Feeding biology of oribatid mites in high alpine regions

Oribatid mites (Acari: Oribatida) are among the most abundant und species rich taxa of the soil living mesofauna. Presumably they play an important role in decomposition and are significantly engaged in nutrient cycles and dispersal of fungal spores. With densities reaching up to several hundred thousand individuals oribatid mites dominate mesofauna-communities. The colonization success is multifactorial, and primarily resource availability, reproduction mode and mobility, and secondarily habitat characteristics have to be considered. According to the present state of knowledge oribatid mites feed mainly on fungi or detritus, but with noteworthy feeding specializations on animal tissue or lichen. Oribatid mites display both sexual and parthenogenetic reproduction; the question why most species reproduce sexually is still an unresolved problem. Potential explanations are evolutionary constraints, e.g. interactions between food source and animal, or limitation of food resources. Species composition in oribatid mite communities has been investigated frequently and is mainly influenced by habitat type. But the mechanisms for habitat specific colonization success of oribatid mite species are largely unknown. The main objective of this project is understanding mechanisms which lead to spatial distribution patterns of oribatid mites. We hypothesize that species composition in oribatid mite communities are primarily caused by biotic interactions, mainly availability of resources, which seems to be the strongest factor determining the growth of populations. Furthermore we assume that the percentage of sexual reproducing species increases with altitude since in higher altitudes food resources are increasingly limited (e.g. fungi), hardly palatable (e.g. lichens) or hardly accessible (nematodes as prey) Sexual reproduction should therefore dominate in higher altitudes, where resources become scarce. We investigate the feeding biology of oribatid mite species along an altitudinal gradient in the Central Alps. The feeding biology of oribatid mites will be analyzed with dual stable isotope analysis ( 13C, 15N). To investigate the resource spectrum of oribatid mites and trophic shifts, food choice experiments will be carried out. Analysis of the gathered data on community structure, reproduction mode and feeding biology of oribatid mites will enhance the knowledge about mechanisms leading to spatial distribution patterns of oribatid mites and their ecological requirements in alpine habitats. This combination of classical and modern methods allows a more detailed insight into the feeding biology, trophic niche differentiation and reproductive mode of oribatid mites than ever, and will substantially contribute to the basic knowledge in soil zoology and soil ecology in general, and particularly in the field of soil food webs and distribution patterns of oribatid mite species.

Oribatid mites are one of the most abundant microarthropods in soil and play a major role as decomposer in terrestrial ecosystems, particularly where macrodecomposers (e.g. earthworms or millipedes) are rare or lacking. Though it is known, that microarthropod-communities are easily disturbed, especially by increasing temperature, basic knowledge of diversity, distribution mechanisms, reproductive mode and feeding biology of oribatid mites in high alpine ecosystems are very limited. Unfortunately, there is a lack of recent studies addressing oribatid mites, although it would be important to monitor possible changes in species distribution, whether to address soil succession, climatic changes or other possible influences.In the completed project we investigated (1) the spatial distribution of oribatid mite species along an altitudinal gradient, (2) reproductive mode of oribatid mite species and (3) their feeding ecology. Additionally we compared the current species distribution with oribatid mite communities in the same area 30 years ago.We were able to show, that each altitude has its own species composition and only four of the totally found 86 species were recorded in all four sites.Generally species richness and abundance decrease with increasing altitude and also reproductive mode correlates with altitude: in 2900m a.s.l. more than 90% of oribatid mites reproduce sexually, while in 2050 m a.s.l. more than 70% are parthenogenetic. These findings are in contrast with proposed hypothesis, that parthenogens are more frequent at high altitude and in habitats with low density of the respective species. This seems to follow the structured resource theory of sexual reproduction, which proposes that sexual reproduction favours the exploitation of resources in habitats were food resources are limited or of low quality.One of the most extraordinary finding is, that several species obviously migrated from comparatively low altitudes to the high alpine zone and apparently established stable populations. A remarkable shift is evident especially at the highest altitude when group spectra of oribatid mite communities in previous studies are compared with recent investigations. Although analyses are still ongoing, it is most likely that changes in oribatid mite communities are related to changes in the environment.