Contrasting responses of millipedes and terrestrial isopods to hydrologic regime changes in forested montane wetlands

Forested montane wetlands are subject to changing disturbance regimes. The factors responsible for their hydrologic degradation include drainage for forestry. Few studies have explicitly addressed the effects of changing hydrologic regimes linked with artificial drainage and disruption of these drainage systems on soil fauna communities. Because millipedes (Diplopoda) and terrestrial isopods (Oniscidea) are an important components of the soil macro-decomposer community and exhibit a wide range of ecological requirements, we examined their response to regimes changes in mountain fen sites of Caltho-Alnetum demonstrating different degrees of alteration of hydrologic conditions and classified into three categories: natural, semi-natural and degraded. Our results showed that hydrologic alteration of mountain fens affected terrestrial isopod and millipede communities much less strongly than soil physico-chemical properties. However, the increasing hydrologic disturbances, due to drainage and lowering of groundwater level also led to differential responses of these two groups of soil macro-decomposer communities, and induced significant change of terrestrial isopod community composition in semi-natural conditions but no response of millipede communities. The observed differences in the responses of these two groups correspond to differences in their sensitivity, resistance and resilience to hydrologic changes in mountain fens. We conclude that the significant response of terrestrial isopod community composition documented in semi-natural conditions reflects their low ability to return to the pre-drying state and indicates their higher sensitivity and limited resilience to disturbances related to groundwater withdrawal compared to communities of millipedes. Besides, our results also suggest that in mountain areas altitude acts as a stronger environmental filter on community composition of terrestrial isopods and millipedes than hydrologic conditions.