Cockles

There are three tasks focusing on cockles: one looking at observations; one reading food chains; and one looking at relationships in a marine ecosystem.

Cockles live in sheltered coastal areas around New Zealand.

cockles

They are a very important food source in some marine ecosystems.

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Question 1

Question 1 is about some of the observations and inferences made by scientists about how cockles are suited to their tidal environment.

Observations are what you notice. What you think about your observation is an inference. Below are observations and inferences that scientists have made about cockles.

Sort the statements into the correct boxes.

Question 1

Question 1 is about some of the observations and inferences made by scientists about how cockles are suited to their tidal environment.

Observations are what you notice. What you think about your observation is an inference. Below are observations and inferences that scientists have made about cockles.

Sort the statements into the correct boxes.

When you open a cockle shell water falls out

Strong waves have damaged cockle shells

Cockles feed on plankton because they cannot move to hunt for their food

Cockles have a sturdy heavy shell

Cockles could dry out when exposed to the air

Cockles are sessile (move very little)

Observations

Inferences

Question

Question 2
Question 2 is about cockles in food chains. In a food chain the arrow shows the direction in which the energy flows. In one food chain cockles eat phytoplankton and are eaten by crabs.

Which food chain below shows this?

Cockles are an important link in the food web of organisms that live in the coastal waters of New Zealand. Imagine what would happen to the organisms of this food web if people took large numbers of cockles for themselves.

Questions 3 & 4 assesses your understanding of the interdependence of organisms in a food web.

Question

Question 3a)

Use the food web to decide all the organisms that will be immediately affected by a decrease in cockle numbers.

- Cod
- Bivalve molluscs
- Crabs
- Rock lobsters
- Shrimp
- Zooplankton
- Phytoplankton
- Seabirds

3b) Choose one organism that you ticked would be immediately affected. Explain how it will be affected.

Question

Question 4a)

Choose all the organisms from the food web that will be affected in the long term by a decrease in the cockle numbers.

- Cod
- Bivalve molluscs
- Crabs
- Rock lobsters
- Shrimp
- Zooplankton
- Phytoplankton
- Seabirds

4b) Choose one organism that you ticked would be affected in the long term. Explain how it will be affected.

Answers/responses:

Question

Answer

1. Below are observations and inferences that scientists have made about cockles. Sort the statements into the correct boxes.

Observations	Inferences
Cockles have a sturdy heavy shell.	Strong waves have damaged cockle shells.
When you open a cockle shell water falls out.	Cockles could dry out when exposed to the air.
Cockles are sessile (move very little).	Cockles feed on plankton because they cannot move to hunt for their food.

2. In one food chain cockles eat phytoplankton and are eaten by seabirds, crabs and rock lobsters. Which food chain below shows this?

3a). Which organisms in the food web will be immediately affected by a decrease in cockle numbers?

crabs, phytoplankton, seabirds.

3b). Choose one organism that you said would be immediately affected and explain how it will be affected.

Crabs and seabirds

Crabs and seabirds eat cockles. A decrease in cockle numbers could reduce the food supply for the crabs and seabirds. With less cockles to eat crab and seabird numbers could decrease.

Phytoplankton

Cockles eat phytoplankton. If fewer cockles eat phytoplankton, the phytoplankton numbers could increase.

4a). Which organisms in the food web will be affected in the long term by a decrease in cockle numbers?

cod, crabs, shrimp, phytoplankton, bivalve molluscs, rock lobsters, zooplankton, seabirds.

4b). Choose one organism that you said would be affected in the long term and explain how it will be affected.

All answers are correct. In the long term all the organisms in the food web could be affected in different ways e.g.,

Organisms that eat the cockles have less to eat.

Crabs, seabirds and cod

With fewer cockles for the crabs and seabirds to eat the crab and seabird populations could reduce in number. With fewer crabs the cod could have less to eat and cod numbers could decrease.

2. Fewer organisms feed on the phytoplankton

Phytoplankton, bivalve molluscs, zooplankton, shrimp, rock lobsters and cod and seabirds

With fewer species [cockles] feeding on the phytoplankton, phytoplankton numbers could increase. More phytoplankton could benefit the species that feed on them, e.g., zooplankton and bivalve molluscs populations could increase. Species in the food web that eat the bivalve molluscs and the zooplankton, e.g., shrimp, rock lobsters and cod populations could also increase. As the cod population increases so could the seabird population.

3. Organisms could eat more of the other species.

Seabirds, cod, bivalve molluscs and shrimp, rock lobsters

Seabirds could eat more cod. This could cause the cod numbers to decrease.
Cod could eat more shrimp and bivalve molluscs, This could cause the bivalve molluscs and shrimp numbers to decrease.
A decrease in the bivalve mollusc population could decrease the rock lobster numbers.
A decrease in the bivalve molluscs and shrimp populations could increase the zooplankton and phytoplankton numbers.

Diagnostic and formative information:

Based on a sample of 36 students completing the task on-line.

Question	Problems/Misconceptions
1. Sort the statements into observations or inferences.	If students are struggling with this distinction go to the first capability in science capabilities for further activities.
2. Which food chain for the crab, cockle and phytoplankton is correct?	Most of the students correctly answered this question. For those who answered it incorrectly the option below was selected most often. Next steps: Discuss that the arrows go from producer to consumer because scientists wanted to show the direction of energy transfer and this was the convention they agreed on.
3a). Which organisms in the food web will be immediately affected by a decrease in cockle numbers? 3b). Choose one that you said would be immediately affected and explain how it will be affected.	Most of the students identified the seabirds and crabs, e.g., numbers would decrease. The phytoplankton which would increase in numbers was the least often identified. Some students read a decrease in a species as the end of another [species that eat this species] e.g., Because the crabs have only eaten cockles so they would have died out so the cod won't have as much food
4a). Which organisms in the food web will be affected in the long term by a decrease in cockle numbers? 4b). Choose one organism that you said would be affected in the long term and explain how it will be affected.	Most of the students correctly identified that any one of the species in the food web would be affected by a reduction in cockle numbers in the long term. Some students made just one connection between their chosen organism and another species in the food web, e.g., Crabs [will be affected] because [cockles are] their main food source. Zooplankton numbers will grow because phytoplankton [numbers] would grow. Others made many connections between their chosen species and other species in the food web, e.g., Seabirds eat cockles. If there is less cockles they will eat more cod, but cod eats more bivalve molluscs and shrimp because there is less crab because there is less cockles.

Next steps:

Food relationships are dynamic and adjust to change. Teachers could help students to build this understanding by:

making students aware that in a change that is detrimental for some species [e.g., decreasing their population], there can be a different effect for others, [e.g., a population increase]. Ask students to think about the possible long term effects on this population [e.g., pressure on food sources].

Teachers could also help students to understand a food web is a diagram that represents only a small part of any organism's feeding relationship by asking them to:

research one of the organisms in the food web and list all the things they eat; and/or
include another of their food sources into the food web with the connections to other organisms in the food web that also eat this food.

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