A food web represents the intricate network of food chains within a single ecosystem. In essence, it’s a comprehensive depiction of “who eats whom” (or what) in a specific environment. Each organism in the ecosystem participates in multiple food chains, showcasing the various pathways that energy and nutrients can take as they circulate. The interwoven and overlapping nature of these food chains forms the food web. Understanding What A Food Web is crucial for grasping the dynamics of an ecosystem.
Understanding Trophic Levels
Within food webs, organisms are categorized into trophic levels. These levels are broadly classified into producers (the first level), consumers, and decomposers (the final level).
- Producers: Occupying the first trophic level, producers, also known as autotrophs, are self-sufficient organisms that create their own food source, independent of other organisms for sustenance. The primary method employed by most autotrophs is photosynthesis, a process that converts sunlight, carbon dioxide, and water into glucose (a nutrient).
Green algae, a type of producer in many aquatic food webs, undergoing photosynthesis.
Plants are the most commonly recognized form of autotrophs, although algae, including seaweed, are also autotrophic. Phytoplankton, microscopic organisms residing in the ocean, are also categorized as autotrophs. Certain bacteria species are autotrophs; for instance, bacteria inhabiting active volcanoes utilize sulfur instead of carbon dioxide to synthesize their food through chemosynthesis.
- Consumers: The subsequent trophic levels comprise animals that consume producers. These organisms are categorized as consumers.
Consumers may be carnivores (organisms that consume other animals) or omnivores (organisms with a diet consisting of both plants and animals). Omnivores, such as humans, have diverse diets encompassing plants like fruits and vegetables, animal products like meat, dairy, and eggs, fungi like mushrooms, and algae in edible seaweeds such as nori (utilized in sushi) and sea lettuce (found in salads). Bears are another example of omnivores, consuming berries, mushrooms, and animals like salmon and deer.
Primary consumers are herbivores, consuming plants, algae, and other producers. These organisms occupy the second trophic level. In grassland ecosystems, deer, mice, and even elephants are herbivores that consume grasses, shrubs, and trees. In desert ecosystems, mice that consume seeds and fruits are primary consumers.
In marine ecosystems, various fish and turtle species are herbivores that consume algae and seagrass. In kelp forests, giant kelp seaweeds provide habitat and nourishment for entire ecosystems. Sea urchins are significant primary consumers in kelp forests, consuming substantial quantities of giant kelp daily.
Secondary consumers feed on herbivores, residing on the third trophic level. For instance, in a desert ecosystem, a snake that eats a mouse is a secondary consumer. In kelp forests, sea otters act as secondary consumers by preying on sea urchins.
Tertiary consumers, occupying the fourth trophic level, consume secondary consumers. In a desert ecosystem, owls or eagles may prey on snakes.
Food chains may include multiple levels of consumers before reaching the top predator, also known as an apex predator. These top-level predators consume other consumers and may exist on the fourth or fifth trophic level. Apex predators have no natural predators other than humans. Lions are apex predators in grassland ecosystems, while fish like the great white shark are apex predators in marine environments. Bobcats and mountain lions are top predators in desert ecosystems.
- Detritivores and Decomposers: Detritivores and decomposers constitute the final phase of food chains. Detritivores are organisms that consume non-living plant and animal remains. For example, scavengers like vultures consume dead animals, while dung beetles consume animal feces.
Decomposers, such as fungi and bacteria, complete the food chain by converting organic waste like decaying plants into inorganic materials like nutrient-rich soil. This process completes the cycle of life, restoring nutrients to the soil or oceans for utilization by autotrophs. This begins new food chains.
Food Chains Within Food Webs
Food webs integrate diverse food chains and trophic levels. Food webs sustain both long, complex food chains and very short ones.
For example, grass in a forest clearing produces food through 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, enriching the soil with nutrients for plants like grass.
This short food chain is part of the forest’s food web. Another food chain in the same ecosystem might involve different organisms. A caterpillar may consume tree leaves, a sparrow eats the caterpillar, a snake preys on the sparrow, and an eagle (an apex predator) may prey on the snake. A vulture then consumes the dead eagle, and bacteria decompose the remains.
Algae and plankton are primary producers in marine ecosystems. Krill (tiny shrimp) consume microscopic plankton, and blue whales prey on tons of krill daily. Orcas prey on blue whales. As whale carcasses sink, detritivores break down the material. The nutrients released provide chemicals for algae and plankton to start a new series of food chains.
Biomass and Food Web Stability
Food webs are defined by biomass (the energy in living organisms). Autotrophs convert solar energy into biomass, but biomass decreases with each trophic level. Lower trophic levels contain more biomass than higher ones.
Due to biomass reduction, healthy food webs have more autotrophs than herbivores and more herbivores than carnivores. Ecosystems cannot support a large number of omnivores without even larger populations of herbivores and autotrophs.
Healthy food webs contain abundant autotrophs, many herbivores, and few carnivores and omnivores. This balance helps ecosystems maintain and recycle biomass.
Every link in a food web is connected to at least two others. An ecosystem’s biomass depends on how balanced and connected its food web is. When one link is threatened, some or all links are weakened or stressed, leading to a decline in biomass.
Plant life loss usually decreases the herbivore population due to drought, disease, or human activity (deforestation and paving grasslands).
Biomass loss on the second or third trophic level can also unbalance a food web. For example, diverting a salmon run reduces biomass. Bears are forced to rely on other food sources, shrinking the ant population. Fewer ants mean fewer nutrients broken down in the soil, which hinders autotrophs, and biomass is lost. Salmon are also predators of insect larvae and smaller fish. Without salmon, aquatic insects may devastate local plant communities, reducing plant life and biomass.
Carnivore loss on higher trophic levels can disrupt food chains. In kelp forests, sea urchins consume kelp, and sea otters prey on urchins. If the sea otter population declines, urchins devastate the kelp forest, creating urchin barrens with no producers, and biomass plummets.
Human activity can reduce predator populations. In 1986, Venezuela dammed the Caroni River, creating a large lake. Hilltops became islands, reducing terrestrial predator habitats. Prey animals like howler monkeys, leaf-cutter ants, and iguanas flourished. The ants destroyed the rainforest, killing trees and plants. The food web surrounding the Caroni River was destroyed.
Bioaccumulation in Food Webs
Biomass decreases as you ascend trophic levels, but some materials, especially toxic chemicals, increase. These chemicals collect in animal fat.
When herbivores eat plants covered in pesticides, the pesticides are stored in their fat. Carnivores then consume these herbivores and ingest the stored pesticides. 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 pollutants. Primary consumers, such as sea turtles and fish, eat the seagrass, using the energy and nutrients but storing the chemicals in their fatty tissue. Predators, 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.
Due to bioaccumulation, organisms in some polluted ecosystems are unsafe to eat. Oysters in New York City’s harbor accumulate pollutants and are unsafe for consumption.
In the 1940s and 1950s, the pesticide DDT was used to kill disease-spreading insects. During World War II, the Allies used DDT to eliminate typhus in Europe and to control malaria in the South Pacific. DDT was largely responsible for eliminating malaria in places like Taiwan, the Caribbean, and the Balkans.
However, DDT bioaccumulates and harms the environment. DDT accumulates in soil and water and decomposes 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 preyed on.
Birds with high amounts of DDT in their bodies laid eggs with extremely thin shells that often broke before hatching.
DDT was a major reason for the decline of the bald eagle. Today, DDT use has been restricted, and the food webs of which it is a part have recovered in most parts of the country.
Conclusion
Understanding what a food web entails is essential for comprehending the intricate relationships within an ecosystem. From producers to apex predators, each organism plays a vital role in maintaining the balance and flow of energy. Disruptions to any part of the web, whether through habitat loss, pollution, or over-exploitation, can have cascading effects throughout the entire system. Protecting and preserving biodiversity is crucial for maintaining healthy and resilient food webs that support life on Earth.
