Availability and quality of food resources limit consumer performance and modulate food webs, which in turn, can affect ecosystem functioning. Availability and quality of food resources can be especially important in streams, where consumers depend on allochthonous organic matter whose availability and quality can differ markedly both spatially and throughout the year. Most studies of the relationships between food quality and detritivore performance have been based on standardized food types and have not been designed to allow consideration of interactions between resource quantity and quality. Our goal was to evaluate the effects of quality and quantity of resource on shredder performance. We raised larvae of the caddisfly Sericostoma vitattum in the laboratory with food taken from a stream reach. We controlled food type (conditioned Alnus glutinosa leaves, natural leaf packs from riffles, and deposits within thick debris jams) and quantity (low: 2 mg individual-1 d-1; high: 10 mg individual-1 d-1). Consumption rate tended to be higher for lower- than for higher-quality materials, and the differences were more marked at high resource quantity. However, this compensatory mechanism was not enough to compensate growth in the case of lowest-quality food. In addition, food quality and quantity interacted to determine the energy allocation to body condition or growth. Our results show that the interaction between resource quality and quantity can influence invertebrate condition and life cycles. Stored organic matter in large jams, despite their poor quality, could enable consumers to survive until the next high-input season.
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Vol. 33 • No. 1