A small supply of high quality detritus stimulates the consumption of low quality materials, but creates subtle effects on the performance of the consumer

Abstract

Riparian forests produce leaf litter of very different quality and originate packs of litter mixtures of very heterogeneous characteristics in adjacent freshwater ecosystems. Decomposition rates of these litter mixtures often show a non-additive response, with high quality leaf litter stimulating decomposition of poor resources. However, little is known about the performance of consumers facing a diet based on a litter mixture. In this study, we assess in a microcosm experiment how a small availability of a high quality resource affects the consumption and performance of the amphipod Echinogammarus berilloni. Resources of varying quality (Corylus avellana, Fraxinus excelsior, Quercus robur and Fagus sylvatica) were offered individually and supplemented (20% of the total mass) by nitrogen-rich Alnus glutinosa leaf litter. Consumption of leaf species significantly differed when offered separately, with Alnus being the most and Quercus being the least consumed. Addition of Alnus showed only a positive net effect on the consumption when in combination with low quality resources (Quercus and Fagus). For these combinations we observed a positive complementarity effect and a negatives selection effect, i.e. the consumption of both materials was increased, but more of the low quality resource. With the offered Alnus amount, a slight increase in growth was observed but lipid content and RNA:body mass were reduced. The observed reduction in body condition was related to the quality of the predominant leaf, high quality resources (Corylus and Fraxinus) showing the largest reductions. Our study shows that a small availability of a high quality resource can stimulate the consumption of low quality organic matter, with small effects on the consumer.