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Thomas Park’s experimental design
Controls and replicates
The experimental design behind the data from Park’s 1954 paper consisted of three kinds of
populations: (1) single species control populations of Tribolium castaneum; (2) single
species control populations of T. confusum; and, (3) experimental two-species competition
populations.
For each single-species control populations (1 and 2), 20 replicates were established for
each species; for the experimental populations, 30 replicates were set up. Thus, there were
a total of 70 populations.
The replicate populations were set up identically and handled in the same way until the end
of the experiment. Thus, differences from one replicate to another provide an estimate of
the variation intrinsic to the biological process being studied. The control replicates differ
from one another due to variation intrinsic to the biological process of population growth,
while the experimental replicates differ due to variation intrinsic to population growth when
two species are reared together.
Why are the single-species populations considered controls? The inference that two
species reared together are in competition with one another is drawn when one observes a
difference between the control replicates taken together and the experimental replicates.
Since each process has intrinsic variation, the difference between the control and
experimental populations must be greater than or exceed the differences between identical
replicates within a control.
Climate treatments
Park went further to investigate the effects of climate (temperature and humidity) on singlespecies population growth and on two-species competition by setting up populations in 6
different climates. He combined three temperatures (T), 260C, 290C, and 340C with two
humidities (H), 30% and 70% that span the range of conditions which allow population
growth in these species. The 6 combinations of T-H approximate the major climatic types,
ranging from Hot-Wet (340C – 70%) to Temperate-Wet (290C – 70%) to Cold-Dry (240C –
30%).
He achieved these climates by using 6 incubators, each maintained at a specific
combination of T and H. For each climate treatment, Park set up 20 replicate singlespecies populations (total 40) and 30 two-species populations, except for the Cold-Dry
climate where he set up only 15 single-species replicates and 20 two-species replicates
(total 50)1.
How was each population set up? A population consisted of eight adult beetles, four males
and four females, in an vial with 8 grams of dry growth medium (95% by weight stone
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How many populations did Park set up counting all populations in all climates? Answer: (5
climates)(20 + 20 + 30 per climate) + (1 climate)(15 + 15 + 20) = 400 populations!
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ground wheat flour and 5% by weight dried brewer’s yeast). If the population was a singlespecies control, then every male and female was a young adult of the appropriate species.
In a two-species population, 4 adults were T. castaneum and the other 4 adults were T.
confusum.
Once a population was assembled, it was placed in the appropriate incubator and the
beetles were left alone for 30 days to eat, mate, lay eggs, hatch and develop. After 30 days,
the eggs, larvae, pupae and adult beetles (or ‘imago’) of each population were sifted out,
counted (except for the eggs) and transferred to new medium (see diagram below).
From Park (1962, Science)
These census data constitute the primary observations and from them we can determine:
(1) how climate affects population growth; and (2) how climate affects competition. A
census of each population was conducted every 30 days for well-over two years.
Bibliography
Park T (1954) Experimental studies of interspecies competition II: Temperature, humidity,
and competition in two species of Tribolium. Physiological Zoology 27(3):177-238 URL:
htp://www.jstor.org/stable/30152164
Park, T. (1962) Beetles, competition and populations. Science 138(3548):1369-1375,
doi:10.1126/science.138.3548.1369, http://www.jstor.org/stable/1709946.
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