A food web consists of all the food chains in a single ecosystem. Each living thing in an ecosystem is part of multiple food chains. Each food chain is one possible path that energy and nutrients may take as they move through the ecosystem. All of the interconnected and overlapping food chains in an ecosystem make up a food web.
Organisms in food webs are grouped into categories called trophic levels. Roughly speaking, these levels are divided into producers (first trophic level), consumers, and decomposers (last trophic level).
Producers make up the first trophic level. Producers, also known as autotrophs, make 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 kinds. Algae, whose larger forms are known as seaweed, are autotrophic. Phytoplankton, tiny organisms that live in the ocean, are also autotrophs. Some types of bacteria are autotrophs. For example, bacteria living in active volcanoes use sulfur, not carbon dioxide, to produce their own food. This process is called chemosynthesis.
The next trophic levels are made up of animals that eat producers. These organisms are called consumers. Primary consumers are herbivores. Herbivores eat plants, algae, and other producers. They are at 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 a primary consumer.
Secondary consumers eat herbivores. They are at the third trophic level. In a desert ecosystem, a secondary consumer may be a snake that eats a mouse. In the kelp forest, sea otters are secondary consumers that hunt sea urchins as prey. Tertiary consumers eat the secondary consumers. They are at the fourth trophic level. In the desert ecosystem, an owl or eagle may prey on the snake.
There may be more levels of consumers before a chain finally reaches its top predator. Top predators, also called apex predators, eat other consumers. They may be at the fourth or fifth trophic level. They have no natural enemies except people. Lions are apex predators in the grassland ecosystem. In the ocean, fish such as the great white shark are apex predators. In the desert, bobcats and mountain lions are top predators.
- Detritivores and Decomposers
Detritivores and decomposers make up the last part of food chains. Detritivores are organisms that eat nonliving plant and animal remains. For example, scavengers such as vultures eat dead animals. Dung beetles eat animal feces. Decomposers, like fungi and bacteria, complete the food chain. Decomposers turn organic wastes, such as decaying plants, into inorganic materials, such as nutrient-rich soil. They complete the cycle of life, returning nutrients to the soil or oceans for use by autotrophs. This starts a whole new series of food chains.
- Ecological Pyramids
The concept of ecological pyramid was developed by Charles Elton; these pyramids are also known as Eltonian pyramids. The pyramids are a graphical representation which depicts the number of organisms, biomass and productivity at each trophic level. All ecological pyramids begin at the bottom with the produces and proceed through different trophic levels. Ecological pyramids begin with the producers at the bottom like plants and they proceed to various trophic levels like herbivores consume plants, carnivores prey on herbivores and so on. The highest level is at the top of the food chain.
Pyramid of Numbers
The number of organisms in a food chain can be represented graphically in a pyramid. Each bar represents the number of individuals at each trophic level (feeding level) in the food chain. The pyramid of numbers usually shows that the number of organisms at each trophic level gets smaller towards the top. This particular case is an exception – one tree provides food for many caterpillars.
It usually takes a large number of plants to provide sufficient energy for the consumers in the food chain.
Pyramid of numbers in Grassland and Forest Ecosystem
In grassland and forest, there is generally a gradual decrease in biomass of organisms at successive levels from the producers to the top carnivores.
Pyramid of Numbers in Pond Ecosystem
In a pond as the producers are small organisms, their biomass is least, and this value gradually shows an increase towards the apex of the pyramid, thus making the pyramid inverted in shape.
Pyramid of Energy
An energy pyramid is a graphical model of energy flow in a community. The different levels represent different groups of organisms that might compose a food chain. From the bottom-up, they are as follows:
- Producers — bring energy from nonliving sources into the community
- Primary consumers — eat the producers, which makes them herbivores in most communities
- Secondary consumers — eat the primary consumers, which makes them carnivores
- Tertiary consumers — eat the secondary consumers
In some food chains, there is a fourth consumer level, and rarely, a fifth.
Net biomass productivity is the difference between gross productivity (production of plant material by photosynthesis) and respiration. So long as the rate of production exceeds that of respiration, the plant will grow.
Net productivity represents the amount of organic material produced by a plant. Net productivity is closely related to a number of environmental factors like climate, soils, and available nutrients. Net biomass production will be highest where there is an ample supply of moisture to meet the needs of plants. Biomass productivity is also high where soils are rich in nutrients and have a positive soil moisture balance. The figure below illustrates this well. With ample rainfall and sunlight, the tropical rain forest ranks the highest in terms of organic matter production.
Primary productivity is a term used to describe the rate at which plants and other photosynthetic organisms produce organic compounds in an ecosystem. There are two aspects of primary productivity:
- Gross productivity = the entire photosynthetic production of organic compounds in an ecosystem.
- Net productivity = the organic materials that remain after photosynthetic organisms in the ecosystem have used some of these compounds for their cellular energy needs (cellular respiration).
Since oxygen is one of the most easily measured products of both photosynthesis and respiration, a good way to gauge primary productivity in an aquatic ecosystem is to measure dissolved oxygen. We cannot measure gross productivity directly because respiration, which uses up oxygen and organic compounds, is always occurring simultaneously with photosynthesis—but we can measure it indirectly. We can measure net productivity directly by measuring oxygen production in the light, when photosynthesis is occurring.
We can also measure respiration without photosynthesis by measuring O2consumption in the dark, when photosynthesis does not occur. Since net productivity = gross productivity − respiration, we can calculate gross productivity