2 Ecosystems and Ecological Processes

Ecosystem Structure

An ecosystem consists of a community of living organisms and the nonliving environment with which they interact. Its biotic components are organisms; its abiotic components include sunlight, water, air, soil, temperature, and minerals.

These components are connected. For example, soil moisture and sunlight affect plant growth, which in turn influences the animals and decomposers able to live in that environment.

Organism Roles

Organisms occupy different roles in ecosystems:

• Producers, also called autotrophs, include plants and algae. They make organic food using sunlight or, in some environments, chemical energy.

• Consumers, also called heterotrophs, obtain energy by eating other organisms. Herbivores eat producers; carnivores eat other consumers; omnivores eat both.

• Decomposers, including many fungi and bacteria, break down dead organisms and waste, returning nutrients to the environment.

Energy Flow and Trophic Levels

Energy enters most ecosystems as sunlight. Producers convert some of that energy into chemical energy stored in organic molecules. When organisms eat, energy is transferred through trophic levels: producers, primary consumers, and then secondary and higher-level consumers. Some ecosystems, such as deep-sea vent communities, rely on chemical energy rather than sunlight.

Energy flows in one direction rather than being recycled. Organisms use energy for life processes, and much of it is released as heat. As a result, less energy is generally available at higher trophic levels, limiting how much life those levels can support. An energy pyramid represents this decline in energy from the base to the top.

Food Chains and Food Webs

A food chain shows one pathway of feeding and energy transfer. 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.

A food web combines many connected food chains. It better represents most ecosystems because organisms often eat several kinds of food and may be eaten by multiple predators. If one population changes, effects can spread through these feeding relationships. Decomposers act on dead material and waste from organisms at every trophic level.

Nutrient Cycles

Unlike energy, matter is recycled. Elements and compounds move between organisms and nonliving reservoirs such as air, water, soil, and rock. These movements are called biogeochemical cycles. Decomposers help return nutrients from dead matter and waste to forms that producers can use.

• Water cycle: Water evaporates from surfaces, condenses in the atmosphere, and returns as precipitation. It may run off into waterways, enter the soil, or be taken up by organisms.

• Carbon cycle: Producers take in carbon dioxide during photosynthesis and incorporate carbon into organic matter. Carbon moves through food webs and returns to the atmosphere or water through respiration and decomposition. Some carbon is stored for long periods in sediments and fossil fuels.

• Nitrogen cycle: Bacteria convert atmospheric nitrogen into forms plants can absorb. Nitrogen moves through food webs and returns to soil and water through waste and decomposition; other bacteria return some nitrogen to the atmosphere.

• Phosphorus cycle: Weathering releases phosphate from rocks into soil and water. Organisms take it up, and it returns through waste and decomposition. Runoff can carry phosphate into aquatic ecosystems.

These cycles are interconnected. For instance, water moving across land can carry nitrogen and phosphorus into rivers and lakes.

Putting the Processes Together

Ecosystems are shaped by interactions among living organisms and their nonliving surroundings. Producers capture energy, consumers transfer it through food chains and food webs, and decomposers help recycle nutrients. Energy flows through ecosystems and is ultimately dissipated as heat, while matter cycles between organisms and the environment.