Abstract: Changes in land cover are one of the most extensive and influential human impacts on fluvial ecosystems. In this study, we assessed how changes in food resource quantity and type linked to forest cover affect mountain stream multitrophic macroinvertebrate community size structure. We tested whether the total organism biomass (size spectrum intercept) and the energy transfer efficiency among trophic levels (size spectrum slope) are altered. Community size spectra slopes remained unchanged with variations in food resource type, while intercept increased with resource quantity. This pointed to an internal compensatory regulation of the community size structure for the maintenance of trophic transfer efficiency, only adjusting the total organism biomass to total food resource quantity through variations in organism abundance. Community size structure consistency was related to detritivore-omnivore substitutions, although the shifts in size structure with variations in the degree of allochthony differed between these feeding groups. Omnivore change was numerical (i.e., increase in large organism density with a decrease in allochthony) not related to body size or taxonomic composition. In contrast, detritivores showed a reduction in body size with the decrease in allochthony that did not coincide with a reduction in total biomass or density, but was related to a taxonomic replacement of larger case-building trichopterans by smaller detritivores. These findings elucidate how the size structure of a multitrophic macroinvertebrate community might respond to changes in forest cover, shedding light on the effects of global environmental change on the structure and stability of stream food webs.

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