These pyramids illustrate the movement of energy from the broad base of producers at the bottom through the decreasing number of consumers up to the top of the pyramid. Food webs illustrate the same information but use lines to connect each consumer to what it eats. First-level consumers, also known as primary consumers, eat producers such as plants, algae and bacteria.
Producers comprise the first trophic level. Herbivores, the first-level consumers, occupy the second trophic level. First-level consumers do not eat other consumers, only plants or other producers. The physical size of first-level consumers varies greatly, ranging from tiny zooplankton to elephants, and all first-level consumers only eat producers.
Secondary or second-level consumers eat primary consumers. Tertiary or third-level consumers eat lower-level consumers and are sometimes called final consumers. Some secondary and tertiary consumers eat plants as well as lower level consumers, making them omnivores.
Humans are good example of omnivorous upper-level consumers; we eat primary producers plants as well as other consumers animals. Food availability may vary seasonally or by time of day. An organism like a mouse might play two roles, eating insects on occasion making it a secondary consumer , but also dining directly on plants making it a primary consumer. A food web of who eats who in the southwest American desert biome might look something like this:.
Keystone Species : In some food webs, there is one critical " keystone species " upon which the entire system depends. In the same way that an arch collapses when the keystone is removed, an entire food chain can collapse if there is a decline in a keystone species.
Often, the keystone species is a predator that keeps the herbivores in check, and prevents them from overconsuming the plants, leading to a massive die off.
When we remove top predators like grizzly bears, orca whales, or wolves , for example, there is evidence that it affects not just the prey species, but even the physical environment.
Apex Predators : These species are at the top of the food chain and the healthy adults have no natural predators. The young and old may in some cases be preyed upon, but they typically succumb to disease, hunger, the effects of aging, or some combination of them.
The also suffer from competition with humans, who often eliminate the top predators in order to have exclusive access to the prey species, or through habitat destruction, which is an indirect form of competition. Decomposers : When organisms die, they are sometimes eaten by scavengers but the remaining tissues are broken down by fungi and bacteria.
In this way, the nutrients that were part of the body are returned to the bottom of the trophic pyramid. Each organism in a food chain occupies what is called a trophic level. Depending on their role as producers or consumers, species or groups of species can be assigned to various trophic levels.
The organisms that consume the primary producers are herbivores: the primary consumers. Secondary consumers are usually carnivores that eat the primary consumers. Tertiary consumers are carnivores that eat other carnivores. Higher-level consumers feed on the next lower tropic levels, and so on, up to the organisms at the top of the food chain: the apex consumers. In the Lake Ontario food chain shown in Figure 1, the Chinook salmon is the apex consumer at the top of this food chain.
One major factor that limits the length of food chains is energy. Energy is lost as heat between each trophic level due to the second law of thermodynamics. Thus, after a limited number of trophic energy transfers, the amount of energy remaining in the food chain may not be great enough to support viable populations at yet a higher trophic level.
The loss of energy between trophic levels is illustrated by the pioneering studies of Howard T. They range in size from tiny insects like ants to large creatures—like people.
Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Image Grasshopper Eating Leaf Grasshoppers are primary consumers because they eat plants, which are producers.
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