Watersheds are often composed of nested tributaries with distinct hydrologic, temperature, and biogeochemical regimes. This heterogeneity can provide a template for food webs that vary in structure and phenology, and which are spatially connected via organism movement and organic matter transport. Theory suggests that these “meta-food-webs” can contribute to ecological stability, but few studies have evaluated meta-food-webs in riverscapes. Here we examine the structure and seasonal dynamics of river food webs in a hydrologically diverse region (southeast Alaska, USA), and we identify features of meta-food-webs that contribute to ecosystem stability and species persistence.
In southeast Alaska, a single watershed can contain a mosaic of glacier-, snowmelt-, and rain-fed streams that have distinct hydrologic, temperature, and biogeochemical regimes. Our research suggests that these stream types also have distinct food webs, which exhibit seasonal asynchronies in resource availability and support consumers via distinct local energy flow pathways. Model analyses suggest that mobile consumers such as trout and salmon can benefit by moving between glacier-, snowmelt-, and rain-fed streams, tracking local peaks in resource availability, and spatially coupling different energy flow pathways (e.g., brown versus green). In turn, spatial-coupling of local food webs via resource tracking may contribute to the stability of the aggregate meta-food-web by reducing the predicted strength of predator-prey interactions. However, as glaciers diminish and precipitation shifts from snow to rain, the physical and chemical characteristics that make glacial or snowmelt streams distinct from rain-fed streams will fade. This hydrologic homogenization could result in the loss of unique food webs promote ecological stability and species persistence.