THE BIOTA OF LONG-DISTANCE DISPERSAL. II. LOSS OF DISPERSIBILITY IN PACIFIC COMPOSITAE.

Comparison of fruits and seeds in the endemic floras of oceanic islands with those of related species of mainland floras reveals that the insular species often have fruits or seeds notable for diminished dispersibility. This observation, which has been offered elsewhere (Carlquist, 1965, 1966) needs further explanation, documentation, and qualification. One way of demonstrating this phenomenon is a survey of the floras of oceanic islands of sufficient age and ecological opportunity to have fostered appreciable evolutionary change. The writer is currently preparing such a survey for the Hawaiian flora. Another means of demonstrating the phenomenon is to examine fruits or seeds of a large family, well represented on islands, notable for rapid evolution, and sensitive adjustment to new environments. This latter method, suggested as a means of analyzing various tendencies on islands (Carlquist, 1966), is particularly suitable for study of loss of dispersibility. Among plants, the family Compositae is an outstanding example of evolutionary plasticity; it is also the largest family of flowering plants. Viewed on other grounds, Compositae are also an excellent group for such a survey. Fruits of Compositae are often provided with exceptionally prominent dispersal mechanisms. Any diminution of these devices can be noticed readily. The excellence of dispersal mechanisms of mainland Compositae have permitted them to range far into the Pacific, so that representation of the family is sufficient for study of autochthonous alteration of fruit morphology on islands. The presence of Compositae on numerous islands is advantageous, for each island or archipelago serves as a replication, so multiple confirmations of loss of dispersibility can be afforded. Moreover, Polynesian Compositae represent not one, but a multiplicity of independent systematic groups within the family, also offering replications of any evolutionary phenomenon induced by insular conditions. Interestingly, Pacific Compositae are overwhelmingly American in affinity, thus providing alien notes in Polynesian floras, which are predominantly Indo-Malaysian in character. The fact that Polynesian Compositae are mostly American in origin derives from phytogeographic circumstance. The western Pacific is poor in Compositae, whereas the family is abundantly developed in the western portions of North and South America. Polynesian Compositae with American affinities can mostly be traced to mainland genera and even species with relative clarity. To estimate the degree to which dispersibility has been lost, one must have an approximate idea of the ancestral fruit form. Pacific species in which dispersal mechanisms are still relatively intact are also important, for they show earlier stages in loss of dispersibility. The peculiar morphology of fruits in certain Pacific Compositae, such as Dendroseris litoralis (Fig. 7G) or Bidens macrocarpa (Fig. 2K) first suggested to the writer a phenomenon which could be designated "loss of dispersibility." In addition, the prominence of loss of flight among insular insects and birds (viz., Zimmerman, 1948) suggested that among insular plants, a similar loss of vagility might occur. Ecological considerations have also proved suggestive. The reasons for loss of dispersibility in insular plants prove not unlike the reasons for ecological shift and loss of vagility in insular insects given by Darlington (1943) and Wilson (1959).