1 of 4
HOW DO ECOLOGISTS GENERATE TESTABLE HYPOTHESES?
Ecology is the study of interactions between organisms and their biotic and abiotic
environments. The goal is simple: to figure out what causes organisms to have a particular
distribution and abundance. Amidst all of the variation and complex interactions that
characterize the natural world, how do ecologists generate testable hypotheses that allow them to
determine what processes have the most important influences on the distribution and abundance
of organisms? During this field lab, we will apply the scientific method to ecological problem
solving—moving from observing patterns in nature, to formally documenting those patterns, to
posing questions and formulating alternative hypotheses, and finally to designing experiments to
test those hypotheses.
First, what is a hypothesis? A hypothesis provides a possible explanation for an observation
in the physical universe. Science advances by rejecting hypotheses, not by proving them. For
example, the hypothesis that all people die cannot be proven by observing a number of people
and documenting that they are not immortal, for the possibility always exists that a person could
come along that never dies. However, if such an immortal person indeed exists, then the
hypothesis could be wholeheartedly rejected.
Rejecting hypotheses often occurs by comparing the hypothesis to a null hypothesis, which is
essentially a hypothesis of no difference or a hypothesis that the effect is just due to chance and
not to some biological factor. For example, one hypothesis might be that grasshoppers reduce
the seed production of goldenrod plants by consuming leaves. The null hypothesis for this
interaction would be that grasshoppers have no effect on goldenrod when they consume leaves.
This use of explicit hypothesis testing dates to Francis Bacon (c. 1620), who proposed that
formulating alterative hypotheses and conducting explicit tests to distinguish among them
resulted in the greatest progress in scientific understanding. Later, Karl Popper (1963)
formalized a version of the scientific method; in his view, accurate hypotheses cannot be proven,
except by eliminating every possible alternative, while incorrect hypotheses can be ruled out by
observations or experiments that contradict them. (“the criterion of the scientific status of a
theory is its falsifiability, or refutability, or testability” (Popper 1963). In a formal Popperian
world, estimates of probability are unfalsifiable, meaning that ideas based on statistical
probability cannot be considered scientific theories (e.g., any biological generalization that has at
least one exception cannot attain theory status). In a Popperian view, hypotheses also must be
mutually exclusive.
There has been quite a lot of philosophical debate about the Popperian tradition in ecology
(e.g., Platt 1964, Strong 1980, Weiner 1995). For the most part, mutually exclusive hypotheses
are hard to come by in ecological systems. For example, one set of hypotheses might include:
(1) Competition affects the abundance of lizards. (2) Predation affects the abundance of lizards.
(3) Climate (e.g., minimum temperature) affects the abundance of lizards. In the real world, all
of these factors likely contribute to the lizard densities in a given habitat, and in fact, these
factors may interact, for example, with predation being more important in some climates and
competition more important in others. Thus, mutually exclusive hypotheses are not very useful
in this case. Many practicing ecologists would describe ecology as a statistical, or probabilitybased, science. The objective is to find a pattern in nature, propose potential causal factors, and
then evaluate their contribution to the pattern. In this type of hypothesis testing, “chance” or
“error” is also evaluated and refers not only to physical randomness, but also to factors that
affect the pattern, but that were not identified by the investigator or included in the hypothesis
2 of 4
testing. This practice of science differs from the Popperian model in that (1) mutually exclusive
hypotheses are not a primary goal and (2) that “the objective of investigation…is not to
determine the single cause of a pattern, as no such cause exists, but rather to assign relative
importances to the contributions of, and interactions between, a number of processes, all known
or reasonably suspected of operating to some degree” (Quinn and Dunham 1983).
Thus, often in ecology the most useful questions ask, “What is the relative effect of factor
X?” To return to the grasshopper example, grasshoppers may only have a very small effect on
goldenrod seed production, while flies that create gall structures on goldenrod stems may cause
very large reductions in seed numbers. Thus, although grasshoppers do affect goldenrod
reproduction, galling flies are a much more important source of ecological variation. It is, then,
the relative importance of grasshoppers as compared to other herbivores that provides insight
into the ecology of the goldenrod-herbivore system.
Observing the natural world: finding patterns
The search for patterns at all levels is one of the most critical parts of the science of ecology today.  Weiner (1995)
We will start by observing the natural world with as little bias as possible. Find an isolated
place in the reserve. Observe the flora and fauna around you…try to create as little disturbance
as possible as this could influence your observations in unintended ways. First, note patterns in
the natural world around you. Some examples include: There are fewer tree seedlings under
hemlock trees than under other tree species. Web-building spiders are more common on plants
with large leaves than on plants with small leaves. Acorns closer to oak trees have more weevil
larvae inside of them than do acorns farther away from oak trees.
Next, write down questions that relate to these patterns, as they come to you, for about 40
minutes. Don’t filter your questions, just write down anything that comes to you about the
natural world around you. Don’t worry about whether your questions have been asked by others.
Your questions might relate to the distributions of plant or animal species, factors affecting
species diversity, patterns of animal behavior, interactions between organisms and their abiotic
environments (soil, water availability), and species interactions (competition, predation,
mutualism), among others. Some examples: Do hemlock roots leak toxic chemicals into the soil
that harm other tree seedlings? Do web-building spiders catch more prey on plants with large
leaves? Do weevils concentrate near parent trees where acorn densities would be greatest? Does
water availability affect the distribution of red maple trees?
After 40 minutes or so, go back through your list of questions. Try to classify your questions
into categories:
Informational—asking for facts or measurements
Functional—asking about how a behavior or process works
Relational—asking about the relationship among components of a system
Evolutionary—asking how relationships or processes changed due to selection pressures
Also, ask yourself how much your questions reflect your own personal background.
3 of 4
The Importance of Alternative Hypotheses
As Platt put forth in 1934…
The moment one has offered an original explanation for a phenomenon which seems
satisfactory, that moment affection for his [or her] intellectual child springs into existence, and as
the explanation grows into a definite theory, his [or her] parental affections cluster about his
offspring and it grows more and more dear to him [or her]… To avoid this grave danger, the
method of multiple working hypotheses is urged. It differs from the simple working hypothesis in
that it distributes the effort and divides the affections…and if a group of hypotheses encompass
the subject on all sides, the total outcome of means and methods is full and rich. (brackets added).
For the next part of the assignment, take what you think are the 5 most interesting
questions on your list. Try to think through all of the possible alternative hypotheses for
the pattern described by your question. Make a list of these alternative hypotheses for
each of the 5 questions. This should take about 15 minutes. For example: Hemlock trees
let less light through the canopy than do other canopy tree species. Hemlock trees support
a fungus that kills tree seedlings of other species. Hemlock trees attract fewer seed
dispersers, such as birds and squirrels, so there are fewer tree seedlings growing near
hemlocks.
Designing Effective Experiments
For the final part of the assignment, choose the single question on your list that you
think is most interesting. Use about 20 minutes to think about how you would test this
question, and how you would rule out viable alternative hypotheses.
Documenting the pattern. First, you will want to figure out ways to formally
document the pattern you observed. Often, one’s impression of a pattern will not stand
up in the face of actual data, and who wants to waste time doing experiments to test a
pattern that doesn’t exist? This is why formal documentation of a pattern is the first step
in designing effective experiments.
Data are usually divided into 2 categories: discrete data occur in distinct units of a
finite number, e.g., yes/no, presence/absence, red/orange/yellow. Continuous data occur
in over an infinite number of intervals, including data such as time, temperature, numbers
of flamingoes, and seed masses of coconuts. Think about what type of data you would be
interested in collecting. In the hemlock example, for instance, you might want to suggest
recording the number of tree seedlings along a transect that starts at the base of a hemlock
tree and runs away from the tree. You could also suggest placing m3 plots under
hemlocks and away from hemlocks and then counting the number of seedlings in each
plot. Both methods acquire continuous data. If there are fewer tree seedlings at the base
of the tree and more seedlings further away AND if this pattern is repeatable for multiple
hemlock trees, then you would have formally documented the pattern. (You don’t have
to collect data for the assignment; just suggest what types of data you would collect, and
how.)
Keep in mind that documenting the pattern does not establish causality. For example,
showing that two species of frogs never occur in the same place does not mean that they
are so distributed because they compete for resources (or did compete for resources).
4 of 4
You would be surprised by the number of practicing ecologists who have made the
mistake of confusing correlation with causation.
“How many hemlock trees need to be sampled?” you might ask. A general rule of
thumb is to have sample sizes of 30+.
Experimental ecology. Next, try to think of ways to manipulate the system to isolate
the factors that you think may be important for determining the pattern you observed.
For the hemlock example, you could plant some seedlings into soil collected from under
hemlock trees and other seedlings into soil collected from under other tree species to
address whether hemlock trees affect tree seedlings through interactions that occur in the
soil (e.g., soil pathogens, toxic chemicals released from the roots). You also might cut
branches from hemlock trees to let more light into the understorey in order to test
whether seedlings are limited by light under hemlock more than under other tree species.
A note of advice: the most effective experiments alter only one variable at a time,
holding all other variables constant. In light of this fact, you may want to think about the
appropriate controls for any manipulations you propose. For example, if you suggest
spraying plants with a water-based insecticide to determine the effects of insect
herbivores on plant growth, an appropriate control would be to spray plants in the control
treatment with water. Or, if you suggest transplant seedlings from one area of the forest
to another, you would want to also transplant seedlings back into their home area as a
control for the disturbance of digging them up and re-planting them.
Group Discussion
After you have had time to make observations, generate questions, think of alternative
hypotheses, and devise some possible experiments, we will re-convene as a group to
review these four steps in hypothesis testing. Be prepared to discuss one of your
questions, some alternative hypotheses, and ideas you have for how to test it.
References Cited
Popper, K. 1963. Conjectures and Refutations; the growth of scientific knowledge.
Routledge and Kegan Paul, London, U.K.
Platt, J. R. 1964. Strong inference. Science 146:347-352.
Quinn, J. F. and A. E. Dunham. 1983. On hypothesis testing in ecology and evolution.
American Naturalist 122:602-617.
Strong, D. R. 1980. Null hypotheses in ecology. Synthese 43:271-285.
Weiner, J. 1995. On the practice of ecology. Journal of Ecology 83:153-158.
5 of 4
***************ASSIGNMENT DUE 24 AUGUST IN LECTURE*************
Write a 1-2 page (single-spaced, 1-inch margin, 12 point Times New Roman font)
summary of this exercise. Start with a paragraph that sets-up your most interesting
question by describing why you think it is interesting from an ecological perspective (3-4
sentences). Next, state the question for which you designed an experiment during the
lab. Also, state the null hypothesis for your question. Then, provide a list of at least 3
alternative hypotheses that could explain the pattern described by your question. Finally,
in a paragraph or list describe the steps you would take to test your hypothesis: both to
formally document the pattern you observed, and to experimentally manipulate the
system in order to isolate a factor or process of interest.