Mites in forest canopies: filling the size distribution shortfall?

Although often unobserved or ignored, mites usually exceed all other arthropods in abundance in forest canopies. Second in species richness only to canopy insects, the arboreal acarofauna is composed of multiple lineages of predators, scavengers, grazers, animal associates, and plant parasites that each have radiated extensively in canopy habitats. The canopy fauna is largely complementary to the mite fauna of the forest floor, suggesting that estimates of more than one million living species of mites are not extreme. Most mites are less than a millimeter in length as adults, and canopy mites tend to be smaller than species from other habitats. Even among mites, however, very small species are relatively rare, and diversity increases with decreasing size only to the penultimate size class (0.316-1 mm). This pattern may be explained by declines in microhabitat diversity or host specificity as the limit of body size in a group of organisms is approached.