Litter decomposition in a temperate and a tropical stream: the effects of species mixing, litter quality and shredders

The current rapid decline in biodiversity has led to concerns about the consequences for stream ecosystem processes, one of which is the decomposition of leaf litter derived from riparian vegetation. We conducted field experiments in a tropical and a temperate stream to test for the effects of mixing leaf species differing in resource quality on the decomposition of leaf litter and on the colonisation of the litter by leaf-shredding invertebrates. The effects of litter mixing were minor compared with the effects of litter quality and the presence or absence of shredders. Low shredder abundance in the tropical stream and poor quality of the tropical leaf species, particularly in terms of phosphorus content and toughness, were associated with low decomposition rates in the tropical compared with the temperate stream. This is especially true when considering the 20 degrees C temperature differences between the two streams. In the presence of shredders, the decomposition rate of a standard litter type, leaves of Alnus glutinosa, was 2.6-fold faster in the temperate stream, whereas rates were similar when shredders were absent. This indicates that differences in environmental conditions other than temperature had a strong effect. Differences in water chemistry, such as higher concentrations of dissolved nutrients in the temperate stream, might account for this effect. In conclusion, despite a lack of clear effects of litter mixing on decomposition, our results highlight the importance of litter identity and environmental conditions for both microbial and shredder-mediated litter decomposition, suggesting that changes in riparian vegetation and other stream characteristics will affect stream ecosystems in the face of widespread environmental change.