Filling the limelight when a star is lost: Compensatory effects of subordinate predators following disease-driven declines of the keystone sea star Pisaster ochraceus
Theory predicts that loss of a keystone predator should greatly affect the structure and functioning of an ecological community. However, the effects of keystone loss may be modulated by the presence of other predators in the system capable of compensatory responses. These so-called “redundant” predators in keystone-dominated ecosystems may only become important when the keystone is removed. In rocky intertidal systems along the U.S. west coast, sea star wasting syndrome has decimated populations of the keystone predator Pisaster ochraceus, which could change community structure notably by reducing control on the dominant space-occupying mussel Mytilus californianus. Classic studies by Paine and others have shown that, in the absence of P. ochraceus, these mussels can invade the low zone of the intertidal and out-compete the existing algal-dominated community. We conducted a field experiment to test whether three important subordinate predators could maintain existing community structure after the depletion of the keystone predator. We removed subordinate whelk (Nucella canaliculata and N. ostrina) and sea star (Leptasterias spp.) predators in a factorial experiment at two sites with high abundance of sessile invertebrate prey along the Oregon coast. We then tracked abundance of prey (barnacles and two species of mussels) and community trajectories over time.
In the absence of the keystone P. ochraceus, subordinate predators had variable effects on prey after 8 months. Cover of the smaller mussel M. trossulus was lower in plots with predators present (26.5% ± 3.4%, vs 33.7% ± 3.7%, p=0.06). This trend was driven by whelk predators rather than by Leptasterias, which were rare in plots. Barnacle abundance differed by site but not by predator presence. Abundance of the dominant mussel M. californianus was low overall (0.6% ± 0.15%), indicating that plots were at a mid-successional stage where M. trossulus dominates. Whelk predators are not likely to be able to control populations of M. californianus through consumption (due to size limitations and feeding preferences). However, we found that whelks may be able to limit M. californianus in the low zone by reducing the abundance of species facilitating its recruitment (M. trossulus and barnacles). Our results indicate that, less than a year after keystone predator decline, subordinate predators are reducing abundance of an important facilitator of the competitively dominant prey species. We predict that the effects of subordinate predators will strengthen over longer time scales, at sites more strongly dominated by top-down processes, and as subordinate predator sizes and populations increase.