Habitat structure provided by living organisms often affects the distribution and abundance of organisms in predictable ways. This study tested the hypothesis that canopies of the kelp, Ecklonia radiata, affects the cover of algae in a boulder reef at West Island in southern Australia. Measurative experiments demonstrated that the percentage cover of encrusting corallines was greater (and articulated corallines less) under a canopy of dense E. radiata (>7 plants/m²) than in areas with no E. radiata. A series of experimental clearances of kelp and reciprocal transplants of boulders between patches of kelp and no kelp was used to test the hypothesis that E. radiata caused the cover of encrusting coralline algae to be greater (and articulated coralline algae less) under canopies of dense E. radiata.

Experimental results revealed that kelp maintains the percentage cover of encrusting coralline algae and reduces the percentage cover of articulated coralline algae. This result rejects the null hypothesis that the spatial correlation of this canopy species (E. radiata) and understorey species (encrusting coralline algae) are accidental co-occurrences of species that share the same component of the reef. Instead, these results support the model that these two sets of taxa are positively correlated because kelp facilitates the maintenance of encrusting coralline algae and removes articulated coralline algae which overgrow encrusting coralline algae. This study demonstrated that habitat provided by kelp affects the distribution of encrusting coralline algae and articulated coralline algae in predictable ways.