The responses of oribatid mites to grazer exclusion in a boreal forest over different time scales

Abstract

Oribatid mites (Acari: Oribatida) represent an especially abundant and species-rich group of soil fauna in northern boreal forests. They contribute significantly to the complexity of soil food webs, nutrient cycling and organic matter decomposition. Thus, their role in ecosystem functioning may be pronounced, and consequently, understanding their responses to environmental changes is important. Northern boreal forests often have patchy ground vegetation – i.e., the dominance of lichens vs mosses and dwarf shrubs varies – and are affected by reindeer (Rangifer tarandus tarandus L.) that can impact the abiotic and biotic environment via eating plants, fertilization, and trampling. Here, we studied how differences in reindeer grazing affect oribatid mite community composition, feeding guilds, and life traits and whether the responses of Oribatida depend on the habitat type. We did this by comparing reindeer grazed areas with short-term and long-term ungrazed areas in sunlit, lichen-dominated and in shaded, moss and dwarf shrub-dominated habitats of an oligotrophic Scots pine forest in NE Finland. Grazing treatments affected oribatid mite species composition and these effects depended on the habitat. For example, in the sunlit habitats, the short-term ungrazed treatment benefitted Chamobates pusillus, while the long-term ungrazed treatment benefitted Carabodes labyrinthicus. On the contrary, grazing treatments did not induce any significant differences in the abundances of feeding guilds and life traits. To conclude, we propose that the cessation of reindeer grazing can affect the species composition of oribatid mites over different time spans by impacting the abundance of less common oribatid species, while ecological functions associated with different ecological and life traits of oribatid mites may remain stable under changing reindeer grazing.