Afood webconsists of all thefood chains in a singleecosystem. Each living thing in an ecosystem is part ofmultiplefood 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.
Organisms in food webs are grouped into categories calledtrophic levels. Roughly speaking, these levels are divided intoproducers (first trophic level),consumers, anddecomposers (last trophic level).
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.
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, inedibleseaweeds 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 agrasslandecosystem, deer, mice, and even elephants are herbivores. They eat grasses,shrubs, and trees. In adesertecosystem, a mouse that eatsseeds andfruits 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 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.
Food webs are defined by theirbiomass. 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 andrecyclebiomass.
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 biomassdeclines.
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. Inkelp 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, biomassplummets. 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 werent 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, 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 animals fat. When a carnivore eats several of these herbivores, it takes in the pesticide chemicals stored in its prey. This process is calledbioaccumulation.
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 harboraccumulate 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 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.