Lab: Becoming an
Ecologist: Investigation into the Life Cycle of the 17-Year Cicada Name______________ Period___ Date Due____________________________ Grade______________ Fix/Finish/Return______________________
Background
An ecologist is someone who tries to explain patterns and relationships among organisms and their environment. Before you begin your ecology unit and begin learning different concepts and content, its important to become familiar with the process ecologists used to create this knowledge. Although there are many ways to explore our natural world there are several important elements to any scientific process.
Note:
The word ‘Hypothesis’ generally has two meanings. It can be a:
1. Tentative explanation based on observations
2. Statement of prediction. Describes in concrete terms exactly what you expect to happen in
your study. In other words, it is a ‘testable’ hypothesis.
Every scientific explanation is tested by dozens and sometimes hundreds of other scientists. An explanation is not accepted until it has been rigorously tested. In science, most explanations change through time as new information accumulates. The objective of this lab is to give you the opportunity to become familiar with the above elements of science as a process. This will be done through your investigation of the 17-year Cicada.
Observations and Questions
The
Life Cycle of the 17 year Cicada. Magicicada septendecim. This cicada spends
the first seventeen years of its life in underground tunnels. After these 17 years, millions of cicadas
emerge from the earth in late May. The
onslaught of these cicadas is memorable as tremendous numbers cloud the sky
echoing with their continual high-pitched sound.
Ecologists noticed that when the seventeen-year cicadas emerge they all feed on the same resource, the sap of trees. Magicicadas can number as many as 20,000 to 40,000 to a tree! What would be the negative aspects of such a strategy? _______________________________________________________________
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Ecologists
also noticed that when Magicicadas begin their
emergence, predators such as birds, will gorge themselves. Any bird can
eat as many cicadas as it can capture because the Magicicadas have no
anatomical defense mechanisms such as stingers,
chemical weapons, or odors. With
no anatomical defense mechanisms how is it that some cicadas survive to
reproduce? __________________________________________________________________________
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One
explanation for this behavior, proposed by some ecologists, is called the
Predator Satiation Hypothesis. Wow!
That’s a mouthful. Lets break it down and find out what it means.
1) A predator
is an organism that does the killing and eating of its prey.
2) Satiation—sounds like
satisfied, right? So the predators,
which are birds, are satisfied
meaning
that they have enough to eat.
Based
on this Hypothesis, what do you predict to be the relationship between the
number of cicadas and the number of cicadas that will be eaten. Which graph below, represents the
relationship and Why?
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Read
the procedure below. If the results of
this activity were to support the Predator Satiation Hypothesis what you would
hypothesize the relationship to be between the number of prey items and the
number of prey items picked up.
Hypothesis
1. On a piece of paper place 30 identical objects (we are going to be using popcorn kernels.) These objects represent the Magicicadas.
2. Select one
person in your group to be the predator.
The same person must be used as the predator for each trial, for
consistent data collection. The predator will have 30 seconds to pick up as
many kernels as he/she can. The
predator has three restrictions
i. Eyes must be closed
ii. He/she can only pick one kernel up at a time
iii. The predator can only use his pinky finger and thumb to pick up the kernels
3. Repeat steps 1 and 2 using larger population
sizes of 45, 60, 75 100 & 125. Magicicadas. Be sure to test each population size at least
twice time.
Table
1. The number of Magicicadas eaten at
each population size
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# Prey Items picked up by predator |
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# of Prey Items |
Trial 1 |
Trial 2 |
Average |
Percent Survival |
30 |
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45 |
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60 |
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75 |
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100 |
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125 |
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Draw a line graph and label both axes with the independent variable on the x-axis and the dependent variable on the y-axis. Use appropriate scales and labels. Title the graph.
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Examine your graph. Describe the relationship between the number of prey items available and the number of prey items captured by the predator. ______________________________________________
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1. Does your data support the Predator Satiation Hypothesis? Explain.
2. Not all scientific explanations are accepted
by the scientific community. Based on
the results of
your experiment, would you support, reject
the Predator Satiation Explanation.
3. What yet to be discovered biotic or abiotic
factors could give strength to or weaken this hypothesis?
4. Describe the difference between the two uses
of the term “hypothesis”. Give an
example of each, as
used in this lab
5. Can you think
of any other examples of organisms in nature for which predator satiation
would be applicable as either a survival
strategy or a reproductive strategy?
6. Organisms in nature have a variety of
different defenses to avoid predation.
For instance, some
organisms can camouflage with their
backgrounds, others like the snuck can secrete a harsh odor.
According to the Predator Satiation Hypothesis, what is the defense Magicicadas use to avoid
predation?
Extension. Day 2
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Hypothesis
Use this adaptation to construct a hypothesis, or tentative explanation
to explain why cicadas are continuing to survive and reproduce.
Test
Use the above procedure to test your newly adapted cicadas. Before you carry out the procedure be sure to
make a prediction or hypothesis about what you expect to happen in your study
Hypothesis (Prediction)
Results
|
# Prey Items picked up by predator |
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# of Prey Items |
Trial 1 |
Trial 2 |
Average |
Percent Survival |
30 |
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45 |
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60 |
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75 |
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100 |
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125 |
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Graph % survival for yesterday’s trials and today’s trials using a line
graph. Use appropriate scales and
labels. Number of prey should go on the independent axis and % survival on the
y-axis Title the graph.