What is Trophic Levels and Food Webs?

Trophic Levels and Food Webs : Energy flows and nutrients are cycled through an ecosystem via organisms that feed on one another. When considering energy, the organisms in an ecosystem are assigned functional levels based upon their relative positions in terms of energy flow. These levels of energy consumption and conversion within a community are called feeding levels, or trophic levels.

As such, the organisms in the first level that make energy available for all the other organisms are called autotrophs. They are also referred to as the primary producers because they produce food or chemical energy that supports all other organisms within the community. In most cases, the primary producers are photosynthetic, and convert sunlight energy into a biochemical form called food. In other situations, the ultimate source of energy comes from chemicals.

All of the other organisms in an ecosystem that depend on the primary producers are called consumers. Consumers are not able to obtain and use sunlight energy to sustain themselves. They must feed on either the primary producers or other organisms. These types of organisms are referred to as heterotrophs. Ecologists make further distinctions between the levels of consumption that occur in ecosystems. Primary consumers eat only primary producers. These are also known as herbivores. An example of a herbivore would be a rabbit. Secondary consumers are those that feed on herbivores - a carnivore, if you will. An example of a carnivore that feeds on herbivores would be a snake. A tertiary consumer would be an organism that feeds on the snake, probably something like a hawk.

The hawk feeds on the snake, which feeds on the rabbit, which feeds on grass. This list of organisms at different trophic levels, linked by consumption, is called a food chain. A food chain is characterized by only one link between each trophic level - each organism feeds on only one other organism. Food webs more closely approximate the complex arrangement of energy flow and nutrient cycling between organisms. That is to say, different types of secondary consumers may consume one type of herbivore. For example, a grasshopper may fall prey to a frog, or a lizard, or bird. The frog, lizard, or bird may be eaten by one of any number of carnivores. Those carnivores in turn may be consumed by a variety of higher level carnivores. Then, if any of these die, their bodies could get eaten or decomposed by any number of other organisms. In this way, a food web can better represent the energy flow and nutrient cycling within an ecosystem.