary productivity measure? What does it tell you about an ecosystem?
2. What point does each type of ecological pyramid emphasize?
3. What does the shape of the energy pyramid indicate?
