Henderson Island has been designated a UNESCO World Heritage Site.
Henderson Island is one the few places in the world with an ecology virtually unaltered by man. This permits the study of the dynamics of insular evolution and natural selection. Despite being reported as "infested with rats" (the Polynesian Rat [Rattus exulans], which is a good deal smaller than the Brown Rat), Henderson is particularly notable for plants and land birds, endemic to the island. Studies of piles of bones left in caves by Polynesian visitors have determined that several species of birds became extinct due to their presence.
There are no native species of land mammal. All four of the island's land birds are endemic: the flight less Henderson rail Nesophylax ater, Stephen's lorikeet Vini stepheni (R), the Henderson fruit dove Ptilinopus insularis, and the Henderson warbler Acrocephalus vaughani taiti. Very little information is available on either the ecology or the status of these four birds. Fifteen seabirds have been recorded, at least nine of which are thought to breed on the island; Murphy's petrel Pterodroma ultima, phoenix petrel P. alba, herald petrel P. arminjoniana, Kermadec petrel P. neglecta, shearwater Puffinus pacificus, masked booby Sula dactylatra, red-tailed tropic bird Phaethon rubicauda, brown noddy Anous stolida, blue-grey noddy Procelsterna caerulea, and fairy tern Gygis alba.Other terrestrial species are also poorly recorded and understood (including lizards and skinks as well as invertebrates), and it is likely that the invertebrate fauna is much larger, including several more endemics. For example, a new species of hawk-moth was identified in 1986, which is significantly different from any described hawk-moth.
Various records of the marine and littoral fauna have been made: Species of particular note include coconut crab Birgus latro (R) (identified from remains collected in 1987), at least two coenobite species (one of which was found to be the commonest crustacean on the island in 1987), and spiny lobster Panulirus penicillatus (CT). Green turtle Chelonia mydas (E) occasionally nests on the island. Collections of marine molluscs and sponges and of as yet unidentified caridean shrimps (mostly Alpheids, probably comprising 5-8 species), were made in 1987. There is a diverse echinoderm fauna. An unidentified holothurian is common on the northern reef flats, and an echinoid Heterocentrotus sp. (possibly H. trigonarius) is locally abundant on the sloping marginal reefs and shallow reef flat of the northern beach. Fish are sparse, with Caranx lugubris being the most common and obvious species.
From the sea Henderson Island appears to consist of a level plateau, about 100 feet high, extending above the cliffs; but in actual fact the surface is cut up by innumerable sharp coral pinnacles, covered with dense undergrowth. The soil consists almost entirely of decayed vegetation and supports a very limited range of trees and plants. A few coconut, lime and orange trees have, however, been planted on the beach by the Pitcairn Islanders, and David Young, a former Chief Magistrate of Pitcairn, who knew the island well, told H. E. Maude, O.B.E. that there were five or six acres of comparatively good soil in the north-east corner, where crops of potatoes had been grown.
Steve Waldren (Botanic Garden Curator/Administrator), Trinity College Botanic Garden, Dublin, who spent three months on Henderson Island in 1991 (and who is returned to the island in June 1997), has sent the following information about the flora of Henderson Island:
A trail cut across the island by a party of amateur explorers (Operation Raleigh in 1986) has been responsible for spreading several species of plants into new habitats. This underlines how easily the environment of the island can be altered by the intervention of irresponsible visitors.
Lobsters are reported as being very plentiful on the surrounding reef at low tide. The island's large hermit crabs have been reported as displaying an "odd..... orderly, methodical, almost tactical behaviour" by a visitor.