The effects of top-down and bottom-up controls on macroinvertebrate assemblages in headwater streams

Headwater stream macroinvertebrates play an important role in processing allochthonous leaf litter, which suggests that bottom-up forces control macroinvertebrates. However, because larvae of stream-breeding salamanders are predators of macroinvertebrates and are abundant consumers in these ecosystems, macroinvertebrates in fishless headwater streams might also be controlled by top-down forces through predation by salamander larvae. The aim of this study was to test if and to what degree taxa richness, abundance, and biomass of macroinvertebrates are affected by bottom-up and top-down forces. We selected headwater streams with high abundances of fire salamander larvae (1.2–2.6 individuals per 1 m of shore length) and manipulated bottom-up and top-down forces on macroinvertebrates by leaf litter addition and by the exclusion of salamander larvae. The amphipod Gammarus fossarum Koch, 1836 was the dominant taxon and responded positively to litter addition. Linear models showed that neither predator exclusion nor leaf litter addition affected richness. However, variation in biomass and density were both explained by the individual and joint effects of bottom-up and top-down forces. These findings suggest that macroinvertebrates in these streams are strongly dependent on the organic matter input and salamander larvae, and headwater streams interact strongly with their adjacent terrestrial areas.