The Latitudinal Spans of Seaweed Species and Their Patterns of Overlap

Geographic ranges of groups of related species may overlap one another to a greater or lesser extent. This paper explores the overlap among the ranges ('latitudinal spans') of shoreline species along a north-south trending coast. A method of measuring the overlap of a group of s such species is devised and the theoretically expected overlaps, under two contrasted null hypotheses, are derived. According to one hypothesis (Hi) the range limits of congeneric species are independently located; according to the other (H2) the ranges themselves, assumed to have their observed lengths, are independently located. The latitudinal spans of 684 species of benthic marine algae, on the western shores of the Atlantic, are then tested for conformity with the hypotheses. Compared with Hi, species overlaps exceed expectation very significantly, but the data give no reason to reject H2. It is concluded that the seaweeds' geographic ranges are unaffected by interspecific competition; and (speculatively) that the zonation patterns of seaweeds on the shore are likely to be less developed in regions where, due to recent disappearance of barriers to dispersal, hitherto isolated seaweed communities are mingling. The variation with latitude of the numbers of seaweed species, and the observed frequency distributions of their latitudinal spans, are also described.