The Creation of Paradise

Marine Geography and Ecology of the Hawaiian Islands

by Ann Fielding
(some parts excerpted from an Underwater Guide to Hawaii)

The Hawaiian Islands are unique in many ways. For the diver or snorkeler interested in natural history the geology and biology are worth learning about. The intent of this article is to give a brief introduction to the geology, geography and biology of this beautiful island chain. We begin with the unusual formation of Hawaii.

Geology

The earth is made up of a hard outer crust and a hot and fluid inner mantle. The crust is broken into sections, called plates, which are moved about slowly by currents generated by the mantle. The edges of the plates rub and push against each other, causing earthquakes and volcanoes. The largest of these plates is the Pacific Plate which underlies most of the Pacific Ocean, and is bordered by North and South America on the east, the Aleutian Islands to the north, and the Marianas Trench, Japan, Papua New Guinea and New Zealand to the west. The moving edges of the Pacific Plate are responsible for so many earthquakes and volcanoes that the entire perimeter of the plate is called the "ring of fire".

The Hawaiian Islands are volcanic, but located in the middle of the Pacific Plate rather than at the active edge. At the site where the island of Hawaii rests scientists believe there lies, deep within the earth's mantle, a stationary 'hotspot'. This hotspot spews lava onto the seafloor, forming volcanic islands. As the Pacific Plate moves slowly to the northwest over the hotspot, a steady succession of new volcanoes has been created that are older to the northwest and younger to the southeast. The island of Hawaii is less than 1 million years old and still vocanically active, while Kauai is about 5 million and Midway, far to the northeast, is 27 million. Many scientists believe that the Emperor Seamount chain, north of the Hawaiian chain, was formed at this hotspot. The oldest age date on one of these seamounts is 70 million years, which means there may well have been an island at this spot in the Pacific continuously for 70 million years. (As you will see shortly, this has had major ramifications on the life forms inhabiting the islands). At the very end of the Emperor Seamount chain the northwesterly movement of the Pacific Plate is forcing seamounts into the Aleutian Trench, where they are recycled into the earth's mantle.

Geography

The Hawaiian Islands are the most isolated group of islands in the world. The main islands (Hawaii, Maui, Molokai, Lanai, Oahu and Kauai) are about 2,500 miles from the Society Islands and Tahiti in the south, 2,500 miles from the West Coast of the USA to the east, 2,500 miles from the Aleutian Islands to the north, and about 2,000 miles from the Marshall Islands to the west. This isolation has had a profound effect on the types of plants and animals found there. Plants taken by botanists on early exploratory trips to Hawaii showed that about 95% of Hawaii's plants were endemic, or found nowhere else in the world. A high rate of endemism was also found in all the native land dwelling animal groups which arrived before humans. These are birds, spiders, insects, land snails and the two native mammals, the hoary bat and monk seal. How did these life forms reach Hawaii? Insects, spiders and some plant seeds and spores could have come on the wind, other plant seeds and the monk seal would have come by water. Hawaii was probably a stop for migratory birds which could have brought seeds in their guts or stuck to their feathers, and land snails also could have come on birds. Non-migratory birds and the bat may have been blown here in huge storms.

Biology

Endemic species evolve when a colonizing life form gets to a place where it is interbreeding within a small population and is cut off from the larger gene pool where it originated. Because of the great distances involved, biologists believe that colonizing organisms reached Hawaii on an average only once every 70,000 years. If the islands have been in place for 70 million years there has been plenty of time for life forms to reach here and for evolution to modify the DNA codes. As new islands were formed the plants and animals already in place would have colonized from the older islands to the newer ones.

There are fewer endemic forms in the marine environment. It is estimated that about 30% of the fishes, 18% of the algae, 20% of the mollusks and 20% of the seastars and brittlestars are endemic to Hawaii. It is obviously easier for marine animals to be distributed through the ocean than for land species to be distributed through the air. However, the center of diversity of marine life found in the the Indonesia-Malaysia region has approximately 3,000 species of fishes and over 500 different species of corals. Hawaii has only 420 species of reef and inshore fishes and about 40 kinds of corals. The diver or snorkeler who has visited Papua New Guinea, the Great Barrier Reef or other islands in the western Pacific, as well as Hawaii, will have noticed that Hawaii has virtually no soft corals, no anemones inhabited by clown fishes, no staghorn or table corals, few mushroom corals, no bright blue damsel fish or large nest-building triggerfishes, and very few native shallow water snappers or groupers. The difference seems to lie in how long the larva of a fish or coral can remain viable as it floats for months in oceanic currents on its way to Hawaii. Another factor may be the water temperature as larvae are carried from warm equatorial waters to the cooler, subtropical waters surrounding Hawaii. Moray eels are common in Hawaii, being the second largest family of Hawaiian fishes with more than 35 species. It is speculated by Dr. John Randall of the Bernice P. Bishop Museum in Honolulu, that their abundance is a result of their long-lived larval stage in combination with the scarcity of their competitors on the reef, the large snappers and groupers.

So, the marine life in Hawaii is unusual in three respects. The first is that there are fewer species here than there are at other tropical islands to the south and west. The second is that there are more endemic species in Hawaii than are at other islands, and several of these endemic fishes are very common, including the saddle wrasse and milletseed butterflyfish. The third is that some species which are relatively uncommon in other areas are quite abundant in Hawaii. These include yellow tang, goldring surgeonfish, the collector and slate pencil sea urchins, and lobe and finger corals.

The differences seen in the marine life of the Hawaiian islands are directly related to the islands' isolation and the fact that they were formed volcanically, rather than being an old continental land mass that slowly moved away from its point of origin. If this had been the case, the marine life that was found around it would have remained as it moved. The Hawaiian Islands are truly unique both geologically and biologically, and worth an in depth exploration by the informed diver and snorkeler.

. . . . . . . . . . . . . . . Ann Fielding