The Stress Gradient Hypothesis (SGH) predicts primary producer
competition and facilitation is related to environmental stress. Despite
being well-documented in terrestrial systems, facilitation is rarely
integrated into our understanding of the dynamics in marine forests.
Critical coastal habitats created by canopy-forming seaweeds have
recently been seen to transition to less biodiverse regimes dominated by
turf algae and populations are predicted to increasingly transition
based on the paradigm that turf algae negatively affect canopy seaweeds.
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, however, these
interactions become increasingly facilitative at shallower depths and
high latitude, supporting the SGH. This work provides new insight into
the dynamics of a highly-studied ecosystem, and emphasizes the need to
re-assess the importance of facilitation when predicting the response of
systems to global change.