Have you experienced seeing tons of sowbugs crawling up when you lifted a log in a park forest? You would have probably freaked out. This trauma makes us often hesitate flipping logs lying down. But why do you think there are so many bugs, especially sowbugs, underneath those logs? We can easily assume that a log is providing a good environment for these sowbugs, by covering where they live or decreasing the exposure to air. However, exactly what factor enhance their survival under logs? Some factors that seemingly influence the survival of sowbugs are temperature, moisture, and light. We designed experiments to test the influence of these factors on the survival of sowbugs. First, we wanted to know if there is an actual difference in the number of sowbugs on top and under logs in a forest. Since the temperature is one of our guesses, we measured the temperature on top of the log and under the log to see whether they are different. Our experiment in Pacific Spirit Park, British Colombia showed a difference in isopod density on top and under the logs. As expected, under the logs had a much higher density. Unlike density, there was no difference in temperature on top and under logs. Thus, we found out that the higher density of isopods under the logs is not due to a temperature difference.
Once we verified the density difference between the top and the bottom, we carried out experiments for temperature, moisture, and light in a laboratory setting. By measuring the preference of isopods to each factor, we tried to explain which one might be related to density of isopods under logs. Although temperature on top and under the logs was not different, it does not mean that these sowbugs have no temperature preference. Thus, we first proceeded experiment on their temperature preference using foil trough half sitting on ice water and the other half on lukewarm water. The result was each bug was found more often on the cold side of aluminum trough.
We tested their light or darkness preference by taping one side of a petri dish to block the light on that side, and they did not show any preference. The last factor left to be tested was moisture. Most of the time, they ended up on the wet side of petri dish overlaid by damp filter paper. We see that the area underneath the log is wet and our laboratory experiment shows they prefer a wet to dry environment. Thus, we can say that moisture influences on the difference in the number of isopods found on top and under logs. One limitation to this conclusion is since we did this experiment in winter, we do not know if there is a difference in density of sowbugs on top and under logs during other seasons. Thus, we will need another experiment! Besides, we do not know biotic factors interacting with the factors we tested on these sowbugs. For example, there are certain bacteria naturally living in the stomach of common sowbugs. We don’t know how these bacteria might affect the sowbug survival. Further studies on potential influence of the bacteria on isopods can help us understand more about life of isopods and their survival. Here is a link to the study on the bacteria living in an isopod species. https://academic.oup.com/femsec/article/56/1/132/562729/Diversity-of-commensal-Bacillus-cereus-sensu-lato