4 Ecosystems and Energy Flow

Ecosystem Components and Organism Roles

An ecosystem consists of a community of organisms and the nonliving environment with which they interact. Its living and nonliving parts are linked: organisms depend on environmental conditions and materials, while their activities also affect the environment.

• Biotic components are living organisms, including plants, animals, fungi, and microorganisms.

• Abiotic components are nonliving conditions and materials, such as sunlight, water, air, soil, temperature, and minerals.

Organisms have different roles in how energy enters and moves through an ecosystem. Producers (autotrophs), such as plants and algae, make organic molecules using sunlight. In some ecosystems, including deep-sea hydrothermal vents, bacteria use energy from chemical reactions instead. Consumers (heterotrophs) get energy by eating other organisms. Decomposers, including many fungi and bacteria, break down dead organisms and waste, releasing nutrients back into the environment for producers to reuse.

Takeaway: Ecosystems include both living and nonliving components, and organisms contribute to energy transfer and nutrient reuse in different ways.

Food Chains and Food Webs

A food chain shows one pathway through which energy passes as organisms eat one another. For example:

grass → grasshopper → frog → snake

The arrows point from the organism being eaten to the organism that eats it, showing the direction of energy transfer.

Most organisms have more than one food source and may be eaten by several kinds of organisms. A food web combines many connected food chains, giving a more realistic picture of feeding relationships. Grass, for example, may feed grasshoppers and rabbits; grasshoppers may be eaten by frogs or birds; and snakes may eat frogs or birds. Decomposers break down dead organisms from every part of the web.

Takeaway: A food chain traces one feeding pathway, while a food web connects multiple pathways.

Trophic Levels

An organism’s trophic level is its feeding position in a food chain or food web. The levels are commonly described as:

1. Primary producers form the first level.

2. Primary consumers eat producers.

3. Secondary consumers eat primary consumers.

4. Tertiary consumers eat secondary consumers. Some food webs include still higher-level consumers.

In the chain grass → grasshopper → frog → snake, grass is a producer, the grasshopper is a primary consumer, the frog is a secondary consumer, and the snake is a tertiary consumer. A species can feed at different trophic levels depending on what it eats. Decomposers act on material from all levels rather than occupying only one step in a chain.

Takeaway: Trophic levels describe feeding positions, but an organism’s position can vary with its diet.

Energy Flow and Energy Pyramids

In most ecosystems, sunlight is the original energy source. Producers capture some sunlight and store it as chemical energy in organic matter. Consumers receive energy by eating producers or other consumers, and decomposers obtain energy from dead material and waste.

Energy transfer is one-way: energy enters mainly as sunlight—or, in some ecosystems, chemical energy—passes among organisms, and eventually dissipates as heat. Organisms use much of the energy they acquire for life processes such as movement, growth, and maintaining their bodies. Not all acquired energy becomes new biomass available to the next trophic level. Consequently, less energy is generally available at higher levels, helping explain why food chains are usually limited in length.

A common rule of thumb is that roughly $10\%$ of the energy at one trophic level becomes biomass available to the next. This is an approximation; the actual fraction varies among organisms and ecosystems. An energy pyramid represents the general decline in available energy, with producers at its broad base and higher-level consumers above them.

Energy and matter behave differently in ecosystems: energy flows through and leaves as heat, while nutrients can be recycled through organisms and the environment.

Takeaway: Energy moves through ecosystems in one direction and becomes less available at successive trophic levels, while nutrients can be reused.