Impacts of biological diversity on sediment erosion in streams

Background/Question/Methods

Over the past decade, an increasing number of studies have shown that biological structures (e.g. roots) have large impacts on sediment erosion. But while it is now recognized that biological structures influence the transport of sediments, work to date has focused primarily on the impacts of individual, usually dominant, species.  Here we ask whether competitive interactions cause multi-species communities to have fundamentally different impacts on sediment mobility than single-species systems.  To address this question, we manipulated the presence or absence of two common species of net-spinning caddisfly larvae (Ceratopsyche oslari, Arctopsyche californica) in experimental streams and measured how their silk nets influence the critical shear stress required to initiate sediment motion.

Results/Conclusions

We show that critical shear stress increases non-additively when species are allowed to interact.  Critical shear stress needed to initiate sediment motion was 31% higher in a multi-species stream compared to a single-species stream, and 14% greater than levels of stability achieved by the species having the single largest impact on sediment motion.  Accompanying behavioral studies suggest that the non-additive increase in critical shear stress likely occurred as competition among species led to shifts in the spatial distribution of the two populations.  When each species was alone in a stream, both taxa built nets at the bed surface.  But when species were placed together, the inferior competitor shifted its distribution and built nets deeper in the sediment, leading to complimentary use of space in the streambed.  Incorporating our findings into a common model of sediment transport in gravel bedded rivers shows that the non-additive increase in critical shear stress that we documented predicts a 50% reduction in the cumulative bed flux during spring floods that are typical of streams in the Sierra Nevada Mountains where caddisflies are abundant and exist in multi-species assemblages. Taken together, our results suggest that species interactions can generate synergies that have unique and lasting impacts on sediment stability, which may improve our ability to accurately quantify the influence of biology on sediment transport conditions.