INTERSPECIFIC AND INTRASPECIFIC COMPETITION I
One of the central questions in ecology is how important is competition among individuals of
the same species (intraspecific) and among individuals of different species (interspecific) in
determining the fitness of individuals in populations, and ultimately the structure of
communities. One way to address this question is to grow organisms in single species groups
and in mixed species groups in the laboratory, and comparing their performance. Such
laboratory experiments can manipulate population density as well as environmental factors
such as nutrients and light, i.e. limiting resources for which individuals compete.
Today, we will initiate the experiment by planting the seeds in pots in the greenhouse. Next
week during a portion of the laboratory, you will census your plants so that you know how
many of each species survived in each treatment. The collard seedlings should be very easy
to distinguish from the radish seedlings. The collards are shorter initially than the radishes
and have smaller cotyledons. Also, the radishes have red stems and hairy first leaves.
This exercise is an analysis of the effects of nutrient availability and population density
on the growth of individuals of two species growing alone and in mixed species
groups. Therefore, in the experiment plant species, nutrient availability, and density
will be varied using a 3 x 2 x 2 factorial design.
A factorial experiment consists of two or more factors (treatments), each with distinct
values or “levels”, and whose experimental units take on all possible combinations of
these levels across all factors. An experiment using factorial design allows one to
examine simultaneously the effects of multiple independent variables as well as their
degree of interaction.
PLANT SPECIES
We will use two plant species, radish (Raphinus sativa) and collard (Brassica oleraceae), as
the experimental units. In the experiment, plants will be grown in monospecific (single
species) pots as well as mixed species pots (1:1 ratio) to quantify the effects of intra and
interspecific competition, respectively. Thus, three levels of plant species will be used in
this experiment: radish alone, collard alone, and radish:collard mix.
DENSITY
The intensity of competition for limited resources is likely to increase as population
density increases. To understand how population density may influence competitive
interactions two levels of population density levels will be examined in this experiment.
Low density pots will consist of 8 individuals per pot, while high density pots will
contain 64 individuals. Each plant mixture (radish alone, collard alone, and the radish
collard mix) will be planted at low and high density. Recall, the radish collard mix is a
1:1 ratio of radish:collard, thus the low density is a mix of 4 radish and 4 collard, along
with 32 radish and 32 collard for the high density mix.
NUTRIENTS
Competition between individuals may occur for one or more limiting resources including
space, nutrients, mates, etc. For plants living in the same location, competition is often
intense for vital nutrients (e.g. nitrogen and phosphorous) which must be obtained from
the soil. In areas of high nutrient availability you might predict that plants would grow
faster, and that competition for light (maximizing photosynthesis and growth) may be
more important. On the other hand, in areas of poor nutrient availability, plants may
grow more slowly, and competition may shift belowground to increase root growth, and
maximize growth via nutrient acquisition. These predictions may or may not be valid for
your experiment. The outcome of competition under high and low nutrient availability
may seem straightforward, but the results may depend on the interaction between nutrient
and density treatments. For example one of the species may perform better at low density
but be severely impaired at high density. It is up to you to investigate the background
literature, and decide for yourself based on available data what you predict will be the
outcome of competition under these experimental conditions.
Two levels of nutrients will be established in this experiment (low and high). The high
nutrient treatment will include the addition of slow-release fertilizer to two replicates of
each species x density treatment (see experimental design below). The low nutrient
treatment will consist of unmanipulated soil. Both low and high nutrient availability
levels will be established at all levels of density and plant species. Be sure to follow the
labeling scheme (Table 1) in order to keep track of each individual species x density
x nutrient treatment.
Important note: The success of this experiment depends on the ability of the
investigator to implement the design in a consistent manner. If you place all of the seeds
in the center of the pot, the actual density that the seedling will experience will be much
greater than what the experimental design intended. Place the seeds equidistant from one
another. In pots where two species are present, alternate species in your planting array.
This will cause each individual of the two species to encounter individuals of the other
species to the same extent. After arranging your seeds on the surface of the soil in the pot,
cover them with a thin layer (1 cm) of soil by sprinkling soil across the top. Be careful
not to bury them too deeply. Be sure to label each of your pots with a tape label (see
labeling scheme in Table 1).
Each week you will census your plants and observe their growth e.g. how many of each
species in each pot have their first true leaves, second true leaves, etc. After five weeks we
will terminate the experiment and collect the above ground portions of the plants. These will
be counted and weighed to determine which species was most successful under which
treatment. The observations that you make each week are very important. The first week's
observations tell you how successful you were at setting up the experiment, how many seeds
germinated successfully and in what spatial array. The subsequent weeks’ observations reveal
the course of the competitive interactions: are all plants growing at the same rate in all
treatments, how are the plants different, etc. These observations are what will allow you to
interpret fully the final biomass results of your experiment.
EXPERIMENTAL DESIGN
In this experiment you are manipulating three variables, each with discrete levels. There are
3 levels of plant species (radish alone, collard alone, and radish:collard mix), 2 levels of
density (low and high) and 2 levels of nutrients (low and high) for a 3 x 2 x 2 design.
Therefore, there are 12 possible treatment combinations. The class will be broken down into
4 groups of students. Each group is responsible for planting TWO REPLICATES of
each of the 12 possible treatment combinations. THUS, EACH GROUP IS WILL
PLANT 24 TOTAL POTS. The replicates from all groups will be pooled at the termination
of the experiment.
Table 1: Experimental design and treatment combinations for plant competition.
PLANT SPECIES
Density
8
64
nutrients
low
low
high
high
low
low
high
high
Replicate
1
2
1
2
1
2
1
2
RADISH
ALONE
Pot label
1R-8-L
2R-8-L
1R-8-H
2R-8-H
1R-64-L
2R-64-L
1R-64-H
2R-64-H
COLLARD
ALONE
Pot label
1C-8-L
2C-8-L
1C-8-H
2C-8-H
1C-64-L
2C-64-L
1C-64-H
2C-64-H
RADISH:COLLARD
MIX
Pot label
1R:C-8-L
2R:C-8-L
1R:C-8-H
2R:C-8-H
1R:C-64-L
2R:C-64-L
1R:C-64-H
2R:C-64-H
Pot label code = replicate# plant species - density - nutrient treatment
Be sure to label all of your pots as indicated in Table 1, and with your group
name and section number.
PLANT COMPETITION LABORATORY SET UP NOTES
There are a few important details about the lab set-up that must be kept in mind in order to
execute a precise and rigorous experiment.
1. For all treatments be sure to space the seeds in a uniform distribution in the pots.
That is, be sure to space the seeds equidistantly from one another. DO NOT PLACE
SEEDS IN A CLUMP IN THE MIDDLE OF THE POT!
2. For the mixed species treatments: Be sure to alternate seed patterns (radish, collard,
radish, collard). YOU DO NOT WANT ALL THE RADISH SEEDS ON ONE
SIDE, AND ALL THE COLLARD SEEDS ON THE OTHER SIDE.
3. For all treatments: be sure to cover the seeds with approximately the same amount of
soil. The seeds in all replicates should sit approximately 1 cm below the soil surface.
THIS IS IMPORTANT, AS YOU WANT ALL THE SEEDS TO GERMINATE
AT THE SAME TIME.
4. Be sure to label your pots with pot label code (see above), group name, and
section number.