Shrinking glaciers drive the reorganisation of river food webs

Glaciers cover ~10% of Earth’s land surface, and their meltwater rivers are home to diverse biological communities that play central roles maintaining water quality, supporting fisheries, and subsidising the diet of terrestrial birds and mammals1,2. However, current unprecedented rates of glacier shrinkage3 are expected to cause major changes to algae and invertebrate biodiversity in rivers worldwide with largely unknown consequences4,5. Whilst knowledge of co-dependencies among biological groups is critical for predicting biodiversity change and driving conservation actions6, the intricate species-interactions shaping glacial-river food webs remain poorly understood4. Here we use >3500 newly observed feeding interactions among diatoms and invertebrates from rivers in the European Alps to demonstrate how decreasing glacier influence leads to increases in food web size as habitats become warmer, more stable, and less turbid. These responses are consistent with mountain-river biomonitoring observations from a new global meta-analysis of >190 studies. With decreasing glacier influence, diatoms became more species-rich and abundant both in real terms and relative to invertebrate consumers, and mean trophic level declined implying more efficient primary production transfer to consumers. We show this reorganisation is predictable from constituent species’ body mass and abundance, allowing projections to be made for rivers where species interactions remain unstudied. By demonstrating predictable links between species composition, traits, food web structure, and shrinking glaciers, this study significantly advances our ability to forecast ecological responses in rapidly changing mountain environments.