Teacher's Section: Food Web Unit

Food Web Review Outline

Introduction

What will be crucial in understanding how the marine ecology maintains its flow of energy and of matter will be this two week unit on trophic relationships, food webs and primary production. Often students only have a limited knowledge of these topics to their local area, much less of how they are organized in the marine environment. This unit will relate what students know from their local environment, from basic biology and from their own experiences to form a knowledge base on marine food web ecology.

Objectives

identify the trophic relationships in the marine environment and how they relate to each other.

draw and describe a typical marine food web.

describe, name and give the importance of marine primary producers.

describe and discuss the factors that affect primary production.

Food Web Unit Student Activities

Day One

Have a class discussion to review trophic relationships. Remind the students of the function and importance of photosynthesis and cellular respiration. As a class, come up with specific lists of producers, consumers (herbivores and carnivores) and decomposers from your local ecology. (Go outside and have some fun with this!) Still working as a large group, allow the students to organize those organisms into a food web. If time permits, have the students come up with the same categorical lists of organisms, but as they apply to the marine environment. This list will be used for a jumping off point tomorrow.

Days Two and Three

Do the food web activity, using the lists of organisms the class generated the period before. Evaluate according to the concept map rubric.

Days Four--Nine

Do the primary production learning cycle activity.

On the first day, have a class discussion listing what types of organisms are primary producers in the sea. Lead into a discussion talking about what factors might affect the rate of photosynthesis in the ocean. List all the possibilities on the board.

Possible Discussion Questions

What is a primary producer? What does it do?

What are some primary producers? Are they all plants? Why or why not?

What factors might affect the rate of primary production? Why?

What else is necessary for photosynthesis besides light? Why?

How could these necessary factors be affected in the ocean?

On the second day, allow time for the students to formulate their hypotheses and predictions and to write their procedures.

The next day set up the experiments and allow them to sit over the weekend.
The following week, have students gather data, present their results to the class and write their lab reports. Evaluate the lab reports according to the lab report checklist.
On the last day, follow up with a discussion/lecture covering anything not addressed by the labs, especially focusing on the role of the thermocline and the availability of nutrients.

Food Web Notes

A. Trophic Relationships

1. defined as what an organism eats and what eats it

2. all organisms require matter (for building blocks) and energy (for metabolism) from their nourishment

3. transfer of matter and energy results in an interdependence of three categories

Producers: autotrophic organisms capable of building high-energy organic substances. Primary producers are on the first trophic level.

Consumers: heterotrophic organisms that depend upon autotrophs for nourishment

Herbivores eat producers and are on the second trophic level.

Carnivores eat animals and are on the third or higher trophic level).

Decomposers: exist on detritus (excrement, waste products and dead remains)

B. Food Chains

1. Defined as the paths that nutrients and energy follow through the living portion of ecosystems

2. Can be grazing food chains (succession of grazers and predators) or detritus food chains (based on the death of grazing food chains)

3. Usually arranged in a pyramid to illustrate the decrease in available energy and material from lower to higher trophic levels

4. communities seldom have straight-line food chains. A more descriptive term for the complex feeding relationships are called food webs. Often feeders are opportunistic and may occupy a different level at different times can become quite complex and confusing when it takes competition into account

5. Symbiosis is when there is an intimate and prolonged relationship between two or more organisms in which at least one organism obtains some benefit from the relationship.

Commensalism benefits the symbiont without affecting the host.

Mutualism benefits both symbiont and host.

Parasitism benefits the symbiont at the expense of the host.

6. Competition

a. The upper limit of species distribution is restricted by the species’ ability to cope with environmental stresses and other physical factors, such as temperature or desiccation, whereas a species’ lower vertical range is limited by biological factors, especially competition with superior species.

b. In regions where physical factors equally permit species to survive, competing groups interact by dominating the available attachment space or by overgrowing their competitors and monopolizing the resources available from the water.

c. Physical processes (such as wave battering or seasonal die-offs) and biological processes (such as predation) clear off patches for future resettlement and competition.

(See lessons 80 and 81 from The Marine Biology Coloring Book (Niesen, 1982)).

C. Primary Production in the Sea

1. mostly accomplished by pelagic phytoplankton. Standing crop of benthic organisms only very little (5-10% of producer mass).

2. Factors that affect primary production

a. population size—oscillatory depending upon limiting factors such as light, nutrient availability and herbivore grazing. (Fig. 5.3)

b. light penetration—photic zone determined by water transparency, solar angle and atmospheric absorption. (Fig. 5.4)

c. nutrient requirements—nutrient regeneration (detritus cycle) and upwelling dependent upon physical factors such as water depth, habitat type and the thermocline. (Fig. 5.16—5.19)

d. herbivore grazing—population sizes respond to feedback mechanisms in oscillatory patterns and can be increased in rate due to arctic melting and herbivore migrations.

(figures 2.10, 11; 5.3, 4, 16-19 from: Sumich, James L. Marine Life. Wm. C. Brown Pub, 1992.)