Does pre-exposure to mild drought enhance desiccation resistance in Collembola?

Species of springtails (Collembola) have a stratified distribution along the soil/air profile and present an opportunity to study adaptive traits relating to a gradient of fluctuating humidity. The habitat of eu-edaphic species (found in deeper litter and soil layers) secures a relatively stable humidity, whereas hemi-edaphic (found in upper litter layers) and epigeic springtails (found on soil surface or vegetation) can experience an increased range of moisture fluctuation with much more rapid shifts between extremes. We hypothesized that the capacity for acclimation to drought conditions is better developed in species associated with the soil surface than in soil-dwelling species. To test this hypothesis, we measured the osmotic response of two epigeic, two hemi-edaphic and three eu-edaphic species to mild drought acclimation (3 d at -1.5 MPa; the average wilting point of plants) and compared the drought tolerance of drought acclimated animals with controls. Epigeic species had no or very little osmotic regulation during mild drought acclimation and unexpectedly became less tolerant to subsequent acute drought stress than controls. The hemi-edaphic and eu-edaphic species significantly increased body fluid osmolality in response to mild drought acclimation. However, none of these species became more drought-tolerant after exposure to mild drought. In conclusion, we found that the osmotic response to drought acclimation was indeed related to the vertical position of a species in the soil/air profile, however, the relationship was opposite to our hypothesis.