Urban‐driven homogenization of aquatic subsidy size structure cascades to riparian predator communities

The export of emergent aquatic insects is a critical energy subsidy for terrestrial food webs. While urbanization is known to alter stream communities, its effects on the size structure of these insect subsidies and the subsequent consequences for riparian predators remain poorly understood. Yet, body size, a key dimension of subsidy quality, can strongly shape resource use by terrestrial predators, given their size‐dependent foraging. Here, we investigated how land impervious cover affects the body‐size distribution of emergent insects and riparian spider communities along the entire length of two urban streams. We sampled emergent insects to characterize community composition and two size‐structure metrics (i.e. mean body size and size range), and used stable isotopes to assess reliance of spiders on aquatic prey. Impervious cover was the strongest driver of the emergent community structure, overriding effects of longitudinal position. Increasing impervious surface cover was associated with community homogenization, a pronounced shift toward smaller individuals and a contraction of body‐size range. Notably, total exported biomass did not change significantly, indicating that the influence of surface imperviousness manifests primarily in qualitative rather than quantitative terms. Those changes led to higher reliance on aquatic prey from riparian spiders. Our work highlights that the homogenization of aquatic prey size distribution is a powerful driver of change within riparian food webs and underscores the importance of integrating body‐size composition into assessments of land‐use change to better capture its consequences for cross‐ecosystem resource flows.