The role of flow shaping the functional and structural invertebrate diversity at a high altitude tropical stream

Background/Question/Methods

The absence of wide temperature changes in tropical streams suggests that the variations of benthic assemblages are related to other environmental variables. At high altitude tropical streams, the temperature is usually below 10 ºC along the year, which is a harsh environment for the development of aquatic insects. In this context, long developmental times would be expected with more dependence on hydrological variables. Our objective was to characterize the hydrological variation of a high altitude Andean stream and to relate this variation to the composition and diversity of taxa and traits of aquatic insects. Our hypothesis is that the hydrological variation plays a major role shaping the functional and structural diversity in this type of stream. We studied the Saltana creek located in the Western Andean ranges of Ecuador at 3800 masl. We took twelve random Hess benthic samples monthly from April 2009 to April 2010. From every sample we recorded the velocity, substrate and depth. With these variables we also calculated the Reynolds and Froude numbers. Stream flow was followed every hour by a depth logger located in a fixed transect in the stream. We related the composition, abundance and diversity (taxa and traits) to flow and hydraulic variables.

Results/Conclusions

Annual mean flow was 130 l/s. We found two distinct hydrological periods along the year. One with high flows and common spates (spate defined as values larger than 1.5 the annual mean = 185 l/s, when full bank flow is present) during April to September, and other more stable with no spates from September to March. Taxonomical richness and abundance varied inversely to flow. Months with similar flow regime had similar benthic community composition. Also, flow was related to density changes of certain taxa (e.g. Andesiops ardua density decreased as flow increased), and also to certain traits (e.g. proportion of tarsal hooks were higher in high flows). At microhabitat level the effect of hydraulic variables was strong. We found that high velocity, flow turbulence and large substrates were significantly related to individuals of small sizes (length, width and height) and also found effects on the distribution of the different taxa found in the stream (e.g. Gigantodax sp. presented a strong direct relationship with velocity and substrate). Based on these results we conclude that the effect of the hydrological variation on the benthic insects was strong shaping the community composition and constraining the sizes of the individuals at microhabitat level.