Vertical stratification of oribatid (Acari: Oribatida) communities in relation to their morphological and life-history traits and tree structures in a subtropical forest in Japan

To clarify the effect of tree structure on the diversity of oribatid mites (Acari: Oribatida), we collected a total of 16,325 oribatids in 181 morphospecies from the leaves, branches, and trunk bark of trees and from the forest-floor soil and litter in a subtropical forest, Okinawa, Japan, and tested three predictions: (1) moisture stress in arboreal habitats would lead to larger body size (not supported); (2) morphological traits related to gripping a surface (number and size of claws) would be more developed in arboreal species (supported); and (3) advantages in colonization (no cost for searching mates and doubled population growth) would favor parthenogenesis in the arboreal oribatid communities (not supported). We observed vertical stratification among the five habitats in terms of mite density, species diversity, and species composition, but found no difference in the body length of oribatid species between the arboreal and forest-floor habitats. However, (homo-)tridactylous species predominated in the arboreal habitat, suggesting that this claw morphology facilitates adherence to and movement on arboreal substrates. Sexual species were common in the arboreal oribatid communities, whereas about half of the dominant species collected from the forest floor were likely to be parthenogenetic. It is unclear how these different reproductive systems may be advantageous for oribatid mites in arboreal and forest-floor habitats. Nevertheless, the structural complexity provided by trees appears to enhance and maintain the diversity of oribatid communities through vertical stratification in this subtropical forest.