Price, J.P. & Elliott-Fisk, D. (2004) Topo- graphic history of the Maui Nui complex, Hawai’i and its implications for biogeog- raphy. Pacific Science, 58, 27–45. Wilson, J.T. (1963) A possible origin of the Hawaiian Islands. Canadian Journal of Physics, 41, 863–870. Zimmerman, E.C. (1948) Insects of Hawaii, Volume I Introduction. University of Hawaii Press, Honolulu (reissued 2001). Editor: Robert Whittaker doi:10.1111/j.1365-2699.2006.01629.x Dispersal and vicariance in Hawaii: submarine slumping does not create deep inter- island channels Reply to a comment by G. Nelson on R.H. Cowie & B.S. Holland (2006) Dispersal is fundamental to biogeography and the evo- lution of biodiversity on oceanic islands. Journal of Biogeography, 33, 193–198. The Hawaiian Islands form sequentially as the Pacific plate moves over a stationary volcanic source beneath the crust (Clague, 1996). The volcanoes grow rapidly, but portions collapse catastrophically, gener- ating giant landslides and tsunami, or fail gradually, forming submarine slumps (Moore et al., 1989; Eakins et al., 2003). This well-studied, clearly understood tectonic–volcanic progression has pro- duced a chain of islands sequentially older to the north-west. A growing body of evidence suggests that many endemic terrestrial species on the present islands evolved from lineages that initially col- onized older islands (Carson & Clague, 1995; Price & Clague, 2002). Many Hawaiian lineages show phylogenetic cohesion among islands, and it is perti- nent to ask what has driven distributions that span multiple islands separated by marine channels: vicariance, in which once contiguous distributions on histori- cally larger islands are fragmented by erosion, subsidence and sea-level changes; or dispersal, in which ancestral species crossed ocean channels on the wind, at- tached to birds, or by over-ocean drift. We contend that both forces operate in the Hawaiian Islands (Cowie & Holland, 2006). Nelson (2006) suggests the possibility of a continuous above-water Hawaiian Ridge that once connected the islands of Kauai and Oahu and of Maui Nui and Hawaii (the ÔBig IslandÕ), thereby permit- ting vicariant explanations for biotic distri- butions that span the deep marine channels between these islands. However, high- resolution bathymetry and the well-under- stood geological processes by which these islands form, erode, abrade and subside preclude the existence of such connections. Massive landslides, known as slope fail- ures, have indeed been detected on or near to the Hawaiian Ridge (Moore et al., 1989; Smith & Wessel, 2000), and the sources, composition, fates and ages of the resulting debris fields have been determined, documented and mapped. Such events, while catastrophic, have not resulted in the ÔdisintegrationÕ of islands, historical island connections or land brid- ges. This seems to be the key misinterpret- ation in Nelson’s commentary: he confounds slope failures with the presence of deep marine channels between islands. One respected marine geophysicist states that ÔI have never seen anyone try to argue for land bridges between Oahu/Kauai and Maui Nui/Big IslandÕ, and that it is Ôcom- mon knowledge to the geologic community that there were no land bridges between the two sets of islandsÕ (J.R. Smith, personal communication). Furthermore, contrary to Nelson’s portrayal of the bathymetry around the current islands as ÔdubiousÕ, state-of-the- art multibeam sonar systems, mounted on GPS-navigated research vessels, have revealed the sea floor in stunning detail (Eakins et al., 2003). Submarine processes moving enough material to clear a chan- nel more than 1600 m deep would leave evidence in enormous swaths. Nelson’s commentary represents just the kind of view we had in mind when we wrote our article (Cowie & Holland, 2006). While we respect his standing in the field, his perspective on the importance of vicariance versus dispersal in shaping the Hawaiian biota seems to be an attempt to impose a vicariant hypothesis on a situa- tion for which the geological evidence manifestly precludes it. All the geological evidence, gathered, interpreted and agreed upon by the marine geology community without controversy, shows that, while Oahu was indeed once con- nected to Maui Nui, as evidenced by the shallow submarine terrace now connect- ing them (Price & Elliot-Fisk, 2004), the deep channels between Maui and Hawaii and between Oahu and Kauai were never bridged. As we stated (Cowie & Holland, 2006), we accept a role for vicariance between islands that were once con- nected, but not between islands that have never been connected. Maui and Hawaii were indeed once only 15 km apart, although 50 km today, but the deep channel between them has been in place since their formation. Similarly, the deep channel between Oahu and Kauai was once narrower than it is today, but these islands have never shared a terrestrial connection. Invoking vicariant island fragmentation processes to explain ter- restrial biotic distributions spanning these two channels is simply not supported by the geological evidence. While we welcome iconoclastic, imagi- native and unconventional suggestions, these must follow the scientific process and acknowledge rigorously established mecha- nisms. We see Nelson’s commentary as an attempt to force a vicariant perspective onto a situation that cannot sustain it. The evi- dence strongly supports the view we pre- sented regarding the geological history of the Hawaiian islands, inferred from solid bathymetric data. Nelson’s hypothesis is not supported scientifically and simply seems like an unjustified attempt to negate the role of dispersal. In our view it is time for island biogeography to move on, finally leaving unbalanced, vicariance-only thinking behind. We thank Dr John R. Smith, Science Program Director and Marine Geophysical Specialist, Hawaii Undersea Research Lab at SOEST, University of Hawaii for discussion of the key ideas expressed herein. Supported by NSF grant DEB-0316308. Brenden S. Holland and Robert H. Cowie Center for Conservation Research & Training, Pacific Biosciences Research Center, University of Hawaii, 3050 Maile Way, Honolulu, HI 96822, USA *E-mail: bholland@hawaii.edu; cowie@hawaii.edu REFERENCES Carson, H.L. & Clague, D.A. (1995) Geology and biogeography of the Hawaiian Islands. Hawaiian biogeography: evolution on a hotspot archipelago (ed. by W.L. Wagner and V.A. Funk), pp. 14–29. Smithsonian Institution Press, Wash- ington, DC. Clague, D.A. (1996) The growth and sub- sidence of the Hawaiian–Emperor vol- canic chain. The origin and evolution of Journal of Biogeography 33, 2154–2157 2155 ª 2006 The Authors. Journal compilation ª 2006 Blackwell Publishing Ltd Correspondence