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It is quite simple to describe the terrestrial biodiversity of the Marshall Islands, for it is just that, quite simple. But that doesn't mean it is uninteresting or insignificant. Far from being a barren wasteland, however, the dry land of these atolls is covered with all sorts of life.
That term - dry land - nevertheless, has to be used in a relative sense for most of the Marshalls. Because of the high humidity, high rainfall, and continual salt spray, only during the infrequent periods of windless drought that the land could truly be called dry. Often, there is so much moisture that a brown alga as well as Nostoc sp. (a type of cyanobacteria, similar to an alga), grow, showing how un-dry the dry land is.
As previously discussed, this close relationship between the marine environment and the land strongly influences the biota, especially as to what species were able to survive apart from human intervention. It is figured that only about 80 vascular plants - less than 20% of all the plants documented in the entire country - are native. More species are found in the southern atolls where there is more rainfall.
There are probably only about 700 native species of land animals. As with the plants, introduced species greatly outnumber the native. By far the largest category of land animals are the insects. It is hard to determine with any certainty, but likely only about 150 of the close to 600 species of insects are natives. That may seem like a lot of species compared to the 18 species of reptiles, or the 5 native land birds, but the entire order of insects contains close to 1,000,000 species worldwide, so it is remarkably small. Other native land animals include snails and crabs, but no mammals, not even the bats and flying foxes found on nearby high Micronesian islands.
Among these few species, there are endemics. Most of these are among the insects, with 25 species and subspecies endemic to the Marshalls, and another 45 which do not range beyond Micronesia. There are 8 species of endemic longhorn beetles, a group of animals which are gaining attention in many places around the world as being both endangered as well as endemic - and sometimes pests. The Majuro longhorn beetle is found nowhere else in the world.The most famous endemic land animals are the birds, the extinct Wake Rail and the two subspecies of Micronesian pigeons.
The endemic plants of the Marshalls are admittedly unimpressive to most people. There are two species of endemic grasses, one on Bokak and Wake, and another (possibly a recent hybrid) on Enewetak and Kwajelein Atolls and Rota in the Mariana Islands. There are also 3 species of regionally endemic liverworts.
But some endemic plants are intriguing because of their history of cultivation. Pandanus is well acknowledged to have been prehistorically cultivated to the degree that many useful clones were developed. There is also an endemic lily species that is theorized to be of cultivated origin. Yet it is a variety of cotton found only on Wake that is the most perplexing. It is not entirely certain whether it should be called native or not. Cotton in general is considered native to the American continents. However, it appears that an endemic variety somehow became established about 10,000 miles away from the place of origin of other cottons, on the little isolated atoll of Wake.
Most tropical areas have extensive wetlands and freshwater environments. These are also found in the Marshall Islands, but, as expected, to a limited degree. The few true mangrove and mangrove-like groves are small embayments and hollows, not the extensive forests that surround other nearby Micronesian islands, such as Kosrae and Pohnpei. With no mountains, there are of course, no true rivers, but there are a few freshwater ponds with their own ecosystems. Even tree stumps provide serious ecosystems on these atolls, having their own assortment of species, including endemics.
Human activity has greatly increased the number of species on the land and in freshwater. Now there are terrestrial mammals and many, many more plants and insects. These numbers will undoubtedly continue to grow as more species are brought in, both intentionally and inadvertently.