A food web is a complex diagram illustrating the movement of energy through an environment by showing how organisms are connected by what they eat. Every organism requires energy to survive, grow, and reproduce. Consumers represent organisms that must obtain their energy by feeding on other organisms. They drive the flow of energy across the ecosystem by ingesting organic matter.
Defining Consumers and Producers
Organisms in an ecosystem are broadly separated into two groups based on how they acquire energy: producers and consumers. Producers, also known as autotrophs, create their own food, typically by capturing light energy from the sun through photosynthesis. Plants, algae, and certain bacteria form the base of nearly every food web because they convert inorganic materials into organic energy-rich compounds.
Consumers are classified as heterotrophs because they cannot manufacture their own food. They must ingest or absorb organic material from other sources, meaning they eat either producers or other consumers. This reliance places consumers in a position to receive energy originally captured by the producers. Producers convert external energy into biomass, while consumers transfer that stored biomass energy throughout the rest of the ecosystem.
The Trophic Levels of Consumption
Consumers are organized into distinct feeding positions, called trophic levels, based on what they eat. Primary consumers occupy the second trophic level, feeding directly on producers, and are commonly known as herbivores. Examples include grazing mammals like deer, insects that eat leaves, and zooplankton that consume microscopic algae in aquatic systems.
The next level consists of secondary consumers, which prey on primary consumers, often making them the first level of carnivores. Tertiary consumers then feed on secondary consumers. A small fish that eats zooplankton or a snake that eats a mouse are examples of secondary consumers.
In some food webs, a fourth level consists of quaternary consumers, which are predators that eat tertiary consumers. The top of the food web is often occupied by an apex consumer, a predator that typically has no natural predators in its own ecosystem, such as an orca or a bald eagle. An omnivore, like a bear or a human, eats both producers and other consumers, allowing it to occupy multiple trophic levels simultaneously.
The Functional Role of Consumers in Ecosystems
Consumers perform the primary function of transferring energy through the food web. This energy transfer is highly inefficient, with only about 10% of the energy from one trophic level being converted into biomass at the next level. The remaining 90% of the energy is lost, mostly as heat, or used by the organisms for metabolic processes, which is why food webs rarely have more than four or five consumer levels.
Consumers also play a regulatory role by controlling the populations of organisms at lower trophic levels. Herbivores prevent plant populations from growing unchecked, and predators keep herbivore populations from overgrazing the producers, a process that helps maintain ecological balance. When these organisms die, their stored energy and nutrients are made available to decomposers, such as fungi and bacteria. Decomposers break down the dead organic matter, recycling essential nutrients back into the soil or water for producers to use.