• food web
    Corals and fishes and sharks, oh my.

    Photograph by James Abernethy

    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.

    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.

    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 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.

    Trophic Levels

    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
    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.

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

    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 as prey.

    Tertiary consumers eat the secondary consumers. They are at the fourth trophic level. In the desert ecosystem, an owl or eagle may prey on the 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 people. 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.

    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.


    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 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 forests 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. A hawk, another apex predator, may prey on the eagle. Yet another bird, a vulture, consumes the body of the dead hawk. Finally, bacteria in the soil decompose the remains.

    In a desert ecosystem, an autotroph such as a cactus produces fruit. Herbivorous insects, such as flies, consume the cactus fruit. Birds such as the roadrunner consume these insects. Detritivores such as termites eat the roadrunner after it dies. Bacteria and fungi help decompose the remaining bones of the roadrunner. The carbon in the bones enriches the desert soil, helping plants like cactuses develop.

    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.

    Biomass

    Food webs are defined by their biomass. Biomass is the energy in living organisms. Autotrophs, the producers in a food web, convert the suns 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 few carnivores and omnivores. This balance helps the ecosystem maintain and recycle biomass.


    In addition, smaller animals are more numerous than larger ones. Tigers and ants are both consumers in a tropical food web. However, it takes much more biomass to support a tiger population than a colony of ants. Tigers consume more food and take up a much larger space. There are many more ants than tigers in the food web of a tropical ecosystem.

    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.

    As the number of plants and other autotrophs is reduced, the rest of the food web is forced to adapt or die. Deer have fewer plants to eat. Butterflies and bees have fewer flowers to pollinate. In turn, predators like mountain lions have fewer deer to consume. Birds like kingfishers have fewer insects to eat.

    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 many food chains, which make up the ecosystems food web. Unable to eat salmon, omnivores like bears are forced to rely more heavily on other food sources, such as ants. The areas 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 the kelp forest, 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. About 1,000 hilltops turned into islands in this lake. With their habitats reduced to tiny islands, many terrestrial predators, such as jaguars, armadillos, and weasels, 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 rain forest, 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, 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 animals 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 and not allowed to be harvested. Oysters in the harbor of New York City, for instance, are unsafe to eat. The pollutants in the harbor accumulate in 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. In the 1950s, 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 may not decompose for 20 years. Worms, grasses, algae, and fish accumulate DDT. Apex predators, such as eagles, had the highest amount 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. Thin shells also made it easier for predators such as snakes and other birds to consume the eggs and embryos inside.

    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.

  • Term Part of Speech Definition Encyclopedic Entry
    abundance Noun

    large amount.

    accumulate Verb

    to gather or collect.

    adapt Verb

    to adjust to new surroundings or a new situation.

    algae Plural Noun

    (singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

    Allies Noun

    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.

    apex predator Noun

    species at the top of the food chain, with no predators of its own. Also called an alpha predator or top predator.

    aquatic Adjective

    having to do with water.

    autotroph Noun

    organism that can produce its own food and nutrients from chemicals in the atmosphere, usually through photosynthesis or chemosynthesis.

    Encyclopedic Entry: autotroph
    bacteria Plural Noun

    (singular: bacterium) single-celled organisms found in every ecosystem on Earth.

    Balkan Adjective

    having to do with the Balkan Peninsula in southeastern Europe.

    bioaccumulation Noun

    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.

    biomass Noun

    living organisms, and the energy contained within them.

    carbon Noun

    chemical element with the symbol C, which forms the basis of all known life.

    carbon dioxide Noun

    greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.

    carnivore Noun

    organism that eats meat.

    Encyclopedic Entry: carnivore
    chemosynthesis Noun

    process by which some microbes turn carbon dioxide and water into carbohydrates using energy obtained from inorganic chemical reactions.

    colony Noun

    group of one species of organism living close together.

    consist Verb

    to be made of.

    consumer Noun

    organism on the food chain that depends on autotrophs (producers) or other consumers for food, nutrition, and energy.

    dam Noun

    structure built across a river or other waterway to control the flow of water.

    DDT Noun

    (dichloro-diphenyl-trichloroethane) toxic chemical used as an insecticide but illegal for most uses in the U.S. since 1972.

    decay Verb

    to rot or decompose.

    decline Verb

    to reduce or go down in number.

    decomposer Noun

    organism that breaks down dead plant material.

    desert Noun

    area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

    Encyclopedic Entry: desert
    detritivore Noun

    organism that consumes dead plant material.

    devastate Verb

    to destroy.

    divert Verb

    to direct away from a familiar path.

    drought Noun

    period of greatly reduced precipitation.

    Encyclopedic Entry: drought
    earthquake Noun

    the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.

    ecosystem Noun

    community and interactions of living and nonliving things in an area.

    Encyclopedic Entry: ecosystem
    edible Adjective

    able to be eaten and digested.

    eliminate Verb

    to remove.

    energy Noun

    capacity to do work.

    enormous Adjective

    very large.

    farm Noun

    land cultivated for crops, livestock, or both.

    fat Noun

    material found in organisms that is colorless and odorless and may be solid or liquid at room temperature.

    feces Noun

    waste material produced by the living body of an organism.

    filter feeder Noun

    aquatic animal that strains nutrients from water.

    food chain Noun

    group of organisms linked in order of the food they eat, from producers to consumers, and from prey, predators, scavengers, and decomposers.

    Encyclopedic Entry: food chain
    food web Noun

    all related food chains in an ecosystem. Also called a food cycle.

    Encyclopedic Entry: food web
    forest Noun

    ecosystem filled with trees and underbrush.

    fruit Noun

    edible part of a plant that grows from a flower.

    fungi Plural Noun

    (singular: fungus) organisms that survive by decomposing and absorbing nutrients in organic material such as soil or dead organisms.

    glucose Noun

    "simple sugar" chemical produced by many plants during photosynthesis.

    grassland Noun

    ecosystem with large, flat areas of grasses.

    harbor Noun

    part of a body of water deep enough for ships to dock.

    Encyclopedic Entry: harbor
    harvest Noun

    the gathering and collection of crops, including both plants and animals.

    herbivore Noun

    organism that eats mainly plants.

    Encyclopedic Entry: herbivore
    inorganic Adjective

    composed of material that is not living, and never was, such as rock.

    kelp Noun

    type of seaweed.

    kelp forest Noun

    underwater habitat filled with tall seaweeds known as kelp.

    krill Noun

    small marine crustacean, similar to shrimp.

    landslide Noun

    the fall of rocks, soil, and other materials from a mountain, hill, or slope.

    Encyclopedic Entry: landslide
    larva Noun

    a new or immature insect or other type of invertebrate.

    levee Noun

    bank of a river, raised either naturally or constructed by people.

    Encyclopedic Entry: levee
    lumber Noun

    precisely cut pieces of wood such as boards or planks.

    malaria Noun

    infectious disease caused by a parasite carried by mosquitoes.

    marine Adjective

    having to do with the ocean.

    multiple Adjective

    many.

    nori Noun

    red algae that is often dried and used to wrap sushi.

    nutrient Noun

    substance an organism needs for energy, growth, and life.

    Encyclopedic Entry: nutrient
    omnivore Noun

    organism that eats a variety of organisms, including plants, animals, and fungi.

    Encyclopedic Entry: omnivore
    organic waste Noun

    used or excess material that might be broken down to be used again. Also called biodegradable waste.

    oyster Noun

    type of marine animal (mollusk).

    pesticide Noun

    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.)

    photosynthesis Noun

    process by which plants turn water, sunlight, and carbon dioxide into water, oxygen, and simple sugars.

    phytoplankton Noun

    microscopic organism that lives in the ocean and can produce its own food through photosynthesis.

    plant Noun

    organism that produces its own food through photosynthesis and whose cells have walls.

    plummet Verb

    to fall sharply.

    pollinate Verb

    to transfer pollen from one part of a flower (the anther) to another (the stigma).

    pollutant Noun

    chemical or other substance that harms a natural resource.

    prey Noun

    animal that is hunted and eaten by other animals.

    primary consumer Noun

    organism that eats plants or other autotrophs.

    producer Noun

    organism on the food chain that can produce its own energy and nutrients. Also called an autotroph.

    rain forest Noun

    area of tall evergreen trees and a high amount of rainfall.

    recycle Verb

    to clean or process in order to make suitable for reuse.

    reduce Verb

    to lower or lessen.

    remains Noun

    materials left from a dead or absent organism.

    remarkable Adjective

    unusual and dramatic.

    runoff Noun

    overflow of fluid from a farm or industrial factory.

    Encyclopedic Entry: runoff
    salmon run Noun

    area or path of a river where salmon return to spawn every season.

    scavenger Noun

    organism that eats dead or rotting biomass, such as animal flesh or plant material.

    Encyclopedic Entry: scavenger
    seagrass Noun

    type of plant that grows in the ocean.

    sea lettuce Noun

    seaweed with large, flat leaves.

    sea urchin Noun

    marine animal (echinoderm) with a circular, spiny shell.

    seaweed Noun

    marine algae. Seaweed can be composed of brown, green, or red algae, as well as "blue-green algae," which is actually bacteria.

    secondary consumer Noun

    organism that eats meat.

    seed Noun

    part of a plant from which a new plant grows.

    shrub Noun

    type of plant, smaller than a tree but having woody branches.

    stress Verb

    to strain or put pressure on.

    sulfur Noun

    chemical element with the symbol S.

    sunlight Noun

    visible radiation from the sun.

    survive Verb

    to live.

    sushi Noun

    bite-sized rolls or balls of sticky rice topped with seafood or vegetables.

    terrestrial Adjective

    having to do with the Earth or dry land.

    tertiary consumer Noun

    carnivore that mostly eats other carnivores.

    top predator Noun

    species at the top of the food chain, with no predators of its own. Also called an alpha predator or apex predator.

    toxic Adjective

    poisonous.

    trophic level Noun

    one of three positions on the food chain: autotrophs (first), herbivores (second), and carnivores and omnivores (third).

    tropical Adjective

    existing in the tropics, the latitudes between the Tropic of Cancer in the north and the Tropic of Capricorn in the south.

    typhus Noun

    highly infectious and sometimes deadly disease with symptoms of itching sores and severe headache, caused by lice.

    urban area Noun

    developed, densely populated area where most inhabitants have nonagricultural jobs.

    Encyclopedic Entry: urban area
    urchin barren Noun

    site of a former kelp forest that has been destroyed by sea urchins.

    vegetable Noun

    plant that is grown or harvested for food.

    volcano Noun

    an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

    Encyclopedic Entry: volcano
    World War II Noun

    (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.)

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