Trophic Level - Overview

Overview

The three basic ways in which organisms get food are as producers, consumers and decomposers.

• Producers (autotrophs) are typically plants or algae. Plants and algae do not usually eat other organisms, but pull nutrients from the soil or the ocean and manufacture their own food using photosynthesis. For this reason, they are called primary producers. In this way, it is energy from the sun that usually powers the base of the food chain. An exception occurs in deep-sea hydrothermal ecosystems, where there is no sunlight. Here primary producers manufacture food through a process called chemosynthesis.

• Consumers (heterotrophs) are animals which cannot manufacture their own food and need to consume other organisms. Animals that eat primary producers (like plants) are called herbivores. Animals that eat other animals are called carnivores, and animals that eat both plant and other animals are called omnivores.

• Decomposers (detritivores) break down dead plant and animal material and wastes and release it again as energy and nutrients into the ecosystem for recycling. Decomposers, such as bacteria and fungi (mushrooms), feed on waste and dead matter, converting it into inorganic chemicals that can be recycled as mineral nutrients for plants to use again.

Trophic levels can be represented by numbers, starting at level 1 with plants. Further trophic levels are numbered subsequently according to how far the organism is along the food chain.

• Level 1: Plants and algae make their own food and are called primary producers.
• Level 2: Herbivores eat plants and are called primary consumers.
• Level 3: Carnivores which eat herbivores are called secondary consumers.
• Level 4: Carnivores which eat other carnivores are called tertiary consumers.
• Level 5: Apex predators which have no predators are at the top of the food chain.

• Second trophic level
  Rabbits eat plants at the first trophic level, so they are primary consumers.
• Third trophic level
  Foxes eat rabbits at the second trophic level, so they are secondary consumers.
• Fourth trophic level
  Golden eagles eat foxes at the third trophic level, so they are tertiary consumers.
• Decomposers
  The fungi on this tree feed on dead matter, converting it back to nutrients that primary producers can use.

In real world ecosystems, there is more than one food chain for most organisms, since most organisms eat more than one kind of food or are eaten by more than one type of predator. A diagram which sets out the intricate network of intersecting and overlapping food chains for an ecosystem is called its food web. Decomposers are often left off food webs, but if included, they mark the end of a food chain. Thus food chains start with primary producers and end with decay and decomposers. Since decomposers recycle nutrients, leaving them so they can be reused by primary producers, they are sometimes regarded as occupying their own trophic level.