Further Thoughts on Dioecism and Islands

Abstract

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 thai 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, estabHshment, 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 jjerennial 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 spectively.But making a quick diagnosis of the breeding systems of seed plants and, more re-breeding system is not so easy with insect-polcently, this has been concerned with the role of linated species, particularly in the tropics.Bedioecism in the origin and maintenance of ad-cause both staminate and pistillate flowers must aptation to various environmental conditions attract the same visitors, they tend to be similar (OmdufT, 1966; H.