A food web represents the interconnected network of food chains within a single ecosystem. In essence, it illustrates all the possible pathways that energy and nutrients take as they move through the environment. Every living organism in an ecosystem participates in multiple food chains, and the overlap of these chains creates the intricate structure we call a food web.
Trophic Levels Explained
Organisms within a food web are categorized into trophic levels. These levels broadly consist of producers (the first trophic level), consumers, and decomposers (the final trophic level).
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Producers (Autotrophs): The Foundation of the Food Web
Producers occupy the first trophic level. They are also known as autotrophs because they produce their own food, not depending on other organisms for nutrition. The majority of autotrophs utilize photosynthesis, a process that converts sunlight, carbon dioxide, and water into glucose, a nutrient-rich sugar.
Plants are the most recognizable autotrophs, but various other types exist. Algae, including seaweed, are autotrophic. Phytoplankton, microscopic organisms inhabiting oceans, are also autotrophs. Certain bacteria are autotrophs as well. For instance, bacteria residing in active volcanoes employ chemosynthesis, using sulfur instead of carbon dioxide to synthesize food.
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Consumers: Herbivores, Carnivores, and Omnivores
The subsequent trophic levels are comprised of animals that consume producers, known as consumers.
Consumers are further classified as carnivores (animals that eat other animals) or omnivores (animals that consume both plants and animals). Omnivores, like humans, have diverse diets. Humans consume plants such as fruits and vegetables, animals and their products like meat, milk, and eggs, fungi such as mushrooms, and algae, including edible seaweeds like nori and sea lettuce. Bears are also omnivores, consuming berries, mushrooms, salmon, and deer.
Primary consumers are herbivores. They feed on plants, algae, and other producers, and occupy the second trophic level. In a grassland ecosystem, herbivores include deer, mice, and elephants, all of which consume grasses, shrubs, and trees. In a desert ecosystem, a mouse that eats seeds and fruits is a primary consumer.
Ocean ecosystems are home to numerous herbivorous fish and turtles that feed on algae and seagrass. Kelp forests feature giant kelp, a seaweed providing shelter and food for an entire ecosystem. Sea urchins are primary consumers in kelp forests, eating large quantities of giant kelp daily.
Secondary consumers eat herbivores. They reside at the third trophic level. In a desert ecosystem, a snake that eats a mouse is a secondary consumer. In kelp forests, sea otters are secondary consumers that prey on sea urchins.
Tertiary consumers prey on secondary consumers. They occupy the fourth trophic level. In a desert ecosystem, an owl or eagle may prey on a snake.
Consumer levels can extend beyond the fourth trophic level before reaching the top predator. Top predators, also known as apex predators, eat other consumers and may be at the fourth or fifth trophic level. They have no natural predators except humans. Lions are apex predators in grassland ecosystems. Great white sharks are apex predators in the ocean. Bobcats and mountain lions are apex predators in the desert.
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Detritivores and Decomposers: Nature’s Recyclers
Detritivores and decomposers constitute the final component of food chains. Detritivores consume nonliving plant and animal remains. Vultures, for example, are scavengers that feed on dead animals. Dung beetles eat animal feces.
Decomposers, such as fungi and bacteria, complete the food chain. They convert organic waste, including decaying plants, into inorganic materials, such as nutrient-rich soil. Decomposers complete the cycle of life by returning nutrients to the soil or oceans for use by autotrophs, initiating new food chains.
The Interconnectedness of Food Chains
Food webs connect numerous different food chains and trophic levels, which can range from long and complicated to very short.
Consider a forest clearing where grass produces its own food via photosynthesis. A rabbit eats the grass, and a fox eats the rabbit. When the fox dies, decomposers like worms and mushrooms break down its body, returning nutrients to the soil for plants like grass.
This short food chain is one component of the forest’s food web. Another food chain within the same ecosystem might involve completely different organisms. A caterpillar may eat the leaves of a tree. 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 vulture consumes the body of the dead eagle, and bacteria in the soil decompose the remains.
In marine ecosystems, algae and plankton are the primary producers. Tiny shrimp called krill consume the microscopic plankton. The blue whale, the largest animal on Earth, preys on tons of krill daily. Apex predators like orcas prey on blue whales. When the bodies of large animals such as whales sink to the seafloor, detritivores like worms break down the material. The nutrients released by the decaying flesh provide chemicals for algae and plankton to initiate new food chains.
Biomass: The Energy Pyramid
Food webs are defined by their biomass, which is the energy contained within living organisms. Autotrophs, the producers in a food web, convert the sun’s energy into biomass. Biomass decreases with each ascending trophic level. Lower trophic levels always have more biomass than higher ones.
Due to the decrease in biomass at each trophic level, a healthy food web always has more autotrophs than herbivores and more herbivores than carnivores. An ecosystem cannot sustain a large number of omnivores without supporting a larger number of herbivores and an even larger number of autotrophs.
A healthy food web features an abundance of autotrophs, many herbivores, and relatively few carnivores and omnivores. This balance helps the ecosystem maintain and recycle biomass.
Each link within a food web connects to at least two others. The biomass of an ecosystem hinges on the balance and interconnectedness of its food web. When one link in the food web is threatened, some or all of the links are weakened or stressed, leading to a decline in the ecosystem’s biomass.
For instance, the loss of plant life usually results in a decline in the herbivore population. Plant life can decline due to drought, disease, or human activities like deforestation for lumber or paving grasslands for development.
The loss of biomass at the second or third trophic level can also disrupt a food web’s equilibrium. Consider what happens when a salmon run is diverted. Salmon runs, rivers where salmon swim, can be diverted by landslides, earthquakes, or the construction of dams and levees.
Biomass is lost when salmon are cut off from the rivers. Unable to eat salmon, omnivores such as bears are forced to rely more heavily on other food sources, such as ants, shrinking the ant population. Since ants are scavengers and detritivores, fewer nutrients are broken down in the soil, hindering its capacity to support autotrophs, thus losing biomass. Salmon themselves are predators of insect larvae and smaller fish. Without salmon to control their population, aquatic insects can devastate local plant communities, resulting in fewer surviving plants and a loss of biomass.
A loss of organisms at higher trophic levels, such as carnivores, can also disrupt a food chain. In kelp forests, sea urchins are the primary consumer of kelp, and sea otters prey on urchins. If the sea otter population decreases due to disease or hunting, urchins decimate the kelp forest. Without a community of producers, biomass plummets, and the entire kelp forest disappears, resulting in urchin barrens.
Human activities can reduce the number of predators. In 1986, officials in Venezuela dammed the Caroni River, creating a vast lake. Hundreds of hilltops became islands. With their habitats reduced, many terrestrial predators were unable 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 plants. The food web surrounding the Caroni River was destroyed.
Bioaccumulation: The Toxin Trap
While biomass decreases with each ascending trophic level, certain materials, particularly toxic chemicals, increase. These chemicals typically accumulate in the fat of animals.
For instance, when an herbivore eats a plant covered in pesticides, the pesticides are stored in the animal’s fat. When a carnivore eats several of these herbivores, it consumes the pesticide chemicals 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. 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, utilizing the energy and nutrients provided by the plants but storing 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 humans, it may be storing a significant amount of bioaccumulated toxins.
Because of bioaccumulation, organisms in some polluted ecosystems are unsafe to eat and cannot be harvested. Oysters in New York City’s harbor, for instance, are unsafe for consumption due to the accumulation of pollutants.
In the 1940s and 1950s, DDT (dichloro-diphenyl-trichloroethane) was widely used to kill insects that spread diseases. Allies used DDT to eliminate typhus in Europe and control malaria in the South Pacific during World War II. Scientists believed they had discovered a miracle drug. DDT played a significant role in eliminating malaria in Taiwan, the Caribbean, and the Balkans.
Unfortunately, DDT bioaccumulates in an ecosystem and damages the environment. DDT accumulates in soil and water and decomposes slowly. Worms, grasses, algae, and fish accumulate DDT. Apex predators, such as eagles, accumulate high amounts of DDT in their bodies from the fish and small mammals they prey on.
Birds with high amounts of DDT in their bodies lay eggs with extremely thin shells that often break before the chicks are ready to hatch.
DDT was a major factor in the decline of the bald eagle, an apex predator that primarily feeds on fish and small rodents. Today, the use of DDT has been restricted, and the food webs have recovered in most parts of the country.
Conclusion
Understanding food webs is crucial for comprehending the intricate relationships within ecosystems. From producers converting sunlight into energy to decomposers recycling nutrients, each trophic level plays a vital role in maintaining balance. Disruptions to food webs, whether from habitat loss or bioaccumulation, can have cascading effects on entire ecosystems. Recognizing these connections is essential for conservation efforts and ensuring the health of our planet.
