Morphological Uniformity Among Widely Disjunct Populations of the Rare "Copper Moss," Scopelophila cataractae (Pottiaceae)

Abstract

Eight morphological characters of the gametophyte generation and three of the sporophyte generation were measured in 15 plants from each of 76 populations of the rare and geographically disjunctive moss, Scopelophila cataractae, in order to assess whether subtle differences exist between plants from widely separated populations. Plants were variable in all morphological traits, but most of the variation occurred among individual stems within populations and among populations within broad geographic regions. In spite of statistically significant differences between plants from widely disjunct regions (including different continents) for eight of the 11 morphological traits, the percentage of the total variation attributable to regional differences was minimal. Little regional variation in individual traits was corroborated by multivariate analyses including UPGMA clustering and principal component analysis. Furthermore, the degree and pattern of correlations among morphological traits were remarkably similar in populations of plants separated by thousands of miles. The general rarity and habitat specificity of S. cataractae throughout its range suggest that widely disjunct populations have been continuously separated. This, combined with the present analysis of morphology, suggests that morphological diversification may occur slowly in this moss. Relative to seed plants, most species of have broad geographic ranges. Less than 25% of the of the eastern United States, for example, are endemic even in the broad regional sense, with most taxa occupying remarkably broad ranges that include Europe and/ or Asia (Crum and Anderson 1981; Schuster 1983). Even in floristically rich areas such as the Ozarks, 90% of the are widespread elsewhere in the eastern United States and 70% occur also in the Old World (Crum and Anderson 1981). Indeed, on a worldwide basis, endemism at the specific level is low in mosses. It is common knowledge among moss monographers that many species occur disjunctively on several continents and that there is often little or no morphological differentiation between widely disjunct populations. These systematic and biogeographic observations have led some bryologists to hypothesize that many species of are geologically old and that presently disjunct populations of such species have undergone little or no morphological evolution over vast amounts of time (Crum 1972; Crum and Anderson 1981; Schuster 1983). Arguments in this regard often cite the observation that many intraspecific disjunctions in correspond to the same patterns that occur at the generic level in seed plants. Such patterns (e.g., eastern Asia-eastern North America) have been widely accepted as having derived from fragmentation of the Arctotertiary flora (Herzog 1926; Crum 1972). The likelihood of more recent long-range dispersal has also been discussed for some species (e.g., van Zanten 1984; van Zanten and Pocs 1981). In spite of the common perception of monographers that little morphological differentiation exists between widely disjunct populations of many mosses, there have been few quantitative studies to determine whether subtle patterns of regional variation do in fact occur. The ecologically unique mosses provide an especially good opportunity to address this issue. The copper include about ten species of and liverworts that exhibit an ecological association with substrates with higher than average concentrations of copper or other metals (Persson 1948, 1956; Shacklette 1967). In addition to the unusual habitat of the copper mosses, these species share the characteristic of having remarkably broad and highly disjunctive geographic distributions. Furthermore, most of the copper do not reproduce sexually (as evidenced by sporophyte formation) over large portions of their ranges. One such copper moss, Scopelophila cataractae (Mitt.) Broth., occurs disjunctively in central America and Mexico, the United States, Europe, the Himalayan Mountains, and southeast Asia including the Philippine Islands northward to Japan (Zander 1967). The species is extremely rare throughout its range. In the United States, for example, it occurs at one locality each in