Pacific Ocean: plants and animals

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Various geographic factors affect the characteristics of organisms and the types of plants and animals that colonize islands. Some factors are subject to change, notably so in respect to land connections and sea barriers, and to some extent there is evidence of climatic modifications.

A moderate drop in sea level could bring about a great change in the geography of the distribution of land and water in the Pacific area. A lowering of the ocean by a few hundred feet would unite Australia and New Guinea, would join Java, Sumatra, and Borneo to Asia, and would make an isthmus of Bering Strait. Such drops in sea level occurred during the glacial period, and during that time plants and animals could migrate across what is now water. Australian plants and some animals invaded New Guinea, and Asiatic species spread into Indonesia. Celebes contains mostly representatives from Asia along with a few from Australia, showing that at intervals it has been connected with both continents.

Wallace's line, named after Alfred Russel Wallace, an English contemporary of Charles Darwin, separates, in general, Asiatic from Australian flora and fauna. It was originally drawn between Bali and Lombok and north between Borneo and Celebes. Later studies by naturalists seem to show that the boundary between the zones in which Asiatic or Australian-Papuan life forms predominate should be farther east, and what is called Weber's line has been tentatively drawn starting on the north between Celebes, which has an estimated 20 to 40 per cent Papuan forms, and the northern Moluccas on the east, which possess 80 to 90 per cent Papuan elements. Southward the authorities have difficulty in deciding the exact location of Weber's line, but it is now generally placed to the east and south of Timor. Wallace's line is still recognized as a faunal boundary. Tigers, squirrels, and other mammals found in Bali are absent from Lombok, and the land birds differ greatly on opposite sides of the Lombok Strait. Borneo has a rich Asian fauna, but the Australian marsupial, phalanger, is found on Celebes. Probably the best way to look at the matter is to consider that the islands between Wallace's and Weber's lines represent a transition zone between Asiatic and Australian-Papuan faunas and floras since much of the evidence is contradictory. Farther east there are successively other "lines" that divide the Asian and Australian floras and faunas from the Oceanic ones.

Animals and plants that colonized oceanic islands have been subjected to varied environments, and the longer the elapsed time since their introduction, the more the local conditions bring about modifications in the flora and fauna. For example, on a high island, temperature, moisture, and soil conditions vary with the altitude and exposure to the winds from the ocean, and representatives of the same family, by adaptation to different local conditions, are modified into new species, each of which is best fitted to thrive in a certain environment. In general, oceanic islands have few families represented in the flora and the fauna but have many species in each family. Some of these species may be endemic (peculiar to one island only); for example, the species of land snails may vary from island to island and even from valley to valley on the same high island.

Insects are very numerous in the Pacific islands; it has been estimated that there are over 100,000 existing species. Malaria mosquitoes (Anopheles) are found in New Guinea, New Britain, the Solomons, New Hebrides, and some other places. They are absent from Fiji and Polynesia. Other species of mosquitoes transmit dengue fever and the trypanosomes that cause elephantiasis, and the yellowfever mosquito (Aedes aegypti) is common in some islands although the disease has not been imported. In Melanesia and New Guinea burrowing mites cause scrub itch, and stinging flies, hornets, poisonous centipedes, and scorpions may make life miserable for human beings. On some islands, plant insect pests, some of which have been carelessly introduced by man, injure many planted crops and trees.