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Scientists have attempted to explain how living things that are not native to the Hawaiian Islands were able to reach the islands from distant places. The way in which birds reached the Hawaiian Islands is obvious enough. Some of the plants that probably came with them had seeds that readily attached to feathers, about 7 percent of the Hawaiian nonendemic (nonnative) seed plants probably arrived in this way. The Hawaiian insects, too, arrived by air. Entomologists have used airplanes and ships to trail fine nets over the Pacific at different heights and have trapped a variety of insects, most of which, as would be expected, are light-bodied. These types also predominate in the Hawaiian Islands (an indication of their airborne arrival), although heavier dragonflies, sphinx moths, and butterflies are also found there. The influence of the winds in providing colonists is shown by the fact that, although flowering plants are far more common than ferns in the world as a whole, their diversity in Hawaii is more evenly balanced: 225 immigrant flowering plants and 135 immigrant ferns. The relatively greater success of the ferns is probably due to the fact that their spores (reproductive structures) are much smaller and lighter than the seeds of flowering plants. Of the nonendemic seed plants of the Hawaiian Islands, about 7.5 percent almost certainly arrived carried by the wind, while another 30.5 percent have small seeds (up to three millimeters in diameter) and thus may also have arrived this way. One of the most interesting plants that probably arrived as a wind-borne seed is the tree Metrosideros. It is unusual because its seeds are relatively tiny, and this has allowed it to become widely dispersed through the Pacific islands. It is able to form forests on lowland lava with virtually no soil – a great advantage on a volcanic island. Metrosideros shows great variability in its appearance in different environments, from a large tree in the wet rain forest, to a shrub on windswept ridges, to as little as 15 centimeters high in peatlands, and it is therefore the dominant tree of the Hawaiian forest. The different forms are not distinct species, and intermediates are found where two different types are adjacent to one another. Probably the single most important method of entry of seed plants to the Hawaiian Islands has been as seeds within the digestive systems of birds that have eaten their fruit (e.g. blueberry, sandalwood), about 37 percent of the nonendemic seed plants of the islands probably arrived in this way. Significantly, many plants that succeeded in reaching the islands are those that, unlike the rest of their families, bear fleshy fruits instead of dry seeds, such as the species of mint, lily, and nightshade found in Hawaii. Dispersed by sea accounts for only about 5 percent of the nonendemic Hawaiian seed plants. As well as the widespread coconut, the islands also contain Scaevola toccata, this shrub has white, buoyant fruits and forms dense hedges along the edge of the beach. Another seaborne migrant is Erythrina, most species of this plant have buoyant, beanlike seeds. On Hawaii, after its arrival on the beach, Erythrina was unusual in adapting to an island environment, and a new endemic species, the coral tree E sandwichensis, has evolved on the island. Unlike those of its ancestors, the seeds of the coral tree do no float – an example of the loss of its dispersal mechanism often characteristic of an island species. The successful colonists of the Hawaiian Islands are the exceptions, many groups, both plants and animals, have failed to reach the islands by natural processes. There are no truly freshwater fish and no native amphibians, reptiles, or mammals (except for one species of bat), while 21 orders of insect are completely absent. As might be expected, most of these are types that seem in general to have very limited powers of dispersal. For example, the ants, which are an important part of the insect fauna in other tropical parts of the world, were originally absent. They have, however, since been introduced by humans, and 36 different species have now established themselves and filled their usual dominant role in insect faunas. This proves that the obstacle was reaching the islands, not the nature of the Hawaiian environment.