Much of the coast is lined by mangrove swamp, succeeded inland by nipa palm (Nypa fruticans) in brackish waters. Large stands of sago palm are scattered farther inland, particularly along the valleys of the larger rivers in the north and along the deltas of the south coast. Primary lowland rainforest covers much of the island up to elevations of approximately 3,300 feet (1,000 metres). The forest is characterized by a large number of species, by the absence of pure stands of any one species, by fairly distinct layering of the forest into two or three levels, by the limited development of undergrowth, and by the small amount of human impact upon it.

Dense undergrowth is usually a sign of human interference, except on soils that are particularly poor or where the low height of the forest allows sunlight to penetrate to the soil surface. In those lowland areas where drier conditions prevail, agriculture and the burning of vegetation to facilitate hunting have long sustained a grassland environment (for example, in the Sepik and Markham river valleys).

In the uplands above 3,300 feet, stands of single species become more common, and trees such as oaks, beeches, red cedars, and pines become increasingly dominant. Above about 6,500 feet (2,000 metres), cloud or moss forest characterized by conifers, tree ferns, and a wealth of fungi and epiphytes (such as mosses and orchids) appears. Papua New Guinea possesses a rich variety of reptiles, marsupials (animals that carry their young in pouches), native freshwater fishes, and birds but is almost devoid of large mammals.

Dense undergrowth is usually a sign of human interference, except on soils that are particularly poor or where the low height of the forest allows sunlight to penetrate to the soil surface. In those lowland areas where drier conditions prevail, agriculture and the burning of vegetation to facilitate hunting have long sustained a grassland environment (for example, in the Sepik and Markham river valleys).

In the uplands above 3,300 feet, stands of single species become more common, and trees such as oaks, beeches, red cedars, and pines become increasingly dominant. Above about 6,500 feet (2,000 metres), cloud or moss forest characterized by conifers, tree ferns, and a wealth of fungi and epiphytes (such as mosses and orchids) appears.

Papua New Guinea possesses a rich variety of reptiles, marsupials (animals that carry their young in pouches), native freshwater fishes, and birds but is almost devoid of large mammals.

This has assisted the evolution of some 40 species of birds-of-paradise. The largest animals are the cassowaries (large flightless birds) and crocodiles. For roughly the past 55 to 56 million years, New Guinea and Australia, to which it was joined, have been isolated from other landmasses by the sea, and the former land bridge between New Guinea and Australia explains the presence of marsupials, tree kangaroos, and echidnas on the island—species it has in common with the continent. New Guinea’s animals have evolved in isolation since the end of the last ice age (about 12,000 years ago), when sea levels rose and covered the land bridge between the two landmasses.

New Guinea shares with the Indonesian archipelago many species of insects, carried by winds. Likewise, New Guinea has been a centre of dispersal for many plants to all neighbouring regions. The extraordinary profusion in Papua New Guinea of plants such as orchids, figs (genus Ficus), and false beech (Nothofagus) and of such animals as cassowaries, birds-of-paradise, parrots, butterflies, and marsupials—including tree kangaroos and cuscus (a type of phalanger)—gives the island an unparalleled biogeography.

Papua New Guinea’s unique biological species have long been sought by collectors throughout the world, but the government has established several conservation and protection measures. The export of birds-of-paradise is banned, and hunters thereof are restricted to the use of traditional weapons. 

Similarly, the export of many other birds and butterflies and of crocodile skins is strictly regulated. Other policies encourage the controlled expansion of selected exports of “farmed” orchids and crocodiles and of “cultivated” butterflies and other insects. A number of conservation projects, encouraged by foreign environmental groups, provide a small income for a few local landowners.