Teeraina has an unusual age structure; nearly half the population (44%) is aged under 15 and one in five people (19%) are children under five years. There are more boys than girls in almost all age groups except the elderly.
As regards its physical characteristics, this is one of the most interesting islands in the Pacific. It is a raised coral atoll, but it has not filled up with sand and soil, yet still retains a significant remnant of the former lagoon. The lake, however, is only just barely perceptibly brackish, as its only significant source is the plentiful rain. The lake is only a few feet (around 1–2 meters) deep for the most part, though the supposed maximum depth is nearly 10 meters (33 feet). Being only about 520 km (320 mi; 280 nmi) away from the equator, Teeraina is inside the ITCZ; its climate is thus extremely humid, making it one of the "wet" Pacific islands.
The western inland is made up by peat bog, which is still flooded much after heavy rains, and constitutes infilled former lakebed. It is not clear in what way the western lake or lakes - there are now 2 main areas of bogland, which may correspond to former lake basins - were connected to the remaining waterbody. One bog is immediately adjacent to the lake's western end, the other is halfway between that and the island's NW tip. Canals have been cut into the bogs, for punting, rowing and motor boats transporting people and produce. There is some removal of peat and sediments to stem the lake's ongoing infilling; in addition it seems that in recent times, the lake's level is slowly rising again so that the eastern bog's area has receded somewhat. The peat reaches thicknesses of about 1-1.5 meters (some 3–5 ft), much of which is located above sea level.
It is also not precisely known where the last connection of the inland waters to the ocean were, and when they closed. The SE end is more likely however, as the island is in the Equatorial Counter Current which runs west to east, and drifting coral and other reef-builder larvae as well as flotsam would therefore predominantly land at the island's western side. Thus, it is to be expected that land build up faster there. This also agrees with the eastern location of the remaining lake. In any case, the canal network now opens to the sea south of Tangkore, and there is a direct connection from the lake to the ocean at Teeraina's eastern tip.
Ecologically, Teeraina is also highly interesting, for several reasons. First, as is readily apparent from its peculiar geographical and geological features, it possesses a combination of ecosystems that is quite unique in the entire world. Second, it holds the world's largest population of a rare bird species though it is over 2,500 km (1,600 mi; 1,300 nmi) from that bird's original home. Furthermore, it was until fairly recently home to some enigmatic dabbling ducks which are now extinct. Last, the island's biodiversity seems to prove quite conclusively that, probably by about 1200 AD or so, the island was temporarily inhabited by a significant number of humans.
At present, there is no formal protection for the islands' ecosystems or species, but it has been suggested to legally protect key habitat, namely the boglands. Though it is globally endangered, the Rimatara lorikeet (Vini kuhlii) does not appear to be in need of formal protection; it actually benefits from human land use change and the feral cats. The former provides the birds with more habitat, while the cats have so far managed to keep Teraina completely free of black rats (Rattus rattus) which due to their tree-climbing habits would seriously jeopardize the species' existence, should they become established in numbers. Given the negative experience e.g. from Rennell Island, maintenance of a vigorous tilapia fishery would seem to be advisable. These fish certainly represent a valuable source of protein on Teraina, and in fact were originally introduced for that purpose.
Over 30 species of flowering plants are known from the island, but most seem to be not originally native. Cocos nucifera, the coconut palm, is the most conspicuous tree on Teeraina. It is found planted, but also constitutes one of the dominant forest trees. The palms occur in wet forest around the bogs, mixed with Pandanus (screwpine), and an undergrowth dominated by the ferns Asplenium pacificum and Phymatosorus scolopendria. In more elevated places near the beach, Pisonia (catchbird tree) atoll forest is found, though Teeraina has not that much of this ubiquitous Pacific ecosystem for its size.
Among the local crops, sugar-apple (Annona squamosa), breadfruit (Artocarpus altilis), papaya (Carica papaya), bananas (Musa cultivars, including Fe'i bananas) and apple guava (Psidium guajava) are the most significant, apart from the coconuts. Frangipani (Plumeria) and hibiscus are popular as ornamental plants.
Though numerous seabirds nest on Teeraina, for many such species the limited habitat makes it a less important rookery than other, similar-sized raised atolls. About 10 species of seabirds breed here, most significantly tree-nesters like the little white tern (Gygis microrhyncha) and the red-footed booby (Sula sula). The eastern reef egret (Egretta sacra), widespread throughout the region, can also be found on Teraina.
Among migratory birds, the ruddy turnstone (Arenaria interpres), sanderling (Calidris alba), bristle-thighed curlew (Numenius tahitiensis), Pacific golden plover (Pluvialis fulva), and grey-tailed (Tringa brevipes) and wandering tattlers (T. incana) use Teeraina as stopover location or winter quarters on a regular basis. Other shorebirds, gulls, and occasionally ducks of North American and East Asian species may occur as vagrants.
In historic times, two species of landbirds and one subspecies of duck have been recorded. The latter, Coues' gadwall (Anas strepera couesi), was the only distinct subspecies of the widespread gadwall. It is surrounded by considerable mystery, mainly as regards the origin of the population, the age and therefore validity of the subspecies (it is sometimes disputed to be significantly distinct), and the causes and date of its disappearance. Only two specimens are known - a couple that is not fully mature, and therefore only limited information can be gleaned from it. What is certain is that there was a duck population of some size in the mid-1870s, while in 1900 all were gone.
The bokikokiko (Acrocephalus aequinoctialis) is Kiribati's endemic reed-warbler. This small greyish passerine is well-known, due to its bold and inquisitive habits, and its song, a series of alternating higher and lower squeaks after which it is named.
The Rimitara lorikeet (Vini kuhlii) is present with about 1,000 birds. It is a tiny parrot with brilliant plumage and an endearing and highly social behavior, owing to which they were known to late 19th century settlers as "love-birds". This species was originally native to the Austral and Cook Islands in SE Polynesia, a long distance to the south. They were treasured by the natives of the Society Islands and were an item of high value in inter-island trade; these birds were kept as pets, and in addition their feathers were used in crafts and art. In any case, there is considerable evidence from Ancient Hawaiʻi that Polynesian seafarers travelled between southeastern Polynesia and Hawaiʻi with some regularity, perhaps as early as 400 AD, but certainly by about 1200 AD. Since it is almost inconceivable - given the prevailing winds, ocean currents, known trade routes, and the difficulty with which these birds can be kept alive during voyages - that Vini kuhlii was transported from the Austral or Cook Islands to the Gilbert Islands and from there to Teeraina, the presence of the birds there is best explained by them having been introduced by SE Polynesian travelers. Today, the species is extinct on many islands on which it formerly occurred, while the Teeraina population, considered crucially important for its survival, contains some 60% of the remaining global wild population. Ironically, the reason for the Rimitara lorikeet thriving on Teeraina is the replacement of native forest with coconut plantations; these birds feed mainly on the nectar of coconut palm flowers and nest in old coconut shells or husks.
As it thus seems clear that there was prehistoric human activity of some degree on Teeraina, it is also likely that birds became extinct consequently, like on all such Outer Pacific islands for which research has been conducted. David Steadman in his comprehensive review lists several such hypothetical taxa for Kiribati as a whole. For Teeraina specifically, considering the habitat and what birds still exist, one or more rails (Gallirallus and/or Porzana), an imperial-pigeon (Ducula), and maybe a Todiramphus kingfisher or an Aplonis starling make the most likely candidates for birds gone extinct prehistorically. It must be considered, however, that given the lack of fieldwork it is not quite clear what effect changing sea levels would have had on Teraina. If the sea level were only half a meter (c. 2 ft) higher it is certainly possible that the forest and freshwater lake would be replaced by shrubland or dunes and a brackish lagoon. That notwithstanding, it is well possible that a Prosobonia sandpiper related to or identical with the mysterious taxon from Kiritimati one lived on Teeraina.
As with most outer Pacific islands, there are no native land mammals.
Polynesian rats (Rattus exulans) are present on Teeraina, apparently since prehistoric times. They may have arrived with flotsam after storms further west, or accidentally or deliberately (as food) been introduced by prehistoric seafarers. Their present-day impact on the bird population is minor, but if rails were once present on Teeraina, the rats had probably some role in these birds' disappearance, and maybe in that of any other birds gone extinct in prehistoric times too. If a Prosobonia sandpiper once bred on Teeraina, it is almost certainly those rats that are responsible for these birds' disappearance; only a single taxon of Prosobonia remains today, precariously holding its own on atolls that are devoid of any rat species.
Feral dogs, cats and pigs occur in varying numbers on Teraina; the cats especially are responsible for some decline in the number of ground-nesting seabirds. On the other hand, as noted above, the cats have thus far kept the rat population at bay.
Lacustrine species reported from Teeraina include fish. and some unspecified "shrimp", i.e. (in all probability) a member of the Crustacea. Teeraina's freshwater fish include the marbled eel (Anguilla marmorata), a Caranx freshwater trevally, and Oreochromis tilapias and the milkfish (Chanos chanos). The latter two, and perhaps also the trevally, were introduced in recent times. The eels were already established by 1877; like many Anguillidae they are catadromous and able to migrate some distance on dry land. Thus it may be presumed that the lake is continuously being restocked from the Pacific, though apparently no actual field data exists on the habits of the eels of Teeraina.
A few green turtles (Chelonia mydas) nest on the beaches. It is not a very important nesting site however and the clutches have a rather low probability of success, as Teeraina is one of those Kiribati islands where turtle egg collecting is permitted.