08 Ecosystems and Energy Flow

Ecosystem components and roles

An ecosystem includes a biological community—the populations of different organisms in an area—and its abiotic surroundings. Sunlight, water, air, soil, and temperature interact with living things. These components are connected by the movement of energy and the cycling of matter.

Organisms have different roles in these relationships. Producers make organic molecules, usually by capturing sunlight through photosynthesis; some bacteria use chemical energy instead. Consumers gain energy by eating organisms or organic matter. Decomposers break down dead organisms and wastes, returning nutrients to the environment. Detritivores, such as earthworms, consume dead organic material.

Following feeding relationships

A food chain traces one pathway through which energy and nutrients pass as organisms feed on one another. For example, grass can be eaten by a grasshopper, which can be eaten by a frog, which can be eaten by a snake.

The arrows in a food chain point from the organism or material being eaten toward the organism that eats it. They show the direction of energy transfer. In natural communities, an organism may eat several species and may itself be eaten by several others. Many connected food chains therefore form a food web, a more realistic picture of feeding relationships.

Trophic levels and energy transfer

A trophic level is an organism’s feeding position in a food chain or web. Producers occupy the first level. Organisms that eat producers are primary consumers; organisms that eat primary consumers are secondary consumers. Higher-level consumers may be called tertiary consumers or top predators. Omnivores can feed at more than one level, while decomposers obtain material from organisms across the web.

Energy generally enters an ecosystem as sunlight. Producers convert it into chemical energy stored in organic matter. When one organism eats another, some energy becomes part of the consumer’s body and can pass to the next level. Much of the energy is used for life processes and released as heat through respiration; some remains in uneaten parts and waste.

A common teaching approximation is that about $10\%$ of the energy available at one trophic level becomes available to the next. This is a rule of thumb, not a fixed law: transfer efficiency varies among organisms and ecosystems. For example, if producers store $10{,}000$ energy units, the next level might receive about $1{,}000$, and the level after that about $100$.

Energy pyramids and matter cycling

An energy pyramid represents the decline in available energy at successive trophic levels. Its levels show energy transferred over a given area and time, with less energy available higher in the pyramid. Because energy transfer is incomplete at each step, food chains usually have only a limited number of levels.

Energy and matter behave differently in ecosystems. Energy flows through the ecosystem and ultimately dissipates as heat. Matter is recycled: decomposers release nutrients that producers can use again.

Takeaway: Producers capture energy, consumers transfer it through feeding relationships, and decomposers return nutrients to the environment. Energy diminishes as it flows through trophic levels, while matter cycles.