Food Web (2024)

Afood webconsists of all thefood chains in a singleecosystem. Each living thing in an

ecosystem

is part ofmultiple

food chains

. Each

food chain

is one possible path thatenergyandnutrients may take as they move through the

ecosystem

. All of the interconnected and overlapping

food chains

in an

ecosystem

make up a

food web

.

Trophic Levels

Organisms in

food webs

are grouped into categories called

trophic levels

. Roughly speaking, these levels are divided intoproducers (first

trophic level

),consumers, anddecomposers (last

trophic level

).

Producers

Producers

make up the first

trophic level

.

Producers

, also known asautotrophs, make their own food and do not depend on any other organism for nutrition. Most

autotrophs

use a process calledphotosynthesisto create food (a

nutrient

calledglucose) 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 asseaweed, are

autotrophic

.Phytoplankton, tiny organisms that live in the ocean, are also

autotrophs

. Some types ofbacteriaare

autotrophs

. For example,

bacteria

living in activevolcanoes usesulfur, not carbon dioxide

, to produce their own food. This process is calledchemosynthesis.

Consumers

The next

trophic levels

are made up of animals that eat

producers

. These organisms are called

consumers

.

Consumers

can becarnivores (animals that eat other animals) oromnivores (animals that eat both

plants

and animals).

Omnivores

, like people, consume many types of foods. People eat

plants

, such asvegetables and fruits. We also eat animals and animal products, such as meat, milk, and eggs. We eatfungi, such as mushrooms. We also eat

algae

, inedible

seaweeds

likenori (used to wrapsushirolls) andsea lettuce(used in salads). Bears are

omnivores

, too. They eat berries and mushrooms, as well as animals such as salmon and deer.

Primary consumers areherbivores.

Herbivores

eat

plants

,

algae

, and other

producers

. They are at the second

trophic level

. In agrassland

ecosystem

, deer, mice, and even elephants are

herbivores

. They eat grasses,shrubs, and trees. In adesert

ecosystem

, a mouse that eatsseeds and

fruits

is a

primary

consumer

.

In an ocean

ecosystem

, many types of fish and turtles are

herbivores

that eat

algae

andseagrass. Inkelp 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 itstop predator.

Top predators

, also calledapex 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 animalremains. For example,scavengers such as vultures eat dead animals. Dung beetles eat animalfeces.

Decomposers

, like

fungi

and

bacteria

, complete the

food chain

.

Decomposers

turnorganic wastes, such asdecaying

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 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 calledkrilleat 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

.

Biomass

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 anabundanceof

autotrophs

, many

herbivores

, and relatively few

carnivores

and

omnivores

. This balance helps the

ecosystem

maintain andrecycle

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 orstressed. 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 todrought, disease, or human activity.

Forests

are cut down to providelumberfor 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 asalmon runisdiverted. A

salmon run

is a river where salmon swim.

Salmon runs

can be

diverted

bylandslides andearthquakes, as well as the construction ofdams andlevees.

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,aquaticinsects maydevastatelocal

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 calledurchin barrens.

Human activity can reduce the number of predators. In 1986, officials in Venezuela

dammed

the Caroni River, creating anenormouslake 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.

Bioaccumulation

Biomass

declines

as you move up through the

trophic levels

. However, some types of materials, especiallytoxicchemicals, increase with each

trophic level

in the

food web

. These chemicals usually collect in thefatof animals.

When an

herbivore

eats a

plant

or other

autotroph

that is covered inpesticides, 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.Runofffromurban areas orfarms can be full ofpollutants. 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 aremarkableamount of bioaccumulated toxins.

Because of

bioaccumulation

, organisms in some polluted

ecosystems

are unsafe to eat and not allowed to beharvested.Oysters in theharborof 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

calledDDT(dichloro-diphenyl-trichloroethane) was widely used to kill insects that spread diseases. DuringWorld War II, theAlliesused

DDT

toeliminate typhusin Europe, and to controlmalariain 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

bio

accumulates

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.

USDA: The Soil Food Web

Environmental Protection Agency: Great Lakes Monitoring—Fish Indicators

National Geographic: BioBlitzNational Geographic Ocean: Marine Food Chain