The meta-ecosystems framework was established to explore how the movement of organisms and materials between ecosystems affects local ecosystem dynamics. However, little empirical work has assessed such dynamics in natural landscapes. Here we ask how anthropogenic land use, habitat fragmentation, and terrestrial biodiversity affect leaf litter processing by amphipods, a regionally-dominant macroinvertebrate which are an important food source for fish and other fauna as well as contributing to carbon and nutrient recycling. We established transects of sampling points in 10 independent freshwater streams with varying land use characteristics across Eastern Switzerland. Points were sampled four times over the course of a year to assess amphipod communities, benthic leaf litter availability and diversity, stream habitat characteristics, and water chemistry. These seasonal surveys were paired with laboratory mesocosm experiments measuring breakdown rates of different leaf types by native and non-native amphipods, providing a framework for assessing decomposition in multiple ecosystem pairs.
In-stream leaf litter biomass and diversity varied substantially between sites and seasons, with biomass increasing eight-fold from spring and summer to fall and staying high through winter. Species richness of benthic litter doubled after fall leaf drop, meaning that terrestrial food resources increased not only in quantity but possibly in quality. Meanwhile, amphipod abundance and identity was relatively stable within and between catchments over the course of the one-year sampling period. More than half of sites saw no change in presence/absence. In most catchments only one amphipod species was present, with species identity shifting along the north-south regional gradient. The laboratory experiments showed that this pattern has major implications for leaf processing because the two most common species, the native Gammarus fossarum and non-native Gammarus roeseli, have different feeding rates on some leaf litter types. Although breakdown rates are similar for beech, which dominates upland forests, comparable densities of G. roeseli break down riparian leaf litter up to twice as quickly, creating more secondary growth and increasing carbon and nutrients provisioned through recycling. Thus the slow species turnover within the dendritic aquatic network interacts with terrestrial land use patterns to yield important effects on ecosystem functioning.