Teacher preparation:

Overarching learning goals

Students will understand what is meant by food chain, food web, trophic level and trophic relationships, and they will recognise that marine ecosystems are comprised of unique food webs and trophic relationships.

Teacher content information

Food chains are a linear way to show the feeding relationships in an ecosystem. An animal grazes on the plant. Another, usually larger, animal eats the animal browser, and so on in a chain. The plant gets its energy using sunlight to make food. The browsing animal gets its energy from eating plants. The predators get their food from eating animals. We can use an arrow to show this relationship. It is important to note that the arrow always points to the animal that has just eaten something. This is because it shows the flow of energy. For example, the browsing animal eats the plant so the energy from the plant flows to the browsing animal, thus the arrow would point at the animal.

But nature isn’t as simple as a food chain. Relationships in nature are complex and involve all sorts of interconnections and energy flows that are best shown as a food web.

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Student and classroom organisation:

Step 1.

Begin this activity by sharing and discuss the following information with your students (these images are also available on the student worksheet).

A food chain shows the feeding relationships between plants and animals. A food chain is organised into a linear form and shows the flow of energy between organisms. Plants are at the beginning of the food chain and they get their energy from sunlight. The browsing animal gets its energy from eating the plants. The predators get their food from eating other animals. We can use an arrow to show these relationships. It is important to note that the arrow always points to the animal that has just eaten something, as this shows the flow of energy. For example, the browsing animal eats the plant so the energy from the plant flows to the browsing animal, thus the arrow would point at the animal.

But nature isn’t as simple as a food chain. Relationships in nature are complex and involve all sorts of interconnections and energy flows that are best shown as a food web. The picture below shows an Antarctic food web.

You can see from this food web that the flows of energy go in all sorts of directions and weave and wind around each other. You can also see that generally the smaller animals at the bottom of the food web have a greater number of predators, and the number of predator-prey relationships decreases the further up the food web you go.

Step 2.

Show the flow of energy. In this part of the activity students can work from the student worksheet.

Ask students to experiment with identifying energy flows in the bush food web on their Student Worksheet. They should ensure that the arrows point in the correct direction (answers at end of the Teacher Worksheet).

Discuss the food webs. Consider the following questions (answers included where appropriate):

Which parts of the food web did they find difficult or challenging?
Which species are at the top of the food chain, and which are at the bottom - is there a pattern?
- Suggested Answer: Organisms at the bottom of the food chain tend to be smaller and are consumed by many species. Those at the top tend to be carnivores with few predators and many types of prey.
- Organisms on a food web are generally classified as being a producer, consumer or a decomposer. Producers are organisms that produce energy, generally through photosynthesis (such as plants and algae). Consumers are those consume energy through food (all animals, including humans). Decomposers are those that break down plant and animal wastes and release it again as energy (such as bacteria and fungi).
Which organisms on the food web are producers, consumers and decomposers?

Also ask students to think about how indigenous and introduced species affect food webs.

What is an indigenous organism?
- An indigenous organism is a plant or animal that occurs naturally in an ecosystem.
What is an introduced organism?
- An introduced organism (also known as ‘feral’) is one that is brought into an area to which it doesn’t naturally belong.
What role do indigenous organisms play in the food web?
- Indigenous species have evolved with the ecosystem for thousands of years and are critical to the health and balance of food webs. For example, an indigenous fish will eat indigenous crustaceans and be eaten by indigenous marine mammals (like a seal).
What role do introduced organisms play in the food web?
- Introduced species upset the balance of food webs by producing energy that can’t be absorbed or by consuming energy that is required by other organisms. For example, the Crown-of-Thorns Starfish has both sharp spines and a chemical defence that repels many local species. It is also fast-growing and fast-moving and has managed to colonise large areas of coral reefs, stripping the reef bare and limiting food sources for local species. The starfish thrive when pollution run-off from the land enters the reef system - the starfish appear to feed off this pollution.

Step 3.

What kinds of food webs might exist at your local beach or marine habitat area? Break the class into groups of 5 or 6. Through independent research online, or by visiting your local marine resource centre or zoo, groups should investigate what food webs exist in their local area. (If you don't have a beach locally, consider investigating an aquatic ecosystem in your area, or vote as a class on a marine ecosystem to investigate.)

Students should consider and include:

The role of crustaceans, molluscs, fish, mammals, cetaceans;
Introduced species;
Migratory species;
The trophic level of the organisms they investigate; and
The role of humans in the food web.

Step 4.

Using the organisms identified in your local area ask groups to create their own food web. Instead of creating a picture of a food web with arrows, ask students to create a short film or animation (1 to 2 minutes) that presents their food web. The film should tell the story of one organism's (e.g. seal) role in the food web, and how this affects all the other organisms in the food web. The film can be either a documentary or a narrative. It should however, include common names for each organism in their food web and should indicate the flow of energy in the food web. Film making tips.

Step 5.

Ask students to present their food web films for discussion. Consider the following questions:

Which organisms were the easiest to place in the food web?
Which were the hardest?
What do they think the role is of introduced species in the marine food web?
How healthy do they consider their local marine food web to be?
How could the health of their local marine food web be improved?

Assessment/Reflection

Ask students to reflect on the animations of other groups using the Peer Assessment Rubric for Film.

Extension

1. Conservation status

Ask students to see if they can find out the conservation status of any of the species they looked at. Add this information to the food web films. Links below:

2. Habitat protection

Ask students to investigate the strategies implemented by governments or environmental groups to maintain your local marine habitat areas. Students should briefly describe:

The purpose of these strategies and actions;
Which species are these actions targeted towards; and
Who is responsible for these actions.

Limit the number of actions to 2 or 3. If no local actions exist, ask students reflect upon why this might be and instead look at a number of national actions and strategies.

Seal the Loop - Marine Food Webs

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