Evolution of Hawaiian Ferns and Fern Allies in Relation to Their Conservation Status

Abstract

Evolutionary and conservational differences between Hawaiian pteridophytes and angiosperms involve differences in life histories (free-living generations, fertilization, and spore dispersal). Very high base chromosome numbers characterize the homosporous pteridophytes. Long-distance spore dispersal took place mainly from Old World and pantropical ancestors, accounting for some 80% of the taxa. The ratio of native pteridophyte to angiosperm taxa in Hawai'i averages roughly 1: 6, much higher than in continental floras with 1: 14. Two hundred twenty-four pteridophyte taxa, including hybrids and naturalizations, are known in Hawai'i. The 170 native orthospecies include endemics (highly variable taxa with polymorphies involving one or more characters, monophyletic species swarms, and solitary endemics) as well as nonendemics. Hybrid nothospecies compose an important additional component, as do naturalized orthospecies. Most of the hybrids are sterile intermediates that propagate by vegetative means; sexual hybrids are rare. The percentage of naturalized species is only one-fourth that of angiosperms. Hawaiian pteridophytes have evolved much more slowly than the angiosperms, as shown by lower endemism (75% versus 91% overall and relatively fewer one- or two-island endemics) and much smaller species swarms (average 1.5 versus 16.0 descendants from each introduction in the 20 most species-rich genera, respectively). Anticipated listing of Hawaiian rare and endangered fern species will probably comprise ca. 17% of the natives, including four believed to be extinct. Naturalized species compose only one-fourth of the percentage in angiosperms, and very few are pests. Habitat destruction by humans and feral mammals is the major conservation problem. Although artificial spore banks and whole-plant culture may help save some rare pteridophytes, the most promising procedure is establishment of natural preserves.