The complex net effect of reciprocal interactions and recruitment facilitation maintains an intertidal kelp community

Abstract

SummaryTheoretical and empirical ecology has transitioned from a focus on the role of negative interactions in species coexistence to a more pluralistic view that acknowledges that coexistence in natural communities is more complex, and depends on species interactions that vary in strength, sign, and reciprocity, and such contexts as the environment and life‐history stage.We used a whole‐community approach to examine how species interactions contribute to the maintenance of a rocky intertidal macroalgal canopy–understorey assemblage. We determined both the types of interactions in this network, and whether interactions were sensitive to environmental gradients.Focusing on a structurally dominant canopy kelpSaccharina sessilis, and its diverse co‐occurring understorey assemblage, we evaluated the role of the understorey in controllingS. sessilisrecruitment and quantified the reciprocal effect of theS. sessiliscanopy and understorey on one another using a removal experiment replicated across 600 km of coastline. We determined the sensitivity of interactions to natural variation in light and nutrient availability (replicated among four regions on the N.E. Pacific coast), and under different wave conditions (three wave regimes).Surprisingly, species interactions were similar across sites and thus not context‐dependent. Unexpectedly, the understorey community had a strong positive effect on theS. sessiliscanopy, whereby the adult canopy decreased dramatically following understorey removal. Additionally,S. sessilisrecruitment depended on the presence of understorey coralline algal turf. In turn, the canopy had a neutral effect on the coralline understorey, but a negative effect on non‐calcifying algal turfs, likely eventually generating positive indirect canopy effects on the coralline understorey. Density‐dependent intraspecific competition betweenS. sessilisadults and recruits may moderate this positive feedback between theS. sessiliscanopy and coralline understorey.Synthesis. Our research highlights the importance of positive interactions for coexistence in natural communities, and the necessity of studying multiple life‐history stages and reciprocal species interactions in order to elucidate the mechanisms that maintain diversity.