2) Historical Case Study: Zebra Mussels – Teaching Notes
i.
The Historical Case Study: Zebra Mussels falls under the category of Basic Concepts of
Science.
ii.
Ministry Curriculum Expectations:
Overall Expectations:
F3. Demonstrate an understanding of concepts related to population growth, and explain
the factors that affect the growth of various populations of species.
Specific Expectations
F3. Understanding Basic Concepts:
F3.1 Explain the concepts of interaction (e.g., competition, predation, defence mechanism,
symbiotic relationship, parasitic relationship) between different species.
F3.2 Describe the characteristics of a given population, such as its growth, density (e.g.,
fecundity, mortality), distribution and minimum viable size.
F3.3 Explain factors such as carrying capacity, fecundity, density, and predation that cause
fluctuation in populations, and analyse the fluctuation in the population of a species of
plant, wild animal, or microorganism.
F3.5 Explain how a change in one population in an aquatic of terrestrial ecosystem can
affect the entire hierarchy of living things in that system (e.g., how the disappearance of
crayfish from a lake causes a decrease in the bass population of that lake; how the
disappearance of beaver from an ecosystem causes a decrease in the wolf population in that
ecosystem).
iii.
DiGiuseppe, M. (2003). Biology 12. (1st ed.). Toronto (ON): Nelson Thompson Learning.
iv.
Teaching notes:
a. Hand out one assignment to each student.
b. Ask one student to read the introductory paragraph aloud.
c. Students are to work independently to answer all assignment questions.
d. Scrap paper may be used as needed.
e. Allow 30 minutes of class time initially; papers are due the following day.
f. Grade according to marking scheme indicated below.
Historical Case Study: Zebra Mussels in Lake Ontario
Background:
Zebra mussels were an exotic species to the Great Lakes until the 1980’s. It is thought that they entered
our water system through bilge waters released by ships from the Caspian Sea, where they are an
indigenous species. Advantages of zebra mussels in Lake Ontario include their role as a food source for
certain fish and water fowl, and their ability to filter certain water pollutants. Disadvantages of their
presence include how rapidly they consume food and space, their ability to cluster and clog pipelines
and cling to boats, and how quickly they reproduce: females can lay 30,000-4,000 eggs per year.
Knowledge/Understanding:
1. Define the following terms:
a. Exotic Species (2 marks)
b. Indigenous Species (2 marks)
c. Carrying Capacity (2 marks)
d. Food Web (2 marks)
2. The carrying capacity of zebra mussels in Lake Ontario is unknown. What abiotic and biotic
factors, positive or negative, might affect their population? (6 marks)
Abiotic Factors
Biotic Factors
3. Review the following food web. Is the zebra mussel a producer or a consumer? (2 marks)
Thinking/Investigation
4. Zebra mussels have the capability of filtering water-borne pollutants that can be toxic to certain
predators. In Lake Ontario, catfish occupy the same ecosystem as zebra mussels. Suppose the
zebra mussels were harmed by these toxins and lost their ability to filter pollutants. How might
this impact the above food web? Why would you see these changes? (4 marks)
5. The following population concentration of zebra mussels has been observed on water intake
pipes in Lake Ontario since the species discovery in the 1980’s:
Year
Pop m2
1991
400
1992
520
1993
676
1994
879
1995
1142
1996
1485
1997
1930
1998
2509
1999
3262
2000
4241
a. Graph the above population curve. Remember to label both axes. (5 marks)
b. The zebra mussels’ growth rate (r) was calculated to be 0.4/day. If the population of mussels
on water intake pipes in 2000 was 4241, calculate the instantaneous growth rate. (4 marks)
c. Calculate the time it will take for the zebra mussel population to double (Td). (4 marks)
Application:
6. Zebra mussels are known to clog water pipelines, impacting Ontario’s water production
industry. What are the possible socio-economic impacts of this occurrence? (6 marks)
Historical Case Study: Zebra Mussels in Lake Ontario – Answer Key
Background: Ask a student to read the following paragraph aloud:
Zebra mussels were an exotic species to the Great Lakes until the 1980’s. It is thought that they entered
our water system through bilge waters released by ships from the Caspian Sea, where they are an
indigenous species. Advantages of zebra mussels in Lake Ontario include their role as a food source for
certain fish and water fowl, and their ability to filter certain water pollutants. Disadvantages of their
presence include how rapidly they consume food and space, their ability to cluster and clog pipelines
and cling to boats, and how quickly they reproduce: females can lay 30,000-4,000 eggs per year.
Knowledge/Understanding: Students are to work independently:
7. Define the following terms:
a. Exotic Species (2 marks)
A species that is not native to a certain ecosystem.
b. Indigenous Species (2 marks)
A species that is native to a certain ecosystem.
c. Carrying Capacity (2 marks)
The maximum number of organisms sustainable by available resources over a given time span.
d. Food Web (2 marks)
A sequence of linked organisms which feed on each other as producers and consumers. Energy flows
between the trophic levels through each additional consumer.
8. The carrying capacity of zebra mussels in Lake Ontario is unknown. What abiotic and biotic
factors, positive or negative, might affect their population? (6 marks)
-
Abiotic Factors
Presence of water pipelines to attach
Pollutants in the water
Trapping/catching by nets
-
Biotic Factors
Increased number of predators (gulls)
Decreased number of consumers
(phytoplankton)
Changes to reproductive capacity
9. Review the following food web. Is the zebra mussel a producer or a consumer? (2 marks)
The zebra mussel is both a producer and a consumer. It is a food source for organisms higher on the food
web (crabs, whelk, lobster) and it uses other organisms as a food source (phyloplankton, zooplankton).
Thinking/Investigation
10. Zebra mussels have the capability of filtering water-borne pollutants that can be toxic to certain
predators. In Lake Ontario, catfish occupy the same ecosystem as zebra mussels. Suppose the
zebra mussels were harmed by these toxins and lost their ability to filter pollutants. How might
this impact the above food web? Why would you see these changes? (4 marks)
If zebra mussels could not filter pollutants, the water and surrounding air could become more polluted.
This may affect the food web at many levels. E.g. Lake basin may not support growth of seaweed, water
toxicity may not support life of plankton, lower level food web producers may die off, limiting food supply
for consumers. Zebra mussel population could grow exponentially, increasing food supply for consumers
(gulls, fish). Zebra mussels may lose food supply (plankton).
11. The following population concentration of zebra mussels has been observed on water intake
pipes in Lake Ontario since the species discovery in the 1980’s:
Year
Pop m2
1991
400
1992
520
1993
676
1994
879
1995
1142
1996
1485
1997
1930
1998
2509
1999
3262
2000
4241
d. Graph the above population curve. Remember to label both axes. (6 marks)
Zebra Mussel Population
5000
4000
Number
3000
Zebra Mussel
Population
2000
1000
0
Year
e. The zebra mussels’ growth rate (r) was calculated to be 0.4/day. If the population of mussels
on water intake pipes in 2000 was 4241, calculate the instantaneous growth rate. (4 marks)
dN/dt
f.
Td
= rN
= (0.4)(4241)
= 1696.4 per day
Calculate the time it will take for the zebra mussel population to double (Td). (4 marks)
= 0.69 / r
= 0.69 / 0.4
= 1.725 days
Application
12. Zebra mussels are known to clog water pipelines, impacting Ontario’s water production
industry. What are the possible socio-economic impacts of this occurrence? (6 marks)
-
Difficulty accessing water supply
Possible price increases for water access
Increased pollutants in water
Lower SES residents unable to afford clean water
Pricing wars similar to gas shortage
Premium pricing bottled water