Lumbricus terrestris: Dispersion
Patterns at the UCA Nature Preserve
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Introduction
As a group, we decided to study
Lumbricus terrestris—the common
earthworm.
Some earthworm facts: Earthworms
were reintroduced to the soils of
North America, are most numerous in
grassland & mull soils, are
detritivores (improving soil with their
castings & helping to eliminate
erosion), and are 70% protein (a
major food source for many
secondary consumers). Links:
www.nysite.com &
www.eap.mcgill.ca
This was an easy organism to study &
was given as an example by Dr.
Larson at the beginning of the
experiment.
Methods
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The general hypothesis was suggested
by Dr. Larson—dispersion patterns
would be “random.”
Each member of the group chose a
random direction to start each plot of
the study.
We decided that the plot size should
be 1 ft x 1 ft.
We removed the plant litter from the
surface soil & counted the number of
worms in each plot.
We recorded the data for thirty
random plots.
Some predictions made during the
experiment: worms were believed to
be found in wetter soils and worms
were believed to be mating.
Results
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In thirty random plots within the
ecology site, we found 36
earthworms.
Average= 1.2 individuals per plot
We expected 9 plots with 0
individuals, but we found 13 plots. In
addition, we found less plots with 1-3
individuals than expected by the
Poison Probability.
The results depict a contagious
dispersion (compare to Figure 4C.5 in
Lab Manual).
Our variance-to-mean ratio suggests
random dispersion with a high
tendency toward contagious
dispersion (Figure 4C.6 in Lab
Manual).
Number in
Plot
Observed
Frequency
Observed
Probability
Poisson
Probability
0
1
2
3
4
5
13
6
4
6
1
0
0.43
0.20
0.13
0.20
0.03
0.00
0.301
0.361
0.217
0.087
0.026
0.006
Probabilities of Spatial Distributions
Observed & Poisson (Random) Probabilities
0.500
0.450
Proportion of Plots Containing X Individuals
0.400
0.350
0.300
Observed Probability p(X)
Poisson Probability P(X)
0.250
0.200
0.150
0.100
0.050
0.000
Number of Individuals in a Plot, X
Conclusion
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Our study supports the hypothesis that
earthworms are randomly distributed in the
soil. There are several reasons this might be
true. According to Edwards and Lofty in
Biology of Earthworms there are three factors
that may attribute to the random dispersion of
earthworms in the soil. The first is physicochemical factors which include soil pH,
moisture, temperature, inorganic salts, aeration,
and soil texture. Edwards and Lofty explain that
the only two of these factors truly important in
earthworm population dispersion are moisture
and temperature. Moisture is important because
90% of an earthworms weight is actually water
and water is directly correlated to earthworm
fecundity. Reproduction rates are better in
wetter environments. Temperature is relevant to
the vertical dispersion of earthworms. At
different times of year earthworms are more
likely to be found in higher or lower levels of
soil. Usually in high heat earthworms are more
likely to be found deeper in the soil (below 7.5
cm). However, recent lower temperatures and
frequent rain may have brought the earthworms
closer to the surface.
Conclusion Continued
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A second important factor in earthworm
population dispersion is food
availability. According to Edwards and Lofty
this generally points toward aggregations rather
than random or regular patterns. Due to the
abundance of herbage litter in our plots,
however, it is likely that we found a random
dispersion because all of our plots contained
equal amounts of available food and caused a
more random pattern.
The third factor is reproductive potential
and dispersive powers. Immature earthworms
tend to be more aggregated and adult
earthworms tend to be more random. At times
when reproduction is high one would most likely
find aggregations of worms because of the age of
the worms. However, if it is not a regular mating
season or conditions are not right for mating
(such as too high temperatures which reduces
earthworm activity) then the patterns should be
more random because the majority of the
population would be adults.
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A possible hypothesis for the randomness of
the dispersion of the earthworm population in
Jewel E. Moore Nature Reserve could be the
widespread availability of food and
unseasonable wetness and cooler temperatures.
Another possible hypothesis is that the worms
were actually in aggregations due to moisture for
reproduction and unseasonably cool
temperatures and our sample plots were too
small and close together for an accurate
depiction of the dispersion patterns.
A third possible hypothesis is that the
earthworms took advantage of the moisture and
coolness a few days prior to our samples and had
the reproductive potential and dispersive powers
to cause the immature worms to have already
moved far enough apart for random dispersal.
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Edwards, C.A., and J.R. Lofty. Biology of
Earthworms. London: Chapman and
Hall, 1977.