Communities and Ecosystems

Energy Flow in a Community

Within a community, food chains and food webs are models used to describe how energy flows as it first begins with self-feeding autotrophs, or producers, and ends with high-level consumers such as carnivores.
A community is a group of interacting species living in the same area. Energy flows through a community through a series of feeding interactions referred to as food chains or food webs. A community remains relatively stable as long as drastic changes, such as natural disasters, do not occur. Factors contributing to community stability include the cycling of energy and nutrients obtained from external sources. Scientists observe the flow of energy through a community by using a pyramid model. A food chain describes the relationship between different organisms based on which organism eats or is eaten by another organism. Any community can consist of many food chains. A food web is a combination of food chains interacting with each other within an ecosystem.

Food Chains Within a Food Web

Food webs and food chains are different ways to model how energy flows through an ecosystem. The food chain describes the relationship between different organisms based on what they eat. Thus, for example, grass obtains energy from the sun via photosynthesis, which helps it grow, and grasshoppers feed on the grass for nutrients and energy. Rats eat grasshoppers and owls eat rats. Both the rat and owl obtain nutrients and energy in this manner. A food web is constructed of multiple, interacting food chains.
A food chain resembles a sequence of events. Within this sequence there are different positions or levels. Each feeding position within a food chain or food web is referred to as a trophic level. At each level, organisms obtain energy necessary for survival. A given food chain usually consists of two or three trophic levels; however, it is not uncommon for a food chain to have five trophic levels.
  • A producer, also known as an autotroph, is an organism in the first trophic level that makes its own food, typically by absorbing energy from sunlight via photosynthesis. Plants, some bacteria, and algae are examples of producers.
  • A primary consumer is an organism in the second trophic level that obtains its energy by consuming plant matter. Organisms in this category are called herbivores. Examples of primary consumers include sheep, grasshoppers, and koalas.
  • A secondary consumer is an organism in the third trophic level that obtains its energy by feeding on organisms that eat producers. Organisms in this category are called carnivores. Examples include wolves and snakes.
  • A tertiary consumer is an organism in the fourth trophic level that obtains its energy by feeding on secondary consumers. Examples include owls and seals.
  • A quaternary consumer is an organism usually at the top of the food chain that obtains its energy by feeding on tertiary consumers. An example is a hawk that eats an owl, which is a tertiary consumer.

Because all organisms eventually die, scavengers, detritivores, and decomposers exist. A scavenger, such as a vulture, is an animal that feeds on dead plants or animals or on refuse. A detritivore, such as an earthworm, is an organism that gets its energy by feeding on dead organic material, particularly plants. A decomposer is an organism, such as a bacterium or fungus, that breaks down dead materials and organic wastes. All of these organisms break down decaying matter and return nutrients and minerals to the ecological community.

It is not uncommon for consumers to feed at more than one trophic level. For example, when humans eat only plants, such as tomatoes, they are classified as primary consumers. However, humans can be secondary consumers if they eat beef from cattle.

Trophic Levels

In a food chain, trophic levels are used to describe the feeding positions of organisms and show how energy flows. Each trophic level has its own unique characteristics. The arrows signify the flow of energy through the community with the sun serving as the primary energy source. A small amount of energy passes from one trophic level to the next; thus, organisms at the higher trophic levels must feed more to obtain sufficient energy for survival.