Salamander diversity alters stream macroinvertebrate community structure

Summary Salamanders are abundant consumers in many fishless stream ecosystems, but few studies have explicitly examined their ecological role. Stream‐dwelling salamander larvae are generalist predators of aquatic macroinvertebrates and may play an important role in structuring macroinvertebrate communities. The potential for emergent effects of multiple predator species suggests that changes in salamander diversity could alter their effects on macroinvertebrate communities, but this has not been tested. We used in‐stream enclosures to manipulate the presence and diversity of the two most abundant salamanders, Eurycea wilderae (EWIL) and Desmognathus quadramaculatus (DQUA), in a southern Appalachian Mountain headwater stream to examine the impacts of salamander predation on macroinvertebrate communities. We were particularly interested in testing for potential diversity effects by comparing the effects of each species in monoculture to polycultures of both species (BOTH). Salamanders reduced macroinvertebrate abundance compared to the control treatment (CONTROL), but only when both species were present together. The general pattern of macroinvertebrate abundance among treatments was: CONTROL = EWIL = DQUA > BOTH, although no treatment significantly reduced the abundance of epibenthic taxa. The BOTH treatment also significantly altered macroinvertebrate community structure and reduced taxon richness by c. 57%. The effects of salamanders were particularly pronounced for chironomids (Diptera: Chironomidae), which comprised the majority of macroinvertebrate total abundance. Increased salamander diversity substantially altered macroinvertebrate communities, suggesting that niche complementarity or facilitation occurred despite the apparent functional similarity of these two species. Therefore, changes in salamander diversity may alter the effects of salamander communities on macroinvertebrates, which may cascade through food webs to affect stream ecosystem function.