Endemism in the Hawaiian Islands
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Located some 2,400 miles (4,000 km) from the nearest continental shore, the Hawaiian Islands are the most isolated group of islands on the globe. The plant and animal life of the Hawaiian archipelago is first the result of early, very infrequent colonizations by arriving species and the slow evolution of those species—in isolation from the rest of the world's flora and fauna—over a period of at least 70 million years. As a consequence, Hawai'i is home to a large number of endemic species. Indeed, the radiation of species described by Charles Darwin in the Galapagos Islands, and so critical to the formulation of his ideas leading to the Theory of Evolution, is far exceeded in the more isolated Hawaiian Islands.
The relatively short time that the existing main islands of the archipelago have been above the surface of the ocean (less than 10 million years) is not the time period available for biological colonization and evolution in the archipelago. High, volcanic islands have existed in these waters far longer, extending in a chain to the northwest, once mountanous islands now reduced to submerged banks and coral atolls. Midway Atoll, for example, formed as a volcanic island some 28 million years ago. Kure Atoll, a little further to the northwest, is near the Darwin Point—defined as waters of a temperature that allows coral reef development to just keep up with isostatic sinking. And extending back in time before Kure, an even older chain of islands spreads northward nearly to the Aleutian Islands; these former islands, all north of the Darwin Point, are now completely submerged as the Emperor Seamounts.
There are other factors as well. The islands are well known for the remarkable diversity of environments that occur on a high mountain in a trade winds field. On a single island, the climate can differ around the coast from dry tropical (< 20 in or 50 cm annual rainfall) to wet tropical; and up the slopes from tropical rainforest (> 200 in or 500 cm per annum) through a temperate climate into alpine conditions of coldness and dryness. And the rain climate impacts on soil development, which largely determines ground permeability, which effects the distribution of streams, wetlands, and wet places—and on and on and on.
Distance—remoteness—is a filter. And of those germs of life that occasionally made it through, how many found their way into a suitable microclimate? Whether a seed or spore attached to a lost migrating bird's feather or an insect falling out of the high winds, put there by a storm in its homeland—of these, how many found a place to survive and whatever else was needed to reproduce? This narrowing of the gene pool meant that at the very beginning the population of a colonizing species was a bit different from that of the far away contributing population. And now, in these islands, the rules for survival are not the same. There are different competitors, or maybe none; perhaps no predators either. Distances are compressed, and yet over those distances: a great variety of suitable and unsuitable conditions. Natural selection—different environmental factors working on a skewed gene pool—leading each founding population slowly away from clear identity with that in the old homeland, enhancing characters that contribute to survival, dropping those that do not.
And what of the neighboring island? Another filter; another separating divide.
Arrival of Humans
Human contact, first by Polynesians and later by Europeans, has had a significant impact. Both the Polynesians and Europeans cleared native forests and introduced non-indigenous species for agriculture (or by accident), driving many endemic species to extinction. Fossil finds in caves, lava tubes, and sand dunes have revealed an avifauna that once had an endemic eagle, two raven-size crows, several bird-eating owls, and giant ducks known as moa-nalos.
Today, many of the remaining endemic species of plants and animals in the Hawaiian Islands are considered endangered, and some critically so.
A partial list of endemic species of Hawai‘i
- Nene (Branta sandvicensis) - vulnerable
- Po'ouli (Melamprosops phaeosoma) - critically endangered
- Yellow hibiscus (Hibiscus brackenridgei) - endangered
- Loulu – (Pritchardia fan palms)
External links
- Article on invasive species in Hawaii (http://biology.usgs.gov/s%2Bt/SNT/noframe/pi179.htm) from the U.S. Geological Survey
- Hawaii's endemic birds (http://biology.usgs.gov/s+t/noframe/t018.htm) from the U.S. Geological Survey
- Online multimedia Encyclopedia of Flora and Fauna of the Northwestern Hawaiian Islands (NWHI) Coral Reef Ecosystem Reserve (http://noaa.stage.webthink.com/onms/Park/Parks/?pID=12) from NOAA