Divergent Functional Roles of Shredder Size: Interspecific Versus Intraspecific Effects on Aquatic Leaf Litter Decomposition

ABSTRACT Biodiversity underpins ecosystem functioning, and higher diversity of taxa and traits often enhances efficiency. However, such relationships may vary, as intraspecific variation—including differences across ontogenetic stages—can modulate a taxon's contribution to ecosystem functioning. We conducted laboratory experiments to examine the effects of shredder identity, diversity, and ontogenetic stage on leaf litter decomposition and fine particulate organic matter (FPOM) production. Three caddisfly shredder taxa ( Allogamus , Potamophylax , and Sericostoma ) were collected in two different months and fed with highly decomposable alder ( Alnus glutinosa ) leaves individually and in shredder combinations during 1‐week incubation periods. The experiments reflected distinct larval instars and were conducted with a time interval of 2 months. We hypothesized that decomposition and FPOM production would vary with taxon identity and ontogeny, with diversity enhancing processing rates. We further expected higher processing rates in younger larvae due to their greater metabolic demands. Results showed that both shredder identity and ontogenetic stage significantly affected decomposition, FPOM production, and particle size distribution. The largest taxon, Potamophylax , had the highest decomposition and FPOM production rates and produced the largest FPOM particles, while the smallest taxon, Allogamus , had the lowest rates and produced the smallest particles. Within taxa, younger and smaller larval stages exhibited higher rates than their older conspecifics. These results highlight the importance of shredder identity and ontogenetic stage in shaping both the magnitude and timing of key ecosystem processes.