Embedded Files
Outcomes:
generate a hypothesis based on background information
select the appropriate graph to display information
draw conclusions based on interpreted data
AtL
Reflective skills: Consider ethical, cultural and environmental implications
In the 1960s, Robert Paine, an ecologist at the University of Washington in the United States, conducted an experiment to determine the effect of predators on the stability of an ecosystem. He, and other ecologists at the time, wondered if the removal of an important 'top' predator from an ecosystem would have a positive impact on the ecosystem, and result in a greater variety of species, and a healthier ecosystem; or if the absence of the top predator would have a negative impact on the organisms, resulting in fewer species and an unbalanced and unhealthy ecosystem.
Figure 1.1 Predators are consumers that hunt, kill and eat other animals as their source of food.
In order to answer these questions about the impact of top predators on the health of an ecosystem, Paine studied the species' interactions on the rocky shoreline of Washington. For his experiment, Paine removed the top predators from a portion of the rocky shoreline. The top predator that he removed and returned to the ocean was a type of starfish, Pisaster ochraceus, which preys, or feeds on, other organisms, such as snails, mussels and barnacles, that live in the ecosystem. In the area of the shoreline next to the removal site, he left the ecosystem in its original state as a control group, with the top predator starfish still feeding on the other species.
For the next 10 years, Paine kept the experimental area of the ecosystem free from any Pisaster that migrated there. Throughout the time of the experiment, he made observations and collected data about the numbers and types of different species that lived in the experimental area, and compared that to his observations and data from the control area. His results surprised and impressed not only him, but many members of the scientific community. Indeed, his findings and conclusions about the role of top predators in an ecosystem have formed the foundation of ecological principles on species interactions.
So, what is your hypothesis? Considering that a healthy ecosystem is one that has a diverse variety of species interacting, what impact do you think removing top predators has on an ecosystem? Will removing top predators help the ecosystem to be more diverse, stable and healthy, or will removing the top predators make the ecosystem less diverse, unbalanced, and unhealthy? In this activity, you will process some of Paine's data and present it in a graph. You will then be able to analyse the results to determine, as Paine did, the impact that top predator removal has on an ecosystem.
Activity:
Will removing top predators help the ecosystem to be more diverse, stable and healthy, or will removing the top predators make the ecosystem unbalanced and unhealthy? What makes you say that? Write your ideas down, and share them with a partner.
Table 1.1 Average number of species per year in coastal ecosystems with Pisaster present and with Pisaster removed.
Activity:
Use the data in Table 1.1 to make two graphs to:
show the pattern in the number of species from 1963 to 1973
compare the average number of species in the two areas by the end of the experiment.
Some guidelines to help you in making your graphs.
Before you begin, decide what type of graph you should make in order to best show the relationship in the data.
Some questions to consider in deciding the type of graph are:
Which is the best type of graph to show:
a trend over time
a percentage or fraction of a whole and
an average of final results?
What does the data (Table 1.1) from Paine's experiment on Pisaster show?
There can be more than one correct answer.
a trend over time
a percentage or fraction of a whole or
an average of final results?
Therefore, what type of graph(s) should you create in order to:
show pattern in the number of species from 1963 to 1973
compare the average number of species in the two areas by the end of the experiment?
What will you put on the y-axis? Remember, the y-axis should be the dependent variable, or the 'results' that are counted, measured or calculated in the experiment.
What will you put on the x-axis? Remember, the x-axis can be the independent variable, or the factor the experimenter changes or manipulates; or, the x-axis can show the time during which the results are being recorded.
Activity:
After you make your graphs, answer and discuss these questions:
What is the effect of removing Pisaster from the rocky shoreline ecosystem? What data from the graphs supports your answer?
Does removing Pisaster result in more or less species diversity? What data from the graphs supports your answer?
Does removing Pisaster result in a more or less stable ecosystem? What data from the graphs supports your answer?
What can you conclude from the effect of top predators on an ecosystem? Do they contribute to or take away from the diversity, stability, and health of an ecosystem?
How does your hypothesis compare to the results of the experiment? Was your prediction and reasoning correct or incorrect? Why?
How effective was Paine's method in determining the effect of removing a top predator from an ecosystem? In other words, what aspect of Paine's experiment made his results reliable or 'trustworthy'? What might he have done differently to improve the experiment?
Assessment opportunities
This activity can be assessed using Criterion C: Processing and evaluating.
In this activity we have practiced the skill of processing data and reporting the results. We used
a summary of Paine's data to make graphs and interpreted the meaning of the results. We then shared our understanding of the results with the class.
SUMMARY REFLECTION
We have processed and analysed data from a foundational experiment in the field of community ecology. We have seen how removing a top predator from an ecosystem can result in an ecosystem that is less diverse, less stable and less healthy.
EXTENSION
AtL
Information literacy skills: Access information to be informed and to inform others.
Paine conducted his experiments many years ago. What have experiments shown since then? Is there more support for and examples of the crucial role top predators have in maintaining a balanced ecosystem? Perhaps you can research about your local ecosystem to learn if there have been any investigations about species interactions in the natural areas where you live.
Can you find more current studies and information about the role of predators in ecosystems? Use these search terms: top predator, role, ecosystem.
SUMMARY REFLECTION
With your partner or the class, discuss the following:
How do the findings from Paine's and others' research compare to your predictions of the effect of top predators?
Why might the top predator have such an impact on the ecosystem? Suggest some reasons.
Citations
Davis, Andrew, and Patricia Deo. “Biology: MYP by Concept 4 & 5.” Biology: MYP by Concept 4 & 5, Hodder Education, an Hachette UK Company, 2016, pp. 224–227.
Kirkhart, Jerry. “Pisaster Ochraceus (Purple Sea Star or Ochre Sea Star).” Wikimedia Commons, 24 Nov. 2007, commons.wikimedia.org/wiki/File:Pisaster_ochraceus_(purple_sea_star_or_ochre_sea_star)_(2132256087).jpg.
Mussels, Barnacles, Mytilus, Watt Area. pixabay.com/en/photos/mussel%20shells/.
taqplayer.info/machine-gears-clipart.
Page updated
Report abuse