NRE 509 Schueller Lecture 2:
Experiments in Ecology
1. From observation to results –
testing hypotheses
2. What are the essential elements of
experiments?... Of real-world
adaptive management?
3. Different experimental approaches:
balance between realism and
precision/control
Debate with your neighbor…
• What is science, beyond hypothesis
testing?
• How is science a human process?
• Is neutral science possible?
• Why should we rely on science for
policy?
• What is scientific consensus?
OBSERVATION
Pattern: Different organisms
occur in zones in the
intertidal zone
Chthamalus
stellatus
Balanus
balenoides
Barnacles zonation
See also Fig. 23.20
NUMBER OF INDIVIDUALS
OBSERVATION
Balanus
Chthamalus
HEIGHT IN INTERTIDAL RELATIVE TO
LOW TIDE WATER LEVEL
Top
Question, hypothesis,
prediction
Question: Why is Chthalamus
restricted from lower intertidal
zones?
Hypothesis:
Prediction:
If (hypothesis), then
(prediction).
If competition with Balanus restricts
the distribution of Chthalamus, then
when Balanus is removed Chthalamus
will expand its distribution.
If competition restricts the distribution of Chthalamus, then when
Balanus is removed Chthalamus will expand its distribution.
Numbers
Balanus
Chthamalus
Numbers
HEIGHT IN INTERTIDAL RELATIVE TO
TIDE WATER LEVEL
HEIGHT IN INTERTIDAL RELATIVE TO
TIDE WATER LEVEL
What I see
LOW
What I predict
LOW
Testing hypotheses
If this IS what happens, have I proven my
hypothesis?
We never prove a hypothesis!! We can be
right for the wrong reasons (Every time
we make a prediction we also make
other ASSUMPTIONS about the system
that may not be true)
Testing hypotheses
Prediction not met: we REFUTE or REJECT
our hypothesis
Prediction met: our hypothesis is
SUPPORTED but not proven
A hypothesis is always subject to new tests and
the possibility of being rejected.
Beware of :
“Scientists have proven that…
On the Other Hand…
“Not proven” ≠ No understanding
“As a result of deep-rooted social and political
feelings concerning the collapse of the sardine
off California, many conflicting hypotheses
have arisen, in spite of the completion of a vast
amount of research.”
- Ludwig, D. (2001). "The era of management is over." Ecosystems
4: 758-764 [510 Reading]
• Which hypotheses are best supported?
Scientific Method
1.
2.
3.
4.
5.
Observe
Ask questions
Generate a testable hypothesis
Test hypothesis: collect data
Evaluate hypothesis: Do data support or
refute?
6. Make more observations, Generate new
hypothesis
7. Test hypothesis
8. And on and on and on and on…..
See also Stiles Fig. 1.10 & “The Field Ecologists Scientific Method”
on Canvas
NRE 509 Schueller Lecture 2:
Experiments in Ecology
1. From observation to results –
testing hypotheses
2. What are the essential elements of
experiments?...
Creativity: Experiments in
Ecology
What are the essential
elements of ecological
experiments?
An experiment measures the effect of
changing one variable on another variable.
Response variable is
what you
measure =
dependent
variable (y axis)
Categorical
Without
competitors
Performance of
individual plant
Manipulated variable
is what you
change or choose
= independent
variable (x axis)
Performance of
individual plant
Either variable can be:
With
competitors
Or Continuous
Density of competitors
What is the effect of rabbit herbivory on plants?
Treatment
Control
Replicates of controls and
treatments.
How many?
Why?
Response Variable
What is the effect of rabbit herbivory on plants?
Individual: growth, survival, reproduction,
behavior, morphology
Population: birth rates, death rates, growth, sex
ratios, age structure, distribution in space
Community: species composition, % abundance,
diversity
Ecosystem: productivity, decomposition, energy
flux, nutrient and water dynamics
Why measure more than
one response variable?
Example
What is the effect of parasites
on house martins (bird)?
Treatment = With blood parasite,
Control = without
Response variables:
Clutch size (# of eggs laid)
Brood size (# birds) at hatching
Brood size at fledging
Weight at fledging
Fig. 15.7
Which showed an effect?
Can measure effects alone and
together
Zaneveld et al.
2016. Overfishing
and nutrient
pollution interact
with temperature
to disrupt coral
reefs down to
microbial scales.
For your graph:
1. What is the dependent (response)
variable? Is it continuous or
categorical?
2. What is the independent (manipulated)
variable? Is it continuous or
categorical?
3. What was the QUESTION being tested?
% defoliation
(= leaf loss due to cankerworms)
1
Fig. 11.2b
Root Dry Weight (g)
Root Dry Weight (g)
2
Watered, Unwatered
Days
Days
So.. Experiments have:
1. Treatment(s) – manipulated
variable
2. Control – something to compare to
3. Replicates
4. Response variable(s)
How does this differ from trial and error?
“Bad science”
Trial and error: may change more than one
variable at a time and have no replication
Anecdotal: conclusion based on only one or
a few observations
Pseudoscience: scientific-sounding claims
that are not supported by unbiased
(agenda?), methodical scientific studies.
Bias: Sampling, measuring, or reporting in a
way that skews the results, even
unintentionally (see assigned reading on
predator control methods)
NRE 509 Schueller Lecture 2:
Experiments in Ecology
1. From observation to results –
testing hypotheses
2. What are the essential elements of
experiments?... Of real-world
adaptive management?
Adaptive Management/
Experimental Design
How do you figure out the effectiveness of:
• Management/policy choices
• Technological approach
• Environmental education program
• Efforts to improve community environment
• ….
From assigned reading:
“…restoration projects costing
$70 billion have attempted to
restore or re-create 7.4 million
acres…But hardly anyone does a
proper job of measuring the
results.”
Adaptive management/
Experimental Landscape
design
Strategies are experiments
http://www.gcdamp.gov/
“Tianjin Qiaoyuan Wetland Park in Tianjin,
China, has terraced ponds that incorporate
designed experiments to monitor benefits.”
Adaptive Experimentation
“Manipulative
experiments are
rarely used in
studies of human–
environment
interactions, despite
the key role that
experimentation
usually plays in
science.”
Cook et al 2004. Learning to roll with
the punches: adaptive
experimentation in humandominated systems
NRE 509 Schueller Lecture 2:
Experiments in Ecology
1. From observation to results –
testing hypotheses
2. What are the essential elements of
experiments?... Of real-world
adaptive management?
3. Different experimental approaches:
balance between realism and
precision/control
Experiments to test a hypothesis in Ecology
Natural
Field
Semi-field
Trade-off
REALISM
Lab
Simulation
Model
PRECISION
AND
CONTROL
Natural Experiments
Change in manipulated variable chosen but NOT
caused by ecologist
Before
After
e.g., Volcanic eruptions, hurricanes, drought,
different locations/habitat types, islands
Most 509 labs & Citizen Science programs
are Natural Experiments
Compare/relate
existing variation
Natural Experiments
REALISM
Advantages:
• Very high realism and generality
• Large spatial and temporal scales
•
Limitations:
• Very low control
•
PRECISION
AND
CONTROL
Field Experiments
Ecologist changes manipulated variable –
can still be large scale!
Field Experiments: Whole-lake manipulation
David Schindler,
Ontario,
1974; See Fig. 27.5
Field Experiments
REALISM
Advantages:
• High realism and generality
•
Limitations:
•
PRECISION
AND
CONTROL
Semi-Field Experiments
“experimental garden or mesocosm”
Semi-Field Experiments
REALISM
PRECISION
AND
CONTROL
Advantages:
• Better control, especially initial conditions
• Lower variation among replicates
• Mimic some natural conditions
• Can do treatments not possible or desirable
in the field (e.g. toxins).
Limitations:
• Loss of some realism and generality
Laboratory Experiments
REALISM
Advantages:
• Very high control, precision
• Low variation among replicates
• Many treatments and replicates
Limitations:
• Very low realism and generality
•
PRECISION
AND
CONTROL
Question: Why are dung beetles declining?
Prediction
then beetles raised in dung from treated
animals (independent variable or
treatment) will have higher mortality rates
(measurable response variable) than those
raised on dung from untreated animals
(control/comparison).
Hypothesis
If dung beetles are declining (observation)
because of the use of antiparasitic drugs in
livestock (proposed explanation),
Question: Why do cacti have thorns?
….In groups of 3: Write an “If, then” (with labels) and hand it
in. Hypothesis can be ecological or evolutionary.
Simulation Models
Climate change models
Compare different hydrologic
scenarios – inform Everglades
restoration
Models
Advantages:
• Considers many manipulated factors in very
complex systems.
• Can provide tests not possible through other
experimental approaches
Limitations:
• Realism of model assumptions
And What is METAanalysis?
After class: Fill in this table with 2 new friends
high, low, med?
Control over
variables
manipulated?
Variation among
replicates?
Many replicates
possible?
Realism?
Example
Natural
Field
Semi-field
Lab
Model
Moral of the Story
• No single, best approach to studying
ecology!
• Each has advantages and limitations!
• Using a combination of approaches is
a powerful strategy!
What about statistical
analysis?
• T-test
• Chi-square
• ANOVA
• Regression analysis
• And many more…..
Are the differences
between treatment
and control due to the
treatment, or just a
product of random
variation or chance?
Self assess
• Match the following with each of the bracketed portions of this
hypothesis and prediction: independent variable, hypothesis,
observation, predicted response variable, control: “If [dung
beetles are declining] [because of the use of antiparasitic drugs
in livestock], then [beetles raised in dung from treated animals]
will have [higher mortality rates] than [those raised on dung
from untreated animals].
• Do results ever PROVE a hypothesis?
• In what way might an experimental approach differ from a trialand-error or managagement approach (i.e. what aspects of
experiment might be absent)? How does this relate to
evidence-based decision-making?
• Fill in a table that describes the advantages and disadvantages
as well as an example of a natural, field, semi-field, lab and
modeling experiment.