Overview

All organisms in an ecosystem occupy a trophic level in the food chain. The lowest level consists of primary producers, which synthesize their food from either solar or chemical energy. Each subsequent level obtains energy from the levels below. Detritivores can occupy any of the levels above primary producers.

Food Chains

Within an ecosystem, energy and nutrients flow through organisms in a sequential order known as a food chain. Each organism occupies a trophic level or a specific place within that food chain. The lowest trophic level consists of autotrophic organisms. Autotrophic organisms synthesize their food by transforming energy either from chemicals—chemoautotrophs—or the sun—photoautotrophs. Organisms in the subsequent levels—heterotrophs—obtain energy from other organisms in the levels below. The second trophic level includes primary consumers, herbivores, which consume autotrophs—primary producers—while the third and fourth levels consist of secondary and tertiary consumers, the carnivores. Apex predators occupy the top trophic level of this chain. Decomposers, or detritivores, usually bacteria or fungi that consume dead organisms, can exist on any heterotrophic level and play a critical role in the recycling of nutrients and matter in the ecosystem. These organisms secrete digestive enzymes that break down organic material, converting it into inorganic compounds that can be utilized again by primary producers.

Food Webs

Food chains do not always accurately depict the interactions between species in an ecosystem. Predators and prey can exist at multiple trophic levels. A food web—a holistic model in which all interactions are taken into account—may better depict the complexity of relationships in an ecosystem.

Procedure

Species within an ecosystem are organized into a food chain, a sequential order in which energy and nutrients are passed through different organisms. Each species in the chain represents a specific position known as a trophic level.

The lowest end consists of primary producers called autotrophs, organisms that obtain the energy to synthesize new organic matter from an abiotic energy source, such as sunlight.

All of the levels above consist of heterotrophs, which must obtain energy from other organisms. The first level of heterotrophs are primary consumers, organisms that feed directly on primary producers. Next up, secondary consumers obtain energy from feeding on primary consumers, and they are subsequently eaten by tertiary consumers.

However, organisms are not always exclusive to these categories. For example, a hawk can be both a secondary consumer feeding on seed eating birds, or a tertiary consumer feeding on snakes which may also feed on the seed eating birds.

Decomposers, including fungi, bacteria and detritivores, make up the final trophic level. These organisms feed on the dead remains of other organisms, and can be placed anywhere within the food chain.