Next steps: species interactions, global change, and regime shifts
Natural or anthropogenic disturbances may create situations in which turfs could alternately compete with or facilitate canopy-species recovery (Table 1). In particular, non-coralline turfs tend to be more resilient to disturbances such as El Niño (Filbee-Dexter & Wernberg 2018) and other disturbances (eutrophication, sedimentation, ocean acidification) actually favor the growth of non-coralline turfs over other taxa (Connell & Russell 2010; Muth et al. 2017). If turf or crustose algae inhibit canopy recruitment, then the marine forest may never recover after disturbance. However, if facilitation occurs, canopy species may recover from a disturbance more quickly in environments with stronger or more frequent positive interactions. Previous global change predictions for marine forests were based on a negative effect of turfs on canopy-forming species (Harley et al. 2011). Undeniably, competition with turfs contributes to the globally-observed regime shift away from canopy-dominated ecosystems (Petraitis & Dudgeon 2004; Filbee-Dexter & Scheibling 2014; O’Brien & Scheibling 2018). However, in many ecosystems, facilitation creates a positive feedback that maintains contrasting system states (Kéfi et al. 2016). Given the frequency at which we observe turf and crust facilitation of canopies in this study, these positive effects may also contribute to alternative stable states in marine forests. Future work should explicitly test for positive interactions in these systems, and examine the role of facilitation in amplifying or dampening the observed global regime shift to turf- and crust-dominated marine forests.
We found that interactions in this system formed a continuum from competitive to facilitative, across a predictable stress gradient. We used latitude as a geographic proxy for temperature, as few studies in this meta-analysis included environmental data alongside interaction estimates. In the future, abiotic data, and especially temperature, should be reported and possibly experimentally manipulated in studies of marine forest interactions. We found a stress gradient effect when we combined intertidal and subtidal studies in the same analysis, though few empirical studies treat these as one system with an “elevational” gradient. We encourage more empirical work integrating the subtidal and intertidal ecosystems, especially given environmental changes that may drive species to deeper (warming) or shallower (sea level rise) water. Finally, the interactions included in this study are only a few of the diverse set of trophic and non-trophic interactions in marine forests. Herbivory, competition, facilitation, predation, and indirect effects are all interactions that shape community structure and play a role in regime shifts (Petraitis & Dudgeon 2004). Linking the effect of the canopy on turfs and crusts with the interactions in this study would be a feasible first step in understanding the emergent impact of all these interactions (Bennett et al. 2015; O’Brien & Scheibling 2018). Our results highlight the importance of testing for facilitative interactions where competition is thought to dominate, shedding new light on a well-studied interaction involved in a global regime shift.