A food web consists of all the food chains in a single ecosystem. Each living thing in an ecosystem is part of multiple food chains. Each food chain is one possible path that energy and nutrients may take as they move through the ecosystem. All of the interconnected and overlapping food chains in an ecosystem make up a food web.
Organisms in food webs are grouped into categories called trophic levels. Roughly speaking, these levels are divided into producers (first trophic level), consumers, and decomposers (last trophic level).
Producers make up the first trophic level. Producers, also known as autotrophs, make their own food and do not depend on any other organism for nutrition. Most autotrophs use a process called photosynthesis to create food (a nutrient called glucose) from sunlight, carbon dioxide, and water.
Plants are the most familiar type of autotroph, but there are many other kinds. Algae, whose larger forms are known as seaweed, are autotrophic. Phytoplankton, tiny organisms that live in the ocean, are also autotrophs. Some types of bacteria are autotrophs. For example, bacteria living in active volcanoes use sulfur, not carbon dioxide, to produce their own food. This process is called chemosynthesis.
The next trophic levels are made up of animals that eat producers. These organisms are called consumers.
Consumers can be carnivores (animals that eat other animals) or omnivores (animals that eat both plants and animals). Omnivores, like people, consume many types of foods. People eat plants, such as vegetables and fruits. We also eat animals and animal products, such as meat, milk, and eggs. We eat fungi, such as mushrooms. We also eat algae, in edible seaweeds like nori (used to wrap sushi rolls) and sea lettuce (used in salads). Bears are omnivores, too. They eat berries and mushrooms, as well as animals such as salmon and deer.
Primary consumers are herbivores. Herbivores eat plants, algae, and other producers. They are at the second trophic level. In a grassland ecosystem, deer, mice, and even 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 types of fish and turtles are herbivores that eat algae and seagrass. In kelp forests, seaweeds known as giant kelp provide shelter and food for an entire ecosystem. Sea urchins are powerful primary consumers in kelp forests. These small herbivores eat dozens of kilograms (pounds) of giant kelp every day.
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 the kelp forest, sea otters are secondary consumers that hunt sea urchins.
Tertiary consumers eat the secondary consumers. They are at the fourth trophic level. In the desert ecosystem, an owl or eagle may prey on a snake.
There may be more levels of consumers before a chain finally reaches its top predator. Top predators, also called apex predators, eat other consumers. They may be at the fourth or fifth trophic level. They have no natural enemies except humans. Lions are apex predators in the grassland ecosystem. In the ocean, fish like 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 are organisms that eat nonliving plant and animal remains. For example, scavengers such as vultures eat dead animals. Dung beetles eat animal feces.
Decomposers, like fungi and bacteria, complete the food chain. Decomposers turn organic wastes, such as decaying plants, into inorganic materials, such as nutrient-rich soil. They complete the cycle of life, returning nutrients to the soil or oceans for use by autotrophs. This starts a whole new series of food chains.
Food webs connect many different food chains, and many different trophic levels. Food webs can support food chains that are long and complicated, or very short.
For example, grass in a forest clearing produces its own food through photosynthesis. A rabbit eats the grass. A fox eats the rabbit. When the fox dies, decomposers such as worms and mushrooms break down its body, returning it to the soil where it provides nutrients for plants like grass.
This short food chain is one part of the forest's food web. Another food chain in the same ecosystem might involve completely different organisms. A caterpillar may eat the leaves of a tree in the forest. A bird such as a sparrow may eat the caterpillar. A snake may then prey on the sparrow. An eagle, an apex predator, may prey on the snake. Yet another bird, a vulture, consumes the body of the dead eagle. Finally, bacteria in the soil decompose the remains.
Algae and plankton are the main producers in marine ecosystems. Tiny shrimp called krill eat the microscopic plankton. The largest animal on Earth, the blue whale, preys on thousands of tons of krill every day. Apex predators such as orcas prey on blue whales. As the bodies of large animals such as whales sink to the seafloor, detritivores such as worms break down the material. The nutrients released by the decaying flesh provide chemicals for algae and plankton to start a new series of food chains.
Food webs are defined by their biomass. Biomass is the energy in living organisms. Autotrophs, the producers in a food web, convert the sun's energy into biomass. Biomass decreases with each trophic level. There is always more biomass in lower trophic levels than in higher ones.
Because biomass decreases with each trophic level, there are always more autotrophs than herbivores in a healthy food web. There are more herbivores than carnivores. An ecosystem cannot support a large number of omnivores without supporting an even larger number of herbivores, and an even larger number of autotrophs.
A healthy food web has an abundance of autotrophs, many herbivores, and relatively few carnivores and omnivores. This balance helps the ecosystem maintain and recycle biomass.
Every link in a food web is connected to at least two others. The biomass of an ecosystem depends on how balanced and connected its food web is. When one link in the food web is threatened, some or all of the links are weakened or stressed. The ecosystems biomass declines.
The loss of plant life usually leads to a decline in the herbivore population, for instance. Plant life can decline due to drought, disease, or human activity. Forests are cut down to provide lumber for construction. Grasslands are paved over for shopping malls or parking lots.
The loss of biomass on the second or third trophic level can also put a food web out of balance. Consider what may happen if a salmon run is diverted. A salmon run is a river where salmon swim. Salmon runs can be diverted by landslides and earthquakes, as well as the construction of dams and levees.
Biomass is lost as salmon are cut out of the rivers. Unable to eat salmon, omnivores like bears are forced to rely more heavily on other food sources, such as ants. The area's ant population shrinks. Ants are usually scavengers and detritivores, so fewer nutrients are broken down in the soil. The soil is unable to support as many autotrophs, so biomass is lost. Salmon themselves are predators of insect larvae and smaller fish. Without salmon to keep their population in check, aquatic insects may devastate local plant communities. Fewer plants survive, and biomass is lost.
A loss of organisms on higher trophic levels, such as carnivores, can also disrupt a food chain. In kelp forests, sea urchins are the primary consumer of kelp. Sea otters prey on urchins. If the sea otter population shrinks due to disease or hunting, urchins devastate the kelp forest. Lacking a community of producers, biomass plummets. The entire kelp forest disappears. Such areas are called urchin barrens.
Human activity can reduce the number of predators. In 1986, officials in Venezuela dammed the Caroni River, creating an enormous lake about twice the size of Rhode Island. Hundreds of hilltops turned into islands in this lake. With their habitats reduced to tiny islands, many terrestrial predators weren’t able to find enough food. As a result, prey animals like howler monkeys, leaf-cutter ants, and iguanas flourished. The ants became so numerous that they destroyed the rainforest, killing all the trees and other plants. The food web surrounding the Caroni River was destroyed.
Biomass declines as you move up through the trophic levels. However, some types of materials, especially toxic chemicals, increase with each trophic level in the food web. These chemicals usually collect in the fat of animals.
When an herbivore eats a plant or other autotroph that is covered in pesticides, for example, those pesticides are stored in the animal’s fat. When a carnivore eats several of these herbivores, it takes in the pesticide chemicals stored in its prey. This process is called bioaccumulation.
Bioaccumulation happens in aquatic ecosystems too. Runoff from urban areas or farms can be full of pollutants. Tiny producers such as algae, bacteria, and seagrass absorb minute amounts of these pollutants. Primary consumers, such as sea turtles and fish, eat the seagrass. They use the energy and nutrients provided by the plants, but store the chemicals in their fatty tissue. Predators on the third trophic level, such as sharks or tuna, eat the fish. By the time the tuna is consumed by people, it may be storing a remarkable amount of bioaccumulated toxins.
Because of bioaccumulation, organisms in some polluted ecosystems are unsafe to eat and not allowed to be harvested. Oysters in the harbor of the United States' New York City, for instance, are unsafe to eat. The pollutants in the harbor accumulate in its oysters, a filter feeder.
In the 1940s and 1950s, a pesticide called DDT (dichloro-diphenyl-trichloroethane) was widely used to kill insects that spread diseases. During World War II, the Allies used DDT to eliminate typhus in Europe, and to control malaria in the South Pacific. Scientists believed they had discovered a miracle drug. DDT was largely responsible for eliminating malaria in places like Taiwan, the Caribbean, and the Balkans.
Sadly, DDT bioaccumulates in an ecosystem and causes damage to the environment. DDT accumulates in soil and water. Some forms of DDT decompose slowly. Worms, grasses, algae, and fish accumulate DDT. Apex predators, such as eagles, had high amounts of DDT in their bodies, accumulated from the fish and small mammals they prey on.
Birds with high amounts of DDT in their bodies lay eggs with extremely thin shells. These shells would often break before the baby birds were ready to hatch.
DDT was a major reason for the decline of the bald eagle, an apex predator that feeds primarily on fish and small rodents. Today, the use of DDT has been restricted. The food webs of which it is a part have recovered in most parts of the country.
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.
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.
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.
to gather or collect.
to adjust to new surroundings or a new situation.
(singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.
alliance of countries that opposed the Axis during World War II. The Allies were led by the U.S., the United Kingdom, and the Soviet Union.
species at the top of the food chain, with no predators of its own. Also called an alpha predator or top predator.
having to do with water.
organism that can produce its own food and nutrients from chemicals in the atmosphere, usually through photosynthesis or chemosynthesis.
(singular: bacterium) single-celled organisms found in every ecosystem on Earth.
having to do with the Balkan Peninsula in southeastern Europe.
process by which chemicals are absorbed by an organism, either from exposure to a substance with the chemical or by consumption of food containing the chemical.
living organisms, and the energy contained within them.
chemical element with the symbol C, which forms the basis of all known life.
greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.
organism that eats meat.
process by which some microbes turn carbon dioxide and water into carbohydrates using energy obtained from inorganic chemical reactions.
group of one species of organism living close together.
to be made of.
organism on the food chain that depends on autotrophs (producers) or other consumers for food, nutrition, and energy.
structure built across a river or other waterway to control the flow of water.
(dichloro-diphenyl-trichloroethane) toxic chemical used as an insecticide but illegal for most uses in the U.S. since 1972.
to rot or decompose.
to reduce or go down in number.
organism that breaks down dead organic material.
area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.
organism that consumes dead plant material.
to direct away from a familiar path.
period of greatly reduced precipitation.
the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.
community and interactions of living and nonliving things in an area.
able to be eaten and digested.
capacity to do work.
land cultivated for crops, livestock, or both.
material found in organisms that is colorless and odorless and may be solid or liquid at room temperature.
waste material produced by the living body of an organism.
aquatic animal that strains nutrients from water.
group of organisms linked in order of the food they eat, from producers to consumers, and from prey, predators, scavengers, and decomposers.
all related food chains in an ecosystem. Also called a food cycle.
ecosystem filled with trees and underbrush.
edible part of a plant that grows from a flower.
(singular: fungus) organisms that survive by decomposing and absorbing nutrients in organic material such as soil or dead organisms.
"simple sugar" chemical produced by many plants during photosynthesis.
ecosystem with large, flat areas of grasses.
part of a body of water deep enough for ships to dock.
the gathering and collection of crops, including both plants and animals.
organism that eats mainly plants and other producers.
composed of material that is not living, and never was, such as rock.
type of seaweed.
underwater habitat filled with tall seaweeds known as kelp.
small marine crustacean, similar to shrimp.
the fall of rocks, soil, and other materials from a mountain, hill, or slope.
a new or immature insect or other type of invertebrate.
bank of a river, raised either naturally or constructed by people.
precisely cut pieces of wood such as boards or planks.
infectious disease caused by a parasite carried by mosquitoes.
having to do with the ocean.
red algae that is often dried and used to wrap sushi.
substance an organism needs for energy, growth, and life.
organism that eats a variety of organisms, including plants, animals, and fungi.
used or excess material that might be broken down to be used again. Also called biodegradable waste.
type of marine animal (mollusk).
natural or manufactured substance used to kill organisms that threaten agriculture or are undesirable. Pesticides can be fungicides (which kill harmful fungi), insecticides (which kill harmful insects), herbicides (which kill harmful plants), or rodenticides (which kill harmful rodents.)
process by which plants turn water, sunlight, and carbon dioxide into water, oxygen, and simple sugars.
microscopic organism that lives in the ocean and can convert light energy to chemical energy through photosynthesis.
organism that produces its own food through photosynthesis and whose cells have walls.
to fall sharply.
to transfer pollen from one part of a flower (the anther) to another (the stigma).
chemical or other substance that harms a natural resource.
animal that is hunted and eaten by other animals.
organism that eats plants or other autotrophs.
organism on the food chain that can produce its own energy and nutrients. Also called an autotroph.
area of tall, mostly evergreen trees and a high amount of rainfall.
to clean or process in order to make suitable for reuse.
to lower or lessen.
materials left from a dead or absent organism.
unusual and dramatic.
overflow of fluid from a farm or industrial factory.
area or path of a river where salmon return to spawn every season.
organism that eats dead or rotting biomass, such as animal flesh or plant material.
type of plant that grows in the ocean.
seaweed with large, flat leaves.
marine animal (echinoderm) with a circular, spiny shell.
marine algae. Seaweed can be composed of brown, green, or red algae, as well as "blue-green algae," which is actually bacteria.
organism that eats meat.
part of a plant from which a new plant grows.
type of plant, smaller than a tree but having woody branches.
to strain or put pressure on.
chemical element with the symbol S.
visible radiation from the sun.
bite-sized rolls or balls of sticky rice topped with seafood or vegetables.
having to do with the Earth or dry land.
carnivore that mostly eats other carnivores.
species at the top of the food chain, with no predators of its own. Also called an alpha predator or apex predator.
one of three positions on the food chain: autotrophs (first), herbivores (second), and carnivores and omnivores (third).
existing in the tropics, the latitudes between the Tropic of Cancer in the north and the Tropic of Capricorn in the south.
highly infectious and sometimes deadly disease with symptoms of itching sores and severe headache, caused by lice.
developed, densely populated area where most inhabitants have nonagricultural jobs.
site of a former kelp forest that has been destroyed by sea urchins.
plant that is grown or harvested for food.
an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.
World War II
(1939-1945) armed conflict between the Allies (represented by the United States, the United Kingdom, and the Soviet Union) and the Axis (represented by Germany, Italy, and Japan.)