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

Abstract

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. Oomycetecolonised leaves were consumed at similar rates but were of lower food quality than fungicolonised leaves. 5. We argue that direct strain-specific attractant or repellent effects of fungi and oomycetes on consumption by G. roeselii are not important. However, we found indirect strainspecific role operating via effects on leaf parameters.

Bottom-up effects of streambed drying on consumer performance through changes in resource quality

“…The reduced consumption of DRY leaves was expected due to their lower conditioning (Cummins and Klug 1979;Graça et al 2001), the result of reduced microbial metabolism in terrestrial habitats (Molles et al 1995). Among aquatic detritivores, leaf palatability has been inversely related to leaf toughness and positively related to fungal biomass and N concentration (Irons et al 1988;Hladyz et al 2009;Aßmann et al 2011), which were significantly lower in our DRY leaves. More surprising, however, was the reduced consumption rates of Stenophylax when fed on an INT diet in comparison to the PERM diet, given the similarities between these types of leaves.…”

“…Because effects on microorganisms increase with drying duration (Langhans and Tockner 2006), changes in fungal activity or assemblage composition favouring more resistant or terrestrial species cannot be excluded. Several studies have shown that the consumption rates of shredders vary according to the fungal species in leaf assemblages (Arsuffi and Suberkropp 1984;Chung and Suberkropp 2009;Jabiol and Chauvet 2012), whereas lower microbial activity during the terrestrial phase (Langhans et al 2008;Larned et al 2010;Bruder et al 2011) might have prevented the degradation of repellent secondary metabolites or more recalcitrant C, such as cellulose, lignin or condensed tannins, resulting in increased leaf toughness and thus, less palatable leaves in the INT than PERM treatments (Irons et al 1988;Hladyz et al 2009;Graça and Cressa 2010;Aßmann et al 2011). As expected, the increased consumption of PERM leaves translated into higher growth rates, which is consistent with the bigger size at eclosion found for an insect detritivore reared on continuously submerged resources (Aspbury and Juliano 1998).…”

“…compensatory feeding), thus maintaining their homeostasis and growth (Frost et al 2005;Fink and Von Elert 2006;Flores et al 2014). Nevertheless, reduced microbial conditioning and food quality have also been observed to dampen detritivore ingestion rates when fed on low quality leaves (Aßmann et al 2011;Danger et al 2012;Graça and Poquet 2014). Therefore, differences in the responses of organisms to stream drying can be expected.…”

Effects of two fungicide formulations on microbial and macroinvertebrate leaf decomposition under laboratory conditions

“…Lower nutritional value can either reduce [34] or increase [7] leaf consumption by amphipods. For example, leaf parameters (e.g., protein and polyphenols) are affected by fungal and oomycete strains and by leaf conditioning strategies (stream, tap-water, autoclaving) and are strongly correlated with leaf consumption by amphipods [34]. Unexpectedly, however, leaf decomposition rates were similar for unconditioned control and control leaves.…”

“…This was somewhat unexpected because the composition of aquatic microbial communities (fungi, bacteria, and funguslike eukaryotes known as oomycetes), and hence food quality, can change with temperature and season [4,34,44,45]. Amphipods fed control leaves that had been stream-conditioned in May (8-12 8C, QUILT experiment) grew at similar rates and achieved similar size as amphipods fed control leaves that had been stream-conditioned in July (20-23 8C, PRISTINE experiment).…”