<b>A Complex Web of Interactions in a Rocky Intertidal System: Intraguild Predation or Competition?</b>

Intraguild predation (IGP) is considered an important process in most communities. However, theoretical and empirical research reach contrasting conclusions on the strength and stability of IGP. Theory suggests IGP is fragile, requiring that the IG prey is a more effective consumer of shared prey than the IG predator. Empirical examples find the opposite, that the IG predator is more efficient, and that competition between the IG predator and IG prey dominates. Effects of competition and IGP were experimentally tested in an intertidal predator guild using small, isolated islets. Using an orthogonal design, large sea star <i>Pisaster ochraceus</i> and whelks (genus <i>Nucella</i>) were removed, after which size and density changes in the small sea star <i>Leptasterias hexactis, </i>whelks, and <i>Pisaster </i>were monitored. Competition, not IGP was the primary interaction between <i>Pisaster </i>and the small predators. Contra prior results, <i>Pisaster </i>affected <i>Leptasterias </i>density, not body size. Twelve months post-removal, whelk abundance overshot pre-removal levels, likely due to absence of adult whelk competition with whelk hatchlings. The inconsistency with theory is likely due to web complexity, habitat heterogeneity, size differences of predators, and the system openness. These traits characterize most ecosystems, raising the question of the importance of IGP to community dynamics.