Amidst the global extinction crisis, climate change will expose ecosystems to more frequent and intense extreme climatic events, such as heatwaves. Yet, whether predator species loss-a prevailing characteristic of the extinction crisis-will exacerbate the ecological consequences of extreme climatic events remains largely unknown. Here, we show that the loss of predator species can interact with heatwaves to moderate the compositional stability of ecosystems. We exposed multitrophic stream communities, with and without a dominant predator species, to realistic current and future heatwaves and found that heatwaves destabilised algal communities by homogenising them in space. However, this happened only when the predator was absent.Additional heatwave impacts on multiple aspects of stream communities, including changes to the structure of algal and macroinvertebrate communities, as well as total algal biomass and its temporal variability, were not apparent during heatwaves and emerged only after the heatwaves had passed. Taken together, our results suggest that the ecological consequences of heatwaves can amplify over time as their impacts propagate through biological interaction networks, but the presence of predators can help to buffer such impacts. These findings underscore the importance of conserving trophic structure, and highlight the potential for species extinctions to amplify the effects of climate change and extreme events.

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For migratory species, attaining enough large size before migration is a key mechanism of individuals for their success in risky migration. Since the smaller migrants suffer from high mortality during migration, prospective migrants with smaller size should grow better than larger ones before migration. To test this prediction, we investigated size-dependent patterns of the two growth mechanisms (i.e., growth rate and duration) of juvenile masu salmon (Onchorynchus masou) before their oceanic migration. Masu salmon exhibit a partial migratory strategy, in which single population consists of oceanic migrants and riverdwelling residents. Our individual mark-recapture survey and assessment of the river-descending timing by PIT-tag antenna-reader system revealed that patterns of growth rate in the pre-migration period and the timing of migration correspond with our predictions. For around half-year before outmigration (i.e., between after decision of migration and before start of migration), the prospective migrants showed the size-dependent growth rate, in which individuals with smaller size exhibited higher growth rate than those with larger size, but the residents showed the size-independent growth rate. In addition, the prospective migrants showed the size-dependent timing of outmigration, in which individuals with smaller size delayed the migration timing than those with larger size, to lengthen the duration of pre-migration period.These results suggest that size-selective mortality during migration has shaped size-dependent adjustment of the pre-migration growth in migratory masu salmon. Conditional changes in growth rate and duration of pre-migration period may be an adaptive tactic for the migratory animals.