Food Chains and Webs



Food Chains, Webs, Trophic Levels, and Biomass Reading

Every organism needs to obtain energy in order to live. For example, plants get energy from the sun, some animals eat plants, and some animals eat other animals. A food chain is the sequence of who eats whom in a biological community (an ecosystem) to obtain nutrition. Food webs connect many different food chains. Food webs can support food chains that are long and complicated, or very short. A food web starts with the primary energy source, usually the sun or boiling-hot deep sea vents. The next link is an organism that makes its own food from the primary energy source -- an example is photosynthetic plants that make their own food from sunlight (using a process called photosynthesis) and chemosynthetic bacteria that make their food energy from chemicals in hydrothermal vents. These are called autotrophs.

Next come organisms that eat the autotrophs; these organisms are called herbivores-- an example is a rabbit that eats grass. The next link in the web are animals that eat herbivores - an example is a snake that eats rabbits. In turn, these animals are eaten by larger predators -- an example is an owl that eats snakes. Each food web ends with a top predator and animal with no natural enemies (like an alligator, hawk, or polar bear).

Consumers can be carnivores (animals that eat other animals) or omnivores (animals that eat both plants and animals). Omnivores, like people, consume many types of foods. People eat plants, such as vegetables and fruits. We also eat animals and animal products, such as meat, milk, and eggs. We eat fungi, such as mushrooms. We also eat algae, in edible seaweeds like nori (used to wrap sushi rolls) and sea lettuce (used in salads). Bears are omnivores, too. They eat berries and mushrooms, as well as animals such as salmon and deer.

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Trophic Levels:

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). The trophic level of an organism is the position it holds in a food chain.

1. Primary producers are the base of every food chain

a. 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.

2. Primary consumers are animals that eat primary producers

a. 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.

3. Secondary consumers eat primary consumers.

a. 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.

4. Tertiary consumers eat secondary consumers.

a. 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.

5. Quaternary consumers eat tertiary consumers.

Detritivores and decomposers make up the last part of food chains. When any organism dies, it is eventually eaten by detritivores (like vultures, worms and crabs) and broken down by decomposers (mostly bacteria and fungi), and the exchange of energy continues. 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 food web.

Some organisms' position in the food chain can vary as their diet differs. For example, when a bear eats berries, the bear is functioning as a primary consumer. When a bear eats a plant-eating rodent, the bear is functioning as a secondary consumer. When the bear eats salmon, the bear is functioning as a tertiary consumer (this is because salmon is a secondary consumer, since salmon eat herring that eat zooplankton that eat phytoplankton, that make their own energy from sunlight). Think about how people's place in the food chain varies - often within a single meal!

Numbers of Organisms:

In any food web, energy is lost each time one organism eats another. Because of this, there have to be many more plants than there are plant-eaters. There are more autotrophs than heterotrophs, and more plant-eaters than meat-eaters. Each level has about 90% less energy available to it because some of the energy is lost as heat at each level. Although there is intense competition between animals, there is also interdependence. When one species goes extinct, it can affect an entire chain of other species and have unpredictable consequences.

Equilibrium:

As the number of carnivores in a community increases, they eat more and more of the herbivores, decreasing the herbivore population. It then becomes harder and harder for the carnivores to find herbivores to eat, and the population of carnivores decreases. In this way, the carnivores and herbivores stay in a relatively stable equilibrium, each limiting the other's population. A similar equilibrium exists between plants and plant-eaters.

Biomass:

Food webs are defined by their biomass. Biomass is the energy in living organisms. Autotrophs, the producers in a food web, convert the suns 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.

Food Chains, Webs, Trophic Levels, and Biomass Questions NAME:_________________________

1. What is used to indicate the flow of energy in a food chain or web? ______________________________

2. What travels through a food chain or web. _____________________________

3. What is the ultimate energy for all life on Earth? _________________________

4. The 1st organism in a food chain must always be what type of organism? _____________________________

5. Name 2 food making processes.______________________________ and _____________________________

6. Where do chemosynthetic bacteria get their energy? _____________________________________________.

7. Herbivores are also called ___________________________________________.

8. Secondary consumers are eaten by larger _______________________________.

9. __________________________________ consumers eat secondary consumers.

10. Describe the difference between a food web and a food chain.

11. What happens to the amount of energy as we move from step to step in a chain or web? __________________

12. Describe a trophic level.

13. The 1st trophic level consists of _______________ producers called _________________.

14. Name the 2nd trophic level (both names). _________________________ and _________________________

15. What is the 3rd trophic level called? __________________________________

16. What is the 4th trophic level called? ___________________________________

17. At the 5th trophic level would be _____________ consumers that eat _____________ consumers.

18. Give an example of 3 detrivores. _______________________________________

19. What organisms feed on dead plants and animals?_________________________________________

20. Can an organism fill more than one trophic level --- yes or no? ___________Give an example.

21. In food chains and webs, what trophic level must you have more of than others?_________________________

22. Each trophic level has how much LESS energy? __________________

23. Describe Biomass:

24. Describe a Carnivore:

25. Describe an Omnivore:

26. Describe how energy enters and ecosystem and how it exits.

27. Describe how nutrients are recycled.

28. Use the image below to fill in the table:

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|Producers |Primary Consumer |Secondary Consumer |Tertiary Consumer |

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29. Add decomposers to the food web above. Indicate their location by adding arrow in the correct location.

30. The arrows indicate the direction the ________________________ is flowing.

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Trophic Level Pyramid

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