Further Thoughts on Dioecism and Islands

Non-dioecious taxa are the floral majority, both in island floras and in those of the mainlands. This is true at all latitudes. Temperate zone islands have very low percentages of dioecious species. The percentages are somewhat higher in subtropical islands. Relatively high percentages of dioecism are found only in moist tropical islands or in floras that have had a moist tropical influence on their composition. Low islands in the tropics and rather arid islands in the sub-tropics and tropics have only small numbers of dioecious species in their floras. A multiple regression analysis shows that latitude and maximum height of islands together account for 82% of the variation in percentage of dioecism in our samples. Although autochthonous development of dioecism in an hermaphrodite immigrant cannot be ruled out yet, the percentage of dioecism in an island flora appears to be related to the percentage in a probable source flora in a comparable climatic zone. This equivalence could result from parallel evolution but probably depends more upon the long-distance dispersal, establishment, and radiation of taxa, including dioecious ones. This may be facilitated for the dioecious taxa by "leakage" in the dioecious breeding system, by dispersal of multiple-seed units, by longevity and vegetative propagation of perennial plants, and by an ability to utilize wind or unspecialized pollinators. In tropical regions, bird-dispersal of seeds particularly may have been involved in the stocking of forests on islands. Lately, there has been increased interest in the breeding systems of seed plants and, more recently, this has been concerned with the role of dioecism in the origin and maintenance of adaptation to various environmental conditions (Ornduff, 1966; H. G. Baker, 1976; Freeman et al., 1976, 1980; Lloyd & Webb, 1977; Webb, 1979; Willson, 1979, 1982; Bawa, 1980, 1982; Bawa & Beach, 1981; Beach, 1981; Givnish, 1980,1982; Lloyd, 1980,1982; Cox, 1981, 1982, 1983; Thomson & Barrett, 1981; Ross, 1982). Among these considerations has been the significance of dioecism in those flowering plants that make up the floras of islands (Bawa, 1980, 1982). Traditionally, it was temperate floras whose reproductive biology was studied, but as more attention is being given to islands in warmer climes, it is important that we do not go too far in the other direction and assume that tropical islands are typical of islands in general. There is no difficulty in spotting dioecism or monoecism in wind-pollinated temperate zone trees and herbs, for the morphological differences between staminate and pistillate inflorescences are striking, being related to the very different functions of casting pollen into the wind and sieving pollen out from an aerial suspension, respectively. But making a quick diagnosis of the breeding system is not so easy with insect-pollinated species, particularly in the tropics. Because both staminate and pistillate flowers must attract the same visitors, they tend to be similar in appearance, sometimes even to the extent of having pistillate flowers that produce non-functional pollen grains (e.g., Actinidia chinensis, Schmid, 1978; Solanum spp., Anderson, 1979). Some of the difficulty that this has produced in attempts to quantify the occurrence of dioecism in tropical floras is shown by estimates of dioecism in the Hawaiian flora. Carlquist (1965) judged it to be 27.7% while Gilmartin (1968), basing her analysis on the flora written by Hillebrand (1888), reported it to be only 5%-a striking discrepancy. Both authors claimed not to include gynodioecious and other deviant species in the percentage. Possibly the truth lies somewhere between these extremes, but we use the Carlquist figure because Gilmartin, herself, suggested that Hillebrand (1888) may have had difficulty in identifying dioecism from imperfectly preserved specimens or rarely collected species. Also, Carlquist (1965) made studies of many species and did not rely solely on the lit-