The most isolated islands

More than any other factor, isolation has shaped the community of organisms present on the Hawaiian Islands. At least this is a reasonable prediction, if we apply principles of island biogeography. According to this classic theory developed by ecologists Robert MacArthur and Edward O. Wilson, species composition on islands is determined by patterns of colonization and extinction over time. Overall species diversity on an island is affected by its distance from a colonizing source (e.g., a mainland) and by the island’s size. Small islands and isolated islands tend to have lower diversity.

Pacific ocean basin bathymetry/topography with the WNW to ESE trending
Hawaiian Island chain and the N to S trending Emperor Seamounts. Base
map from NOAA, NCEI. Source.

The isolation of Hawaii is due to the nature of how the islands were formed geologically. The Hawaiian chain sits in the middle of the vast Pacific crustal plate underneath the largest ocean basin in the world. For tens of millions of years, a geologic hotspot below the crust has continuously burped up magma to the crustal surface, forming some of the tallest mountains in the world. Because of their birth from an undersea hotspot underneath the Pacific plate, the islands have never been connected to the mainland of any continent. Other hotspots dot the planet, but Hawaii is remarkably distant from all other land masses. One of the nearest islands is Kirimati (Line Islands) at about 2000 km away; the distance from Hilo to San Francisco is over 3700 km.

For terrestrial plants and animals, successful colonization of Hawaii came only by long-distance dispersal over thousands of kilometers of ocean. For example, plant colonists may have had seeds that were highly tolerant of salt water, or capable of hitchhiking on birds that landed on the islands. Seed studies suggest that about one third of Hawaiian species arrived there by drifting or rafting over the surface of the ocean; the remaining species probably came with birds. For ferns (which reproduce by small spores, not seeds) many species probably came as winds carried their lightweight spores long distances through the atmosphere.

By looking at the kinds of native organisms present in the terrestrial flora and fauna of a very isolated archipelago like Hawaii, we can infer something about differences in the dispersal ability of those organisms. For example, the native biota of the islands is missing some major groups of animals commonly present on continents: ants, termites, reptiles and amphibians. Mammals are also very rare in Hawaii – restricted to bats and the endangered monk seals. These groups of organisms simply have never naturally colonized Hawaii because the distances are too great, the oceanic environment too hostile, or chance was never in their factor.

Relationship between number of native plant species and
island area for 10 of the Hawaiian Islands (8 main islands
+ Nihoa + Necker). Data are from Evenhuis and Eldredge
 (2004) and Gustafson et al. (2014). The Big Island is some-
what of an outlier, but this may be reflective of its relative
youth in the island chain.

Isolation has led to high rates of endemism on the Hawaiian Islands. Endemic species are those found in a single location, but no where else. Today Hawaii has an estimated 1207 species of native vascular plants, and a remarkable 88% of them are endemic to the island chain. Some of the individual islands also have local endemics. For instance Kauai has 219 endemic plant species while the younger islands of Oahu and Maui have 140 and 89 endemic respectively.

Isolation has affected which species have been able to reach the islands, but other factors more local to the islands have subsequently influenced the evolution of the successful colonists. Each island in the Hawaiian archipelago has a life cycle of perhaps 5-20 million years from birth to erosion and subsidence to its final days as an atoll. The Big Island is youngest at no more than 1 million years; Kauai and Nihau are the oldest of the major islands at about 6 million years old. So, for some of the earliest colonizing lineages of organisms, their long-term persistence on the archipelago may be due to an ability to hop from island to island. As an older island finally sinks back into the Pacific during its old age, species that can colonize a younger island would persist. Interestingly, many of the species present on the islands today appear to have evolved from colonists that arrived not more than about six million years ago (about when Kaua’i formed) suggesting that island hopping isn’t particularly common.  

The honeycreeper, Vestiaria coccinea, on Acacia koa
(Fabaceae). Photo by Ludovic Hirlimann, CC BY-SA 2.0
license. Source.

The Hawaiian biota is a good example of adaptive radiation, the evolutionary process that results in a large and diverse group of species diverging from a single successful colonizing species. Good examples of adaptive radiation include the silversword plants and the Hawaiian honeycreepers (birds).

Studies of similarities and differences among DNA sequences – a powerful tool to discern relationships among organisms – has also shed some insight into the history of colonization on the islands. For the approximately 1200 native plants currently growing in Hawaii, it is believed that historically there were about 375 separate successful colonizing events. Because colonization and local extinction of species on islands is a continuous process, there were likely more successful colonizations over the geologic history of the islands, but some of those lineages went extinct. We can also speculate that there were probably many more near misses where colonizing seeds or spores arrived on the shores of Hawaii but failed to become established, perhaps because they didn’t arrive in densities high enough to successfully reproduce.

Of the nearly 400 plant colonizations resulting in the contemporary Hawaiian flora, about two thirds have left us with only a single living species. The rest have radiated into groups of related plants. For example, the lobeliad plants now comprise 6 genera and 141 species, each derived from what is believed to be a single colonizing event 13 million years ago. The lobeliad radiation has resulted in a diversity of plant types from succulents to shrubs to trees. Hawaiian radiations have sometimes occurred with little genetic differentiation among the species but much ecological differentiation. An example would be the ohia lehua tree (Metrosideros), which I wrote about in a previous post, a species that varies considerably in size and morphology. In contrast, other radiations can result in much genetic diversity among a group of ecologically-similar species. For instance, a single colonizing event for the genus Cyrtandra has resulted in 59 different endemic species of forest plants, all of which live as forest understory trees or shrubs, and may therefore be pretty similar ecologically.

Isolation and endemism – some of the features that make the Hawaiian biota a remarkable laboratory of evolution – also are coupled with significant threats to biodiversity on the islands. With so much unique biological richness there is much to lose. I’ll discuss threats to Hawaiian conservation in a later post.  

References

Evenhuis NL and Eldredge LG. (eds) 2004. Natural History of Nihoa and Necker Islands. Bishop Museum Press, Honolulu, HI, 220 pg.

Gustafson RJ, Herbst DR, Rundel PW. 2014. Hawaiian Plant Life. Vegetation and Flora. University of Hawaii Press, Honolulu, HI.

Price J. 2009. Hawaiian Islands, Biology. In: Encyclopedia of Islands, Gillespie RG and Clague DA (eds), University of California Press, Berkeley, CA, p.397-404.