Zooplankton species in high alpine lakes face harsh conditions in extreme environments, with low productivity and short ice-free periods. Due to warming in alpine regions these hostile environments are rapidly changing, with earlier ice out, increasing water temperatures, and potential changes in productivity and accessibility. This has the possibility to both create habitats more amenable to lowland zooplankton species and to provide more opportunities for lowland species to colonize these habitats, leading to alternate species composition in the lakes. Within a transdisciplinary project, potential barriers to colonization (competitive exclusion, harsh environments), as well as the concept of species sorting within the zooplankton community of the lakes of the National Park Hohe Tauern, Austria, have been investigated, with the following hypotheses: 1. When large zooplankton species are already present in a lake, they can preclude invasion by new lowland species through competitive exclusion. 2. The hostile environment (low temperature and productivity, high turbidity) of some high alpine lakes is limiting the colonization success of invading species. 3. Species sorting occurs, in that the species or clones found in a lake are those best adapted to the habitat in which they were found. Based on the results of the long-term monitoring of 18 lakes of the National Park Hohe Tauern, Austria, model lakes were chosen and the water and species from these lakes used to run experiments in the lab under controlled conditions. The results of the first experiments testing our hypotheses will be presented.