Alpine activity patterns of Mitopus morio (Fabricius, 1779) are induced by variations in temperature and humidity at different scales in central Norway

Abstract

Our research addresses questions about how micro-climate affects activity abundance of a common and widespread harvestman in an alpine ecosystem. Activity patterns of the Harvestman Mitopus morio (Fabricius, 1779) were studied along different alpine gradients in the central Norwegian Scandes. Within a nested design, we surveyed 18 alpine habitats with pitfall traps and micro-climatological equipment along oceanic-continental, two elevational, and (fine-scaled) microtopographic gradients. Sites in the oceanic region of the Scandes showed generally higher abundance of M. morio than sites in the continental region. Furthermore, along the elevational gradient, middle-alpine sites showed higher abundances than low-alpine sites. These general patterns are best explained by higher humidity in the oceanic region and in the middle-alpine belt. Focusing at a finer scale, i.e. one elevational level within each region, revealed partly opposing activity patterns within relatively short distances. While in the western middle-alpine belt these patterns were best explained by humidity-related measures but now with higher activity abundance during drier conditions, in the drier eastern middle-alpine belt heat sums rather than humidity were found to be the best explanatory variables for the observed patterns. Hence, our results imply a pronounced different reaction of the two populations towards climatic variables that partly even contradict the previously described general pattern. Regardless whether these differences in activity abundance in M. morio are a form of phenotypic plasticity or adaptation, our findings stress the importance of detailed autecological knowledge combined with fine-scaled climatic measurements when aiming at predictions about possible future ecosystem structures and spatio-temporal phenomena. M. morio proves to be an ideal biogeographic model organism for understanding spatio-temporal responses of alpine ecosystems under modified climatic conditions.