Nearly every speck of land in the Pacific, no matter how isolated, has ultimately become a home for plants and animals. The value to man of the Pacific islands is related to many natural factors, for example, size, relief features, soil, climate, and minerals; but among the leading factors of environment are the plants and animals that have been introduced, both by nature and by man himself.
The Pacific Ocean itself has many marine faunas and floras, which are adapted to different food supplies, temperatures, pressures, etc. Even the deposits made at great depths on the ocean floor are named from the relative abundance of certain organisms, many of which are microscopic in size. The geologic history and the present distribution of life in the Pacific area are intimately related.
SPREAD OF LIFE IN THE PACIFIC
The geologic history of the Pacific area has to be largely worked out by inference, because exposures of bedrock above sea level are few compared with the vast extent of water. The conclusions of geologists have been reached by studying the types and character of the rocks, surveying the relief features and depths of the ocean floor, and considering the evidence shown by fossils and the existing distribution of the flora and fauna.
It is generally agreed that in the past there were land links by which Indonesia and probably Australia and other adjacent islands were joined to Asia, and in addition a land connection existed to the north, probably at Bering Strait, which tied North America and Asia together for long periods. Conversely, some present isthmuses were once covered by the sea; for example, several straits in the past divided central America into an archipelago. At intervals there may have been land bridges from the supposed south Pacific continent to South America, possibly via Antarctica, over which primitive plants and animals may have migrated.
Without land connections, winds and currents are the principal means by which living forms reach islands. Usually the number of species present in the flora and the fauna of remote islands is small compared to the number in mainland locations and in islands close to continents. Oceanic islands have no native mammals except bats that can fly, seals and the like that swim, and pigs, rats, and mice that intentionally or accidentally were introduced by man. Sea birds have wide distribution because of their powers of flight, but land birds, lizards, and snails have curious locations, apparently from some happening in the past. It is conceivable that great storms of strong upward draft and high, powerful winds may carry land birds, insects, seeds, and the eggs or very young of toads and small reptiles considerable distances. Small mammals and reptiles might survive by floating on logs drifting to a new island. Migratory birds may introduce plants by dropping seeds from fruit that they have eaten, and water fowl may carry spores and seeds of water plants in the mud sticking to their feet. However, the transportation of plants and animals from the continents over the wide ocean to the Pacific islands is difficult to account for by known means, and some scientists therefore fall back upon the possibility of land connections in the past. Most geologists, however, are skeptical of past land connections for oceanic islands that rise from great depths and are remote from continents. In this connection it should be remembered that numerous high islands have disappeared by erosion or been reduced to little atolls, and hence in the past organisms might not have had to migrate as long distances across the ocean as a cursory examination of the present map would seem to indicate as necessary.
The absence of large modern-type mammals from all the native fauna of the oceanic islands indicates that such islands either have never been connected with continents or have been separated at least from the early Tertiary when such forms originated. Australia has a heavy preponderance of primitive marsupials among its mammals, and New Zealand includes in its native fauna only lizards, birds, and aquatic mammals. Until trout were recently introduced, even the rivers of New Zealand lacked edible fish other than eels, which, being catadromous (descending rivers to spawn), migrate from their spawning grounds in the ocean. Judging from the animal life, Australia has been isolated from Asia since the Miocene period, and New Zealand much longer, probably from the late Mesozoic era.