Station 17

Interactions Through Trophic Levels

Reporting Category 5

Interdependence Within Environmental Systems

Essential Question – Bell Ringer

Why is it necessary for the number of prey to be greater than the number of predators in order for an ecosystem to be balanced?

•The number of prey needs to be greater than the number of predators so that enough prey will survive to reproduce and keep the ecosystem in balance.

Pre-Review Question

The series of steps in which a large fish eats a small fish that has eaten algae is:

a. Food web

b. Food chain

c. Pyramid of numbers

d. Biomass pyramid

Pre-Review QuestionThe diagram below shows a food web in a large park. Each circle represents a different species in the food web. Which of the organisms in the food web could be referred to as primary consumer(s)?

a. 7 only

b. 5 and 6 only

c. 2, 3 and 4 only

d. 2, 5 and 7 only

1 – Create a food chain out of each card set and write it in your journal

Take at least 10 cards and place them in a food web using the arrows (see the example below).

Using your food web, draw an ecological pyramid in your journal and show the 10% rule. (see the example)

100%

0.1%

1%

10%

Discuss with partner

Explain the 10% rule of an ecosystem using the information from the ecological pyramid and the

glossary.

The 10% rule is a general rule that helps explain that the amount of energy in an ecosystem is reduced by 90% when it flows from one trophic level to the next. For example, in the ecological pyramid, the grasses (producer level) provide only 10% of the energy they receive to the grasshopper (primary consumer level). The other approximately 90% of the energy in the grass is not lost—it’s used for other purposes. Each successive trophic level in an ecosystem receives only about 10% of the energy available from the previous level.

Discuss with partnerWhich example, a food chain or a food web, provides

a more realistic picture of the flow of matter and energy in an ecosystem? Support your answer.

A food web provides a more realistic representation of the interactions that occur in an ecosystem. For example, the food web in Question 6 shows that the mockingbird has three sources of food: the butterfly, the house fly, and the grasshopper. The food chain from Question 3, which shows only the butterfly as a source of food for the mockingbird, gives a much more simplified view of the flow of matter and energy in the ecosystem.

• Organisms in an ecosystem interact in ways that can be shown in food chains and food webs.

• Ecological pyramids are used to illustrate how organisms in an ecosystem transfer matter and energy from one trophic level to another.

• Approximately 10% of the available energy in a trophic level is passed on to the next trophic level. The remaining energy, approximately 90%, is used for metabolic functions or dissipated as heat.

• Sunlight—radiant energy—is transferred to plants through photosynthesis. Organisms that feed on plants are able to use about 10% of the energy that was available to the plants. The transfer and dissipation of energy continue from one trophic level to the next.

I need to remember………

Post Review Question

Energy pyramids show the most available energy at the base and the least amount of energy at the top. What accounts for the difference in the energy levels in these two parts of the energy pyramid?

a. With each succeeding level, most of the energy is used by the organisms for life processes and some of the energy is “lost” as heat.

b. Some of the energy is destroyed.

c. The energy is used by decomposers.

d. The energy is continually recycled in the biosphere.

Post Review Question

Looking at the energy pyramid above, which best explains why a pyramid is used to

represent the energy flow within an ecosystem?

a. Available energy increases when moving up an energy pyramid

b. Available energy decreases when moving up an energy pyramid

c. The size of the organism decreases when moving up the energy pyramid

d. The population size of the organisms increases when moving up an energy pyramid

Post Review Question

In the biomass energy

Pyramid to the right, which

Level contains 10% of the

energy from the producer?

a.Oak tree

b.Blue tit

c.Caterpillar

d.Sparrowhawk