Biodiversity and riparian forests are mutual biological drivers of ecosystem functions in temperate and tropical streams

Fluxes of energy, matter, and organisms sustain linkages and functions within and between ecosystems. Yet, how biological drivers influence interactions and functions at the interface between aquatic and terrestrial environments (i.e., aquatic-terrestrial ecosystem functions) locally and across regions has received little attention. To test the relative importance of biological drivers on multiple aquatic-terrestrial ecosystem functions, we subsidised local terrestrial detritus in forested and non-forested stream sites in a temperate and tropical region. We also manipulated leaf litter diversity (horizontal biodiversity of resources) and macroinvertebrate access (vertical biodiversity of consumers). We measured secondary production of aquatic fungi, in-stream leaf litter nitrogen loss, and decomposition rates. The simultaneous provision of all three ecosystem functions (i.e., multifunctionality) was positively driven by vertical biodiversity and riparian forests in both regions. In both tropical and temperate streams, nitrogen loss was associated with vertical biodiversity. Decomposition rates were also enhanced by vertical biodiversity and linked to other ecosystem functions. These results reveal strong and consistent effects of biodiversity and riparian forests on aquatic-terrestrial ecosystem functions in freshwater detrital food webs in both temperate and tropical headwater streams. Thus, disentangling the drivers of ecosystem functions in these systems requires an understanding of underlying mechanisms beyond ecosystem borders.