BIOL 4240L Field Ecology Lab -- Ecological Data (Lab 1) -- 2012 -- Name:
Goal
The purpose of this lab is to provide a realistic/authentic activity with which to introduce students to the concepts of sampling schemes, data collection and descriptive statistical analyses.
Objectives
Aftercompleting this lab activity, students should be able to:
- Determine an appropriate sampling scheme for evaluating leaf morphology of an American Sycamore tree (Platanus occidentalis)
- Describe the doubled rope technique (DRT) technique of canopy access
- Collect a good statistical sample population of leaves
- Calculate various descriptive statistics including measures of central tendency and estimate of error
- Create a frequency distribution of the data set
Terms/vocabulary
Be sure that you are comfortable with the following terms. For those you do not know… use your text and other resources to come up with a definition.
Population
Variable
Data
Statistic
Sample
Descriptive statistics
Inferential statistics
Mean
Mode
Median
Range
Variance
Standard variation
Standard error
Introduction
Trees must allocate resources in an effort to maximize energy acquisition. A primary organ for capturing light energy is the leaf. A tree that is easily encountered in urban and suburban areas is the American Sycamore tree (Platanus occidentalis). Because of this trees growth form and accessibility, we will use it as our study species.
Would you expect leaves to vary in size among species and locations?
If so, what might explain this variation (list at least 3 variables)?
How about leaves on a single tree?
Working in your research team, articulate a question regarding leaf sizes.
Q
If possible, create hypotheses that can be tested with the data you will collect.
Ha
H0
Discuss in your groups ideas for how to collect 80 leaf samples. Share your group’s ideas with the class. Discuss the pros and cons of each in order to agree on a standard approach for the class.
Outline the sampling scheme that you selected in the space below. You may use diagrams/sketches as necessary
It is important that ecologists understand the various “populations” that can be studied. List the 3 types of populations and then describe the type that you are studying in this lab.
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Now that you have a set of leaves (make sure they are labeled!!!) You have to determine a method to measure the variable of interest… in this case, leaf size. Use the space below to briefly describe the method you will use to measure this variable.
Data sheets
Use an appropriate spreadsheet application to create a data sheet in which to record your values. After you have measured AND recorded your data, submit it to the professor so that it can combined with results from other lab groups. Once all groups have contributed data, obtain the complete data set from your instructor.
Data set
Your data set consists of a series of repeated measures of a variable(s). There is ALWAYS some amount of variation in any data set.
Arrange the sizes from smallest to largest
Does the size distribution appear to be random?
Restate potential sources for leaf size variation…
You likely observed a clumping of values near some “average” value with fewer leaves having size extremes (really small or really large). This is called the central tendency of values. What are three common measures of central tendency or “average” values (define each in the space below)?
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Using your data set, determine the value for each of the measures above.
Are any of the leaves the same value as the above measures?
How many were greater in size?
How many were smaller in size?
Are each of the 3 common measures the same value?
Take the smallest value of data set and replace it with a value that is 10% of the original value. Recalculate each measure of central tendency.
Which measure was affected the most?
Which of these measures is best at describing variation within the data set?
There are four commonly used measures of the variation within a data set. These include the range, variance, standard deviation and standard error.
Determine the range of your data
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Could two data sets have the same mean but different ranges? If so, explain…
How about different means but the same range… is this possible?
Calculate the variance(S2) of the data set.
Steps:
1) Determine how much each sample deviates from the mean
2) Square the deviation of each sample
3) Sum the squared deviations for all the samples
3) Divide the sum of squares by samples minus 1
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Other commonly used measures of variation in a data set are standard deviation(SD)and standard error(Sx).
You can “standardize” your deviation by taking the square root of the variance.
Calculate the standard deviation of the data set
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Calculate the standard error of the data set
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Another way to evaluate a data set is to inspect it graphically. This is often done with a frequency distribution graph.
In your groups determine the appropriate size categories and then create a frequency distribution for the class data set.
How are leaf sizes distributed?
Can you see visual evidence of a central tendency?
Are the data “normally” distributed?
If not, how do they differ?
Are the mean, mode and median the same?
Since population statistics are nearly never known exactly the µ must be estimated by x.
What other measure we calculated determines how much confidence you have that the two values are very similar?
We can calculate what range of values should cover the TRUE value with a 95% confidence by using the following equation
The appropriate student’s t value is determined by the degrees of freedom and the confidence level you are interested in (in this case 95%).
Calculate what the true value of leaf size must fall between with a confidence of 95% and then again with 80%
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Search the literature to determine what is known about leaf size of this species and/or similar species. Print off the 1st page of the article.
How did your lab findings compare to the results described in the literature?
What modifications would you make to your methods to better answer your original question?
Describe a suitable follow-up question/project