Forest floors are active places consisting of intertwined life cycles among various organisms. The main predators in this web of activity, wandering spiders, do not actually spin webs themselves. Among their prey are the large wingless bugs of the order Collembola. More commonly known as springtails, Collembola graze on the fungi growing on the forest floor. And the fungi, in turn, decompose fallen leaves, completing a cycle that is "essential to life," explains lead author Janet Lensing. The fungi decomposers return the nutrients back to the plants growing in the forest.
Lensing and her colleague David Wise cordoned off plots in a deciduous forest in central Kentucky. For three years, they manipulated the rain that fell on these plots based on predictions of Kentucky's weather from climate change models. The researchers built large shelters over plots and irrigated them with rainwater so they received either more or less rain than the 100-year regional average. "We were just interested in how rainfall affects this food web," Lensing says.
Drought usually slows down the fungi's ability to decompose plants--after all, fungi do not like dryness and grow more slowly as a result. But Lensing and Wise discovered that decomposition actually occured faster during a drought than during excessive rains.
Wandering spiders, it seemed, were helping out. In times of drought, springtails overgraze on the fungi and reduce their numbers. But no matter the weather, the spiders feed on the springtails. When spiders gobble up those bugs during a drought, it takes pressure off the fungi, allowing the latter to grow and quickly cycle nutrients back to the plants despite the drought, the researchers report in the current issue of the Proceedings of the National Academy of Sciences USA. Lots of rain, however, had no effect on the rate of leaf litter decomposition, perhaps because the fungi are not overly stressed and actually thrive when springtails munch on them a bit, the researchers speculate. Predatory spiders may offset the boosted fungal activity in this case, leading to no change in decompositon rate or possibly depressing it.
Because of the sheer complexity in the way parts of an ecosystem work together, just how places like the forest floor will be influenced by climate change will most likely remain difficult to predict, Lensing says. Not every spot will have spiders to take up the slack.