What is a primary consumer in an ecosystem?

Primary consumers are organisms that eat producers. They occupy the second trophic level in a food chain.

Examples are zooplankton, which eat phytoplankton, mountain gorillas, which eat various parts of about 142 different plant species, and grasshoppers, which eat the leaves of grasses like wheat and rice.

$\hspace{2em}$ producer $\rightarrow$ primary consumer $\rightarrow$ secondary consumer $\rightarrow$ tertiary consumer

As you can see in the food chain above, producers are the first trophic level in a food chain, using photosynthesis to make carbon compounds that provide matter and energy for growth and survival. Consumers cannot photosynthesise, and so must obtain carbon compounds from other organisms. Primary consumers eat producers, and are eaten by secondary consumers. They, in turn, are eaten by tertiary consumers. Primary consumers play an important role in regulating the population sizes of both the primary producers they eat and the secondary consumers that eat them.

Because consumers feed in such different ways, we have specific terms to describe them. The following terms are most relevant in IB ESS:

• Herbivores are primary consumers that eat plants, e.g. koalas eat eucalyptus leaves.

The trophic level of the consumers below is determined by the trophic level of the organisms they eat.

• Predators eat prey; a predator such as an owl is a secondary consumer if its prey is a primary consumer, such as a mouse that eats corn, but a tertiary consumer if its prey is a secondary consumer, such as a frog that eats grasshoppers that eat grass.
• Parasites such as mistletoe and tapeworms live in or on another organism and harm it as they feed on it, but do not usually kill it.
• Scavengers like vultures eat dead organisms that were killed by other organisms or died of natural causes.
• Decomposers such as soil bacteria, fungi and earthworms break down dead organisms or their parts as they feed on them.
• Detritivores, such as earthworms, are decomposers that eat decomposing parts of organisms from all trophic levels, as well as their faeces.
• Saprotrophs such as fungi are decomposers that break down decomposing parts of organisms by excreting enzymes onto them and then absorbing the products.