There's a certain mystique about the word 'biodiversity' that seems to be associated with images of steamy jungles or wondrous new medicines, but the word usually refers to the number of species or 'species richness' of an area. One reason why tropical areas are so fascinating is that they contain the highest numbers of plant and animal species found anywhere on earth.

American Samoa sits squarely in the tropics, so we should have a high biological diversity here, but we do and we don't. There is a sharp contrast between the number of plant and animal species that live on land here (few) versus those that live in our coastal waters (many). Most small islands in the South Pacific share this characteristic.

To start at the beginning, when our islands emerged as fiery volcanos above the sea surface, they were devoid of plants or animals. As time passed and the terrain became more hospitable, life for organisms became possible, but the plants and animals still had to cross major ocean barriers to get here from someplace else.

A quick look at a map will show one reason why few land species got here. We are really quite isolated in the Pacific Ocean, far from potential sources of plants and animals. To reach our shores, organisms would either have to blow in on the wind, drift for hundreds or thousands of miles on some piece of floating debris, or be carried in by another organism like plant seeds in a bird's stomach. The species that were successful probably got here by 'island hopping' across the Pacific, spreading from island to island over the course of many thousands or millions of years.

The difficulty in getting here is best illustrated by the sparse representation of native mammal species. Over the past 1.5 million years that Tutuila Island has existed, only 3 mammal species (all bats) got here and established viable populations. Our native species list also includes about 471 flowering plants and ferns, 24 resident land and water birds, 20 resident seabirds, 7 skinks, 4 geckos, 2 sea turtles, 1 snake, and occasional other visitors (this list does not include all the introduced non-native species like rats, dogs, pigs, toads, myna birds, and many weeds).

There's a second reason for our low diversity on land -- the small size of our islands. In general, the smaller the island, the fewer the species on it. For example, tiny Rose Atoll (0.4 sq mi) supports only 5 native plant species, 21 birds (virtually all seabirds), 2 geckos, and 2 sea turtles.

So, although American Samoa technically has 'tropical rainforests' due to our high level of rainfall (200-300 inches per year in some mountainous areas), we lack the high species richness found in the jungle rainforests of Indonesia, Africa or South America that are filled with hooting monkeys, poison dart frogs, pythons and flesh-eating piranhas.

On the other hand, because of our isolation, some terrestrial species in Samoa have evolved over many thousands of years to such an extent that they have become distinctly different species found nowhere else but here. For example, 1% of our plant species occur only in American Samoa; 32% of our plant species and the Samoan starling (fuia) occur only in the Samoan archipelago (which includes western Samoa); and the Samoan fruit bat occurs only in the Samoan and Fijian islands. So, our rainforest may lack diversity, but it contains some species found nowhere else on earth.

Turning to our marine environment, we find the opposite situation. There is an incredibly diverse ecosystem just beneath the waves. Coral reefs are among the most species-rich ecosystems in the world. We have, for example, 890 nearshore fish species which is an amazingly high number compared to many other coastal areas. To get a sense of this species-rich environment, if you were to dive on our reefs once a week, you could in theory see a new fish species on every dive for 17 years.

Although coral reefs are limited to shallow waters, mostly around the fringes of islands, most coral reef species have eggs and larvae that can survive for weeks or months in the open ocean and get dispersed by ocean currents to new locations. As a result of this genetic exchange of marine organisms between islands, there are probably few marine species that are unique to the Samoan islands.

Finally, superimposed over the South Pacific region is a larger-scale pattern of species distributions. Most of our marine and land species can be traced back to the same or related species inhabiting mainland and insular southeast Asia. From that center of remarkably high diversity, rainforest and coral reef species radiated out, spreading eastward across the South Pacific islands. But like ripples in a pond, the farther away one gets from that 'center', the fewer the species (see figure). This pattern applies to corals, fishes, sea turtles, seagrasses, mangroves, land birds and plants. Very few species reached here from the opposite direction (South America) probably due to the much greater distance and fewer islands in that direction to facilitate 'island hopping'.

P. Craig, NPS

High terrestrial and marine biodiversity in Region 1 decreases across the Pacific towards Region 6.

Graph after R. Whittaker. 1998. Island biogeography: ecology, evolution, and conservation. Oxford University Press (NY) 285p.