Consumer is a category that belongs within the food chain of an ecosystem. It refers predominantly to animals. Consumers are unable to make their own energy, and instead rely on the consumption and digestion of producers or other consumers, or both, to survive.
Consumers are found in food chains, where they are joined by two other groups – producers and decomposers. All plants are producers as they produce their own energy from sunlight and nutrients via photosynthesis. Plants make up the primary trophic level of the food chain.
Herbivores – animals which only eat plants – consume vegetation from which they are able to produce energy. Herbivores are unable to make their own energy and are known as consumers. As herbivores only ever eat producers, they are primary consumers in the second trophic level of the food chain.
Omnivores – animals which eat both plants and other animals – are also consumers. The animals they eat do not produce their own energy, and are therefore not classed as producers under any circumstances. Carnivores – animals which eat only meat – hold various positions within different food chains. They may also be part of a predator-prey relationship. Decomposers, which will not be discussed further in this article, turn the dead material of producers and consumers into nutrients which are then used by producers (plants) to grow.
Food chains do not need to include whole animals. In the human food chain, vegetarians who eat dairy are omnivores, and vegans who eat neither dairy nor eggs are the equivalent of primary consumers. Categorization within a food chain may also be transient, as with calves that drink their mother’s milk as omnivores, then become herbivores after weaning.
The position a consumer holds within the food chain can be manipulated by disease, deforestation, the seasons, biodiversity, human encroachment into natural habitats, and many other variables. Additionally, multiple species can be found within each category and trophic level. When multiple species are involved, a simple food chain can become a complex food web.
The two simplified food chains pictured above show terrestrial and marine progression from producer (dandelion and phytoplankton) to quaternary consumer (hawk and shark).
Consumer examples are plentiful, as every animal must consume food in order to live. Consumers are grouped into four categories – primary, secondary, tertiary, and quaternary. The category in which an animal is situated is defined by its food source within a specific food chain or food web, and not necessarily by its species or habits. For example, grizzly bears only have access to salmon at certain times of the year, while in the early spring diets are largely root-based and herbivorous. Depending on the available food source(s), a single species might be placed in different categories. The simple diagram below shows how simple it is to upset the flow of the trophic cascade of a food chain.
Examples of primary consumers are zooplankton, butterflies, rabbits, giraffes, pandas and elephants.
Primary consumers are herbivores. Their food source is the first trophic level of organisms within the food web, or plants. Plants are also referred to as autotrophs. Autotrophs produce their own energy from sunlight and basic nutrients via photosynthesis; in any ecosystem, the terms producer and autotroph have the same meaning. The herbivorous diet does not only include leaves, branches, flowers, fruits and roots of plants, but also other autotrophic sources such as nectar and phytoplankton.
Primary consumers feed exclusively on autotrophs. Any organism that must eat in order to produce energy is both a heterotroph and a consumer. Rather confusingly, primary consumers are located in the second trophic level of the ecosystem. A trophic level is the position any organism occupies within any food chain. As vegetation is the most basic food source, plants are to be found at the first trophic level. Herbivores are positioned on the next rung of the trophic ladder, and are therefore primary consumers at the second trophic level.
Examples of secondary consumers are earwigs, ants, badgers, snakes, rats, crabs, hedgehogs, blue whales (their diet is primarily composed of phytoplankton-eating krill and zooplankton, and phytoplankton), lions, and humans.
Secondary consumers nearly always consume both producers and primary consumers and are therefore usually classed as omnivores. Secondary consumers make up the third trophic level of the food chain and are – as are all consumers – heterotrophs.
Examples of tertiary consumers are hawks, snakes, crocodiles and some big cats.
Tertiary consumers can be either omnivorous or carnivorous. They feed on primary and secondary consumers, and may also eat producers (plants). For a food chain to have a tertiary consumer, there must be a secondary consumer available for it to eat.
It is interesting to note that different organisms in different situations or at different times may occur at similarly different trophic levels. For example, human vegans are primary consumers of the second trophic level, but a large proportion of the human race are omnivores. Another example can be found in beef consumption before and after bovine spongiform encephalophathy (BSE) legislation, where it was eventually decided to stop beef cows from being fed meat- and bone meal. Before legislation was passed, human consumption of beef would class us as tertiary consumers, as cows eating an omnivorous diet are themselves classed as secondary consumers. After the link between bovine spongiform encephalopathy (BSE) and meat-based feeds, farms were only permitted to feed their herds plant-source diets. This means that humans currently eat beef as secondary consumers, as farms are only authorized to produce beef from primary consumers.
Examples of quaternary examples are the white shark, polar bear and alligator.
Quaternary consumers are not necessarily apex predators. An apex predator is at the top of the food chain in which it exists, and is not the living prey of any other organism. A quaternary consumer is simply a consumer which preys upon a tertiary consumer. To be classed as a quaternary consumer within a food chain or food web, there must be a tertiary consumer available for the quaternary consumer to prey upon. Quaternary consumers are found in the fifth trophic level and are not to be found in every food chain. The higher up the consumer ladder one goes, the more the energy required to support it. This is explained in the graphic below, where the size of each layer of the trophic pyramid indicates the ratio of each species to each other within a healthy food chain.
Carnivorous Plants – Producers or Consumers?
Are carnivorous plants, such as the Venus fly-trap and pitcher plant, producers or consumers?
Carnivorous plants are found predominantly in heavily forested regions, notorious for low levels of nutrients. These plant types are the perfect example for showing how the different levels and categories of a food chain are in constant motion and never absolute.
Evolution has produced plants which can collect additional nutrients (mainly nitrogen) through the slow digestion of insects caught via ingenious traps. However, carnivorous plants are able to survive without catching prey, although their rates of growth will be negatively affected. This shows that carnivorous plants are predominantly producers, but able to step outside of this box and simultaneously act as consumers.