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.