Accepted version of this manuscript can be found here: https://doi.org/10.1111/gcb.15862The distribution of abundance and biomass within ecological communities is related to trophic transfer efficiency from prey to predators. While it is considered to be one of the few consistent patterns in ecology, spatiotemporal variation of this relationship across continental-scale environmental gradients is unknown. Using a database of stream communities collected across North America (18-68° N latitude, -4 to 25°C mean annual temperature) over 3 years, we constructed 162 mass-abundance relationships (i.e. size spectra). Size-spectra slopes declined (became steeper) with increasing temperature. However, the magnitude of change was relatively small, with median slopes changing from -1.2 to -1.3 across a 29°C range in mean annual temperature. In contrast, total community biomass increased 3-fold over the temperature gradient. Our study suggests strong conservation of abundance size-spectra in streams across broad natural environmental gradients. This supports the emerging use of size-spectra deviations as indicators of ecosystem health.

Organisms undergo substantial ecological changes throughout development, such as shifts in habitat use, trophic position, and predation risk. However, developmental variation is rarely considered in natural food webs. We manipulated fish predation to test the hypothesis that cross-ecosystem subsidies are regulated by stage-structured predation in a freshwater food web. Fish reduced the emergence of adult stages of aquatic insects to the terrestrial ecosystem by 85 ± 0.04% (mean ± sd DM) leading to a ~40% reduction in terrestrial insectivorous spider abundance. By contrast, fish had weaker effects on larval aquatic insects, reducing dry mass by 63 ± 72%. These stage-specific effects were explained by a stage-structured food web in which fish ate similar prey taxa but different prey stages. Since organisms with complex life-histories are widespread, we suspect that this type of stage-structured predation, and its regulation of cross-ecosystem subsidies, is a common phenomenon beyond freshwater food webs.