Nonlinear Effects of Intraspecific Competition Alter Landscape-Wide Upscaling of Ecosystem Function

Abstract

A major focus of ecology is to understand and predict ecosystem function across scales. Many ecosystem functions are only measured at local scales, while their effects occur at a landscape level. Here, we investigate how landscape-scale predictions of ecosystem function depend on intraspecific competition, a fine-scale process. Specifically, we experimentally investigated the effect of intraspecific density of shredding macroinvertebrates on associated leaf litter decomposition, a key function in freshwater ecosystems. Across two species, we found that leaf processing rates declined with increasing density following a power law, likely due to interference competition. To demonstrate consequences of this nonlinearity, we upscaled estimates of leaf litter processing from shredder abundance surveys in 10 replicated headwater streams. In accordance with Jensens inequality, applying density-dependent consumption rates reduced estimates of catchment-scale leaf consumption up to 60-fold versus using density-independent rates. Our work highlights the need for spatially-explicit upscaling which accounts for intraspecific interactions.