Lumbricus terrestris : Dispersion Patterns at the UCA Nature Preserve

1. Lumbricus terrestris: Dispersion Patterns at the UCA Nature Preserve • Introduction • As a group, we decided to study Lumbricusterrestris—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.

2. Methods • 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.

3. 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).

4. Probabilities of Spatial Distributions

5. 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. 

6. 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. Conclusion Continued