Ecosystem engineering by larvae of net-spinning stream caddisflies creates a habitat on the upper surface of stones for mayfly nymphs with a low resistance to flows

1. Field and laboratory experiments were conducted to examine the effect of larval retreats of the net‐spinning stream caddisfly Hydropsyche orientalis on abundances on flow exposed stone surfaces of nymphs of the mayfly Ephemerella setigera, which prefer slow flow conditions, and to determine whether the construction of retreats ameliorated the habitat and made it more suitable for E. setigera. 2. In a field experiment, artificial substrates with retreats of H. orientalis had higher E. setigera abundances than substrates lacking retreats. In addition, abundances of E. setigera nymphs increased significantly with those of H. orientalis larvae on the upper surface of boulders in streams. 3. The drift loss of E. setigera from plates, with and without retreats, was investigated along a current velocity gradient in a laboratory channel experiment. Nearly all E. setigera nymphs remained on the plates with retreats, even at the highest current velocity. In contrast, on the plates without retreats, the drift loss of E. setigera nymphs increased as the current velocity increased. 4. These results suggested that the habitat amelioration by H. orientalis retreats provided a refuge location for E. setigera nymphs and increased their abundances on stone surfaces exposed to flow forces.