Drill abundance relative to elevation and cover of coarse substrate (boulder and cobble). Each point represents a quarterly transect survey (N = 37) at one of the four sites where drills were present, and points are jittered to reduce overlap. The three clusters of points surrounded by an oval correspond to sites with an average drill density below one drill per 0.25 m2, a level at or below which Olympia oyster and Atlantic drill populations have been observed in coexistence. The two solid ovals are Hilarita Ave., and the dashed oval is Blackie’s Pasture.