All the interconnected and overlapping food chains of an ecosystem form a food web. Organisms in food chains are grouped into categories called trophic levels. A food web consists of all food chains in a single ecosystem. Every living thing in an ecosystem is part of multiple food chains.
Each food chain is a possible path that energy and nutrients can take as they move through the ecosystem. All the interconnected and overlapping food chains of an ecosystem form a food web. Trophic levels Organisms in food webs are grouped into categories called trophic levels. Broadly speaking, these levels are divided into producers (first trophic level), consumers and decomposers (last trophic level).
Producers/producers constitute the first trophic level. Producers, also known as autotrophs, produce 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 types.
Algae, whose larger forms are known as seaweed, are autotrophic. Phytoplankton, small organisms that live in the ocean, are also autotrophic. Some types of bacteria are autotrophic. For example, bacteria that live in active volcanoes use sulfur, not carbon dioxide, to produce their own food.
This process is called chemosynthesis, consumers. The following trophic levels are made up of animals that feed on producers. These organisms are called consumers. Consumers can be carnivores (animals that feed on other animals) or omnivores (animals that eat both plants and animals).
Omnivores, like people, consume many types of food. People eat plants, such as vegetables and fruits. We also eat animals and animal products, such as meat, milk and eggs. We eat mushrooms, like mushrooms.
We also eat seaweed in edible seaweed such as nori (used to wrap sushi rolls) and sea lettuce (used in salads). They eat berries and mushrooms, as well as animals such as salmon and deer, and the main consumers are herbivores. Herbivores eat plants, algae, and other producers. They are in 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 the main consumer. In an ocean ecosystem, many types of fish and turtles are herbivores that eat algae and seagrass.
In seaweed forests, seaweed, known as giant algae, provides 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 algae every day, secondary consumers eat herbivores. They are in the third trophic level.
In a desert ecosystem, a secondary consumer can be a snake that eats a mouse. In the kelp forest, sea otters are secondary consumers who hunt sea urchins, tertiary consumers eat secondary consumers. They are in the fourth trophic level. In the desert ecosystem, an owl or an eagle can hunt a snake.
There may be more levels of consumers before a chain finally reaches its main predator. Major predators, also called supreme predators, eat other consumers. They may be in the fourth or fifth trophic level. They have no natural enemies, except for humans.
Lions are the main predators in the grassland ecosystem. In the ocean, fish such as the great white shark are supreme predators. In the desert, wildcats and pumas are the main predators. Detritivores and decomposers make up the last part of food chains.
Detritivores are organisms that feed on the remains of non-living plants and animals. For example, scavengers, such as vultures, eat dead animals. Dung beetles eat animal feces, decomposers, such as fungi and bacteria, complete the food chain. Decomposers convert organic waste, such as decaying plants, into inorganic materials, such as nutrient-rich soil.
They complete the life cycle, returning nutrients to the soil or oceans for use by autotrophs. This starts a whole new series of food chains, food chains. Food networks connect many different food chains and many different trophic levels. Food networks can support food chains that are long and complicated, or very short.
For example, the grass of a forest clearing produces its own food through photosynthesis. When a fox dies, decomposers, such as worms and fungi, break down its body and return it to the soil, where it provides nutrients to plants such as grass. This short food chain is part of the forest food chain. Another food chain in the same ecosystem could involve completely different organisms.
A caterpillar can eat the leaves of a tree in the forest. A bird like a sparrow can eat the caterpillar. A snake can then hunt the sparrow. An eagle, a supreme predator, can hunt the snake.
However, another bird, a vulture, consumes the dead eagle's body. Finally, soil bacteria break down debris, algae and plankton are the main producers of marine ecosystems. A tiny shrimp called krill eats microscopic plankton. The largest animal on Earth, the blue whale, feeds on thousands of tons of krill every day.
Supreme predators, such as orcas, feed on blue whales. As the bodies of large animals, such as whales, sink to the seabed, detritivores, such as worms, break down the material. The nutrients released by decaying meat provide chemicals for algae and plankton to start a new series of food chains. Food networks are defined by their biomass.
Biomass is the energy of living organisms. Autotrophs, the producers of a food web, convert energy from the Sun into biomass. Biomass decreases with each trophic level. There is always more biomass at lower trophic levels than at higher trophic levels.
Since 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 greater number of herbivores and an even greater 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 a link in the food web is threatened, some or all of the links weaken or become stressed.
Ecosystem biomass decreases. The loss of plant life usually causes a decline in the population of herbivores, for example. Plant life may decline due to drought, disease, or human activity. Forests are cut down to provide wood for construction.
Grasslands are paved for shopping malls or parking lots. The loss of biomass in the second or third trophic level can also unbalance a food web. Consider what can happen if a salmon race goes astray. A salmon track is a river where salmon swim.
Landslides and earthquakes can divert salmon streams, as well as by building dams and levees. Biomass is lost as salmon is extracted from rivers. Being unable to eat salmon, omnivores, such as bears, are forced to rely more on other food sources, such as ants. Ants tend to be scavengers and detritivores, so fewer nutrients break down in the soil.
Soil cannot withstand so many autotrophs, so biomass is lost. Salmon themselves are predators of insect larvae and smaller fish. Without salmon to control their population, aquatic insects can devastate local plant communities. Fewer plants survive and biomass is lost.
The loss of organisms at higher trophic levels, such as carnivores, can also disrupt the food chain. In kelp forests, sea urchins are the main consumer of algae. If the population of sea otters shrinks due to disease or hunting, hedgehogs devastate the algae forest. In the absence of a community of producers, biomass collapses.
These areas are called hedgehog wastelands. Human activity can reduce the number of predators. In 1986, Venezuelan authorities dammed the Caroni River, creating a huge lake approximately twice the size of Rhode Island. Hundreds of hills turned into islands in this lake.
With their habitats reduced to small islands, many terrestrial predators couldn't find enough food. As a result, prey animals such as howler monkeys, leaf-cutting ants and iguanas flourished. The ants became so numerous that they destroyed the rainforest, killing all the trees and other plants. The food web that surrounded the Caroní River was destroyed, bioaccumulation and biomass decrease as trophic levels rise.
However, some types of materials, especially toxic chemicals, increase with each trophic level of the food web. These chemicals usually accumulate in animal fat. When an herbivore eats a plant or other autotroph covered in pesticides, for example, those pesticides are stored in the animal's fat. When a carnivore eats several of these herbivores, it absorbs the chemical pesticides stored in its prey.
This process is called bioaccumulation. Bioaccumulation also occurs in aquatic ecosystems. Runoff from urban areas or farms can be full of pollutants. Small producers, such as algae, bacteria and seagrass, absorb tiny amounts of these pollutants.
Major consumers, such as sea turtles and fish, feed on seagrass. They use the energy and nutrients provided by plants, but store chemicals in their adipose tissue. Predators from the third trophic level, such as sharks or tuna, eat fish. By the time people consume tuna, they may be storing a significant amount of bioaccumulated toxins.
Due to bioaccumulation, organisms in some contaminated ecosystems are not safe to eat and are not allowed to be collected. Oysters in the harbor of New York City in the United States, for example, are not safe to eat. Port contaminants accumulate in its oysters, which are fed by filtration. In the 1940s and 1950s, a pesticide called DDT (dichlorodiphenyltrichloroethane) was widely used to kill insects that spread diseases.
During World War II, the Allies used DDT to eliminate typhus in Europe and control malaria in the South Pacific. Scientists thought they had discovered a miracle drug. DDT was largely responsible for eliminating malaria in places such as Taiwan, the Caribbean, and the Balkans. Unfortunately, DDT bioaccumulates in an ecosystem and causes damage to the environment.
DDT accumulates in soil and water. Some forms of DDT break down slowly. Worms, grasses, algae and fish accumulate DDT. Supreme predators, such as eagles, had large amounts of DDT in their bodies, accumulated from the fish and small mammals they feed on.
Birds with large amounts of DDT in their bodies lay eggs with extremely thin shells. These shells often broke before the pups were ready to hatch. DDT was one of the main reasons for the decline of the bald eagle, a supreme predator that feeds mainly on fish and small rodents. Nowadays, the use of DDT has been restricted.
The food networks of which it is part have recovered in most parts of the country. A common metric used to quantify the trophic structure of the food web is the length of the food chain. In its simplest form, the length of a chain is the number of links between a trophic consumer and the base of the web. The average chain length of an entire network is the arithmetic average of the lengths of all the chains in the food chain.
The food chain is a diagram of energy sources. The food chain starts with a producer, who is consumed by a primary consumer. The primary consumer can be consumed by a secondary consumer, who in turn can be consumed by a tertiary consumer. Tertiary consumers can sometimes become prey to major predators known as quaternary consumers.
For example, a food chain could start with a green plant as a producer, which is eaten by a snail, the main consumer. The snail could then fall prey to a secondary consumer, such as a frog, which in turn can be consumed by a tertiary consumer, such as a snake, which in turn can be consumed by an eagle. Plants are called producers because they can use light energy from the Sun to produce food (sugar) from carbon dioxide and water. These must then be intertwined and glued together to form a chain of species in which one eats the other.
Environmentalists have formulated and tested hypotheses about the nature of ecological patterns associated with the length of the food chain, such as the increase in length with the size of the ecosystem, the reduction of energy at each successive level, or the assertion that long food chains are unstable. Food chains were first introduced by the Arab scientist and philosopher Al-Jahiz in the tenth century and were later popularized in a book published in 1927 by Charles Elton, who also introduced the concept of a food web. Chemosynthetic bacteria and archaea use hydrogen sulfide and methane from hydrothermal vents and cold seeps as an energy source (just like plants use sunlight) to produce carbohydrates; they form the basis of the food chain. A food chain differs from a food web because the complex network of food relationships between the different animals adds up and the chain only follows a linear and direct route of one animal at a time.
When only one element is eliminated from the food chain, it can cause the extinction of a species in some cases. The length of a food chain is a continuous variable that provides a measure of the passage of energy and an index of ecological structure that increases through links from the lowest trophic (food) levels to the highest. Most animals are part of more than one food chain and eat more than one type of food to meet their food and energy needs. There can't be too many links in a single food chain because the animals at the end of the chain wouldn't get enough food (and therefore energy) to stay alive.
The length of the food chain is important because the amount of energy transferred decreases as the trophic level increases; usually, only ten percent of the total energy from one trophic level goes to the next, since the rest is used in the metabolic process. . .
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