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THE
EXPERIMENT
This lab is the second of several for
investigating biodiversity. The report, which you may write alone or
with one partner is due Monday. You will do an
experiment to test the oldest
law of ecology
[This link is to a reference in Science
Magazine; you need to use a library terminal if you want to
verify the documentation]:
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von Humboldt's Law: |
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Larger areas have more species
than smaller ones. |
Before you say, "Well, of course, there's more
room," you should realize that the law applies to equal sizes of
samples from areas of different sizes. For example, 1000 square meters
anywhere within a huge forest would have more kinds of trees than 1000
square meters within a similar, but smaller, forest. Both samples
might have the same number of tree, but the sample from the smaller
forest might have 50 oak trees and 50 hickory trees while the sample
from the larger forest could have 40 oaks and 30 hickories and 10
maples and 20 dogwoods--the same total number of trees but more
species and thus higher species richness and more biodiversity.
In today's lab, you will be counting microscopic critters instead
of trees. You will be given samples from two containers or creeks or the
rocks or sticks in them, like last week only better. Using a microscope,
you will try to find at least 50 critters in a sample and the same
number from a different sample, from an area of a larger size. As you find the critters
on your slides, use a table or some other
method like last week to "virtually sort" them into
different species, describing or drawing or naming the different
types. Out of 100 critters in both samples, you may have only one or two species, or
you may find more types of critters. No matter how many
species you may have on your slide, you should include only the first
50 individual critters you come to, even if they're all in the same
species. Include algae, but not detritus or dead exoskeletons or
fibers.
Before you begin, think through your planned experiment and try to
figure out how to remove any sources of bias and all possible
variables (except the size of the habitats or areas). Remember,
your experimental group and your control group (or in this case, the
sample from the big area and the sample from the small area) should be
treated exactly the same.
If you have time or a partner, you
can repeat this procedure for both areas so that you have
"sorted" more than 200 critters, but be sure to examine the
samples from the two areas exactly the same. Why? If you're
working with a partner, you should start with different samples from
two different areas until you count 50 each. Then switch to
samples from your partner's area. Why do you think this is
better than having one look at one area and the partner look at a
totally different sample?
Do not tabulate critters smaller than 50 µm (=0.05 mm) long; in
other words, ignore critters which look like nothing more than a dot
at the lowest power of your microscope.
Identifying the species you find:
The goal is to figure out how many species you have in each
sample. It's not necessary to identify or name each species, but you
do need to try our critter ID key to
figure out at least general categories, like "insect" or
"protozoa" or "algae." Then if you have two
different types of protozoa, you could just record them as
"protozoa a" and "protozoa b." If you
have time and skill, however, you could use the reference sources on
the internet (linked to the critter ID
key) to get more specific.
THE REPORT
- The report, which you may write alone or with one
partner, is due Monday.
- The report must have the "Results" and
"Conclusion" sections in standard
report form format (column on the right).
- The "Results" section must have
- a heading
- at least one sentence
- (most important) a graph (not a table) comparing the
number of species in the samples from two different
locations.
- The "Conclusion" section must have
- a heading
- A statement like this:
"I reject [or
do not reject] this hypothesis because . . ."
If you need
help, REVIEW the chapter 1 study
guide or practice some of the problems from Lab
1
HOW YOUR
GRAPH WILL BE GRADED
DETAILS FOR MAKING A GRAPH, IN DESCENDING ORDER OF IMPORTANCE
- Does it make its point effectively? Ooops. Do you have a
point to make? Do you think you found some pattern or difference in diversity?
Sometimes playing with the graph is what reveals the point. Then you might want to
revise your graph to make sure that it emphasizes the point you want to make.
Are your data from Lab 2 "summarized" and organized so that
the graph clearly and accurately shows a comparison of species diversity (or variety
of kinds), not just how many critters you saw?
- Is everything labeled correctly?
- title
- both axes?
- all units of measurement? (remember, units must be metric!)
- the sources of any data you didn't collect personally?
- anything else which would not make sense unless labeled?
- your name?
- Have you followed good conventional practice?
- Did you put the dependent variable (=the "results" of your analysis) on the
"y" (vertical or left) axis? Scientists and other scholars expect to see the
important results as differences in "height" on a graph. There
are exceptions like the graph on text page 11, but unless your circumstances are truly
exceptional you should do your graphs with the results on the vertical axis.
- Have you chosen reasonable scales and ranges so that your data are not
scrunched up in one little corner of the graph?
- Have you identified all abbreviations (except
metric units) and calculations? A good place to do so is just after
the title.
- Have you avoided exaggeration or other misrepresentation of data? (You might want to attach your original data if your graph looks too good to be
true.) A graph which has been criticized is the one reproduced
as fig. 51.12b on
text page 999 (vol. 2). (It's in almost every science textbook published
these days.) What could be changed to make it more
"honest"?
Be sure to attach
your conclusion to your graph for the final Lab 2 grade.
SOME OPTIONAL HINTS
While your grade won't be lowered by hand-drawing the graph (even on
note-book paper), using a computer spreadsheet program like Excel or Lotus 123 could help
you avoid errors (or, at least, easily correct your errors without redrawing the whole
graph). It will also save you time in the long run, since making graphs is a marketable
career skill that you'll get to practice throughout this course. If you first summarize
and organize your data on a spreadsheet table, it's usually easy to turn it into a
spreadsheet graph. You may want to get help before lab time in the computer center. If we
have time, we'll help you during the lab period.
- Whether you should use a bar graph or a histogram or an "x-y" graph depends
mostly on how you summarize and organize your original data. Don't worry if your graph
looks different from someone else's. There are thousands of correct
possibilities.
- Look at the graphs in your textbook. Most of them are
great examples, especially these: p. 13, 20, 64, 77, 213
(especially "b" with scales on both axes), 276, 281
- In the lab are some books with examples of good graphs. Here are links to other
examples:
- http://www.consecol.org/Journal/vol4/iss1/art15/figure6.html.
This is a typical scientific graph like almost anybody could make using Excel
or other spreadsheet software. Or you could draw it with a pencil. Notice that
it has almost everything a graph is supposed to have (title, labels, scales, legend) and
technically it is more or less correct. But does it make its point clearly? Can you
think of "tricks" which could improve this graph?
- http://www.consecol.org/Journal/vol4/iss1/art15/figure8.html.
Another typical scientific graph--technically more or less ok. Notice that
it's ok to put bars and "connect-the-dots" on the same picture. In fact,
if the turtle data were presented as bars just like the shrimping data, we would have more
of a problem seeing the point. The point seems clearer than in the previous graph,
but a skeptic would wonder about the averages presented in both these graphs; was there a
lot of variation in the data? Are the differences between the pairs of bars really
as clear-cut as the graph implies?
- http://www.consecol.org/Journal/vol4/iss1/art1/figure6.html
from "Toward a Panther-centered View of the Forests of South
Florida" (http://www.consecol.org/Journal/vol4/iss1/art1/index.html
)
Notice
- The actual results are the circles; and the curve was calculated (not eye-balled) to
emphasize the trend and to emphasize the point.
- the title is very descriptive
- both axes labeled and marked in reasonable scales and ranges
- units (%) are on the vertical axis, but none on the horizontal axis because
"D" is like a ratio.
- http://www.consecol.org/Journal/vol4/iss1/art1/figure7.html
Here the panther authors want you to be able to compare two
perspectives, "A" and "B." They put them side by side, and use
the same vertical axis labels for both perspectives
- http://www.consecol.org/Journal/vol4/iss1/art1/figure10.html.
Here they put the two perspectives on one graph. Notice the details in
the title and the label on the vertical axis.
back to top
What
you need to know from this lab for future tests
- How to interpret almost any graph in our textbook
and other graphs in conventional scientific formats.
- You could be asked to identify or discuss the
possible sources of bias in the graph you drew or in other graphs.
- On upcoming quizzes and tests and lab reports
throughout the course, you must be able to nterpret and summarize
experiments and research news stories.
- Be able to explain what von Humboldt's Law really means.
- Be able to define these terms: species, species richness,
biodiversity, protozoa, algae, metric system, µm and mm (both
measures of length; 1000 µm = 1 mm; 50 µm (=0.05 mm) is the size of a dot at the lowest
power of your microscope).
- You could be asked to identify or discuss the possible sources of
experimental error or bias in the procedures for collecting and counting critters.
Additional References for independent experiments (not needed
for Lab 4):
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