Galled leaves as an improved resource for benthic detritivores

Abstract

Galled leaves are very frequent in southern beech (Nothofagus pumilio) forests. We compared the consumption of leaves (galled and ungalled) by two detritivores and the effect of this consumption on detritivore nutrient content. The amphipod Hyalella curvispina and larvae of the caddisfly Monocosmoecus hyadesi were fed with two diet treatments for 30 days: (1) ungalled leaves and (2) galled leaves. Fungal biomass, carbon (C), nitrogen (N), and phosphorus (P) content were measured in leaves and detritivores, and C:N:P molar ratios were calculated. We hypothesized that galls produce changes in litter nutrient content, affecting invertebrate consumption and nutrient content. Galled leaves had higher N and ergosterol content, and lower C:N ratios than ungalled leaves. Galled leaf consumption was higher than the ungalled for both detritivores. The two species differed in the content of N and C (caddisfly > amphipod) and P (caddisfly < amphipod). The N:P ratio of the amphipods was similar to the ungalled leaf ratio and increased when fed on galled leaves. In contrast, the N:P ratio of the caddisflies was considerably higher than the ungalled leaves and decreased when fed on galled leaves. Galls alter nutrient content in leaves and increase fungal biomass. Both detritivore species were highly homeostatic with regard to C:N ratio, but caddisflies could overcome the limited P content by increasing consumption. We highlight the impact of galls on aquatic detritivores, which in turn are relevant in forested streams for recycling organic matter and cascading effects through brown detritus-based aquatic food webs.