Global warming threatens to further impoverish communities of naturally fishless alpine lakes already stressed by introduced trout. However, dispersal of tolerant regional species, such as those from warmer montane lakes, may mediate the effects of non-native trout and higher temperatures on alpine lake communities. We compared the zooplankton community of an alpine lake when non-native trout were absent and present across a 24-year period of climatic variation and conducted an experiment to test the hypotheses that (1) size-selective predation by non-native trout and higher temperatures suppress large zooplankton while small species are resistant to both, and (2) species imported from a diverse array of lakes can functionally compensate for resident alpine species suppressed by predation and warming. Both non-native trout and higher temperatures decreased large zooplankton but surprisingly, warming suppressed small herbivores, not large Daphnia. Declines of local species in stressed alpine communities increased the establishment of regional zooplankton, including a diversity of montane species under warmed conditions. Despite this, regional species did not alleviate the negative effects of size-selective predation and warming on zooplankton biomass. Further research is needed to assess the influence of regional dispersers on stressed alpine lake communities over ecologically relevant spatio-temporal scales.