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Food Webs



I can identify the types of relationships that occur among populations in a community.

Learning Standard:

LS-14:  Explain the roles and relationships among producers, consumers, and decomposers in the process of energy transfer in a food web.

LS-15:  Explain how dead plants and animals are broken down by other living organisms and how this process contributes to the system as a whole.

LS-16:  Recognize that producers (plants that contain chlorophyll) use the energy from sunlight to make sugars from carbon dioxide and water through a process called photosynthesis.  This food can be used immediately, stored for later use, or used by other organisms.

Main Ideas:

Food Chain

The sequence of the transfer of food energy from one organism to another in an ecological community. A food chain begins with a producer, usually a green plant or algae that creates its own food through photosynthesis. In the typical predatory food chain, producers are eaten by primary consumers (herbivores) which are eaten by secondary consumers (carnivores), some of which may in turn be eaten by tertiary consumers (the top carnivore in the chain).

Energy moves through an ecosystem in the form of food.

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.



Trophic Levels

Organisms in food webs are grouped into categories called trophic levels. 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.

In an ocean ecosystem, many types of fish and turtles are herbivores that eat algae and seagrass. In kelp forests, seaweeds known as giant kelp provide shelter and food for an entire ecosystem. Sea urchins are powerful primary consumers in kelp forests. These small herbivores eat dozens of kilograms (pounds) of giant kelp every day.

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.




Food Chain:

Food Web:

Trophic Level:

Producers:  organisms that capture energy from the sun.  Producers use sunlight and CO2 to make, or produce, their own food through the process of photosynthesis.

Consumers:  organisms that obtain energy by feeding on producers or other consumers.  Consumers must consume other organisms for food because they cannot make their own food from sunlight like producers.

Decomposers:  organisms that obtain energy when they break down the bodies of dead organisms.  Decomposers help break down dead organisms and separate components back into basic elements to be recycled and reused in nature.






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