The main idea of the presented thesis is to assess the relevance of taxonomic and functional diversity of detritivores (shredders) in the ecosystem process of organic matter, decomposition. The thesis also includes comparison of functional traits of several shredder species, which have a great importance in leaf litter breakdown rate in aquatic ecosystems. In our experiments we focused on ponds that are an important element in the carbon cycle and which support high diversity of invertebrates. We investigated breakdown rate of black alder (Alnus glutinosa) leaves in microcosms with different combinations of shredder taxa. Moreover, we compared leaf-consumption traits among shredders using gut content analysis. Our results show that individual species belonging to the same functional feeding group may functionally differ from each other and have different effects on the decomposition rate and ecosystem functioning. Our results showed that simple measures of taxonomic and functional diversity are inefficient in explaining variability of leaf litter breakdown rate. Instead, identity of particular species and interspecific interactions play a major role in the process of decomposition.