BOT 332, Plant Ecology
Lab Exercise IV: Competition1
Objectives:
1. Become familiar with the basic tenets of experimental design, analysis, and
interpretation.
2. Set up an experiment to measure the relative effects of inter- and intra-specific
competition on plant growth and size hierarchies for plants grown at different
densities and proportions.
Background:
Increased density of individuals is expected to increase competition among plants
for resources such as space, water, light, and nutrients. Competition may be defined by
the mechanism, i.e. the capacity for resource capture and rapid growth when resources
are readily available; or defined by the outcome, i.e. ability to maintain growth and
reproduction when resources are limited, due to a low equilibrium resource requirement.
Generally speaking, competition can be classified as either intraspecific (within a species)
or interspecific (among species). Both within and among species, high competitive
ability can lead to skewed size distributions, decreased average growth, reduced or absent
reproductive output, or even mortality of other plants.
There is no doubt that some species are better competitors than others, yet when
we look outside we do not see a world dominated by a single dominant species. Why
hasn’t the “best” competitor replaced all the other species? Clearly, a species’
competitive ability, as well as the population/community level effects of competition will
be dependent upon numerous ecological and environmental factors. The intensity of
effects of competition can be influenced by plant density, spatial arrangement, soil
nutrient heterogeneity, intensity of other ecological factors (e.g. herbivory), resource
availability, and numerous other factors and processes.
We will use a multiple deWit replacement design to test for competition within and
between species. The multiple replacement design varies both density of plants and
proportion of plants of each species. Growing each species in monoculture at different
densities allows us to measure the strength of intraspecific competition. Growing pairs of
species in different proportions allows us to measure the strength of interspecific
competition. Comparing intra- to inter- specific competition in this way allows us to
estimate their relative importance.
The use of a multiple deWit replacement design will also allow us to test how planting
density affects size hierarchies of individual plants within each pot. Many studies have
found that as plants grow and compete for resources, the distribution of plant sizes
becomes increasingly skewed, with a few large individuals and many smaller individuals.
This effect has been hypothesized to be caused by asymmetric competition, where the
few large individuals have disproportionate competitive effects as they prevent their
smaller neighbours from obtaining essential resources such as light.
We will grow each species alone, and in different proportions with the other species.
Each density/proportion treatment will be replicated. Throughout the semester we will
measure a variety of response variables, including plant height and final aboveground
1This lab was designed by Elizabeth Elle, Simon Fraser University and modified for this course.
http://www.sfu.ca/biology/faculty/elle/
biomass. These data will be used to construct DeWit plots, estimate size hierarchies at
different densities, and test the relative importance of intra- and inter-specific
competition.
Write-up:
You will write-up the results of this experiment as a scientific paper. In doing so,
you will review results of other competition experiments done on other species and
compare your results with those obtained by other workers. Consult your lab manual
(p.18-25) for instructions on how to write a scientific paper, and follow Ecology format
when preparing your lab report. The report should be 10-12 pages in length
(doublespaced). See the course web page for a link to detailed information on how to
prepare this report.
You should address the follow issues in your write-up:
1. Compare the strength of inter- vs. intra-specific competition, size hierarchies
and growth rates for our two species at:
i.
different planting densities in monoculture
ii.
different planting densities in two species 'communities'
iii.
different relative abundances
2. For each of these different questions, discuss whether and how the responses
you observed differed depending on how size was measured (height vs.
biomass) or the time of measurement (height at 3 weeks vs. 5 weeks).
3. Discuss the relevance of your findings with respect to previously published
studies and theory. How are the strength of inter- vs. intra-specific
competition, size hierarchies or growth rate predicted to vary with time,
density or relative abundance? Did your findings support these predictions?
4. Give a critique of the experimental method and suggest ideas for further
research.
Readings:
For your write-up, you will need to include a minimum of five papers from the
primary literature. Chapter 10 of the course textbook provides a good introduction to the
study of competition in plant ecology and will point you in the right direction for finding
more studies, but note that textbooks and websites are not considered primary literature
sources!
Experimental Design and Setup:
We will use a multiple deWit replacement design to test for competition within
and between species. The multiple replacement design varies both density of plants and
proportion of plants of each species. Growing each species in monoculture at different
densities allows us to measure the strength of intraspecific competition. Growing pairs of
species in mixture allows us to measure the strength of interspecific competition.
Comparing intra- to inter- specific competition in this way allows us to estimate their
relative importance.
We will grow each species alone, and in mixture with the other species. Each
density/proportion treatment will be replicated multiple times. At the end of the
semester, we will measure our response variables, plant height and above-ground
biomass, and use this information to construct deWit plots, measure size asymmetry and
perform the statistical analyses documented here.
Basic design:
Density = 4
0 % Bean : 100 % Oat
25 % Bean : 75 % Oat
50 % Bean: 50 % Oat
75 % Bean: 25 % Oat
100 % Bean: 0 % Oat
Density = 8
0 : 100
25 : 75
50 : 50
75 : 25
100 : 0
Density = 16
0 : 100
25 : 75
50 : 50
75 : 25
100 : 0
Methods:
A. Preparation
Fill 15 pots to the lip with the soil provided. Label each pot with your group name, the
species names, pot density and the Bean:Oat proportion.ch density, for one
native/invasive pair.
For example, one label might read:
Bean:Oat
25:75
density = 4
Ted & Alice
Transplant seeds into the pots, as designated on your labels. Make a small hole in the
soil, insert the seed, and gently push over the hole. Seeds can be haphazardly arranged in
the pots. Bring the pots up to the greenhouse, and place on the designated bench.
B. Growth Measurements
To estimate plant size, we will take measurements repeatedly during the semester.
We will calculate the average values for individuals of a species within a pot. Individuals
within a pot are not independent, and so although we are interested in how individuals
respond, we must determine the average response. You will be assigned the pots to
measure so that all students have some low and high density pots to work with. Be sure
that you can distinguish the different species within pots, and be careful in recording data
and in transferring the data to the class data sheet to avoid any errors.
For each pot, note the ID, and copy the species names and number of individuals
of each species planted in the pot from the label (some plants may die over the
experiment, so you will need to note how many you actually measure as well). Please be
careful that you measure EVERY PLANT. Measure the length of the longest leaf to the
nearest mm (the length of the main stem to the apical meristem for the bean). Be
GENTLE with the grasses; if you damage them, it may affect your results! Make a list of
all your lengths and leaf numbers in the data sheet provided for you in lab. From these
numbers, you will calculate averages for each variable for each species in each pot
C. Final Measurements
In addition to one final size measurement, we will measure final plant biomass for
plants in each pot. You will be assigned pots to work with. After recording the species
and measuring the height of each plant, cut the plant off at the soil surface, and note
whether each individual is still alive (note that an ‘individual’ is one rooted genet;
multiple stems are not multiple individuals). Using the balances in the lab, get a fresh
biomass reading for each plant within each pot (remember that some pots have
individuals from just one species present). Write your data in the table provided for you
in the lab.
D. Data analysis
Your TA's will provide you with a copy of the pooled class data in spreadsheet form.
These data can be analyzed both descriptively and statistically. Basically, you are
interested in whether the average size of plants, size hierarchies within a pot, or plant
growth rate change with density and with whether plants are grown in monoculture or
mixture. To visualize and interpret the data, you will construct DeWit plots and plots of
growth rate, and calculate coefficient of variation of biomass within each plot. To
determine whether apparent differences on your plots are significant, you will perform
both t-tests and ANOVAs.
1. DeWit plots
For each species pair and density, you will construct a DeWit plot. Density is on the X
axis (for the purposes of these plots, ignore variation in survival), and mean mass per pot
and species is on the Y axis (traditionally termed ‘yield’). There are two lines plotted per
species pair/density combination. You should also draw the expected line if interspecific
and intraspecific competition are equal—this line joins zero to the yield of each species in
monoculture. Your graphs will look like this:
12
Species J
Species I
10
Yield per pot (g)
8
6
4
2
0
0.0
0.5
1.0
Proportion Species J
If yield is above the expected line, then interspecific competition is likely weaker than
intraspecific competition (the case for species I). If yield is below the expected line, then
interspecific competition is likely stronger than intraspecific competition (the case for
species J). This remains a tentative conclusion until you perform t-tests to determine
whether your data are significantly different from the expected value, at which point you
can state them with confidence. See the discussion of competition in your textbook for
more information regarding the interpretation of DeWit plots.
2. Size hierarchies
Symmetrical
Low CV
Asymmetrical
High CV
You will calculate how asymmetrical the size distributions of plants are at
different planting densities using the coefficient of variation (CV) of plant sizes in each
pots. CV for a given pot is calculated as:
CVpot = Standard Deviationpot / Meanpot
In this case, the mean is the average height or biomass of all plants in the pot, and the
standard deviation is a measure of the variability of plant heights or biomasses in that pot.
As the size hierarchy within a pot becomes stronger (a few large plants dominate
numerous smaller plants), the CV will increase. You will test for differences in CV of
height and biomass among different planting densities using an ANOVA, with density as
the independent variable and CV in each pot as the dependent variable.
3. Growth Rate
A simple way to visualize the change in plant size over time is to plot your size
variables over time. It’s best to do each species within each treatment separately. You
can also calculate growth rate using the equation for population growth, with lambda
defined as the relative growth rate:
lambda = N t+1 / N t
In this case, N t+1 will be the mean height for the plants the second time we measured
them, and N t will be their height the first time we measured them
To estimate the effect of interspecific competition on growth rate, subtract lambda
for each species in mixture from lambda from that species in monoculture. To estimate
the effect of intraspecific competition, subtract lambda at high density from lambda at
low density.
4. Statistical Analysis
We will do several different types of analyses of our data, ranging from qualitative
assessments of figures to statistical tests. Your TAs will provide you with a spreadsheet
with the raw data and statistical tests set up for you. The specific details of these tests will
be discussed when the data spreadsheet is provided. You will use t-tests, ANOVAs, and
plots of your data to examine the effects of density and inter- vs. intra-specific
competition on:
a. Yield in monoculture/mixture at different densities (deWit plots, t-tests,
ANOVAs)
b. Size hierarchies (histograms, CV, ANOVAs)
c. Growth rates (plots, t-tests, ANOVAs)
The spreadsheet will be set up with examples of all of these tests; it’s important that
you understand both how to perform them and how to interpret your results!!
Please do not hesitate to contact your TAs if you have any questions.