On the ocean floor, cracks in the Earth’s crust spew superheated water and nutrients. These cracks are called hydrothermal vents. In other places, pressure forces fine nutrient-rich sediment through vents to produce submerged structures called mud volcanoes. Scientists believe Earth’s earliest organisms used these nutrients as a source of energy in a process called chemosynthesis (“chemo”—chemical, “synthesis”—to make).

Most of the energy in present-day ecosystems, however, comes indirectly from sunlight. Green plants, algae, and some bacteria use light-harvesting chlorophyll and other pigments to fuel the conversion of carbon dioxide and water into energy-rich sugar in a process called photosynthesis.

Making or obtaining sugar and storing it, however, is only the start to meeting a cell’s energy needs. Cells use oxygen to break apart the sugar molecule and release energy in a process called
cellular respiration. Cells use this energy to carry out the functions they need to stay alive.

Energy harnessed by the processes of photosynthesis or chemosynthesis is transferred throughout the entire ecosystem from one organism to another. It does not recycle like most of the matter on Earth but flows in one direction, beginning with the producers. Producers make the chemical energy on which all other organisms depend. Organisms that cannot produce their own food are called consumers. Consumers must eat other organisms to obtain the energy they need and are classified by the types of organisms they consume. Herbivores, also called primary consumers, eat only plants. Carnivores, also known as secondary consumers, eat other animals. Consumers that eat both producers and consumers are called omnivores. Scavengers eat dead or decaying animals. Decomposers feed on dead organisms and organic wastes. All interact within the ecological community.

A food chain is a model that describes how matter and energy are transferred from one organism to another. In an ecological community, there can be many food chains and the feeding relationships between organisms can become complicated. A food web is a model that shows a complex network of food chains in an ecosystem. In the illustration on this page, the polar bear does not eat phytoplankton, which are the producers in this aquatic system, but indirectly depends on them for both matter and energy. Scientists, like Russell Cuhel, need to identify the food chains in an ecosystem before they can construct a food-web model.

Organisms within an ecosystem are grouped together in a category called a trophic level based on the food they eat. Energy flows from the producers to the primary consumers, and then to the secondary and higher consumers. However, only a fraction of the original energy is transferred from one trophic level to the next. The reason for this is that some of the energy is used by the organisms during the process of cellular respiration. In addition, not all the herbivores eat every blade of grass in the ecosystem and not every predator catches and eats every prey.

Food Web
Below the frozen sea of the Arctic, feeding relationships are complex. How many food chains can you identify?

Models that represent energy transfer among trophic levels are shaped like a pyramid, with producers on the bottom, primary consumers on the next level, and secondary and higher consumers at the top. The least amount of energy available in an ecosystem goes to the top predator in the food chain, and usually is represented at the apex, or peak, of the pyramid model. That is why top predators are also known as apex predators. The total amount of biotic material in an area is called the biomass. The biomass of the apex predators is usually small in most land ecosystems, but that may not be the case in all ecosystems.