Consumer responses to resource patch size and architecture: leaf packs in streams

Abstract

Accumulations of leaf litter in freshwaters can vary from individual leaves to large leaf packs. Past studies have demonstrated that decomposition rates decrease as leaf pack size increases. We considered a set of hypotheses that lower breakdown in larger leaf packs occurs as a consequence of diffusion gradients of oxygen and nutrients, or lower accessibility to larger detritivores to leaf tissue in the middle of these leaf packs. We manipulated leaf pack size in a stream to quantify decomposition rates and the abundances of consumers relative to the amount of detrital mass available. Mass loss rates of beech leaves were lower as leaf pack size increased. We found no differences in fungal biomass across our treatment gradients, or when comparing leaves in the middle of the leaf pack versus those on the outside. The lower decomposition of larger leaf packs would thus not result from a lower quality of the resource based on fungal biomass. Invertebrates per unit mass of leaf packs declined exponentially with size of leaf pack. Smaller invertebrates were less abundant per unit of resource as leaf pack size increased, but abundances of larger invertebrates declined even more dramatically than that of smaller ones. The results are consistent with accessibility within leaf packs decreasing as leaf pack size increases, a factor that is important for the estimation of consumer-resource functions for this patchy resource.

Decomposition of leaf litter mixtures in streams: effects of component litter species and current velocity

“…leaf litter incubated individually or in mixtures). Past studies have demonstrated that decomposition rates decrease as leaf pack size increases mainly due to limited physical abrasion, oxygen diffusion and shredders accessibility to leaf tissue in the middle of large leaf packs (Richardson and Chauvet 2019). In our study, most of the alder litter was completely decomposed at the end of the experiment, and therefore, the size of the leaf packs containing oak litter in mixtures were about half the size of monospecific oak leaf litter packs.…”

Early spring food resources and the trophic structure of macroinvertebrates in a small headwater stream as revealed by bulk and fatty acid stable isotope analysis

“…Headwater streams have a significant ecological importance due to their effect on the structure and function of higher-order streams, linking low-order with downstream ecosystems (Gomi et al, 2002). Many of the temperate headwater streams are located within upland-forested watersheds, at least in relatively pristine environments (Richardson & Chauvet, 2019) and falling riparian vegetation (tree leaves) is a major source of energy for their food webs (Allan & Castillo, 2007;Collins et al, 2016;Franklin et al, 2020). Deciduous riparian trees in upland catchments have a major influence on energy flow to the streambed.…”