Population and Community responses along environmental gradients across spatio-temporal scales

Ecology as a science investigates organisms in interaction with their environment. This thesis aims at investi-gating how the environment and species’ traits as well as their interaction affect population and community structure and dynamics on varying spatial and temporal scales. In order to derive predictions of how ecological communities will respond to ongoing environmental change we need to understand (I) community responses to environmental gradients and their variation, (II) drivers of species’ performances and (III) how species’ traits affect species’ success in a particular environment. After a general introduction in chapter 1, I investigate these questions in seven articles subdivided in three chapters. Chapter 2 focusses on the response of ecological communities in terrestrial forest ecosystems to environmental conditions changing along natural gradients. Here, I show the strong effect of a local elevational gradient and the associated changes in environmental conditions on alpha and beta diversity of a multi-taxon community. Additionally, I analyze how organisms of varying taxonomic groups and trophic levels respond to temperate forest succession and the associated changes in forest structural parameters. Finally, I investigate how climatic conditions, geographic space and forest structure affect alpha and beta diversity of fungus-dwelling arthropod communities on a continental scale. The results reveal strong differences in the response to changing environ-mental conditions among taxonomic and trophic groups, underline the importance species turnover across successional stages for the regional species pool and reveal that climatic conditions and geographic space may play only minor roles in particular communities like those inhabiting fungus fruiting bodies. In chapter 3, I investigate how species’ performances are affected by species traits. Here, I test several pro-posed hypotheses aimed at explaining the ubiquitous relationship between local distribution and mean abun-dance in a causal statistical framework. For phytophagous insects I show that habitat availability is the most important driver of local distribution which in turn facilitates species’ mean abundances via population dynam-ic processes. This relationship is further modulated by species’ energy uptake and allocation strategies. Species that successfully compensate for higher energetic costs associated with beneficial morphological traits may reach higher local distributions and mean abundances. Chapter 4 aims at investigating how the effect of a particular trait on species’ success may change with chang-ing environmental conditions. Here, two studies investigate the effect of thermoregulatory relevant functional traits on the occurrence of moth along changing thermal conditions along spatial gradients. The results show that the relationship between species’ traits and species’ performances is highly context and scale dependent, as the presented studies revealed strong intra- and intertaxonic differences in the trait-environment relationships. Identifying important environmental gradients driving community dynamics (I), determining the drivers of species’ performances (II) and analyzing the interactive effect of environment and traits on species’ perfor-mances across scales (III) as pursued in this thesis helps us to identify the relevant drivers of species responses to environmental change on particular spatio-temporal scales for particular communities. Furthermore, these insights broaden our knowledge of the processes generating and maintaining terrestrial biodiversity. However, the results also show that it is debatable whether it is possible in the near future to generate reliable and more importantly generalizable predictions of how future environmental change will affect terrestrial communities as a whole.