Climate change, coupled with the introduction of non-native organisms, represent major threats to the functioning of ecosystems, especially in species-poor communities such as polar terrestrial ecosystems. In this laboratory study, we quantified the impacts of the non-native springtail Folsomia candida and isopod Porcellio scaber on seed germination and growth of the non-native grass Poa pratensis, and ecosystem respiration. The impacts of invertebrate communities of progressively increasing complexity were assessed, starting with the native springtail Cryptopygus antarcticus alone, followed by C. antarcticus in combination with F. candida or P. scaber and, finally, a community including all three species. The impact of these invertebrate communities were studied in a simulation of contemporary Antarctic soil surface conditions (2 °C) and a +5 °C warming scenario over one growing season. Warming resulted in earlier germination (21 d), 10-fold increased plant biomass, N-content (>5-fold), and higher levels (90%) of ecosystem respiration. Warming also resulted in a 350% increase in C. antarcticus abundance. The presence of the woodlouse P. scaber had the strongest impact on the measured soil and plant variables and this impact was largely irrespective of temperature. Impacts included: delay in seedling emergence (4 d), reduced plant emergence (20%), and higher ecosystem respiration (135%). The presence of both C. antarcticus and P. scaber resulted in 30% higher plant leaf N-content and a reduction in C:N ratio from 21 to 17. The experimental communities containing F. candida showed a 37% reduction in plant biomass under warming. The presence of P. scaber reduced C. antarcticus abundance (94%) but F. candida abundance was unaffected. Our data indicate that non-native invertebrates differ in their ecosystem impacts, with potentially significant consequences for ecosystem functioning and community composition of plants and animals in cold biomes.