Gendron
Observation - 1
OBSERVATION AND HYPOTHESIS TESTING IN ECOLOGY
Robert P. Gendron
Indiana University of Pennsylvania
“You see, but you do not observe”.
- Sherlock Holmes to Dr. Watson
Like all scientists, ecologists attempt to understand the natural world through the systematic
and rigorous use of the "scientific method." Actually there is no single scientific method - it is
a process that varies from one discipline to another. In most cases, however, this process
begins with an observation of something that is interesting or unexpected. For example,
Observation: Two plant species, call them A and B, are never found in the same location
even though they seem to thrive in similar habitats.
Next, the investigator proposes a tentative explanation for the observation - the hypothesis.
Hypothesis:
Species A somehow excludes species B, perhaps because sp. A is a better
competitor for limited resources. In addition, there is always an implicit
null hypothesis. In this case the null hypothesis is that there is no
difference in competitive ability.
Anyone can come up with a plausible hypothesis. What distinguishes the scientist from the
dilettante is the third step: a rigorous and controlled test of the hypothesis. Often this involves
an experiment, although comparative and descriptive studies are also common.
Experiment:
a. Select 30 sites that contain sp. A.
b. Remove sp. A from 15 of the sites. This is the experimental treatment.
Each site is a replicate.
c. The other 15 sites will serve as the control treatment
Prediction:
If the hypothesis is true sp. B will probably move into those areas from
which A was removed, but not into the control areas.
Do not even think about doing an experiment like this one unless it is properly controlled!
Usually, control and experimental treatments differ in only one variable, in this case the
presence or absence of sp. A.
If someone did this experiment without the control and sp. B moved into the experimental
areas it would not be valid to conclude that this was because sp. A was absent. Explain.
Observation - 2
Explain why is it necessary to replicate an experiment?
Some experiments do not have one obvious control treatment, and yet are still properly
controlled. For example, to test the hypothesis that population density affects reproduction
you could measure reproductive rate at five different densities. Each density is a treatment.
The experiment is properly controlled as long as all variables except density are identical
between treatments; any differences in reproductive rate will then reflect the effect of density.
It may seem paradoxical, but it is much easier to disprove a false hypothesis than it is to prove
a true one. A hypothesis can be thrown out based on the results of a single experiment. On
the other hand, if an experiment turns out as predicted the hypothesis is not absolutely proven,
although our confidence that it is true does increase. (Typically a scientist will say “the results
of the experiment support my hypothesis.”) The process of testing a hypothesis may continue
indefinitely until eventually the hypothesis is disproved, or is supported by so much evidence
that it is elevated to the status of a "theory."
An Ecological Investigation
There are two options for this lab, one of which will be selected by your instructor. One of
these involves going outside and observing nature directly, developing hypotheses based on
your observations and developing an experimental design. Alternatively, your instructor may
have you watch a nature film and develop your hypotheses based on that. In both cases, the
purpose of this lab is to
1.
improve your powers of observation and perception so that you can better recognize
ecologically interesting patterns, and
2.
give you practice in formulating hypotheses and designing experiments.
This second point is particularly important. Throughout this course we have emphasized that
science is not just a body of facts to be memorized. It is a process that is used to reveal the
secrets of nature. Learning how to do science (the process) is just as important, if not more
so, as learning facts (the products of science). In lab your instructore will discuss the basic
principles of experimental design.
Option 1: Observation in Nature
In the outdoor version of this exercise you will go into the field and look for something that is
ecologically interesting. You will then propose one or more hypotheses to explain this
observation and design an experiment to test these hypotheses.
Observation - 3
Once in the field your instructor will point out some examples of ecologically interesting
patterns and discuss how to test hypotheses in more detail. You will then have the
opportunity to put thiese ideas into practice. You will work in groups initially, and then by
yourself. The following outline illustrates the process.
1.
Find something that seems ecologically interesting.
Examples:
a. a heavy infestation of insects on one plant
b. a weed found by the road but not in the woods
c. a flock of birds foraging together
2.
Propose one or more plausible hypotheses for your observations. These explanations may
involve such things as
a. The adaptations of a species
example: a plant species is found in a particular habitat because it is adapted for high
(or low) light intensities or wet (or dry) soils.
b. Interactions between conspecifics (members of the same species)
example: animals clump together for protection (or spread out to avoid competition).
c. Interactions between different species
example: an insect may be found only on its food plant; a species may be rare because
of high mortality caused by competitors or predators.
3.
Suggest a way of testing the hypothesis. Possible approaches to testing ecological
hypotheses include,
a. Perform an experiment in which you eliminate one species from an area to see what
effect this has on other species.
b. In carefully controlled lab experiments, determine the ability of different plant species
to grow under various physical conditions (e.g. light intensity, soil moisture, soil type,
etc.).
c. Study the behavioral interactions of animals living in groups.
d. Use statistical tests to determine if two species are found together more often than
would be expected by chance alone.
Observation - 4
4.
Clearly state the predicted outcome of the experiment(s). That is, if the hypothesis is
correct what will happen when you carry out the planned experiment? Be sure to explain
what result you are going to measure or record.
Option 2: Observation & Hypothesis Testing in the Lab
In this lab you will view a nature video on some ecological topic (e.g. predators and their
prey). As you watch, make a list of at least 10 questions about the organisms and behavior
you observe. Your instructor may pause the video to discuss some possible examples of
questions.
After the film is over your group will discuss everyone’s questions and pick two (2) to
investigate further. As a group you will propose a hypothesis and develop an experimental
design to test each question. The experiments will be described in detail and must be properly
controlled and replicated. The instructor may also have you complete the worksheet on the
next page.
See the previous section which explains the process of designing an experiment.
Observation - 5
Names _____________________________________________________________________
Observation and Hypothesis Testing
Observing Nature on Video
Your Question
Yout Hypothesis (include the rationale for the hypothesis)
Describe experimental design on back
Make sure that the experiment is properly controlled and replicated. Identify the
dependent and independent variables.
Prediction (if your hypothesis is correct what do you expect the result of the experiment to
be?)
OSERVATION AND HYPOTHESIS TESTING
INSTRUCTOR NOTES
Option 1: In the Field
For the field version of this lab I break it into three separate exercises. In the first exercise the
entire class makes observations and comes up with hypotheses. Typically, I have the students
gather around a moss- and lichen-covered tree and ask each one in turn to make some
observation about the tree and its environment. Sometimes it takes a bit of prodding to get
them past the more mundane observations. The most useful observations have involved the
distribution of the moss and lichens. I then pick one or two observations and ask the students
to propose a hypothesis or (better) several alternative hypotheses. We then talk about
experimental design with an emphasis on the need for control and replicates.
In the second exercise the students work in pairs. This gives them someone to bounce ideas
off of and prevents them from getting too stuck. Each pair repeats the previous exercise based
on some new observation. We then meet and discuss what each pair came up with.
Finally, each student repeats the exercise on their own. At this point I have them write down
their observations, hypotheses, experimental design and predictions. I collect these at the end
of the lab or later for a grade.
It is important to circulate and check on the students. Even when they get stuck they usually
will not come to you, but they will ask questions if you stop by. For this reason you might
want to tell them to stay within a limited area so you have time to talk to everybody.
Option 2: In the Lab
I find that using the video works surprisingly well because the students get to see a lot of
interesting behavior in a short period of time, far more than if they were in the field. Some
students, however, have difficulty coming up with questions. Usually all they need to get
started is to see what other students are writing. To this end, I pause the video after about 5
minutes and ask some of the students to describe one of their questions. I also emphasize that
they should not worry at this stage if their question is good or bad, but just write down
whatever they think of. The time for picking the most interesting question will come later.
After the video I give a brief presentation on experimental design which includes such topics
as, independent & dependent variables, control and replication.
Students then discuss their lists of questions and choose two to develop into hypotheses and
experiments. There are usually a lot of questions about experimental design so this lab keeps
the instructor busy.