Discussion
Globally, canopy-dominated marine ecosystems are decreasing, shifting to “urchin barrens” dominated by crustose algae and herbivores (Ling et al. 2015), or turf-dominated ecosystems (Filbee-Dexter & Wernberg 2018). When canopies are lost, marine forests decline in productivity (Chapman 1981), habitat complexity (Miller et al. 2015), and diversity (Graham 2004). Given the ecological and economic importance of these systems, much attention is being paid to factors that might contribute to their decline or prevent their restoration (Gorman & Connell 2009). In particular, crustose and turf-forming algae are thought to be major barriers to the persistence of marine forests, especially under climate change (Connell & Russell 2010), given the assumption that these turf and crustose taxa compete with and exclude canopy-forming species (Connell et al. 2013).
We tested the widespread hypothesis that turf species in marine forests have negative effects on canopies (O’Brien & Scheibling 2018). We found that, although turf algae do compete with the canopy, competition occurs in a limited number of cases and the effect of the turfs on the canopy was highly variable, often neutral, and even facilitative in many cases (Table 1). Importantly, we found that in subtidal ecosystems, turf species do compete with the canopy (primarily kelps; Fig. 4), which supports the existing conceptualization of competition as a driving mechanism in subtidal kelp forests (Dayton et al. 1984; Reed & Foster 1984; Connell & Russell 2010; Connell et al. 2013). Further, experimental findings thought to contradict the competition paradigm (e.g., Barner et al. 2016) can be situated in a continuum of interactions that is predictable from the SGH (Bertness & Callaway 1994; Bennett et al. 2015): negative subtidal interactions shift to positive, facilitative interactions in the intertidal. The mechanisms underlying the turf-canopy interactions in this study were variable (Table 1). It is beyond the scope of this study to describe these mechanisms in detail (recently reviewed in Edwards & Connell 2012; O’Brien & Scheibling 2018), but despite the differences in the physiological, chemical, and physical processes that generate turf-canopy interactions, generalities emerged in the strength and sign of interactions.