The role of invertebrates on leaf litter breakdown at two altitudinal levels in neotropical streams

Background/Question/Methods

Leaf litter breakdown is a key ecosystem process in streams. Headwater mountain streams are generally covered by riparian vegetation which limits in-stream photosynthesis and contributes to significant organic matter inputs (i.e. mainly leaf litter). Benthic invertebrates, particularly shredders, have been recognized in temperate streams as key contributors to the conversion of coarse particulate organic matter to fine particulate organic matter. However, their importance in neotropical streams is unclear. In this study, we examined the effect of invertebrates on leaf litter breakdown at two altitudinal levels (1,900 and 2,600 m) in forested tributaries of the Otún River in southwestern Colombia, South America. We used coarse-mesh (accessible to invertebrates; 15 mm) and fine-mesh (invertebrates excluded; 200 µm) litterbags to quantify breakdown rates (k, d^˗1) of leaves from two plant species (Critoniopsis ursicola and Palicourea cuatrecasasii) and mixed leaf packs (C. ursicola + P. cuatrecasasii) over a course of 56 days. We measured invertebrate density and biomass through time and compared breakdown rates between streams, plant species, and mesh sizes. Additionally, we used artificial leaves to examine whether particulate organic matter accumulated in litterbags from transported material influenced invertebrate colonization.

Results/Conclusions

Stream altitude and mesh size did not have an effect on breakdown rates; in contrast, there were significant differences in breakdown rates of plant species (P<0.0001). Critoniopsis ursicola had the highest breakdown rate (k = 0.0089 d^˗1), followed by mixed leaf packs (k = 0.0072 d^˗1), and P. cuatrecasasii (k = 0.0052 d^˗1). Invertebrate densities and biomasses were not significantly different between streams or plant species. Total density and biomass were high in leaf litterbags (135-146 individuals g^˗1 and 77-82 mg, respectively) and were low in artificial leaf litterbags (38 individuals g^˗1 and 16 mg, respectively). Collector-gatherers were the functional feeding group with the highest density (387 individuals g^˗1), followed by shredders (50 individuals g^˗1), and predators (13 individuals g^˗1). In contrast, shredders had the highest biomass (138 mg), followed by collector-gatherers (69 mg), and predators (43 mg); nonetheless, there was no significant relationship between remaining dry mass of leaves and invertebrate biomass. Invertebrates had no effect on leaf litter breakdown suggesting that leaves were used mainly as substrate. Low leaf palatability and high hydraulic stress (discharge related to rainfall) might explain the low role of invertebrates in organic matter decomposition in these neotropical streams.