Nonlinear Effects of Intraspecific Competition Alter Landscape-Wide Scaling Up of Ecosystem Function

A major focus of ecology is to understand and predict ecosystem function across scales. Many ecosystem functions are measured only 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, by manipulating intraspecific density of shredding macroinvertebrates and examining effects on leaf litter decomposition, a key function in freshwater ecosystems. For two species, we found that per capita leaf processing rates declined with increasing density following power functions with negative exponents, likely due to interference competition. To demonstrate consequences of this nonlinearity, we scaled up estimates of leaf litter processing from shredder abundance surveys in 10 replicated headwater streams. In accordance with Jensen's inequality, applying density-dependent consumption rates reduced estimates of catchment-scale leaf consumption by an order of magnitude relative to density-independent rates. Density-dependent consumption estimates aligned closely with metabolic requirements in catchments with large-but not small-shredder populations. Importantly, shredder abundance was not limited by leaf litter availability, and catchment-level leaf litter supply was much higher than estimated consumption. Thus leaf litter processing was not limited by resource supply. Our work highlights the need for scaling up, which accounts for intraspecific interactions.