BIO 105, CRN 21684, Spring 2014 – Desert Ecology Lab
DESERT ECOLOGY LAB
In this lab, you will work with a partner to conduct your own, hands-on, scientific investigation, on a
topic of your choice related to the ecology of the Sonoran Desert. You will gain experience and
confidence in your ability to do science and think critically. You will also learn how to communicate
scientific results both in writing and via oral presentation.
CONTENTS:
Page numbers
Assignment descriptions and instructions:
1. List of questions
2. Draft proposal
3. Proposal
4. Data table
5. Written report
6. Presentation
Grading rubrics:
1. Proposal and written report grading rubric
2. Presentation grading rubric
Required proposal and written report format
Sample proposal
Sample written report
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ASSIGNMENTS:
1. LIST OF QUESTIONS (10 pts): Each student will write down three research questions
related to the ecology of the Sonoran Desert that could be answered in two hours of data
collection in the desert at the Pima Community College West Campus and turn them in to
me. Make sure each question can be answered just by analyzing the data you collect (i.e.,
without a literature review). Your questions should be descriptive, “Are…?” questions
about things that you can easily observe. Do not ask “Why…?” questions!!!! For
example, ask, “Are there more holes on one side of a cactus than on the other sides?”
Don’t ask, “Why are there more holes on one side of a cactus than on the other sides?”
Before you can legitimately ask a “Why” question, you need to answer the “Are”
question. (Maybe there aren’t more holes on one side of a cactus than on the other
sides.) You will ask the “Why” question and create multiple hypotheses after you answer
your descriptive question.
Hints for coming up with good, answerable questions:
A. Look for aspects of plant or animal ecology that are readily observable, such as
plant density (number of individuals per square meter), plant species richness
(number of plant species), plant size, locations of rodent burrows or woodpecker
holes, positions of cactus arms, etc.
B. Then, ask how some of these readily observable variables vary over space or are
related to each other.
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BIO 105, CRN 21684, Spring 2014 – Desert Ecology Lab
C. Some of the key things that shape desert ecology and therefore might affect the
variables you are interested in are:
i. Availability of water: We have a wash on campus. Plants in the wash will
receive more water than plants away from the wash, so you could compare
aspects of plant or animal ecology in and away from the wash.
ii. Amount of sunlight received: The south sides of hills and of plants receive
more sunlight than the north sides and therefore tend to be drier and hotter
than the north sides. We have a south-facing slope and a north-facing
slope on campus, and of course, every plant has a south and a north side,
which you could compare.
iii. Competition and facilitation between plants: Plants compete fiercely with
each other for limiting resources such as water. Some plants even engage
in chemical warfare with each other, releasing compounds into the soil
that are poisonous to other plants. On the other hand, areas under large
shrubs or trees are cooler than areas exposed to direct sunlight and can
have more soil moisture as a result. Also, many of the trees and shrubs in
the Sonoran Desert are able to convert atmospheric nitrogen to forms that
plants can use, so that soil under them can have heightened levels of this
important nutrient. Because of these effects, seedlings of other plants may
be better able to establish themselves under shrubs or trees than in more
exposed locations.
iv. Human impacts: Humans alter the environment in a wide range of ways
that can affect plants and animals. For instance, areas immediately
adjacent to roads tend to receive more water than areas away from roads
because of the rainwater that washes off of the roads. These areas also
receive more pollution and are avoided by some animals because of traffic
noise. There are several roads adjacent to desert around the campus that
you could use to look at these effects. Areas near buildings and gardens,
such as the classroom buildings on campus, also often receive more water
than natural areas because of irrigation and water running off of roofs,
sidewalks, etc. These areas will also often receive more pollution as well
as trampling and disturbances caused by people walking around.
2. DRAFT PROPOSAL (10 pts): You will choose a partner, and the two of you will
develop a research proposal. Before you start: Select which roles each person will have.
You will need to collaborate on developing the content of the proposal. However, one
person should take the lead in writing the Introduction and Results sections and the other
person should take the lead in writing the Methods and Discussion sections. You will
need to write these roles on each assignment you turn in and your grade will be based
primarily on the sections for which you took the lead (although a component of your
grade will be based on the entire project). (See the grading rubrics below.)
The final pages of this lab show the correct format for your proposal and an example of a
proposal in the required format. You must follow the format I provide exactly! To
receive full credit on the draft proposal, you need to have filled in each section (including
all the subsections) of the proposal. This draft will give me an opportunity to locate
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BIO 105, CRN 21684, Spring 2014 – Desert Ecology Lab
problems that need to be addressed before you turn in the final proposal. Follow these
steps to develop your proposal:
A. Select a research question to investigate. This will likely be one of the ones that
you wrote down for the first assignment but does not have to be. It does,
however, have to be different from every other group’s questions.
B. Refine your question and make your (empty) data table. Before you go any
further, you need to know exactly what you are asking. That means you need to
know what your explanatory and response variables are and how you would
measure them. The best way to make sure you have done this is to write down
what your data are likely to look like. In other words, create the table you will put
your data into and the table and/or graphs you will use to summarize your data. If
you make a graph, put the explanatory variable on the x-axis and the response
variable on the y-axis. (See the sample proposal for an example.)
C. Select a statistical test you can use to analyze your data. Appropriate tests for
different types of situations will be covered in class. Doing this step early can
prevent you from wasting time on a question that you cannot answer, given the
simple statistics that you know.
D. Explain why you expect your explanatory variable to influence your response
variable. This explanation will form the “Biological Interest” section of your
proposal. Make sure you describe the mechanism by which the explanatory
variable affects the response variable. You may need to do some library or web
research to complete this section. Make sure to provide literature citations for any
resources you use.
i. Example 1: If you were comparing plant density on south-facing and
north-facing slopes, then you should talk about how conditions might
differ on the two slopes (e.g., amount of direct sunlight, evaporation rates,
etc.) and how these conditions might affect the plant density.
ii. Example 2: If you were studying preferences birds have for different kinds
of seeds, then you would need to know how the seeds differ (in nutrition,
size, difficulty to eat, closeness to natural food, etc.) and why this might
cause birds to select one type of seed over the other.
E. Come up with hypotheses and predictions. For the kinds of questions we are
investigating, there will be a null hypothesis (H0) that states that there is no
relationship between the explanatory and response variables and one or two
alternative hypotheses (H1 and H2) that state that there is a relationship between
the explanatory and response variables (and possibly specify what kind of
relationship). For example, your hypotheses could be:
i. H0: There are about the same number of holes on the north and south sides
of a cactus.
ii. H1: There are more holes on the south side of a cactus than on the north
side.
iii. H2: There are more holes on the north side of a cactus than on the south
side.
F. Develop your methods. You have already made a start on this task by figuring
out what your data are likely to look like. Now, imagine collected those data and
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BIO 105, CRN 21684, Spring 2014 – Desert Ecology Lab
try to think of everything you will need to do. The proposal format provided later
in this handout will help you to avoid forgetting important aspects of your
methods. Write down your methods in that format. As you design your methods,
remember the guidelines for designing scientific studies that I have presented in
class!
G. Create a proposal title. Make sure your title is descriptive. Usually your best
option will be to use your research question as your title.
H. Fill in your proposal. Use the sample proposal as a template: I suggest cutting
and pasting the sample proposal into a new document and then just changing the
text within each section. If you have done all the previous steps, this step should
be simple.
3. PROPOSAL (25 pts): Revise your draft proposal until it is as good as you can make it,
and then turn it in as the final proposal. Again, you will need to tell me who took the lead
in writing the Introduction and Results sections and who took the lead in writing the
Methods section. See the proposal and written report grading rubric later in this
document for details on how I shall grade your proposal.
4. DATA TABLE (10 pts): Once you have received my comments on your proposal and
made any needed changes to your methods, you can collect your data. I shall give you
two hours of class time (on two separate days) in which to collect data. Be sure to take
some photographs of your study organism and of you doing the measurements! (You’ll
want them for your presentation.)
After you have collected the data, put it into an Excel workbook in a format that will
make it easy to analyze. Make sure to make your column headings clear enough that I
can understand what the numbers are, and/or include some explanatory text at the top of
the table that tells me what the numbers mean. Then submit the Excel file to me.
5. WRITTEN REPORT (25 pts): Now analyze your data, using the StatisticsFile.xls.
Discuss with your partner what the results tell you about your hypotheses. Also talk
through all of the limitations your data may have that could influence how you interpret
it. You will also identify a new, causal question regarding why you found what you did
that could be explored in future work. Then develop several plausible hypotheses that
answer this question. You may need to do some library or web research to help you
generate your hypotheses. Make sure to provide literature citations for any resources you
use.
You can convert your proposal into the final, written report by revising the Methods
section to reflect what you actually did (and changing it to past tense), re-writing the
Results section so that it presents the data you collected, and writing a Discussion section
that summarizes your findings, discusses limitations in your data, and presents your new
question and hypotheses. Again, to receive full credit, you must follow the format I give
at the end of this document exactly. As before, you will need to tell me who took the
lead in writing the Introduction and Results sections and who took the lead in writing the
Methods and Discussion sections. Keep the same roles as when you wrote your proposal.
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BIO 105, CRN 21684, Spring 2014 – Desert Ecology Lab
See the proposal and written report grading rubric later in this document for details on
how I shall grade your written report.
6. PRESENTATION (30 pts): You will prepare a 7-12 minute Powerpoint presentation
describing your study and present it to the class. Make sure you follow the Tips for
Powerpoint Presentations provided on MyPima! Your presentation will be based on your
written report. As before, you need to tell me who was primarily responsible for
preparing each section. Use the same roles as in the proposal and written report (one
person does the Introduction and Results and the other does the Methods and Discussion).
See the presentation grading rubric later in this document for details on how I shall grade
your presentation. You must email me your slides no later than 9AM on the day of
the presentation, so that I can be sure to get them onto the computer before class begins.
Here is what it should contain (in this order):
A. Title slide: Title, your name(s), your roles, and relevant picture(s)
B. Introduction:
i. Research question
ii. Biological Interest:
1. Get the audience excited to learn what you found out!!
2. Make sure you present the biology behind your research question
in a way that leaves the answer open.
3. Show us pictures of your study subject.
C. Methods: Explain how you collected your data, using photographs to illustrate
what you did.
D. Results:
i. Present and explain tables and graphs that summarize your results (the
same ones you used in your written report).
ii. State whether you found a statistically significant pattern.
E. Discussion:
i. Tentative conclusion: Tell us the answer to your research question, based
on your results.
ii. Limitations to results: Tell us about the limitations to your results and tell
us what you would do differently if you repeated this study.
iii. Causal question [note the spelling!]: Ask why you found what you did.
iv. Multiple hypotheses: State at least two hypotheses to answer your causal
question and explain why each hypothesis is plausible.
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BIO 105, CRN 21684, Spring 2014 – Desert Ecology Lab
PROPOSAL AND WRITTEN REPORT GRADING RUBRICS
Overall quality of the entire proposal or written report
To calculate a grade for this section, I shall add together the scores for content for the
sections written by you and by your partner and divide by 2
Quality of content presented in your sections
EXCELLENT – 10 pts.
You include all the material described in the instructions and format guidelines, you
demonstrate that you understand the scientific method, your results, and the
statistics you used, and you show that you have thought deeply about your research
questions and hypotheses and done online/library research as needed to develop
your hypotheses
GOOD – 8 pts.
You include the material listed in the instructions and format guidelines with minor
omissions at most, you demonstrate a reasonably good understanding of the
scientific method, your results, and the statistics you used and some thought
regarding your questions and hypotheses, but there may be some misunderstandings
or your thinking may be somewhat shallow
POOR – 6 pts.
You leave out important material, demonstrate serious misunderstandings of the
scientific method, your results, or the statistics you used, or demonstrate little
thought regarding your questions and hypotheses
VERY POOR – 4 pts.
There are major omissions and misunderstandings throughout and little evidence
that you have thought about your questions and hypotheses
Organization, writing style and grammar in your sections
EXCELLENT – 5 pts. : You follow the required format exactly and your writing is
easy to understand, with no grammatical or spelling errors
GOOD – 4 pts. : You generally follow the required format, but there may be a few
deviations from the correct format, some writing that is hard to understand, and/or
some grammatical or spelling errors
POOR – 3 pts. : You deviate from the required format in significant ways, your
writing is difficult to understand, and/or there are many grammatical or spelling
errors
10 pts.
10 pts.
5 pts.
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BIO 105, CRN 21684, Spring 2014 – Desert Ecology Lab
PRESENTATION GRADING RUBRIC
I shall place your work into the categories that best describe it and assign points accordingly.
Overall quality of the entire presentation
To calculate a grade for this section, I shall add together the scores for content and use of
powerpoint for the sections developed and presented by you and by your partner and
divide by 3
Quality of content presented in your sections
EXCELLENT – 10 pts.
You include all the material listed in the instructions, in the correct order, you
demonstrate that you understand the scientific method, your results, and the statistics
you used, and you show that you have thought deeply about your research questions and
hypotheses and done online/library research as needed to develop your hypotheses
GOOD – 8 pts.
You include the material listed in the instructions with minor omissions at most, you
demonstrate a reasonably good understanding of the scientific method, your results, and
the statistics you used and some thought regarding your questions and hypotheses, but
there may be some misunderstandings or your thinking may be somewhat shallow
POOR – 6 pts.
You leave out important material, demonstrate serious misunderstandings of the
scientific method, your results, or the statistics you used, or demonstrate little thought
regarding your questions and hypotheses
VERY POOR – 4 pts.
There are major omissions and misunderstandings throughout and little evidence that
you have thought about your questions and hypotheses
Effective use of powerpoint in your sections
EXCELLENT – 5 pts.
Your slides are attractive and easy to understand, you use 20+ font size throughout and
avoid distracting backgrounds and animations, text is in outline format and is
grammatical and spelled correctly, and you illustrate your topic throughout with
relevant pictures and diagrams
GOOD – 4 pts.
Your slides are generally understandable, but you may use too much text at times,
sometimes have text that is too small, sometimes include distracting backgrounds or
animations, have some grammar or spelling errors, or provide few pictures and
diagrams
POOR – 3 pts.
Your slides are generally difficult to understand, for instance because of confusing
wording, too much text, text that is too small, or distracting backgrounds
Your presentation style
EXCELLENT – 5 pts.
You speak to the audience, sound knowledgeable and interested in what you are saying,
and speak naturally (without reading slides or notes verbatim)
GOOD – 4 pts.
You speak clearly and sound knowledgeable but may read some or all of what you
present or fail to communicate enthusiasm for the subject
POOR – 3 pts.
You are difficult to understand and/or sound as though you are unfamiliar with the
information you are presenting
PENALTY FOR SLIDES NOT PROVIDED ON TIME (BY 9 AM)
TOTAL:
10 pts.
10 pts.
5 pts.
5 pts.
-3 pts.
30 pts.
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BIO 105, CRN 21684, Spring 2014 – Desert Ecology Lab
REQUIRED PROPOSAL AND WRITTEN REPORT FORMAT
This format has been established by the scientific community because of its clarity and organization. Each
section must be clearly labeled using the headings and subheadings in bold below.
TITLE
Your name(s) and roles
INTRODUCTION
Research Question: State your research question in one sentence with a question mark at the end.
Biological Interest: Explain why the explanatory variable might affect the response variable. Make
sure you describe the mechanism by which the explanatory variable causes an effect on the dependent
variable.
Hypotheses and Predictions: State your null hypothesis (H0) and your alternative hypotheses (H1,
H2, etc.). (The null hypothesis is the one that states that there is no relationship between the
explanatory and response variables.) For each alternative hypothesis, state the statistical criterion you
will use to evaluate whether that hypothesis is correct.
METHODS [for proposal, this should be in future tense; for written report, this should be in past tense]
Dates and Times: Tell the dates and times when you will collect your data (including the year).
Must show at least 2 hours of data collection.
Location(s): Tell where you will be collecting your data and refer to Figure 1, which should be a map
showing the location. (e.g., “We shall collect our data in the wash and adjacent desert areas on the
Pima Community College West Campus (Figure 1.)”)
Sampling units and sample size: Describe your plots/transects/observation areas, etc., including
their size, shape, and how you will place them. Also tell how many plots/transects/observations you
expect to use.
Data Collection: Tell exactly what data you will collect in each of your sampling units and provide
any other details about what you will do that you have not mentioned elsewhere.
Data Analysis: Specify how all data are to be summarized (totals, averages, etc.) and analyzed (state
which statistics you will use).
RESULTS [or, for the proposal, ANTICIPATED RESULTS]
Write at least one sentence of text summarizing your data and referring to your table(s) and/or
graph(s). For example, “There were more holes on the north than on the south side of cacti (see Table
1 and Figure 2).” (For the proposal: just say, “Table 1 [and Figure 2, etc.] show the kind of data we
plan to collect.”)
Tables and Figures: Each table and figure (e.g., graph or map) should be named (i.e., Table 1, Table
2, ... Figure 1, Figure 2, ... etc.). Each table and figure should contain a caption that includes enough
information to allow the table or figure to stand alone so that someone would understand the data
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BIO 105, CRN 21684, Spring 2014 – Desert Ecology Lab
without reading the rest of the report (e.g., include data units, dates, times, locations, sample size,
etc.). (For the proposal: Put in fake data to show what the data might look like: for instance, will
your data tables contain counts (e.g., 134 holes on south sides of cacti and 120 holes on north sides)
or averages (e.g., 1.2 holes per north side of each cactus, on average, and 0.8 holes per south side of
each cactus), if you have graphs, will they be bar graphs or scatter plots, etc. You can leave the pvalues out.)
DISCUSSION [For the proposal: skip this section]
Summary of results: Write one or two sentences that tell whether you were able to reject your null
hypothesis in favor of your alternative hypotheses. If you had more than one alternative hypothesis,
tell us which one was supported.
Interpretation of results: Write a short paragraph in which you describe what you think your results
mean. Make sure to mention the limitations to your data. (All datasets have limitations!) For
instance, if you did not find a significant effect, is it possible that there actually is a relationship
between the variables that you did not detect because your sample size was too small or because of
other problems with your methods? If your results did support one of your alternative hypotheses, are
there any reasons why we should be hesitant to embrace that hypothesis? For instance, were there
any aspects of your methods that might have biased your results towards that hypothesis? Regardless
of what you found, are there any reasons why the data you collected on campus might not apply to the
Sonoran Desert in general?
Next steps: Tell us a causal question (that is, a “why?” question) regarding why you found what you
did. (Usually, that question will simply be “Why was there a relationship between….?” or, “Why did
we fail to find a relationship between…?”) Then list at least two hypotheses that answer that
question.
LITERATURE CITED
Alphabetized list of sources fully cited (see How to Cite Sources, available on MyPima).
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BIO 105, CRN 21684, Spring 2014 – Desert Ecology Lab
SAMPLE PROPOSAL
(Note: This is made up to illustrate correct format, and its contents are not necessarily accurate.)
How does plant density differ between the north-facing and south-facing slope of Tumamoc
Hill in Tucson, AZ?
Brad Fiero (Introduction and Results) and Carla Essenberg (Methods and Discussion)
INTRODUCTION
Research Question: How does plant density (succulents, shrubs, and trees combined) differ
between the north-facing and south-facing slope of Tumamoc Hill in Tucson, AZ?
Biological Interest: South-facing slopes receive more direct sunlight than north-facing slopes,
thus will differ in the following respects ... . [State the variables that differ between south-facing
and north-facing slopes that could affect plant density. Present relevant plant biology to
understand how plant density may be affected by the variables that differ between the two
locations].
Hypotheses and Predictions:
H0: Plant density will be similar on the north-facing and south-facing slopes of Tumamoc Hill in
Tucson, AZ.
H1: Plant density will be greater on the north-facing than the south-facing slope of Tumamoc
Hill in Tucson, AZ.
D1: The p value for the chi square test will be less than 0.05 and the density will be greater on
north-facing slope.
H2: Plant density will be greater on the south-facing than the north-facing slope of Tumamoc
Hill in Tucson, AZ.
D2: The p value for the chi square test will be less than 0.05 and the density will be greater on
south-facing slope.
METHODS
Dates and Times: On February 10, 2013 from 12:30-1:30 pm we shall sample the north-facing
slope, and on February 15, 2013 from 12:30-1:30 pm we shall sample the south-facing slope.
Locations: North and south sides of Tumamoc Hill located on the West side of Tucson, Arizona
(Figure 1). Tumamoc Hill is a protected site reaching 947 m (3107 feet) elevation (~200 m above
surrounding), and contains natural Sonoran Desert vegetation.
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BIO 105, CRN 21684, Spring 2014 – Desert Ecology Lab
Sampling units and sample size: On the north and south sides of Tumamoc Hill (determined by
compass), we shall start at an arbitrary point 100 meters up from the bottom of the hill, and we
shall lay out five (more if time allows) 50-meter transects (10 transects total), 20 meters apart,
running along the contour of the slope (not up and down the slope). The area sampled will be
everything within 10 meters of each transect line (1000 square meters per transect line).
Figure 1. Aerial photo of Tumamoc Hill, Tucson, AZ, the location of this study. The
approximate locations of the two sampling areas are circled: the one on the north side of the hill
is circled in blue, and the one on the south side of the hill is circled in orange. Photo is from
maps.google.com.
Data Collection: We shall record the number of individuals for all succulents, shrubs, and trees
that are at least 10 cm (4") tall within 10 meters of each transect line.
Data Analysis: We shall sum the data from the five transects on each side of the hill to calculate
density per 5000 square meters. We shall compare densities using the chi square test to
determine if there are significant differences (p<0.05) in plant density between the two sides of
the hill.
ANTICIPATED RESULTS
Table 1 shows the kind of data we plan to collect.
Table 1. Plant density (number of individuals per 5000 m2) on the north- versus south-facing
slope of Tumamoc Hill in Tucson, AZ as observed February 21 and 22, 2012. P-value is from a
Chi-square test.
Plant Density
North-facing Slope
6103
South-facing Slope
5921
p-value
LITERATURE CITED
MacMahon, J. A. 1985. Deserts. Alfred A. Knopf, New York.
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BIO 105, CRN 21684, Spring 2014 – Desert Ecology Lab
SAMPLE WRITTEN REPORT
(NOTE: This is made up just to illustrate correct format. Its contents are not necessarily accurate, and in
particular it does not contain real data.)
How does plant density differ between the north-facing and south-facing slope of Tumamoc
Hill in Tucson, AZ?
Brad Fiero (Introduction and Results) and Carla Essenberg (Methods and Discussion)
INTRODUCTION
Research Question: How does plant density (succulents, shrubs, and trees combined) differ
between the north-facing and south-facing slope of Tumamoc Hill in Tucson, AZ?
Biological Interest: South-facing slopes receive more direct sunlight than north-facing slopes,
thus will differ in the following respects ... . [State the variables that differ between south-facing
and north-facing slopes that could affect plant density. Present relevant plant biology to
understand how plant density may be affected by the variables that differ between the two
locations].
Hypotheses and Predictions:
H0: Plant density will be similar on the north-facing and south-facing slopes of Tumamoc Hill in
Tucson, AZ.
H1: Plant density will be greater on the north-facing than the south-facing slope of Tumamoc
Hill in Tucson, AZ.
D1: The p value for the chi square test will be less than 0.05 and the density will be greater on
north-facing slope.
H2: Plant density will be greater on the south-facing than the north-facing slope of Tumamoc
Hill in Tucson, AZ.
D2: The p value for the chi square test will be less than 0.05 and the density will be greater on
south-facing slope.
METHODS
Dates and Times: On February 10, 2013 from 12:30-1:30 pm we sampled the north-facing
slope, and on February 15, 2013 from 12:30-1:30 pm we sampled the south-facing slope.
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BIO 105, CRN 21684, Spring 2014 – Desert Ecology Lab
Locations: North and south sides of Tumamoc Hill located on the West side of Tucson, Arizona
(Figure 1). Tumamoc Hill is a protected site reaching 947 m (3107 feet) elevation (~200 m above
surrounding), and contains natural Sonoran Desert vegetation.
Sampling units and sample size: On the north and south sides of Tumamoc Hill (determined by
compass), we started at an arbitrary point 100 meters up from the bottom of the hill and laid out
five 50-meter transects (10 transects total), 20 meters apart, running along the contour of the
slope (not up and down the slope). The area sampled was be everything within 10 meters of each
transect line (1000 square meters per transect line).
Figure 1. Aerial photo of Tumamoc Hill, Tucson, AZ, the location of this study. The
approximate locations of the two sampling areas are circled: the one on the north side of the hill
is circled in blue, and the one on the south side of the hill is circled in orange. Photo is from
maps.google.com.
Data Collection: We recorded the number of individuals for all succulents, shrubs, and trees that
were at least 10 cm (4") tall within 10 meters of each transect line.
Data Analysis: We summed the data from the five transects on each side of the hill to calculate
density per 5000 square meters. We compared densities using the chi square test to determine if
there were significant differences (p<0.05) in plant density between the two sides of the hill.
RESULTS
We found that plant density was higher on the north side of the slope than the south side (see
Table 1).
Table 1. Plant density (number of individuals per 5000 m2) on the north- versus south-facing
slope of Tumamoc Hill in Tucson, AZ as observed February 21 and 22, 2012. P-value is from a
Chi-square test.
Plant Density
North-facing Slope
5122
South-facing Slope
4807
p-value
0.002
DISCUSSION
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BIO 105, CRN 21684, Spring 2014 – Desert Ecology Lab
Summary of results: Our first alternative hypothesis, that plant density is greater on the north-facing
slope than on the south-facing slope of Tumamoc Hill in Tucson, AZ, was supported.
Interpretation of results: These results suggest that plant density is higher on the north side than on
the south side of Tumamoc Hill. Plant density may be higher on north-facing slopes in general than
on south-facing slopes in the Sonoran Desert around Tucson, AZ. However, our study only sampled
a single north-facing area and a single south-facing area, and therefore the difference in densities
between these two areas results could have been due to many variables other than aspect. For
example, the steepness of the slope different between the two sites and the minerals underlying the
soil may have been different as well. Samples from many more, separate north-facing and southfacing slopes, which controlled for soil type and steepness, are needed to reach firm conclusions
about plant densities on north- vs. south-facing slopes in Sonoran Desert.
Next steps: For our next step, we would like to investigate why plant density is higher on the north
than on the south side of Tumamoc Hill. Hypotheses to explain that pattern include:
1. The north-facing side retains moisture longer than the south-facing side because it
receives less sunlight.
2. The north-facing side retains moisture longer than the south-facing side because its slope
is less steep.
3. The north-facing slope has deeper soil because its slope is less steep than the south-facing
side, leading to less soil erosion.
LITERATURE CITED
MacMahon, J. A. 1985. Deserts. Alfred A. Knopf, New York.
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