Intraspecific and interspecific competition experiments
Objective:


Determine the effects of both interspecific and intraspecific competition on total
yield, survival, individual growth
Do density-dependent factors play a role these factors?
Background Information (posted on web site):
Williams, A. C. and B. C. McCarthy. 2001. A new index of interspecific competition for
replacement and additive designs. Ecological Research 16: 29-40.
Materials needed:
For first lab (planting):
 Planting trays
 Potting soil
 Seeds (radish and wheat or radish and rye)
For second lab (harvesting):
 Scissors or knife to cut stems
 Balance
Procedure
Plant seeds in the following 9 configurations:
A = radish
B = wheat or rye
1 = 16 seeds alone, 8 when two species planted
2 = 32 alone, 16 in combination
3 = 64 alone, 32 in combination
Treatment name Total # of seeds
A1 B1 AB1
16
A2 B2 AB2
32
A3 B3 AB3
64
Plants will be grown under full sun conditions for two weeks.
At the end of the experiment, count the number of individuals that germinated, harvest
shoot tissue, and measure the wet biomass for each species and treatment.
Data analysis and homework assignment:
Use the calculations described below to analyze your data and discuss whether
interspecific or intraspecific competition occurred in your experiment and, if so, under
which treatments.
Determine the yields for each species under each treatment:
YA = yield of species A in monoculture
YB = yield of species B in monoculture
YAB = yield of species A in the presence of species B
YBA = yield of species B in the presence of species A
YDA = yield of species A grown at an overall density of D (e.g., 16 seeds/pot, 32
seeds/pot, or 64 seeds/pot).
Please note that yield can be defined as the total yield of the pot or can be modified to
represent yield on a per plant basis (divide by the number of seeds planted OR by the
product of density and proportion).
pA= proportion at which species A was sown (1.0 for monoculture or 0.5 for mixed
culture with 50% of the seeds representing species A).
pB= proportion at which species B was sown
Note that:
pA + pB = 1
Calculations:
1. Relative yield (RY) measures the yield of each species when grown in mixed
culture divided by the yield in monoculture, while taking into consideration the
proportions at which each species is sown.
D
RYAD  YAB
( pAYAD )
D


RYBD  YBA
( pBYBD )
Therefore, if there is no interspecific competition effect, RY = 1. If RYA > 1, then
species A does better competing against species B than against itself. In contrast, if
RYA < 1, then competition with species B is detrimental to species A and the
interspecific competition effect is stronger than intraspecific competition. Please see
Fig. 1 in Williams and McCarthy (2001) for a graphical depiction of how to interpret
different RY values.
2. Change in Contribution index (CC)
D
D
D
CCAD  (YAB
/(YAB
 YBA
)) (( pA YAD ) /( pAYAD  pBYBD )) 1

This index reflects the proportion of biomass obtained in mixed culture divided by the
expected proportion from monoculture data. By subtracting one from this value, we
have the proportional change (increase if positive and decrease if negative) in
biomass attained in mixture compared to biomass attained in monoculture.
In order to determine severity of competition based on individual plant biomass, by
determining biomass per individual (W=total biomass divided by the number of
individuals harvested).
Intraspecific competition can be determined by calculating the absolute severity of
competition (ASC):
ASC  log( W0 / Wd )
Where W0 is the biomass per individual (biomass divided by the number of individuals
harvested)) under no competition (treatment 1) and Wd is the biomass per individual
under competition (treatments 2 and 3).