Regional processes mediate the relative importance of facilitation and keystone predation for the maintenance of coexistence in intertidal communities

Background/Question/Methods

Theory suggests that species competing for a common resource can persist indefinitely when stabilizing mechanisms are able to compensate for interspecific differences in competitive ability. These mechanisms include asymmetrical trophic interactions such as keystone predation, whereby a predator preferentially consumes the dominant competitor. However, classical keystone predation does not account for regional-scale processes controlling recruitment. In wave-swept intertidal systems, competition for space is fierce, but adults often facilitate the recruitment of con- and hetero-specifics alike by providing a rugose refuge from disturbance. We incorporated recruitment facilitation and keystone predation into an analytically-tractable patch-dynamic model in order to understand how local and regional processes shape the dynamics and structure of intertidal communities. Our goals were to determine (1) the regional conditions under which facilitation or keystone predation tended to promote coexistence, (2) how facilitation and keystone predation interacted when they co-occurred, and (3) to test these predictions in nature. We modeled competition for space between a dominant and a subordinate species, with the subordinate controlling the dominant’s ability to colonize free space via recruitment facilitation. Here, facilitation represents a form of dependence between the dominant and the subordinate, with full facilitation forcing the dominant to colonize the subordinate only.

Results/Conclusions

When the dominant and the subordinate have the same mortality and colonization rates, facilitation is required to compensate for interspecific differences in competitive ability and allow coexistence. As colonization increases, the rate of facilitation needed to maintain coexistence also increases. When predation is added, coexistence can occur in the absence of facilitation as long as the predator preferentially consumes the dominant. Increasing colonization and facilitation promotes the role of keystone predation by increasing the range of predator preference values over which coexistence occurs. Depending on colonization, facilitation and predation can also have countervailing effects and lead to the extinction of the dominant due to apparent competition, or the persistence of all species even when the predator strongly prefers the subordinate (i.e. “anti-keystone” predation). Predator-exclusion experiments in the field confirmed these predictions, with high colonization sites indicating a joint effect of facilitation and predation, and low colonization sites indicating no effect of predation. Overall, our results suggest that regional processes controlling colonization can mediate the relative importance of keystone predation and facilitation in maintaining coexistence, with low colonization rates promoting the role of facilitation and high colonization rates promoting the role of keystone predation.