Consequences of the colonisation of leaves by fungi and oomycetes for leaf consumption by a gammarid shredder

1. Leaf litter breakdown by shredders in the field is affected by leaf toughness, nutritional value and the presence of secondary compounds such as polyphenols. However, experiments involving the use of single fungal strains have not supported the assumption that leaf parameters determine food selection by shredders perhaps because of a failure to test for high consumption prior to isolation of fungal strains, overrepresentation of hyphomycetes or the potential effects of accompanying bacteria. In this study, we used bacteria-free, actively growing fungi and oomycetes isolated from conditioned leaf litter for which a shredder had already shown high consumption rates. 2. Black alder (Alnus glutinosa) leaf litter was exposed to the littoral zone of Lake Constance in autumn, and subsamples were analysed for leaf parameters and consumption by Gammarus roeselii under standard conditions at regular intervals. On dates with a high consumption rate of the exposed leaves, 14 single strains of fungi and oomycetes were isolated, freed of bacteria and grown on autoclaved leaves. 3. Six of eight measured leaf parameters of exposed leaves were significantly correlated with Gammarus consumption rates, with high colinearity among leaf parameters hampering the identification of causal relations between leaf parameters and feeding activity. 4. When single strains of fungi and oomycetes were grown on autoclaved leaf litter, toughness of colonised leaves was always lower than in the control and the content of protein, N and P were increased. There were pronounced strain-specific effects on leaf parameters. Consumption rates also differed significantly, with nine of fourteen isolates consumed at higher rates than controls and none proving to be a deterrent. Protein and polyphenol content were significantly correlated with consumption rates. Oomycete-colonised leaves were consumed at similar rates but were of lower food quality than fungi-colonised leaves. 5. We argue that direct strain