Lab08_4

Lab Activity 8.4
Marine Food Webs

Study the instructional material below. Be sure to click on each of the photographs for an enlarged view in a separate window. The larger version is necessary to complete the assignment. It opens in a separate window which can be resized by grabbing the bottom right corner and dragging it. It can also be moved by grabbing the top heading bar and dragging it. Be sure to close the extra window by using the X in IBM, or the close box in MAC when you are finished using them.

Introduction

An important aspect of understanding how an ecosystem functions is analyzing the trophic relationships among its living members. By mapping the feeding relationships (trophic structure) scientists can study the passage of energy and materials through the living portion of an ecosystem. This basic knowledge leads to better predictions about what may happen if particular organisms are removed by fishing or how pollution might effect the ecosystem. In this lab activity you will construct a food web diagram for a simplified ocean ecosystem. You will then be asked to make some predictions based upon your knowledge of the feeding relationships in the hypothetical web.

Instruction

Study the general information about food webs. Complete the Ocean Food Web exercise by answering the questions below the Ocean Food Web diagram. You can copy the diagram, questions, and answers into your notebook or print out the chart, fill in the answers, and add it to your notebook.

Be sure to write about what you are learning in the lab section of your notebook. You will be expected to answer questions about the lab activity during the lab self test and lab quiz. It helps to have your text and coloring books open beside you for support.

Supporting Information

Refer to the Assigned Readings Below:
Marine Biology Textbook	Chapter 10, pages 221 to 225
Marine Biology Coloring Book	Plate 14

Food Webs in General
One of the logical tools that an ecologist uses to understand how the organisms of a community live together is to construct a food web. The food web is a diagram representing the feeding relationships among the species in the community; arrows are used to connect the predators with their prey. The food web may then be used as a model of the interconnectedness of the members of the community and as a tool to make predictions of how a change in the abundance of one species may affect the other species.
This is best illustrated by an example. Let's examine an example from a natural community with which we are all already familiar-the garden. Suppose we have a vegetable garden in which we are growing the following: carrots, lettuce, and strawberries. Suppose also that this being a normal garden, there are some animals which occasionally visit to forage on our efforts: your sister's turtle, some grasshoppers, robins, a gopher, snails, and your cat. Now let us consider who eats whom. The gopher feeds on the carrots; the grasshoppers, turtle and snails on lettuce; the robins eat grasshoppers and strawberries; and your cat dines on robins, grasshoppers, and gophers. From this information we can construct the food web below. Note the direction of the arrows indicates the direction of the food from prey to predator.
The usefulness of this device might become apparent if you ask questions about the effect of reducing or increasing the number of certain species in this garden community, and then note the effect on the abundance of other community members.
Question: Suppose that you remove the snails, what effect will this have on the lettuce, turtle and grasshoppers?	Answer:The lettuce would be free to grow and supply more food to the turtle and grasshoppers.
Question: What if you remove the strawberries and grasshoppers-what effect will this have on the abundance of robins in your garden?	Answer:The robins would have no food and would dissappear adversely affecting the diet of the cat.
As you can see, there are many possible alternative situations where changing the abundance of one species can potentially affect the entire community.

Ocean Food Web
	Use the ocean food web diagram to the left to answer the questions below it.
Questions	Answers
1. Suppose phytoplankton decrease in abundance. What changes might occur in the abundance of other members of your web?
2. Could phytoplankton predators switch to different prey species?
3. Suppose phytoplankton increase in abundance. What changes might occur in the abundance of other members of your web?
4. Suppose macrozooplankton decrease in abundance. What changes might occur in the abundance of other members of your web?
5. Could macrozooplankton predators switch to different prey species?
6. Suppose macrozooplankton increase in abundance. What changes might occur in the abundance of other members of your web?
7. Suppose small fish and squid decrease in abundance. What changes might occur in the abundance of other members of your web?
8. Could small fish and squid predators switch to different prey species?
9. Suppose small fish and squid increase in abundance. What changes might occur in the abundance of other members of your web?
10. Suppose large fish decrease in abundance. What changes might occur in the abundance of other members of your web?
11. Could large fish predators switch to different prey species?
12. Suppose large fish increase in abundance. What changes might occur in the abundance of other members of your web?