The importance of unpredictable and short-term environmental extremes for biodiversity in oribatid mites
Tolerance of oribatid mites belonging to twenty-three species has been determined after exposure to short-term unpredictable environmental extremes such as drought, frost, heat, and food shortage. Species having a narrow temperature tolerance were also susceptible to drought, whereas species having a very broad temperature tolerance also tolerated drought. Tolerance classes per environmental condition were defined and used to classify oribatid mites in datasets from undisturbed and clearcut forests, and unmanaged, cut, and cattle-grazed grasslands. Once-only events, like clearcutting, could be shown to markedly influence the oribatid mite fauna. Cutting of grassland (once a year) led to the absence of species intolerant to drought, but higher cutting frequencies did not alter the species composition any further with respect to drought tolerance. Soil compaction due to cattle-grazing in combination with removal of the vegetation led to the absence of species intolerant to drought and to a decrease of small species usually able to avoid drought by entering deeper soil layers. The frequency of extreme conditions is discussed in general and a hypothesis is formulated wherein species diversity, metapopulation dynamics, and `unsaturated' biotopes are results of the frequency of disturbance and recovery of the affected populations.