Biodiversity is a key driver of ecosystem functioning, with increased diversity often linked to greater ecosystem efficiency. However, the strength and direction of such relationships can vary, as intraspecific variation, including differences across ontogenetic stages, may modulate a taxon’s contribution to ecosystem functioning. To explore these effects, we conducted laboratory experiments examining the influence of shredder identity, diversity, and larval 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 assigned to individual and mixed-species treatments using highly decomposable alder (Alnus glutinosa) leaves. The experiments incorporated distinct larval instars, reflecting natural variation in body size and ontogenetic stage, and were conducted in separate one-week incubations with a two-month interval between collection periods. We hypothesized that decomposition and FPOM production would differ between taxa and ontogenetic stages, with species diversity enhancing processing rates, and that younger, smaller larvae would exhibit higher processing rates due to elevated metabolic demands. The results showed that both shredder identity and ontogenetic stage significantly influenced leaf litter decomposition, FPOM production, and particle size distribution. Among taxa, the largest species, Potamophylax, exhibited the highest decomposition and FPOM production rates and produced the largest FPOM particles. In contrast, the smallest taxon, Allogamus, had the lowest rates and generated the smallest particles. Within each taxon, younger and smaller larval stages consistently processed leaves faster than their older conspecifics, highlighting the importance of ontogenetic variation. Mixed-species treatments showed intermediate patterns, suggesting that diversity effects were largely mediated by species identity rather than additive or synergistic interactions. These findings demonstrate that while biodiversity can enhance ecosystem functioning, the specific contributions of species and their ontogenetic stages play a critical role in shaping the magnitude and timing of leaf litter decomposition in aquatic ecosystems.