Molecular Panbiogeography of the Tropics

7. Biogeography of the Hawaiian Islands: The Global Context

313 The Hawaiian Islands have one of the most distinctive biotas of all the Pacific islands, and they have been the subject of many in-depth studies. Most authors now interpret the flora and fauna as classic cases of longdistance dispersal. For these reasons the biogeography of the group is discussed here in more detail. As explained in Chapter 6, the most influential theory of island biogeography , MacArthur and Wilson’s (1967) equilibrium model, applied Matthew’s (1915) center of origin concepts to island taxa. Thus Wilson (2001b: 56) described the process whereby “The Hawaiian Islands are colonized by birds, crickets, wasps, damselflies, beetles, snails, flowering plants and other kinds of organisms arriving as occasional wind-blown waifs. As the first colonists multiply and spread, they evolve in response to the distinctive environment of the islands and hence diverge from the ancestral populations left behind on the mainlands of North America and Asia.” But did the groups really come from North America and Asia? What about the former high islands in the Pacific that are now seamounts and atolls—surely they had a diverse fauna and flora? Many authors have stressed the current isolation of the Hawaiian Islands; they lie 3,900 km from the nearest continent (North America) 7 Biogeography of the Hawaiian Islands The Global Context Some of the material included in this chapter has appeared previously in Systematic Biology (Heads, 2011) and is reproduced here with permission from Oxford University Press. Heads_6480007_Ch07.indd 313 Heads_6480007_Ch07.indd 313 10/20/11 2:38 PM 10/20/11 2:38 PM 314 | Biogeography of the Hawaiian Islands and about the same distance from the nearest high islands, the Marquesas group in southeastern Polynesia. The Hawaiian Islands biota differs in many ways from others, and most authors have attributed its evolution to the “distinctive environment” (Wilson, 2001b) and the isolation of the islands. Vargas et al. (1999: 235) wrote: “The volcanic history, extreme geographic isolation, and disharmonic biota of the Hawaiian archipelago demonstrate that terrestrial life in the islands must have arrived by longdistance dispersal.” Craddock (2000: 2) concluded,“It is incontrovertible that speciation in Hawaii is somehow tied to founder events.” Eggens et al. (2007) agreed: “Because the islands are isolated and volcanic in origin, and have never been attached to the continental mainland, plant colonists could only have arrived by long-distance dispersal.” Although the consensus is now almost universal, some experienced biologists have been more cautious. Springer and Williams (1994: 183), discussing reef fishes, concluded that “The causal factors contributing to Hawaiian Islands’ endemism are undoubtedly complex and remain to be elucidated.” For example, the Hawaiian–Emperor chain has long been interpreted as a hotspot,“conveyor-belt” system and so could provide an example of an area where taxa have survived and evolved as a metapopulation throughout the Cenozoic, constantly colonizing new islands from older ones (Menard, 1986: 214). This concept undermines the center of origin/dispersal model, as presented by Carson and Clague (1995), for example. The metapopulation model has been rebutted by the suggestion that there was a period between 33 and 29 Ma in which no islands (Clague et al., 2010: 1022) or no high islands (Clague et al., 2010: 1031) were emergent. This is discussed further below. There are many problems in reconstructing the paleogeography of the region, and in particular it is difficult to eliminate the possibility of small islands, although these may have held considerable biodiversity. The other main evidence used to support the dispersal model is the young age suggested for many clades in molecular clock analyses, although there are problems with the calibrations in many of these. Some of the key assumptions that the standard model of Hawaiian biogeography is based on are examined next. THE STANDARD MODEL OF HAWAIIAN BIOGEOGRAPHY Assumption 1. The Hawaiian Islands have never been connected to other land masses and so must have received all their biota by long-distance dispersal from the continents (mainly Asia and America). Heads_6480007_Ch07.indd 314 Heads_6480007_Ch07.indd 314 10/20/11 2:38 PM 10/20/11 2:38 PM Biogeography of the Hawaiian Islands | 315 This assumes that the clades on the present Hawaiian Islands have not survived on former land in the vicinity and overlooks the evidence for former high islands around Hawaii.Apart from the atolls in the northwestern part of the chain itself (the Northwestern or Leeward Islands), there are many atolls...