• During the class discussion, orally assess students’ ability to classify symbiotic relationships.
• Collect and check students’ work on Types of Symbiosis handout (S-7-3-3_Types of Symbiosis-Teacher Version.doc).
• Collect and check students’ paragraphs about fig trees and fig tree wasps.

Ask students to explain why they think fig trees and fig tree wasps need each other to survive. If needed, they may use their notes from the previous two lessons to help with their explanation.

Define symbiosis and explain three types of symbiosis: mutualism, commensalism, and parasitism. Have students write the definitions for each type of symbiosis in their notes. Discuss each of the following examples and have students identify each type of symbiosis:

1.      Clark’s nutcracker and white bark pine (mutualism):

The trees offer pine nuts as high-energy food to the Clark’s nutcrackers (birds), and so the birds store the nuts in the ground. The nuts that they don’t eat have a good chance of sprouting new pines. If it wasn’t for the trees, the birds would starve, and if it wasn’t for the birds, the trees could no longer reproduce.

2.      Tubeworms and bacteria (mutualism):

Certain bacteria and tubeworms living at hydrothermal vents deep in the ocean share a symbiotic relationship. The bacteria live in special cells inside the worm. In exchange for a safe place to live, the bacteria give the worm all the nourishment it needs. They do this by absorbing three ingredients—oxygen, carbon dioxide, and hydrogen sulfide—and then using those ingredients to produce food for the worm.

3.      Hookworms and human (parasitism):

Small hookworms live in the intestine of a human. The hookworms feed on nutrients that the human is digesting. They cause the human to become anemic, a condition in which the human does not have enough iron in the bloodstream.

4.      Barnacles and whale (commensalism):

Barnacles live on the skin of a whale. The barnacles feed by filtering microscopic plants from the water as the whale swims. The whale is unaffected by the barnacles.

Distribute the Types of Symbiosis handout and have students complete it individually (see S-7-3-3_Types of Symbiosis-Student Version in the Resources folder).

Explain the symbiosis between fig trees and fig tree wasps. Fig tree wasps are the sole pollinators of fig trees, and fig tree wasps can only breed in figs. The fig tree wasp crawls into the syconium on the fig tree and moves around, dropping pollen from other fig flowers. The wasp seeks the right part of the flower to lay its eggs. When the wasp larvae hatch, they eat the fig in which they live. The cycle repeats. (See Related Resources for more information on this relationship). Have students take notes on this mutualistic relationship.

Then, tell students, “Some species of figs are known as strangler figs. The fig grows around a host plant, slowly enclosing it and taking its nutrients. They begin as seeds dropped near the base of a tree by a bird or mammal. The seed sprouts, and the roots that form wrap around the host. This crushes the host tree, and the strangler fig grows strong. What kind of relationship does the strangler fig have with its host tree?” Elicit that this relationship is parasitic.

If you have computers with Internet access, have groups of 2–3 students read “World’s Oldest Fig Wasp Fossil Proves That If It Works, Don’t Change It” online (available at: www.sciencedaily.com/releases/2010/06/100615191649.htm). If computers are not available, distribute copies of “World’s Oldest Fig Wasp Fossil Proves That If It Works, Don’t Change It” (see S-7-3-3_World’s Oldest Fig Wasp Fossil in the Resources folder). Have each student write a paragraph explaining the evidence that fig trees and fig tree wasps have shared a relationship for over 60 million years.

Extension:

• For students who may need opportunities for additional learning, print the Types of Symbiosis worksheet (S-7-3-3_Types of Symbiosis-Student Version.doc) and have them complete the activity for extra practice. It may be helpful to provide them with the following chart to review types of symbiosis (in which “+” means beneficial, “−” means harmful, and “0” means neutral):

• Students who might be going beyond the standards can create a diagram in addition to writing a paragraph about the fig tree/fig tree wasp relationship. The diagram should show how the wasp reproduces and grows in the fig tree, in the order of its life stages.