THE FOOD CHAIN
In ecology, the food chain illustrates how energy and nutrients are transferred from one living organism to another in the forms of food. The smallest organisms are fed upon by larger ones, which in turn feed still larger ones, and so on. As you’ll learn below, the food chain is an elegant systematic arrangement that builds upon itself so that it can supply all living things with the amounts of energy that they need based on their sizes, activities, and life styles. In the end, the food chain becomes a cycle of life in which all energy sources break down to feed the earth, the cycle begins once again, and life continues. In the natural order things, this is how Mother Nature beautifully orchestrates the natural rhythms of self-sustaining ecosystems. Let’s look at each level of the food chain and how they function.
This is what a food chain looks like.
Producers are organisms that make their own food. In terrestrial food chains, they are plants, trees, bushes, grasses, and anything that has leaves. They are called autotrophs (from Greek, “auto” means “self” and “troph” means “feeding”) because they create energy by absorbing carbon dioxide from the air through their leaves and sucking up water and nutrients from the soil, with the help of fungi and bacteria. They use the sun’s energy to convert carbon dioxide into oxygen and water vapor that they then release through their leaves into the atmosphere. They are responsible for infusing energy into the food chain, making them the first link of the food chain. Without plants, all the other animals in the food chain would not exist.
Producers create energy in the forms of glucose (a form of sugar), amino acids, proteins, and fats, which they store inside new cells that are made using the nutrients (nitrogen, sulphur, and phosphate) they took from the soil. Those nutrients affect how fast plants grow.
Consumers cannot make their own food. They must consume other living things to obtain energy.
Consumers are also called heterotrophs (from Greek, “hetero” means “other” and “troph” means “feeding”).
There are different types of consumers defined by the type of food that they need to consume to produce energy:
Primary consumers eat producers. They are the next link in the food chain.
Primary consumers are usually herbivores—animals that feed on plants. They can be as small as grasshoppers and hummingbirds or as large as deer and cows. They typically have multiple stomach chambers and complex digestive systems, including long intestinal tracts, which are designed to maximize the energy that they can absorb from the plants they consume. Herbivores are important energy sources for secondary consumers who benefit from the energy they produce and store inside their bodies.
Secondary consumers eat primary consumers, making them the next link in the food chain. Secondary consumers are divided into two categories:
- Carnivores—animal that feed on flesh. Their digestive systems do not efficiently or effectively process a plant-based diet. They have less complex digestive systems and shorter intestinal tracts than herbivores.
- Omnivores—animals, like humans, whose diet consists of both plants and animals. They can actually be both primary and secondary consumers because their digestive systems can process both plants and animals. This feeding strategy is a great advantage when food is scarce because omnivores benefit from the greatest variety of food sources.
Tertiary consumers are carnivores that feed on secondary consumers. They are usually larger in size and fewer in population than the consumers they eat. If a carnivore eats another carnivore, it will be a tertiary consumer or higher.
Quaternary consumers eat tertiary consumers. They are usually the last link in the food chain. They typically require a lot of food to meet their energetic needs either because of their size, like humans, or because of the way that they use energy. For example, cheetahs are not that large, but they need a lot of energy resources to run at speeds of up to 35 mph (56 km/h). Since they require a great deal of food, quaternary consumers can also be omnivores, feeding on producers and other consumers. They are typically apex predators that eat all the other levels of the food chain, but are rarely ever preyed upon and eaten themselves.
Not every ecosystem has four trophic consumer levels. Some only have two or three. In rare instances there may be five. The more plant and animal species there are in an ecosystem, the more pathways there are for energy to move up the food chain, and the more trophic levels it can contain.
As energy and nutrients are consumed and stored along the food chain, they eventually need to be recycled back into the ecosystem. This occurs through defecation and death.
What animals, do you imagine, occupy each consumer category?
Why would you place them in that category?
Decomposers are organisms that break down dead or decaying material. Decomposers are the final link that completes the cycle of life, returning nutrients to the soil for use by autotrophs. They work in stages.
The first stage of decomposition is carried out by detritivores—animals like worms and flies—that only feed on already dead and decaying animals or plants, breaking them down for the final stage of decomposition.
In the final stage of decomposition, bacteria and fungi change the structure of nutrients such as nitrogen, phosphorus, and carbon into a form that plants can absorb through their roots. These are called saprovores.
These saprovores are the bacteria and fungi that allow plants to take up the nutrients at the very start of the food chain. Unlike detritivores, they do not consume nutrients and energy. Instead they break down nutrients by oozing enzymes from their bodies. These enzymes change the structure of the organisms into forms that can be absorbed by plants’ roots.
Without decomposers to complete the cycle, dead plant and animal matter would build up and important nutrients would remain locked inside. Soils would become nutrient-poor and unable to support primary producers sufficiently, having negative domino effect up the food chain