Positive interactions in global marine forests increase with stress

Background/Question/Methods

The past decade has seen dramatic declines in marine forest cover. Many disturbed marine forests are shifting from canopy-dominated to systems dominated by turf-forming or crustose algae, an example of alternative stable states. With a suite of abiotic and biotic stressors contributing to the decline of kelp and fucoid forests, their recovery is thought to be prevented, in part, by competition with algal species that in the understory. The assumption of competitive exclusion by the understory forms the basis of recommended restoration practices and is particularly influential in the prediction of future community composition. However, competitive interactions among primary producers have been widely shown to shift to facilitative when conditions become stressful, as predicted by the Stress Gradient Hypothesis. Species interactions among marine and limnal taxa have been shown to shift from competitive to facilitative in 1) drier, more stressful tidal elevations, and in 2) warmer, more stressful latitudes. Despite being well-documented in terrestrial and brackish ecosystems, the SGH has not been tested in marine forests, and instead competition is thought to be the dominant structuring mechanism. In this study, we use a new global dataset of marine turf-canopy interactions to test the SGH.

Results/Conclusions

Meta-analysis was used to estimate the effect of turfs on canopy species using experimental and observational data. We found that turf species do compete with marine canopies in subtidal kelp forests, in agreement with the existing competition-dominated paradigm for subtidal systems. However, these interactions become increasingly facilitative at shallower depths, supporting the SGH. Overall, we found that interactions in this system formed a continuum from competitive to facilitative, across a predictable stress gradient This work provides new insight into the dynamics of highly-studied coastal systems, and emphasizes the need to re-assess the importance of facilitation when predicting the response of systems to global change.