Resource

ENCYCLOPEDIC ENTRY

Resource

ENCYCLOPEDIC ENTRY

Food Web

Food Web

A food web consists of all the food chains in a single ecosystem.

Grades

3 - 12+

Subjects

Biology, Ecology

















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Each living thing is part of many different food chains. A food chain is a path that energy takes throughout a certain ecosystem. Together, all the food chains in an ecosystem make up a food web.

Trophic Levels

Organisms in food webs are grouped into categories. These categories are called trophic levels.

Producers

Organisms in the first trophic level are called producers. A producer is also called an autotrophPlants are the type of producer we know best, but there are many other kinds. Algae and some types of bacteria are also producers.

Each producer makes its own food and does not depend on any other organism for nutrition. Most producers use photosynthesis. This is a series of chemical reactions in which organisms create food from sunlightcarbon dioxide, and water.

Consumers

The next trophic levels are made up of animals that eat producers. These organisms are called consumers.

Consumers can be carnivores or omnivores. Carnivores eat meat, while omnivores eat various organisms, including both meat and plants.

Primary consumers are herbivores, which eat plants, algae, and other producers. In a grassland ecosystem, deer, mice, and elephants are herbivores. They eat grasses, shrubs, and trees. In a desert ecosystem, a mouse that eats seeds and fruits is a primary consumer. In an ocean ecosystem, many fish and turtles are herbivores.

Secondary consumers eat herbivores. They are at the third trophic level. In a desert ecosystem, a secondary consumer may be a snake that eats a mouse. In underwater kelp forests, sea otters are secondary consumers. They hunt sea urchins.

The next level of consumers eat secondary consumers. In the desert, an owl or eagle may hunt snakes.

Top predators are also called apex predators. They eat other consumers, and no predators eat them. Lions are apex predators in the grassland ecosystem. In the ocean, fish such as the great white shark are apex predators. In the desert, bobcats and mountain lions are top predators.

Detritivores and Decomposers

Detritivores and decomposers make up the last part of food chains. Detritivores eat plants and animals that are no longer alive. For instance, vultures eat dead animals.

Some organisms, like fungi and bacteria, are decomposers. They turn decaying plants into soil. Since this soil is rich in nutrients, autotrophs use it to feed themselves. This way, the food chain is able to continue. As an example, grass produces its own food through photosynthesis. A rabbit eats the grass and then a fox eats the rabbit. When the fox dies, decomposers such as worms and mushrooms break down its body. It returns to the soil where it provides nutrients for plants like grass.

Biomass

Biomass is the energy in living organisms. Producers in a food web change the sun's energy into biomass. Biomass decreases with each trophic level. There is always more biomass in lower levels than in higher ones. That means there are always more autotrophs than herbivores in a healthy food web. There are also more herbivores than carnivores. A healthy food web has many autotrophs, many herbivores and few carnivores and omnivores.

The biomass of an ecosystem depends on how balanced and connected its food web is. When one part of the food web is threatened, other parts are threatened, too. Then the ecosystem's biomass decreases.

For example, when there are fewer plants, the number of herbivores usually decreases. Plant life can decrease due to drought, disease, or human activity. Forests are cut down to to get lumber for construction. Grasslands are paved over for shopping malls or parking lots.

Bioaccumulation

Toxic chemicals increase with each level in the food web. When an herbivore eats a plant that is covered in pesticides, the pesticides are stored in the animal's fat. When a carnivore eats the herbivore, it also eats the pesticides. This process is called bioaccumulation.

Bioaccumulation happens in water ecosystems too. Runoff from cities or farms can be full of pollutants. Tiny producers such as algae, bacteria, and seagrass absorb pollutants. Sea turtles and fish eat the seagrass. Then, predators such as sharks or tuna eat the fish. By the time people eat the tuna, it may contain a large amount of bioaccumulated pesticides.

A pesticide called DDT was a major reason that bald eagles began disappearing. In the 1940s and 1950s, DDT, or dichloro-diphenyl-trichloroethane, was used to kill insects. DDT built up in soil and water. Worms, grasses, algae, and fish ate organisms with DDT. Apex predators, such as eagles, ate fish and small mammals that had large amounts of DDT. Birds with high amounts of DDT in their bodies laid eggs with extremely thin shells. These shells would often break before the baby birds hatched.

Today, the U.S. government has banned DDT. Food webs, which include the bald eagle, have come back in most parts of the country.

Fast Fact

Lost Energy
Biomass shrinks with each trophic level. That is because between 80% and 90% of an organism's energy, or biomass, is lost as heat or waste. A predator consumes only the remaining biomass.

Fast Fact

A Million to One
Marine food webs are usually longer than terrestrial food webs. Scientists estimate that if there are a million producers (algae, phytoplankton, and sea grass) in a food web, there may only be 10,000 herbivores. Such a food web may support 100 secondary consumers, such as tuna. All these organisms support only one apex predator, such as a person.

Fast Fact

Out for Blood
One of the earliest descriptions of food webs was given by the scientist Al-Jahiz, working in Baghdad, Iraq, in the early 800s. Al-Jahiz wrote about mosquitoes preying on the blood of elephants and hippos. Al-Jahiz understood that although mosquitoes preyed on other animals, they were also prey to animals such as flies and small birds.

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Writers
Hilary Costa
Erin Sprout
Santani Teng
Melissa McDaniel
Jeff Hunt
Diane Boudreau
Tara Ramroop
Kim Rutledge
Hilary Hall
Illustrators
Mary Crooks, National Geographic Society
Tim Gunther
Editors
Jeannie Evers, Emdash Editing, Emdash Editing
Kara West
Educator Reviewer
Nancy Wynne
Producer
National Geographic Society
other
Last Updated

June 2, 2022

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