Autotrophs, the producers in a food web, convert the sun's 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 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 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.
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. Unable to eat salmon, omnivores like bears are forced to rely more heavily on other food sources, such as ants. The area's 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 kelp forests , 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 , officials in Venezuela dammed the Caroni River, creating an enormous lake about twice the size of Rhode Island. Hundreds of hilltops turned into islands in this lake. 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 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 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 to eat and not allowed to be harvested.
The pollutants in the harbor accumulate in its oysters, a filter feeder. In this way, the nutrients that were part of the body are returned to the bottom of the trophic pyramid. Bioaccumulation : In addition to being less energy efficient, eating higher up the food chain has its risks.
Pesticides and heavy metals like mercury, arsenic, and lead tend to be consumed in small quantities by the primary consumers. These toxins get stored in the fats of the animal. When this animal is eaten by a secondary consumer, these toxins become more concentrated because secondary consumers eat lots of primary consumers, and often live longer too.
Swordfish and tuna are near the top of the aquatic food chain and, when we eat them, we are consuming all of the toxins that they have accumulated over a lifetime. For this reason, pregnant women are advised against eating these foods. Solve the following problems mathematically.
Given: 10 billion people can be fed a basic vegetarian diet that is nutritionally complete. How many people could we feed at the American standard-a tertiary level of consumption 3rd order consumers? If there are million people in the United States most of them eating at the Tertiary 3rd level of consumption, how many people could we feed at the Primary level? Some animals like sharks are 5th order consumers!
These trophic levels consist of organisms called consumers that consume eat other organisms for energy. The level above producers is the primary consumers. Think of deer, insects, and rabbits. These animals eat plants, earning them the name "herbivore". The next level up is the secondary consumers. Secondary consumers eat and get energy from the primary consumers.
Since these animals eat other animals and not plants, they are called "carnivores". Next up, we have tertiary consumers. They receive energy by eating the secondary consumers. As you can see, producers are the starting point of energy for other organisms. While there is energy transfer between these groups, a lot of energy is lost as you move up the levels. Earth's Ferrous Wheel. Alternative Stable States.
Recharge Variability in Semi-Arid Climates. Secondary Production. Food Web: Concept and Applications. Terrestrial Primary Production: Fuel for Life. Citation: Hui, D. Nature Education Knowledge 3 12 Food web is an important conceptual tool for illustrating the feeding relationships among species within a community, revealing species interactions and community structure, and understanding the dynamics of energy transfer in an ecosystem.
Aa Aa Aa. Introduction Food web is an important ecological concept. Basically, food web represents feeding relationships within a community Smith and Smith It also implies the transfer of food energy from its source in plants through herbivores to carnivores Krebs Normally, food webs consist of a number of food chains meshed together.
Each food chain is a descriptive diagram including a series of arrows, each pointing from one species to another, representing the flow of food energy from one feeding group of organisms to another.
Food webs are constructed to describe species interactions direct relationships. Food webs can be used to illustrate indirect interactions among species. Figure 3. Food webs can be used to study bottom-up or top-down control of community structure. Food webs illustrate energy flow from primary producers to primary consumers herbivores , and from primary consumers to secondary consumers carnivores.
This phenomenon is call bottom-up control. Correlations in abundance or productivity between consumers and their resources are considered as evidence for bottom-up control. For example, plant population densities control the abundance of herbivore populations which in turn control the densities of the carnivore populations. Thus, the biomass of herbivores usually increases with primary productivity in terrestrial ecosystems. Food webs can be used to reveal different patterns of energy transfer in terrestrial and aquatic ecosystems.
Patterns of energy flow through different ecosystems may differ markedly in terrestrial and aquatic ecosystems Shurin et al. Food webs i. In a review paper, Shurin et al. A dataset synthesized by Cebrian and colleagues on the fate of carbon fixed by primary productivity across different ecosystems was used to show different patterns in food chains between terrestrial and aquatic ecosystems Figure 5.
On average, the turnover rate of phytoplankton is 10 to times faster than that of grasslands and forests, thus, less carbon is stored in the living autotroph biomass pool, and producer biomass is consumed by aquatic herbivores at 4 times the terrestrial rate Cebrian , ; Shurin et al.
Herbivores in terrestrial ecosystems are less abundant but decomposers are much more abundant than in phytoplankton dominated aquatic ecosystems. In deep-water aquatic ecosystems, with their low standing biomass, rapid turnover of organisms, and high rate of harvest, the grazing food chain may be dominant. References and Recommended Reading Cain, M. American Naturalist , Cebrian, J. Ecology Letters 7 , Elton, C. Nature , Krebs, C. Ecology 75 , Molles, M.
Ecology 60 , Paine, R. The American Naturalist , Paine, R. Journal of Animal Ecology 49 , Pimm, S. Nature , Power, M. Ecology 73 , Schoender, T. Ecology 70 , Shurin, J. Share Cancel.
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