Food Chains

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Food Chains

Food Chain
A food chain describes the flow of food in an ecosystem. Energy in an ecosystem flows from producers to consumers. Images Courtesy U.S. Fish and Wildlife Service

Food Chains

Living organisms must obtain energy in order to sustain life. Animal organisms obtain this energy from the foods they consume. A food chain describes the flow of food in an ecosystem. This flow or feeding structure in an ecosystem is called trophic structure. Each level in this structure is called a trophic level and consists of organisms that are categorized into two main groups: producers and consumers. A food chain charts the movement of energy from one trophic level to the next. Food chains in an ecosystem are often interconnected and are grouped into a larger category known as a food web. Food chains and food webs are found in every land biome and aquatic community in an ecosystem. The cycle of nutrients in an ecosystem ensures that organisms can survive.

Trophic Levels

Producers are the foundation of all food chains in an ecosystem. Because producers generate the energy they need to live without depending on other organisms, they are called autotrophs. These organisms typically use sunlight to generate energy through photosynthesis. Photosynthetic organisms obtain carbon dioxide (CO2), sunlight, and water from their environment and use it to produce sugars and to build biological materials. Examples of producers include plants, phytoplankton, algae, and some kinds of bacteria.

Consumers are organisms in a food chain that are dependent upon producers to obtain nutrients. These organisms are called heterotrophs. The trophic level that immediately follows the producer level is the primary consumer level. Primary consumers eat producers to obtain nutrients. These organisms are called herbivores because they depend upon plants or plant-like organisms (seaweed and phytoplankton) as their main source of nutrients. Examples of primary consumers include cows, rabbits, mice, deer, insects, krill, squid, and zooplankton. Secondary consumers comprise the next trophic level. These organisms are carnivores that consume herbivores. Examples include lizards, small birds, moles, lions, wolves, and small fish. Some secondary consumers are omnivores and derive their nutrition from animals and plants. Bears, gorillas, raccoons, foxes, and humans are examples of omnivores. Tertiary consumers are often carnivores that eat secondary consumers. These organisms include cougars, snakes, alligators, seals, and owls. Some ecosystems contain quaternary consumers. These animals eat tertiary consumers and typically have no natural predators. Examples include sharks, killer whales, eagles, and hawks.

Detritivores are unique consumers that obtain energy by decomposing organic waste and dead organisms. Decomposers, such as bacteria and fungi, help to recycle nutrients in an environment by breaking down organic polymers into smaller units and releasing nutrients back into the ecosystem. Bacteria and fungi absorb the smaller units of the degraded polymers to obtain energy. Some scavengers such as worms, cockroaches, and vultures are also considered detritivores because they consume parts of dead plants or animals.

Energy Flow

Energy in an ecosystem flows from producers to consumers. The available energy in a food chain decreases with each step up the food chain. As organisms harvest the energy stored in food through cellular respiration, most of this energy is used to fuel metabolic processes in the organism. This means that much less energy remains available for consumers in higher trophic levels. It is estimated that only about 10% of the energy available at one trophic level gets transfered to the next level. As such, there is less energy available to support organisms at the top of the food chain. That is why tertiary and quaternary consumers are far less abundant in an ecosystem than organisms at lower trophic levels.

Organisms in a food chain or food web are linked and dependent on one another for survival. If organisms in one trophic level become threatened, it impacts organisms in other trophic levels. Disease, damage, or loss of habitat for producers means less food for primary consumers. This in turn means less primary consumers for tertiary and quaternary consumers to feed on. The plant and animal organisms in such an environment could become endangered species or even become extinct. For this reason, it is vital that an ecosystem remains balanced containing an appropriate proportion of producers and consumers.