An experiment in >1000 river and riparian sites found spatial patterns and controls of carbon processing at the global scale.

Under the ongoing climate change, understanding the mechanisms structuring the spatial distribution of aquatic species in glacial stream networks is of critical importance to predict the response of aquatic biodiversity in the face of glacier melting. In this study, we propose to use metacommunity theory as a conceptual framework to better understand how river network structure influences the spatial organization of aquatic communities in glacierized catchments. At 51 stream sites in an Andean glacierized catchment (Ecuador), we sampled benthic macroinvertebrates, measured physico-chemical and food resource conditions, and calculated geographical, altitudinal and glaciality distances among all sites. Using partial redundancy analysis, we partitioned community variation to evaluate the relative strength of environmental conditions (e.g., glaciality, food resource) vs. spatial processes (e.g., overland, watercourse, and downstream directional dispersal) in organizing the aquatic metacommunity. Results revealed that both environmental and spatial variables significantly explained community variation among sites. Among all environmental variables, the glacial influence component best explained community variation. Overland spatial variables based on geographical and altitudinal distances significantly affected community variation. Watercourse spatial variables based on glaciality distances had a unique significant effect on community variation. Within alpine catchment, glacial meltwater affects macroinvertebrate metacommunity structure in many ways. Indeed, the harsh environmental conditions characterizing glacial influence not only constitute the primary environmental filter but also, limit water-borne macroinvertebrate dispersal. Therefore, glacier runoff acts as an aquatic dispersal barrier, isolating species in headwater streams, and preventing non-adapted species to colonize throughout the entire stream network. Under a scenario of glacier runoff decrease, we expect a reduction in both environmental filtering and dispersal limitation, inducing a taxonomic homogenization of the aquatic fauna in glacierized catchments as well as the extinction of specialized species in headwater groundwater and glacier-fed streams, and consequently an irreversible reduction in regional diversity.

Glacier retreat is a worldwide phenomenon with important consequences for the hydrological cycle and downstream ecosystem structure and functioning. To determine the effects of glacier retreat on aquatic communities, we conducted a 4-year flow manipulation in a tropical glacier-fed stream. Compared with an adjacent reference stream, meltwater flow reduction induces significant changes in benthic fauna community composition in less than 2 weeks. Also, both algal and herbivore biomass significantly increase in the manipulated stream as a response to flow reduction. After the flow reduction ceased, the system requires 14–16 months to return to its pre-perturbation state. These results are supported by a multi-stream survey of sites varying in glacial influence, showing an abrupt increase in algal and herbivore biomass below 11% glacier cover in the catchment. This study shows that flow reduction strongly affects glacier-fed stream biota, prefiguring profound ecological effects of ongoing glacier retreat on aquatic systems.

Summary: 1. The stress‐gradient hypothesis (SGH) states that environmental stress modulates species interactions, causing a shift from negative interactions to net positive interactions with increasing stress.2. Potentially, this modulation of species interactions could in turn influence biodiversity‐ecosystem function (B‐EF) relationships along stress gradients. Although the SGH has been extensively discussed in plant community ecology in the past two decades, it has received little attention from animal ecologists.3. To explore whether the SGH could be applied to animal communities, we conducted a litter decomposition experiment with aquatic detritivorous invertebrates in which we manipulated litter quality and measured species interactions along this resource quality gradient. Litter quality was manipulated by presenting detritivores with leaves of plant species varying in specific leaf area and decomposition rate in streams.4. We found a switch from negative to neutral interactions with increasing resource quality stress, in line with the SGH. However, by re‐examining other published results with aquatic detritivores from the perspective of the SGH, we found that a diversity of patterns seem to characterize detritivore interactions along stress gradients.5. Although the basic pattern proposed by the SGH may not apply to animal systems in general, we show that aquatic detritivore interactions do change along stress gradients, which underlines the importance of incorporating environmental stressors more explicitly in B‐EF research.

Retreating glaciers, icons of climate change, release new potential habitats for both aquatic and terrestrial organisms. High‐elevation species are threatened by temperature increases and the upward migration of lowlands species. Improving our understanding of successional processes after glacier retreat becomes urgent, especially in the tropics, where glacier shrinkage is particularly fast. We examined the successional patterns of aquatic invertebrates, ground beetles, terrestrial plants, soil eukaryotes (algae, invertebrates, plants) in an equatorial glacier foreland (Carihuairazo, Ecuador). Based on both taxonomical identification and eDNA metabarcoding, we analysed the effects of both environmental conditions and age of deglacierization on community composition. Except for algae, diversity increased with time since deglacierization, especially among passive dispersers, suggesting that dispersal was a key driver structuring the glacier foreland succession. Spatial β‐diversity was mainly attributed to nestedness for aquatic invertebrates, terrestrial plants and soil algae, likely linked to low environmental variability within the studied glacier foreland; and to turnover for soil invertebrates, suggesting competition exclusion at the oldest successional stage. Pioneer communities were dominated by species exhibiting flexible feeding strategies and high dispersal ability (mainly transported by wind), probably colonising from lower altitudes, or from the glacier in the case of algae. Overall, glacier foreland colonisation in the tropics exhibit common characteristics to higher latitudes. High‐elevation species are nevertheless threatened, as the imminent extinction of many tropical glaciers will affect species associated to glacier‐influenced habitats but also prevent cold‐adapted and hygrophilous species from using these habitats as refuges in a warming world.