Please answer the intermediate disturbance hypothesis review questions found on page 5 of the lab handout. make sure you use the data provided and include a graph in your answer.
INTERMEDIATE DISTURBANCE HYPOTHESIS
What you should know after this lab:
• What a disturbance is
• The different stages of succession
• The general characteristics of pioneer and climax species
• The basis of the Intermediate Disturbance Hypothesis (IDH)
• How to use transects and quadrats in sampling plant species
• How to distinguish different species based on morphological characteristics
Ecology is the study of relationships between organisms and their environment, which includes physical (abiotic)
and biological (biotic) factors. Examples of abiotic factors are temperature, moisture, soil type, and light
availability. Examples of biotic factors are predation and competition. Both biotic and abiotic factors are important
in determining the organisms that exist in a community. A community consists of all of the different species,
including bacteria, fungi, plants, and animals, that co-occur in a given area and interact with each other.
The biological diversity of a community is of interest to ecologists from both a theoretical and a practical
conservation perspective. Plant diversity is a field of particular interest. Since plants are the primary producers in
terrestrial ecosystems, they are important in determining the other types of organisms that can exist within a
community. As a result, the types of plants that occur in an area are important predictors of the other species and
interactions within a community.
One of the simplest ways to quantify biological diversity is species richness (S), which is the number of species
present. To measure species richness, ecologists simply count the number of different species they find in an area.
Many factors, both biotic and abiotic, contribute to species richness in a particular area, including habitat size,
nutrient availability, and disturbances. A disturbance is a relatively sudden event, such as a wildfire or landslide,
that causes mass mortality in a community or that radically alters the availability of key resources. How frequently
disturbances occur, as well as how intense they are, is a major factor in determining which species can survive in a
community. Following a disturbance, plants re-colonize the area in a somewhat predictable manner. This
predictable re-colonization is called succession. Typically, the first plants to re-colonize a disturbed area are shortlived, small, and produce a large number of very small seeds. Plants with these characteristics, such as annual
grasses, are called pioneer species. Over time, these species are crowded out of the area by longer-lived, larger
plant species, such as oak trees. These species are examples of climax species, because they represent the end-point
of succession. Climax species are very good at monopolizing resources, such as soil nutrients and sunlight, and
once they take hold in a community, they tend to crowd out less competitive species.
Today, we will investigate the Intermediate Disturbance
Hypothesis, or IDH (Joseph Connell, 1978). Your instructor will
discuss the rationale for the IDH, but this hypothesis essentially
posits that the highest species richness is found in areas that are
subject to an intermediate level of disturbance.
Today, we will survey vegetation along the American River
Parkway to examine whether the relationship between disturbance
and plant species richness is consistent with the IDH. Working
with your bench partners, you will sample species richness in three
different areas along the parkway that have been subject to different levels (low, medium, and high) of human
disturbance. In each area, you will count plant species richness in two quadrats along a transect. A transect is a
straight line, usually made with a measuring tape. A quadrat is a square frame used for delineating sampling
locations along a transect. Researchers use these tools by walking along the transect and dropping the quadrat at
one or more pre-determined distances from the beginning of the transect.
Before we drop our quadrats, we need to determine the specific distances at which to drop them along the transect.
We want the locations of our “drops” to be determined randomly, to avoid bias. One way to do this is to use
software that generates random numbers, but a simpler way is to open a book to a random page and use the
resulting page number as your random number. This can be done before leaving the lab room or out in the field.
Determining drop locations:
A. Randomly open a book (any book will do).
B. Choose the last digit of the right-page number. This will be the location along your transect at which to drop
your first quadrat. For example, if you opened the book to page 128, you would drop your first quadrat at the 8-
meter mark along the transect (begin walking along the transect at its beginning, i.e. the 0-meter mark).
C. Randomly open the book a second time, and again choose the last digit of the right-page number. This will be
the location along your transect at which to drop your second quadrat. For example, if you opened the book to
page 93, you would drop your second quadrat at the 3-meter mark. (If the second number happens to be the
same as the first, open your book a third time to obtain a different page number.)
D. Enter these values in the “Drop Distance” columns of the data-collection tables on the next page.
II. METHODS FOR VEGETATION SURVEY
- Each bench team should obtain one 30-meter tape and one quadrat frame. What is the total area of the quadrat?
- Out in the field, your instructor will lead you to the three different disturbance areas. Once you reach the first
area, use your measuring tape to make a 10-m long transect. Make sure the 0-m mark begins at the trail, and lay
the transect perpendicular to the trail. Avoid crossing other teams’ transects. Leave your measuring tape on the
ground (although it does not have to be perfectly flat).
- Walk along the transect to your first drop point, then drop the quadrat. You can drop the quadrat however you
want, but for consistency, drop it the same way each time.
- Within the quadrat, count the total number of species present and record them in the data table. Do not count the
number of individual plants. Just count the number of species you find. Use the morphospecies concept to
identify different species. (Your instructor will explain this concept before we head out into the field.). Only
count species that are rooted within the quadrat, with one exception: If there are tree limbs directly over the
quadrat, include the tree in your species count.
- Pick up the quadrat, walk to the second drop-point along the transect, and drop it again. Drop it the same way as
in the first drop, and again count the number of plants species you find in the quadrat.
- Repeat this procedure in the other two disturbance areas.
- Back in the lab, enter your results into the Excel spreadsheet (you can choose any group number in the
spreadsheet). Your laboratory instructor will make these results available to everyone at the end of the week.
III. DATA COLLECTION
As you count species richness, take notes about the general condition of the areas. Do you see evidence of
disturbance by humans, such as heavy trail use, tire marks, etc.? Also take notes about the types of plants present.
Are they mostly small plants, like grasses and forbs? Are there any shrubs, bushes, or trees present?
High Disturbance area – description and vegetation:
Sampling Point Drop Distance Species richness
Medium Disturbance area – description and vegetation:
Sampling Point Drop Distance Species richness
Low Disturbance area – description and vegetation:
Sampling Point Drop Distance Species richness
IV. LAB SUPPLEMENT ASSIGNMENT FOR THE IDH
Our data will be combined with other lab sections’ data to create a single, large data set. You will be provided with
this spreadsheet at the beginning of next week.
Your task will be to complete the supplemental assignment, which will require that you conduct a similar but
abbreviated, analysis and interpretation as you did for the phenotypic plasticity exercise during Week 1. You will
calculate mean species richness and standard deviations for each disturbance area (low, medium, high), and create a
professional-looking, properly labeled graph depicting your results.
Important: You will be responsible for conducting the statistical analysis using the t-test to generate the p-value.
However, in this lab exercise there were three treatment groups. Therefore, you will need to conduct multiple ttests to determine if there are significant differences between each group. Usually, conducting multiple t-tests is
considered inappropriate, as each comparison inflates the Type I error rate (in our case 5% for each comparison).
However, our study will require three comparisons, resulting in a 15% chance (instead of the usual 5%) that any
difference we detect was due to random chance and not an actual population difference. This might be different if
we had to make 100 comparisons. This minimal inflation is further justified by allowing you to practice a familiar
analysis. The goal here is that you learn to analyze and correctly interpret your results.
Customarily, the statistical test, analysis of variance or “ANOVA,” might be conducted for studies with more than
two treatment groups. Your instructor may choose to explain these alternative tests to you but in short, conducting
this test in previous years without further assessments known as “post-hoc” tests, also was inappropriate.
IDH SUPPLEMENTARY EXERCISE
During our IDH laboratory exercise, data was collected from each group (lab bench) concerning the species
richness of three different disturbance areas along the American River Parkway. These data were entered into an
Excel spreadsheet and provided to you for further analysis. Use the provided data to answer the following
questions regarding the IDH. This assignment must be typed and is due in two weeks.
1.) Write a hypothesis and rationale concerning the study you conducted during this laboratory session.
2.) Use the data provided in the Excel spreadsheet to perform an appropriate analysis, which may or may not
support your hypothesis.
a.) What is your approach? In other words, tell me what you calculated and why.
b.) As part of your analysis, provide an appropriate graph to illustrate your findings. Place the graph at the end
of this page.
3.) Was your hypothesis supported? What is your evidence?
4.) Give two factors you think may have resulted in the outcome you determined.
V. REVIEW QUESTIONS
- Why was it important to determine quadrat-drop locations randomly? Explain in one or two sentences.
- You used the morphospecies concept to identify species in this study. This concept is useful, but it can lead to
over-counting or under-counting the actual number of species present. How might it lead to over-counting
species, and how might it lead to under-counting? Give one explanation for each type of error.
- Suppose you are comparing the species richness of two communities. In each community, you find 13 species.
You conclude that since species richness is the same in both communities, the two communities must be very
similar. What is one major problem with this conclusion? (Hint: What does this measure of diversity NOT
- In the diagram below, draw in bars that show the pattern you expect to see if the pooled data support the IDH.
Choose any values for the y-axis; it’s the pattern that’s important. (Note the x-axis!)
- Now, suppose the pooled data do NOT support the IDH. What patterns might you instead expect to see? Using
the following two diagrams, draw two examples of patterns you might instead see.