Reach-scale manipulations show invertebrate grazers depress algal resources in streams

Abstract

Experimental tools that enable manipulations of organisms at larger scales allow for comparisons of processes across multiple spatial scales and expand our ability to make predictions about ecological processes. We performed reachscale (i.e., 50 m2) manipulations of invertebrate communities in streams using a modified electroshocking technique to non-destructively remove invertebrates. In addition, we conducted a microcosm experiment (i.e., 157 cm2) with different grazer densities that enabled comparison of the strength of grazer-algal interactions at large and small spatial scales. In high-elevation headwater streams, electroshocking reduced total invertebrate abundance by 84% in a 50-m2 reach of stream. Although mobile invertebrates recolonized the manipulated area rapidly, daily electroshocking maintained the density reduction. Electroshocking reduced the density of herbivorous invertebrates 86%, which resulted in a 57% increase in algal biomass, whereas in a stream that was not electroshocked, invertebrate density and algal biomass changed much less, only 16 and 8%, respectively. Comparison of grazer effect on periphyton between microcosm and reach-scale experiments revealed that the per capita interaction strength of grazers on primary producers was three times greater in the reach-scale manipulation than that observed in the microcosm experiments. Reach-scale manipulation of invertebrate grazers in streams provides a powerful method to experimentally test patterns observed in the field at a large spatial scale, with more realism than streamside microcosms or small cages in streams.