Isopod Experiment

Moisture Preferences of Terrestrial Isopods, Porcellio scaber

The terrestrial isopod, Porcellio scaber, prefers moist environments over dry environments.

Null Hypothesis:
The terrestrial isopod, Porcellio scaber has no preference between moist and dry environments.

If the terrestrial isopod, Porcellio scaber prefers moist environments over dry environments, then when given a choice, individuals will spend more time in the moist soil/moss environment than the dry soil/moss environment.

Experimental Design:
My partner and I were assigned the Wilcoxon matched-sign statistical test, which is non-parametric (the data is not normally distributed). For this test, the dependent variable must be continuous, and the independent variable must be categorical and only contain two categories; these categories must also be matched, meaning that individuals will be tested twice in two different conditions. Based on these requirements, we designed the following experiment to test isopod preference of wet vs. dry environments:
Independent Variable:
The environment is what we are manipulating; the conditions include wet soil, dry soil, wet moss, and dry moss.
Dependent Variable:
Time spent (out of a total of 2 minutes) on dry and moist sides of both soil and moss environments.
To begin, eight pillbugs were placed individually in eight Dixie cups to eliminate any bias when we tested them individually. Each individual was tested twice, once in each environment (soil and moss). The environments were created using circular plastic containers, shown below. The first container was filled with soil, and water was added to half of the soil to create a moist but not wet environment. The second container was filled with a thin layer of sand before dry moss was placed in half the container, and wet moss was placed on the other (the sand collected the excess water as to not drown the pillbugs). A wooden dowel was placed in both environments to clarify the separation between moist and dry sides. Testing each isopod individually, each trial began by placing an individual on the wooden dowel. They were given one minute to explore their new environment, and at the end of the minute they were placed back on the dowel, at which time a two-minute period began. The duration of total time spent on either side of the environment was then recorded. Each isopod was run twice, once in each environment; four isopods were run in the soil environment first, while the other four were run in the moss environment first to avoid bias.


Analysis: Wilcoxon Matched Sign Test

  • Variables:
    • x = the least frequent sign (not including 0)
    • N = the number of pairs that showed a difference (+ or -)
    • p = resulting p value looked up in a table.
  • Moss:
  • Soil:

With a confidence level of p=0.05, we have to accept the null hypothesis; there is statistically no significant preference for dry or moist conditions in the moss environment (Wilcoxon matched sign test, p=0.188) or dry or moist conditions in the soil environment (Wilcoxon matched sign test, p=0.656). This is the opposite of what we predicted, as we thought they would display preference for the moist environments. If we were to re-run this experiment, it might be helpful to have a larger sample size.

Leave a Reply

Your email address will not be published.