The Base of the Pyramid
Producers are those organisms that use photosynthesis to capture energy from sunlight, water and carbon dioxide to create carbohydrates. They use the energy to create more complex molecules like proteins, lipids and starches that are crucial to life processes. Producers, which are mostly green plants, are also called autotrophs.
Producers funnel into the ecosystem the energy needed for its biological processes. The carbohydrates and other organic chemicals formed by the producers are consumed and utilized by the heterotrophs, or consumers. First, herbivores -- the primary consumers -- eat the plants. Predators -- the secondary, tertiary consumers -- eat the herbivores. But at each step, much energy is lost. Less than 10 percent of the energy stored in plants is converted to herbivore mass. The loss from herbivore to predator is similar. Thus, energy needs to be added to the ecosystem continuously. This is the producers' role.
Shaping the Ecosystem
The efficiency of the producers in adding energy to the ecosystem determines how robust that ecosystem will be. Efficient producers enable an ecosystem to support secondary, tertiary or even quaternary consumers. The energy provided by less efficient producers will be completely dissipated by the first or second level. Aquatic ecosystems are more diverse and robust than terrestrial ecosystems for this reason -- the aquatic producers, such as algae and other microorganisms, are more efficient energy converters than terrestrial plants.