An ecosystem is made up of interconnected relationships of organisms that rely on one another for food and energy. This dependence can be described as a food chain or a food web. A food chain is one possible path that energy and nutrients may take as they move through an ecosystem. Each living thing in an ecosystem is part of multiple food chains. All of the interconnected and overlapping food chains in an ecosystem make up a food web.
Trophic levels
Organisms are grouped into categories called trophic levels. Trophic levels describe the various stages within ecological food chains or webs. These levels are usually divided into producers, consumers and decomposers.
Energy
The amount of energy, or biomass, at each trophic level decreases as it moves through an ecosystem. As little as 10% of the energy at any trophic level is transferred to the next level. The rest of the energy is lost as heat or waste. The energy lost is so great that food chains rarely involve more than four or five steps.
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Population
With each trophic level, population also decreases. Scientists estimate that if there are a million producers in a food web, there may only be 10,000 primary consumers, 100 secondary consumers, and just one apex predator.
Producers
1 Producers make up the first trophic level. Also known as autotrophs, producers make their own food and do not depend on any other organism for nutrition. They convert the sun’s energy into biomass. Most autotrophs use photosynthesis to create food from sunlight, carbon dioxide and water.
Plants are the most familiar type of producer, but there are many others including algae, phytoplankton and some types of bacteria.
Producers support their entire ecosystem. Their abundance and energy determine the overall energy flow of the ecosystem.
Primary consumers
2 The rest of the trophic levels are made up of consumers. Consumers are classified as heterotrophs. These are organisms that cannot produce their own food. Instead, they consume other plants and animals for energy and nutrients.
The second trophic level is made up of primary consumers. These are herbivores that eat plants, algae and other producers.
Some types of primary consumers include deer, mice, cows and elephants. In an ocean ecosystem, many types of fish and turtles are herbivores.
Secondary consumers
3 Secondary consumers are at the third trophic level. These animals are largely comprised of carnivores that feed on the primary consumers.
Other secondary consumers are omnivores, which are animals that eat both plants and animals. These animals feed on both primary consumers and producers.
Because omnivores feed at various places on the food web, they are difficult to classify in terms of trophic level.
Some types of secondary consumers could include frogs, snakes and sea otters.
Tertiary consumers
4 Tertiary consumers are at the fourth trophic level. They eat the secondary consumers.
In a desert ecosystem, an owl or eagle may be prey on a snake and be a tertiary consumer.
The fourth trophic level may be the top level or there may be more levels of consumers before a chain finally reaches its top predator.
Top predators are also called apex predators. These animals eat other consumers and usually have few or no natural enemies except humans.
Lions and great white sharks are examples of apex predators.
Decomposers
Decomposers complete the last part of the food chain. They turn organic waste into inorganic materials, returning nutrients to the soil or oceans for use by autotrophs. This starts a whole new series of food chains.
Detritivores commonly play the role of decomposers. Detritivores are organisms that eat nonliving plant and animal remains. Scavengers, like vultures, eat dead animals, while dung beetles eat animal feces. Fungi and bacteria are also decomposers.
Classifying animals
The second trophic level in an ecosystem is relatively easy to identify because these organisms all obtain their energy directly from the autotrophs at the first level.
However, after the second level, the situation becomes more complex. Many organisms in higher levels obtain energy from several different sources at different trophic levels.
Bears, for example, are omnivores, eating berries and mushrooms as well as salmon and deer. Foxes consume fruits, small herbivores and small carnivores.
Humans are the most omnivorous of all species as we eat just about anything that is not poisonous. In turn, we are not eaten by many other creatures.
A healthy web
In a healthy food web, there are always more producers than herbivores and more herbivores than carnivores and omnivores. This balance helps the ecosystem maintain and recycle biomass since it decreases with each trophic level.
A disruption on any of the trophic levels could result in an unbalanced food web. If there is loss of plant life at the first level, there would be a decline in the herbivore population, which would then affect the upper levels.
If there is a loss of biomass on the second or third trophic level, omnivores would be forced to rely more heavily on other food sources, which would make that population shrink.
A loss of a carnivore on a higher trophic level could cause their prey to over populate without the predators keeping them under control.
Food chains vs. food webs
A food chain follows a single path of organisms and their feeding relationship. It is a simple representation of who-eats-who.
In nature, no food chain operates on its own. Every food chain is connected to other food chains by the individuals belonging to it. This interconnected network of food chains is called a food web.
A food web shows how plants and animals in an ecosystem are interconnected by different paths and different trophic levels. They are used to determine ecological interactions that define energy flow and predator-prey relationships.
Apex predators
Predators at the top of their food chain are called apex predators. They have few natural predators, and they tend to be the biggest animals in their food chain. The diets of apex predators vary widely, which allows them to switch up prey when other species populations lessen.
Apex predators are a necessary component to maintaining a complex ecosystem. They help keep other species’ populations in check in order to prevent them from monopolizing a limited resource. Here are some examples of apex predators.
Orca
Diet: Fish, squid, seals, sea birds, other whales and sometimes great white sharks
Great white shark
Diet: Fish (including other sharks), stingrays, sea lions and seals
Lion
Diet: Rodents, baboons, hippopotamuses, buffalo, wildebeests, zebras and antelopes
Tiger
Diet: Moose, deer, pigs, cows, horses, buffalo, goats, monkeys and fish
Polar bear
Diet: Seals, walruses, whale carcasses, narwhals, reindeer, fish, seabirds and eggs
Saltwater crocodile
Diet: Crabs, fish, birds, turtles, pigs, buffalo, deer, rabbits, boar, tigers and small sharks
Komodo dragon
Diet: Lizards, insects, snakes, birds, rodents, goats, wild boars, deer and other komodo dragons
Golden eagle
Diet: Rabbits, marmots, squirrels, reptiles, birds, fish, large insects and deer
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