Effects of increasing structural complexity in a tropical degraded stream on macroinvertebrate assemblage structure, trophic guilds and biomass

Background/Question/Methods

In stream ecosystems the removal of riparian vegetation to agricultural land-use has been leading to habitat simplification, which in turn can lead to ecosystem function loss. Since most streams have been degraded due to human activities, restoration has been considered a priority and must take into account the protection of spatial variability that underpins the diversity of biota and ecosystem processes. In this context, the aim of the present study was to investigate if adding wood debris and leaf packs in degraded streams modifies macroinvertebrate assemblage structure, trophic guilds, and biomass.In order to achieve this aim, a degraded pasture stream was selected for an experimental manipulation that simulates structural heterogeneity increments. Two stream stretches were selected: one that remained unmodified (control) and other that was incremented with a wooden structure and leaf-packs. The stretches were sampled before experimental manipulation, on April 2011, and then re-sampled after five months. Macroinvertebrates were sampled with D nets and two electrofishing passes. They were fixed, identified up to family level, and weighed to biomass evaluation; trophic guilds were defined based on invertebrate dietary analysis. Finally, ad libitum underwater observations were conducted to investigate if experimental structures were being used by biota. 

Results/Conclusions

A total of 5,740 individuals of 45 families were sampled. Before increments, the control stretch accounted for 23 families and 942 individuals whereas in the treated stretch 12 families and 786 individuals were registered. After the experimental period, the control stretch remained similar to previous conditions whereas in the treated stretch 34 families and 3,053 individuals were registered, evidencing that wood and leaves are factors structuring macroinvertebrate assemblages. Macroinvertebrate biomass followed the same pattern of variation, with similar values between periods at the control stretch and higher values at the treated stretch after the experiment (3.33 g before; 26.5 g after). Detritivores biomass after the experiment was 73.8% higher at the treated stretch. Despite macroinvertebrates did not feed directly on the leaves (including those usually classified as shredders), they used the wood and leaf-packs as habitat (as observed in loco) and feeding ground, since such structures promoted detritus (FPOM) retention and most taxa were detritivores, especially the most abundant ones, as ephemeropterans and chironomids, suggesting a detritus-based food web. Hence, our results indicate that even in degraded environments, such as the studied stream, increments in structural complexity can increase the variety of taxa, abundance, and biomass of invertebrates.