The "high" islands of Micronesia support a heavy cover of vegetation.
Typically the lowlands and stream courses are covered with dense jungle
vegetation, and the slopes and higher hills are covered with grasses and
brush. The vegetation of the "low" coral atolls and islands is, by
comparison, much less dense. Many shorelines are covered with scant
grasses and shrubs and the interior in many places is dominated by
coconut, betel palms, breadfruit, papaya, and pandanus. References to
papers dealing with plants in the islands of the Pacific may be obtained
in Merrill (1945), who (1945:207) writes, "Botanically, the low islands
are very uninteresting and monotonous. The flora of one is usually quite
the same as that of another, although these islands and islets may be
separated by many hundred and in some cases several thousand miles. The
native vegetation may be scanty or reasonably well developed, depending
on the size of the island, the quality of its soil, and whether or not
it is permanently inhabitated." Of the vegetation on the "high" islands
of the Pacific area, Merrill (1945:209) comments that the vegetation "is
well developed, particularly within the forested areas, but for these
high islands within the Pacific basin as a whole, the number of endemic
genera is relatively small and most of them have definite relationships
with those of Malaysia." Concerning the "high" islands of Micronesia,
Merrill (1945:210) remarks that these islands are smaller and more
isolated than some of the others in Oceania and have fewer individual
species "as compared with what one finds on islands of a similar size
located within limits of the Malay Archipelago. Thus with all of the
islands under Japanese mandate, and including a number of high, but at
the same time relatively small islands, less than 1,300 different
species are known, of which 230 manifestly represent purposely or
accidentally introduced ones. This relatively small flora includes
representatives of approximately 620 genera in 192 families.... Specific
endemism is relatively high, for approximately 460 species are confined
to the islands within the area under consideration. The generic endemism
is very low; about seven endemic genera only are involved for the whole
group." The figures for endemism of plants are comparable to those for
birds. Of endemic birds there are 5 genera, 35 species, and 73
subspecies. The total number of species of birds known from Micronesia
is only 206 as compared with 1,300 plants. Yamada (1926:966) writes that
the number of species of plants that Micronesia has in common with Japan
may be due to the influence of the "Japan Stream."
Many land birds in Micronesia depend directly on the plant life for
food. Possibly the soil (including its mineral content), upon which the
plants themselves depend for development of fruits and other edible
parts, may offer a limiting factor to the distribution of birds in
Micronesia. Possibly the fruits and other edible parts of plants do not
provide the necessary amounts of proteins, carbohydrates, minerals,
vitamins, and other essential food items for species of plant-eating
birds, which have not become established in Micronesia. Possibly some
species of plant-eating birds have reached Micronesia but have failed to
establish themselves because of some dietary deficiency caused by
poverty of the soils on which the plants grow. If a comparison were made
of soils and of the food values of fruits of plants in both the islands
of Micronesia and similarly sized islands in the Malay region, a
difference might be revealed which would partly explain why some
plant-eating birds have not become established in Micronesia.