The monopolization of understorey habitat by subtidal encrusting coralline algae: a test of the combined effects of canopy-mediated light and sedimentation

Encrusting-coralline-algae habitat monopolized the substrata (>74% cover) under canopies of Ecklonia radiata at four sites (~1 km apart) within each of four localities (>100 km apart) spanning >1000 km of continuous South Australian coastline. This monopolization was analyzed experimentally to test whether canopy-mediated shade and sedimentation account for this canopy–understorey association. I tested the hypothesis that initially different habitat types (turf-forming-algae habitat versus habitat dominated by encrusting coralline algae) will converge to become like those under E. radiata if subjected to lower light and accumulation of sediment in the absence of E. radiata. Convergence in the absence of canopies, but in physically similar environments as under canopies, provides strong evidence that understorey habitats are primarily dependent on the manipulated environments. The experiment was provided with sufficient time (338 days) to adequately test for convergence as evidenced by indistinguishable percentage covers of encrusting coralline algae between canopies and treatments of shade on previously unoccupied rock and between habitat types transplanted to canopies. Convergence of habitat types, however, did not occur under treatments of shade and sedimentation. The effect of lowering light accounted for ~52% of the effect of canopies on percentage cover of encrusting-coralline-algae habitat. The effect of reducing sediment accumulation under shade was negligible. The magnitude of unaccounted effects (~48%) highlights a need to assess alternative factors that act to exclude taxa from or include taxa in the understorey assemblage. It is clear that canopies place strong constraints on the presence and abundance of many taxa, but not encrusting-algae habitats which beneficially coexist as understorey. A more complete identification of such positive and negative effects is needed to improve our understanding of the conditions that produce readily recognizable canopy–understorey associations that are repeated with great fidelity on a regional scale.