Food Chains, Food Webs, and Energy Pyramids

Chapter 13, Section 13.4

This is called an energy pyramid. It shows how energy is lost moving from the producers to each subsequent, higher, trophic level.

Objectives

• To describe the structure of a food chain.• To explain how food chains and trophic levels

are related.• To analyze feeding relationships in a food web.• To compare and contrast our place in the food

chain with that of our ancestors.

Vocabulary (all between pp. 408-411)

• Food chain• Herbivore• Carnivore• Omnivore• Detritivore• Decomposer• Specialist• Generalist• Tropic level • Food web

Starter

• I want you to think about and describe a typical meal.

• What do you get from these foods?• How does energy and matter come to be in

these foods? – Draw a food chain or food web showing the

relationship of you to your food.

Food Chains and Food Webs (Main Ideas)

• They are models for determining the relationships among species. – They can become very complex because they can

deal with hundreds, in certain cases thousands, of species.

• We talked about energy in ecosystems in Unit 13.3, now we are going to be studying how that energy is transferred from organism to organism in an ecosystem.

Energy Loss with Increase in Trophic Level

Food Chain – A Simple Model

• A food chain is a model of energy flow in an ecosystem.– It is a sequence that links species by their feeding

relationships. – A food chain is a more simple model than a food

web.– It only follows the connection between on

producer and a single chain of consumers within an ecosystem.

Food Chain

A Prairie Food Chain: Little Bluestem Prairie Grass (Producer) Eaten by Black Tailed Prairie Dog (Primary Consumer) Eaten by Coyote (Secondary Consumer)

Food Chain

• Take a moment to think about a food chain.• Draw it in your notebook.• Share with class.

Types of Consumers

• Herbivores – organisms that eat only plants (like our prairie dogs, Canada goose, goats, huemul, pandas).

Types of Consumers• Carnivores – organisms that eat only animals.– Ex. Jaguar, Philippine Eagle, and army ant.

Army Ants

• https://www.youtube.com/watch?v=UozWJTuhbMQ

• This is a You Tube video of army ants. It discusses their mode of predation.

• Where do army ants fit in a tropical food chain?

Types of Consumers

• Omnivores – organisms that eat both plants and animals (Ex. Crows, coyotes, grizzly bears, Kangaroo rat, New Guinea singing dog).

Types of Consumers

• Detritivores – organisms that eat detritus, or dead organic matter (millipede, buying beetle, woodlouse, etc.)

Types of Consumers

• Decomposers – detritivores that break down organic matter into simpler compounds. Fungi are the classic example of decomposers.

• They are vital to the stability of an ecosystem – returning vital nutrients back into the environment.

Specialists

• Specialists are consumers that primarily feed on one specific organism or a very small number of organisms.

• Examples of specialists: giant pandas, Florida snail kite, koala.

Florida snail kite eats primarily apple snails.

Koalas eat only eucalyptus leaves.

Due to their highly specialized diets, specialists are often endangered. If the population of the organism the specialist depends upon for its food declines, the population of the specialist also declines.

Generalists

• Most species do not rely on a single source of food.

• Generalists are consumers that have varied diets. Examples include raccoons, opossum, coyotes, and crows.

• Generalists are typically very adaptable.

Trophic Levels

• Levels of nourishment in a food chain. • The basic food chain is producer-herbivore-

carnivore. – Carnivores is at the highest trophic level.– Herbivores are at the second trophic level.– Producers are the base – first trophic level.

20

Energy Flows Up the Food Chain

• Energy flows from lowest trophic level to highest.

• Primary consumers are herbivores – they are the first consumer above the producer base trophic level.

• Secondary consumers are carnivores that eat the herbivores.

• Tertiary consumers are carnivores that eat secondary consumers.

Loss of Energy Moving up Food ChainAs you move up a food chain (and a food web – see later slides) energy is lost at each step. We can show the loss of energy conveniently in an energy pyramid (on the left).

This loss of energy is one reason there are more primary consumers (herbivores) than secondary consumers (carnivores) – and so-on-and-so-forth.

Predators are rare compared to their prey.

Loss of Energy Moving up Food Chain

• Tallgrass prairie energy pyramid. As you can see, the loss of energy between trophic levels is severe.

Losses would be, for example, from little bluestem grass to prairie dogs, and prairie dogs to coyotes.

This shows a real food chain.

Where the Omnivore Fits-in

• Omnivores, such as black bears and humans, eat both plants and animals.

• Therefore, they may be listed at different trophic levels in different food chains. – Ex. if you eat your vegies (as you should), you are

a primary consumer.

Quick Test of Understanding

• True or False:• Organisms higher up in a food chain eat

everything that is lower.• That is false• Populations higher in a food chain increase in

number because they deplete those lower in the chain.

• That too is false (if they do, it is a brief imbalance).

Quick Test of Understanding

• The top of the food chain has the most energy because it accumulates up the chain.

• That, once again, is false.

Coral Reef Food Web

Coral reefs are called

the tropical rainforests

of the ocean. They

are extremely

rich in biodiversity.

Food Webs

• A food web is a model that shows the complex network of feeding relationships and the flow of energy within and sometimes beyond an ecosystem (see preceding slide).

• Food webs are more complex models of ecosystems than food chains.– Many organisms, especially generalists, play

multiple roles in an ecosystem and thus are connected to many different levels (ex. a raccoon).

Food Webs

• The stability of a food web depends on the presence of producers (form the base of the web).

• In marine ecosystems, such as the coral reef, algae and phytoplankton are two of the producers that play an important role.

• 90% loss of energy between trophic levels (algae and fish that eat algae, and reef sharks and the algae eating fish).

Review Questions

• Q: Why are food chains especially useful for describing the relationship of specialists?

• A: Specialists have specific diets that include only one type of organism, which produces a simple food chain (Examples: snail kite and apple snail, blue whale and krill).

Review Questions

• Q: What happens to energy as it flows through a food web?

• A: Some energy is stored in the organism but much energy is dissipated into the environment.

Review Questions

• Q: Only a small percentage of all consumers are specialists. What danger does a specialist face that a generalist does not?

• A: If a specialist’s food source becomes scarce or disappears, the population may go extinct. A generalist can always shift to another food.

Review Questions

• Q: How might the stability of an ecosystem be affected if all of the decomposers were suddenly removed?

• A: Without decomposers, vital nutrients would not be returned to the ecosystem. The ecosystem would be unstable.

Review Questions

• Q: How might an oil spill in the ocean affect an aquatic food web? What might happen to the food web on land located near the spill? Explain your answers.

• A: The entire food web would be affected. Oily water would kill off phytoplankton which feeds small fish. The number of small fish would be reduced affecting larger fish (secondary consumers), which would affect the tertiary (third level) consumers. As oil seeps on shore, plants and animals living along the shore would be negatively affected – oil is toxic.

Antarctic Food Web

Yellowstone Food Web

Yellowstone Food Web

Arctic National Wildlife Refuge

More Review Questions (Not to Be Turned In)

• An organism that eats only plants• An organism that eats only other animals• An organism that eats both plants and animals• An organism that eats dead organic matter• An organism that breaks down organic matter into simpler

compounds (like minerals) • What is the first consumer above the producer level called?• What is a carnivore that eats herbivores called (what is its

trophic level)?• What is a carnivore who eats other carnivores called?• What are the levels of nourishment in a food chain called?

More Review Questions (Not to Be Turned In)

• How is a food web different from a food chain?• What happens to energy at each link in a food

web?• What type of organism provides the base of a

food web?• What is the difference between a specialist and

a generalist?• What does an energy pyramid show?