Different threshold responses in spontaneously changing postagrogenic forest and unmanaged grassland. Shifts in small mammal populations and communities triggered by an extraordinary drought.

Due to their resilience, various biological systems under environmental changes typically exhibit nonlinear responses with sudden, abrupt shifts. Although such shifts are theoretically expected, few studies traced state-and-transition dynamics in nature (Liu et al., Science 317:1513-1516, 2007). We analysed 18 years' data to trace biomass patterns, species assemblages and small mammals' population trajectories in spontaneously growing forest on formerly ploughed field, hereafter, the postagrogenic forest, and in unmanaged former pasture, hereafter, the grassland. The clear response at individual, populational and ecosystem scales triggered by extraordinary 2010 drought was observed. In the postagrogenic forest, transitioning to the historical ecosystem state, we found a shift from the grassland type of the small mammals' biomass pattern to the forest type with the abrupt reorganisation of the small mammals' community. In the grassland, a relatively steady novel ecosystem, we revealed only a long-term diminishing of total small mammals' biomass, i.e. a regime shift, while maintaining the same functional structure. The changes were based on population response. The bank vole did not show any population reaction, which testifies the ability of individuals to tolerate the drought. The common shrew experienced a population depression, which in postagrogenic forest resulted in the regimen shift after recovery, but in the grassland in only temporal decline with following return to the initial state. The root vole showed a delayed population response with the general decline of population in the grassland, and population collapse in the postagrogenic forest. Therefore, the same impact triggered various responses among different species and resulted in different effects in the successional and steady ecosystems.