A food web is a comprehensive representation of all the interconnected food chains within a single ecosystem. Every living organism in an ecosystem participates in multiple food chains, each outlining a possible pathway for energy and nutrients as they traverse the environment. The sum of these interwoven and overlapping food chains forms the complex network known as a food web.
Understanding Trophic Levels
Within food webs, organisms are organized into categories known as trophic levels. These levels are broadly categorized into producers (the first trophic level), consumers, and decomposers (the final trophic level).
Producers: The Foundation of the Food Web
Producers form the base of the food web, occupying the first trophic level. These organisms, also known as autotrophs, are self-sufficient, creating their own food and not relying on any other organism for sustenance. The majority of autotrophs utilize photosynthesis, a process that converts sunlight, carbon dioxide, and water into glucose, a nutrient-rich food source.
While plants are the most commonly recognized autotrophs, various other forms exist. Algae, including larger seaweed varieties, are autotrophic. Phytoplankton, microscopic organisms inhabiting the ocean, also fall into this category. Certain bacteria species are autotrophs as well; for instance, bacteria residing in active volcanoes utilize sulfur instead of carbon dioxide to produce food via a process known as chemosynthesis.
Alt text: Microscopic phytoplankton cells under magnification, forming the base of the marine food web.
Consumers: Feeding on Producers and Each Other
Subsequent trophic levels consist of consumers, animals that derive nutrition by consuming producers or other consumers.
Consumers can be classified as carnivores (animals that exclusively consume other animals) or omnivores (animals with a diet consisting of both plants and animals). Omnivores, like humans, have a diverse diet. Humans consume plants such as fruits and vegetables, as well as animals and animal products like meat, milk, and eggs. Fungi, such as mushrooms, and algae, like nori (used in sushi) and sea lettuce (used in salads), are also part of the human diet. Bears are another example of omnivores, consuming berries and mushrooms alongside animals like salmon and deer.
Primary consumers, also known as herbivores, are animals that feed on plants, algae, and other producers. They occupy the second trophic level. In grassland ecosystems, deer, mice, and elephants are herbivores, consuming grasses, shrubs, and trees. In desert environments, mice that eat seeds and fruits are considered primary consumers.
In marine ecosystems, various fish and turtle species are herbivores, feeding on algae and seagrass. Kelp forests offer a prime example, where giant kelp serves as both shelter and food for an entire ecosystem. Sea urchins are significant primary consumers in kelp forests, consuming substantial amounts of giant kelp daily.
Secondary consumers prey on herbivores, residing at the third trophic level. In a desert ecosystem, a snake that consumes a mouse is an example of a secondary consumer. In kelp forests, sea otters act as secondary consumers by hunting sea urchins.
Tertiary consumers feed on secondary consumers and occupy the fourth trophic level. In a desert ecosystem, an owl or eagle preying on a snake exemplifies this level.
Food chains can extend beyond tertiary consumers before reaching a top predator. Top predators, or apex predators, consume other consumers and may be found at the fourth or fifth trophic level. They lack natural predators other than humans. Lions are apex predators in grassland ecosystems, while great white sharks hold this position in the ocean. Bobcats and mountain lions are top predators in desert ecosystems.
Detritivores and Decomposers: The Recyclers of the Ecosystem
Detritivores and decomposers finalize the food chain. Detritivores consume non-living plant and animal matter. Scavengers like vultures, for example, feed on dead animals, while dung beetles consume animal feces.
Decomposers, such as fungi and bacteria, complete the cycle by converting organic waste from decaying plants into inorganic materials, like nutrient-rich soil. This process allows nutrients to be returned to the soil or oceans for use by autotrophs, initiating new food chains.
Alt text: Illustrative diagram depicting a food web with energy flow from sunlight to producers, consumers, and decomposers.
The Interconnectedness of Food Chains
Food webs intricately connect numerous food chains and trophic levels, enabling both lengthy, complex chains and shorter, simpler ones.
Consider a forest clearing where grass produces food through photosynthesis. A rabbit eats the grass, and a fox preys on the rabbit. Upon the fox’s death, decomposers like worms and mushrooms break down its body, enriching the soil with nutrients that support the growth of plants like grass.
This brief food chain represents one aspect of the forest’s food web. Another chain within the same ecosystem may involve different organisms. A caterpillar might consume leaves from a tree, followed by a sparrow eating the caterpillar. A snake might then prey on the sparrow, and an apex predator like an eagle might prey on the snake. A vulture may consume the dead eagle’s body, and finally, bacteria in the soil decompose the remains.
In marine ecosystems, algae and plankton serve as primary producers. Tiny shrimp called krill consume microscopic plankton, while the largest animal on Earth, the blue whale, preys on thousands of tons of krill daily. Apex predators like orcas may prey on blue whales. As the bodies of large animals like whales sink to the seafloor, detritivores like worms break down the material. The released nutrients then provide essential chemicals for algae and plankton to initiate new food chains.
Biomass and Its Role in Food Webs
Food webs are characterized by their biomass, which represents the energy contained within living organisms. Autotrophs, the producers in a food web, convert solar energy into biomass. Biomass decreases as it moves up through the trophic levels, meaning lower levels always possess more biomass than higher ones.
Due to this decrease in biomass, healthy food webs typically have more autotrophs than herbivores and more herbivores than carnivores. An ecosystem requires a larger base of autotrophs to support a substantial population of herbivores, and an even larger base to support a population of omnivores.
A balanced food web thrives on an abundance of autotrophs, a sizable herbivore population, and a relatively smaller number of carnivores and omnivores. This balance facilitates the ecosystem’s ability to maintain and recycle biomass efficiently.
Each element within a food web is connected to at least two others. The overall biomass of an ecosystem is directly influenced by the balance and interconnectedness of its food web. The weakening or stressing of one link within the web can impact other links, leading to a decline in the ecosystem’s biomass.
For example, a decrease in plant life can lead to a reduction in the herbivore population. This decline in plant life can result from drought, disease, or human activities such as deforestation for lumber or paving grasslands for development.
Loss of biomass at the second or third trophic level can also disrupt a food web’s equilibrium. Consider the impact of diverting a salmon run, a river where salmon migrate to spawn. Diversions can occur due to natural events like landslides and earthquakes or human-made structures like dams and levees.
The biomass decreases as salmon are removed from the rivers. Unable to consume salmon, omnivores such as bears must rely more heavily on alternative food sources, such as ants, which can shrink the ant population. Because ants are often scavengers and detritivores, a decline in their numbers results in fewer nutrients being broken down in the soil. This, in turn, reduces the soil’s ability to support autotrophs, resulting in a loss of biomass. Salmon also regulate populations of insect larvae and smaller fish. Without salmon, aquatic insects may overpopulate and harm local plant communities, further diminishing biomass.
The loss of organisms at higher trophic levels, such as carnivores, can also disrupt a food chain. In kelp forests, sea urchins are the primary consumers of kelp, and sea otters prey on these urchins. If the sea otter population declines due to disease or hunting, the urchin population can grow unchecked, leading to the destruction of the kelp forest. Without the producer community, biomass plummets, and the kelp forest disappears, creating what is known as an urchin barren.
Human activities have been shown to reduce the number of predators in ecosystems. In 1986, the damming of the Caroni River in Venezuela created an enormous lake, transforming hilltops into islands. Terrestrial predators on these islands were unable to find sufficient food, causing prey animals like howler monkeys, leaf-cutter ants, and iguanas to proliferate. The ant population grew so large that they destroyed the rainforest, leading to the death of trees and other plants, effectively destroying the food web surrounding the Caroni River.
Bioaccumulation: The Dark Side of the Food Web
While biomass decreases at higher trophic levels, certain materials, particularly toxic chemicals, can increase in concentration. These chemicals often accumulate in the fat tissues of animals, a process known as bioaccumulation.
Alt text: Majestic bald eagle perched on a tree branch, a species affected by bioaccumulation of DDT.
For example, when an herbivore consumes a plant treated with pesticides, these pesticides are stored in the animal’s fat. When a carnivore consumes several of these herbivores, it ingests the pesticides stored within its prey.
Bioaccumulation also occurs in aquatic ecosystems. Runoff from urban areas and farms can introduce pollutants into the water. Tiny producers such as algae, bacteria, and seagrass absorb small amounts of these pollutants. Primary consumers like sea turtles and fish consume the seagrass, utilizing the plant’s energy and nutrients while storing the chemicals in their fatty tissue. Predators at the third trophic level, such as sharks or tuna, then consume the fish. By the time the tuna is consumed by humans, it may contain a significant amount of bioaccumulated toxins.
Due to bioaccumulation, organisms in certain polluted ecosystems are deemed unsafe for consumption and harvesting. Oysters in New York City harbor, for example, are unsafe to eat due to the accumulation of pollutants in their tissues.
In the mid-20th century, the pesticide DDT (dichloro-diphenyl-trichloroethane) was widely used to combat insects that spread diseases. During World War II, DDT was employed to eliminate typhus in Europe and control malaria in the South Pacific. DDT was initially regarded as a miracle drug and played a significant role in eradicating malaria in regions such as Taiwan, the Caribbean, and the Balkans.
Unfortunately, DDT’s bioaccumulation within ecosystems caused significant environmental damage. DDT accumulates in soil and water, and some forms decompose very slowly. Worms, grasses, algae, and fish all accumulate DDT. Apex predators like eagles accumulated high concentrations of DDT in their bodies through the consumption of contaminated fish and small mammals.
Birds with high levels of DDT in their bodies laid eggs with extremely thin shells, which often broke before the chicks were ready to hatch.
DDT was a major contributor to 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 it impacted have largely recovered in most parts of the country.
