Lumbricus terrestris: Dispersion Patterns at the UCA Nature Preserve

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Lumbricus terrestris Dispersion Patterns at the
UCA Nature Preserve

• Introduction
• As a group, we decided to study Lumbricus
  terrestristhe 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.

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Methods

• The general hypothesis was suggested by Dr.
  Larsondispersion 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.

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Results

• 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 13 0.43 0.301
1 6 0.20 0.361
2 4 0.13 0.217
3 6 0.20 0.087
4 1 0.03 0.026
5 0 0.00 0.006
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Probabilities of Spatial Distributions
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Conclusion

• 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 physico-chemical
  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. 

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Conclusion Continued

• 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.

• 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.
• Edwards, C.A., and J.R. Lofty.  Biology of
  Earthworms.  London Chapman and Hall, 1977.