Assumptions and limitations
In addition to variability, clear patterns emerged between experimental
and observational studies moving from neutral effects to facilitation of
turfs on canopy species. However, it is important to note that
observational studies were conducted in intertidal, shallow subtidal
and/or high latitude environments, all factors that influence
facilitation interactions. Thus, the different results for observational
and experimental studies may be attributable instead to these
confounding differences. Here, the included observational studies
generally used a study design where the number of canopy individuals
were counted on a given substrate (e.g., turf, crust, bare rock), scaled
by the availability of that substrate (n = 68 out of 91 observational
studies). For three pairwise interactions in this study, effects were
estimated both observationally and experimentally, yielding similar
effect size estimates (Appendix S3 Fig. S3). More work is needed to
understand the relationship between experimental and observational
effect sizes in non-trophic interactions, but the similarity here in
response ratios is encouraging.
Another limitation of the present study is an imbalance in sample size
among turf functional groups. Of the four groups, there are fewer
estimates of the effect of non-coralline crustose algae on canopies
(nnon-coralline crust = 28,
nnon-coralline turf = 79, ncoralline
crust = 114, ncoralline turf = 102). Estimates of the
abundance and importance of these species vary in canopy-forming
systems: in an Alaskan subtidal system, non-coralline crusts dominate
the substrate relative to other species, while classic studies fail to
mention non-coralline crusts as a component of the community (e.g., Reed
& Foster 1984). The present study includes no estimates of the effect
of non-coralline crusts in deep water (deepest is -5m relative to MLLW)
and our finding that the non-coralline crust effect is more positive at
shallower depths (Fig. 4) should be interpreted with caution.