Résumé
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, 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.