Effects of rainfall manipulations versus a natural aridity gradient on plant litter arthropods in desert and Mediterranean ecosystems

Predicting responses of litter arthropods to future variation in precipitation is critical for understanding decomposition processes under global climate change. A field experiment on litter arthropods extracted from litterbags using Berlese-Tullgren funnels was conducted under simulated drought and increasing precipitation treatments at a semiarid site and a Mediterranean site, and along a natural climatic gradient from arid to mesic Mediterranean ecosystems during three consecutive years. Our objective was to examine the effects of variation in water availability on the abundance of litter arthropods along the aridity gradient. The results demonstrated positive responses of total abundances, including Acari abundance, but negative responses of taxonomic richness to experimentally increased precipitation at the semiarid site and the Mediterranean site. Collembolan abundance responded positively while the Shannon index responded negatively to experimental precipitation at the Mediterranean site only. However, total abundance and collembolan abundance were found to decrease across a mean annual precipitation (MAP) gradient from the arid site to the mesic Mediterranean site. Quantile regression for linear mixed-effects models demonstrated a negative effect of MAP on Acari abundance at the medium level of Acari abundance but weak positive effects of MAP on taxonomic richness and the Shannon index. In conclusion, we found that the distribution of abundance was a poor proxy for predicting the response of the studied litter arthropod assembly to spatial variation in precipitation.