The impact of altered precipitation on spatial stratification and activity-densities of springtails (Collembola) and spiders (Araneae)

1. A field experiment was conducted to determine how short-term changes in moisture can alter activity-densities of spiders and springtails. 2. In a Kentucky forest 10 unfenced 4-m(2) plots were divided into two rainfall treatments. A clear roof over five plots excluded rainfall to simulate severe drought conditions (drought treatment). Water was sprayed on the five uncovered plots at a rate equal to two times the long-term mean in order to establish the high-rainfall treatment. Activity-densities of Collembola and spiders were measured using pitfall traps designed to sample the top, middle, and bottom layers of leaf litter. The experiment ran from 20 July to 23 September 2001. 3. Overall (i.e. litter layers pooled) activity-density (mean number trapped each sampling date) of Collembola was approximate to 60% lower in drought plots than in plots receiving increased precipitation. Surprisingly, overall spider activity-density was approximate to 1.6 times greater in the drought plots. 4. Differences in rainfall affected the spatial stratification of Collembola and spiders in strikingly different ways. Activity-densities of neither group differed between drought and high-rainfall treatments in the bottom litter layer. Collembola activity-density was three times greater in the top and middle litter layers in high-rainfall plots than in drought plots. In contrast, spider activity-density did not differ between treatments in the top layer, but activity-density was decreased by 50% in the middle layer of high-rainfall plots compared with drought plots. 5. Three Collembola families (Sminthuridae, Tomoceridae, and Entomobryidae) accounted for most of the Collembola pattern. The spider response was due to altered activity-density of one family of wandering spider, the Gnaphosidae.