Landscape geological age explains large scale spatial trends in oribatid mite diversity

To understand the overwhelming species richness in soil the focus of attention has traditionally been on local soil conditions, such as physical and chemical characteristics. Regional factors like landscape history have been largely ignored. The aim of our study was to assess the importance of geological site age and local site conditions on oribatid mite species richness in undisturbed forest soils. We wanted to evaluate the processes underlying spatial changes in oribatid species richness at the regional level. We selected 41 sites across the Netherlands with different forest types, located on soils with varying levels of humidity and nutrient richness. The selected sites formed a clear spatiotemporal gradient in geological site age, ranging from Holocene sites along the west coast and rivers towards Pleistocene sites in the east of the country. Five samples were collected at each site. Oribatid mites were counted and identified to the species level. In total 145 oribatid mite species were recorded. We observed that oribatid mite species richness across sites was positively affected by site age. Soil nutrient status, water availability, soil type, or forest vegetation type had rather a local modulating effect on soil mite diversity. The increase in species diversity with geological site age was mainly due to an increase in sexually-reproducing species, with an apparent high competitive ability, but lower reproduction rate. Our results suggest that spatial patterns of soil animal community diversity and composition can be significantly determined by geologic age at the regional level.