The results from my 2015 study on the interactions between spiders, grasshoppers, and rolly-polies. Notably, rolly-polies cause the spider to grasshopper effect on soil nitrogen mineralization (Nmin) to disappear, as indicated by the arrow.
Two high school science fair students and I became interested in the behavioural interactions between grasshoppers, spiders, and rolly-polies. Data from my previous work showed that the strong effect of spiders Pisaurina mira onto grasshoppers Melanoplus femurrubrum disappeared when the rolly-polly Oniscus asellus was present. Normally these spiders scare the grasshopper but don't normally get to eat a lot of them. The data suggested that the rolly-polies were causing the spiders to eat more grasshoppers meaning the strong effect of grasshoppers on soil nitrogen mineralization disappeared.
We wondered why the rolly-polies were making the spiders better at catching grasshoppers. To answer this question we have been conducting behavioural studies examining where and how these spiders and grasshoppers spend their time. We hypothesized that the rolly-polies were scaring the grasshoppers and making it more likely that they ran into spiders.
Looking for grasshoppers and spiders in the leaf litter of a behavioural observation cage. Despite the cage being small these animals are very sneaky and hard to spot!
The results produced another surprise: it was the spiders not the grasshoppers that were moving because of the rolly-polies. The grasshoppers like to hang out high up on the plants during the day, but the spiders prefer lower in the canopy to avoid the hot sun. However, when the rolly-polies were present the spiders were more likely to move upwards and more likely to encounter the grasshoppers. We think this is because these spiders hunt by sitting and waiting in one place. They don't eat rolly-polies, but might be disturbed by them and decide to just climb a little higher to avoid wasting energy reacting to the rolly-polies.