Explaining patterns in abundances of organisms on boulders: the failure of 'natural experiments'

In studies of sessile organisms on intertidal boulders on 2 rock platforms near Sydney, Australia, the green alga Ulva lactuca and the tubeworm Galeolaria caespitosa were usually more abundant on one shore (Cape Banks). Alikely explanation for this pattern was the difference in rocktype between the 2 shores - boulders were sandstone at one shore (Cape Banks) and shale or latente at the other (LongReef). Experiments showed this explanation to be correct, though not complete, for U. lactuca but incorrect for G. caespitosa. Further, a common spirorbid, normally equally abundanton both shores, settled and/or survived better on shale/laterite. These results illustrate the dangers of drawing conclusions about processes based simply on observations of patterns.