Hawaiian angiosperm radiations of North American origin

Abstract

Background: Putative phytogeographical links between America (especially North America) and the Hawaiian Islands have figured prominently in disagreement and debate about the origin of Pacific floras and the efficacy of long-distance (oversea) plant dispersal, given the obstacles to explaining such major disjunctions by vicariance.

Scope: Review of past efforts, and of progress over the last 20 years, toward understanding relationships of Hawaiian angiosperms allows for a historically informed re-evaluation of the American (New World) contribution to Hawaiian diversity and evolutionary activity of American lineages in an insular setting.

Conclusions: Temperate and boreal North America is a much more important source of Hawaiian flora than suggested by most 20th century authorities on Pacific plant life, such as Fosberg and Skottsberg. Early views of evolution as too slow to account for divergence of highly distinctive endemics within the Hawaiian geological time frame evidently impeded biogeographical understanding, as did lack of appreciation for the importance of rare, often biotically mediated dispersal events and ecological opportunity in island ecosystems. Molecular phylogenetic evidence for North American ancestry of Hawaiian plant radiations, such as the silversword alliance, mints, sanicles, violets, schiedeas and spurges, underlines the potential of long-distance dispersal to shape floras, in accordance with hypotheses championed by Carlquist. Characteristics important to colonization of the islands, such as dispersibility by birds and ancestral hybridization or polyploidy, and ecological opportunities associated with 'sky islands' of temperate or boreal climate in the tropical Hawaiian archipelago may have been key to extensive diversification of endemic lineages of North American origin that are among the most species-rich clades of Hawaiian plants. Evident youth of flowering-plant lineages from North America is highly consistent with recent geological evidence for lack of high-elevation settings in the Hawaiian chain immediately prior to formation of the oldest, modern high-elevation island, Kaua'i.

Fig. 1.

Map of the Pacific Basin. The distance from southern California to the closest Hawaiian Island (Hawai‘i) is approx. 3800 km to the south-west, and the distance from the Marquesas Islands (Eiao) to Hawai‘i is approx. 3400 km north across the equator. Map courtesy of D. T. Harbaugh.

Fig. 2.

Scientists who proposed hypotheses on the origins of Hawaiian flora. (A) Heinrich Gustav Adolf Engler (1844–1930), (B) Wilhelm (William) B. Hillebrand (1821–1886), (C) Alfred Russel Wallace (1823–1913), (D) Forest Buffen Harkness Brown (1873–1954), (E) Douglas Houghton Campbell (1859–1953), (F) Carl Johan Fredrik Skottsberg (1880–1963), (G) David Daniels Keck (1903–1995), (H) Francis Raymond Fosberg (1908–1993), (I) Sherwin Carlquist (1930–). A, B and D–H courtesy of the Hunt Institute for Botanical Documentation, Carnegie Mellon University. C courtesy of Smithsonian Institution Libraries, from Wallace (1905).

Fig. 3.

Exemplars of Hawaiian angiosperm radiations (left) and American relatives (right) in Compositae–Madiinae (top) and Umbelliferae (bottom). (A) Argyroxiphium sandwicense subsp. macrocephalum. Image by W. L. Wagner. (B) Madia elegans. Image by B. G. Baldwin. (C) Sanicula mariversa. Image courtesy of J. K. Obata. (D) S. arctopoides. Image by B. G. Baldwin.

Fig. 4.

Exemplars of Hawaiian angiosperm radiations (left) and American relatives (right) in Violaceae (top) and Labiatae (bottom). (A) Viola chamissoniana subsp. robusta. Image courtesy of G. D. Carr. (B) V. langsdorffii. Image courtesy of M. Goff. (C) Phyllostegia floribunda. Image courtesy of J. K. Obata. (D) Stachys chamissonis. Image courtesy of A. Brousseau, copyright © 1995 Saint Mary's College of California.

Fig. 5.

Exemplars of Hawaiian angiosperm radiations (left) and American relatives (right) in Caryophyllaceae. (A) Schiedea obovata. Image courtesy of G. D. Carr. (B) Wilhelmsia physodes. Image by L. Brothers. (C) Silene perlmanii. Image courtesy of J. K. Obata. (D) S. antirrhina. Image courtesy of G. Yatskievych.

Fig. 6.

Exemplars of Hawaiian angiosperm radiations (left) and American relatives (right) in Euphorbiaceae (top) and Geraniaceae (bottom). (A) Euphorbia rockii. Image courtesy of J. K. Obata. (B) E. leucantha. Image courtesy of V. Steinmann. (C) Geranium arboreum. Image courtesy of G. D. Carr. (D) G. richardsonii. Image courtesy of D. Fristrom.

Fig. 7.

Exemplars of Hawaiian angiosperm radiations (left) and American relatives (right) in Cruciferae (top) and Compositae–Coreopsideae (bottom). (A) Lepidium arbuscula. Image courtesy of J. K. Obata. (B) Lepidium oblongum. Image courtesy of J. M. DiTomaso. (C) Bidens cosmoides. Image courtesy of G. Diada. (D) B. pilosa. Image courtesy of J. M. DiTomaso.

Fig. 8.

Total number of Hawaiian angiosperm founders from particular source areas in the New World that gave rise to lineages with different levels of species richness. Conservative estimates = taxa confidently assigned to area based on molecular, morphological and/or distributional data; moderate estimates = conservative estimate + taxa confidently assigned to New World and either equivocally assigned to subregion or of potentially indirect dispersal to the Hawaiian Islands, via the South Pacific; liberal estimates = moderate estimate + taxa possibly originating in the New World based on available data but not verified as New World in origin, such as pantropical taxa. See Appendix 2 for taxon assignments to regions.

Fig. 9.

Total number of Hawaiian angiosperm species resulting from lineages of particular New World source areas with different levels of species richness: Conservative estimates, moderate estimates and liberal estimates, as indicated. See legend to Fig. 8 for method of estimation and Appendix 2 for taxon assignments to regions.