Wood ant nest mounds as biodiversity hotspots: Case studies with oribatid mites

Abstract

The red wood ant (Formica) nest mounds form in forest landscape diversity hotspots by hosting large amount of invertebrate ant associates. The most numerous associate group, the soil-dwelling oribatid mites, have however remained rather unstudied due to their minute size and lack of taxonomic experts. Few studies conducted in Finland have recently lighted up the oribatid’s diversity in ant mounds. First, our study based on 10,600 specimens and 74 identified species, showed that the wood ant F. polyctena nest mounds were inhabited by an equally abundant and diverse oribatid fauna as the surrounding soil. Moreover, the results revealed that predominantly different species inhabited these two habitats. Second, a more extensive study based on 18,600 specimens and 94 identified species, showed that the oribatids occurred predominantly on the surface layer of mounds and that their distribution was positively related to mounds’ surface moisture content. In an ecological framework such observations illustrate the variability in species assemblages within forest landscape, and could be taken into account in regional conservation practices, forest management and land use planning. Forest management practices, such as clear felling, affect the physical properties of mounds which may be harmful not only for ants, but also for their rich associate fauna. Indeed, our next study revealed that the surface layer of F. aquilonia mounds was significantly drier in clear fells than in forest, and mounds were cooler in clear fells indicating that humidity has a function in mound thermoregulation. The impacts of forest clear felling and the carry-over effects of the detected drying of mounds on oribatids were investigated next. The study of 16,500 specimens and 67 species revealed that while the clear felling had no impact on abundance or community composition of oribatids, the species richness was significantly lower in clear fells and was positively related to mounds’ surface moisture content. These results indicate that the oribatid fauna of this distinctive habitat may be considered as a useful bioindicator group in the studies of environmental changes. As the red wood ants build large, dense and long-lived nest mounds in boreal forest, their nests are important factors in maintaining oribatid biodiversity. Many of the red wood ant species have a status as vulnerable or near threatened species in Europe and conservation of these species is of high significance.

1. Elo R, Penttinen R, Sorvari J (2016) A comparative study of oribatid mite communities in red wood ant F. polyctena nests and surrounding soil in a Finnish oak forest. Ins. Cons. Div. 9, 210-223. 2. Sorvari J, Elo RA, Härkönen SK (2016) Forest-built nest mounds of red wood ant F. aquilonia are no good in clear fells. Appl. Soil Ecol. 101, 101-106. 3. Elo R, Penttinen R, Sorvari J (2017). Distribution of oribatid mites is moisture-related within red wood ant F. polyctena nest mounds. Appl. Soil Ecol. In