vegetated solely by low-lying herbs. Coconut palms were introduced to the atoll after it was first discovered by Europeans, and a large plantation still exists on South Islet, with palms growing to a lesser degree on Long Islet and Nake Islet. In 2014 the Kiribati government established a fishing exclusion zone extending to 12 nautical miles around each of the southern Line Islands—Caroline, Flint, Vostok, Malden, and Starbuck. Caroline Island is an important breeding site for several species of seabirds, most notably the sooty tern (Onychoprion fuscata), numbering around 500,000—a colony of sooty terns dominates the eastern islets—and the great frigatebird (Fregata minor), numbering over 10,000, according to estimations made in 1980. Caroline Island and Flint Island host one of the world's largest populations of the coconut crab (Birgus latro). Other native animals include the Tridacna clam, which is abundant in the central lagoon, hermit crabs, and multiple species of lizards. Giant clam populations reach densities up to four per square foot (43 per m2) in parts of the lagoon. The most common species is the "small giant clam" Tridacna maxima, and the largest clam species, Tridacna gigas, is also found in the lagoon. The lagoon is a nursery habitat for fish species, including important and heavily exploited species such as the blacktip reef shark (Carcharhinus melanopterus) and the endangered Napoleon wrasse (Cheilinus undulatus). The atoll is designated as a wildlife sanctuary for turtle nesting. The endangered green turtle (Chelonia mydas) nests on the beaches of Caroline Island, but there have been reports of poaching by homesteaders. Human contact has caused the introduction of about twenty non-native species of flora to Caroline Island, including the vine Ipomoea violacea, which has begun to proliferate. Domestic cats and dogs introduced alongside a small homestead have driven the seabird population away from the islet of Motu Ana-Ana. Environmental issues Caroline Island is low-lying, with no land greater than above sea level. Regional sea surface temperatures have risen by around 0.1 °C per decade since the 1950s, and in 2006, the United Nations rated the island as among those most vulnerable to rising sea levels. The UNEP reported in 2006 that Caroline could disappear "within the next 3050 years". Caroline Island has recovered from destruction caused by settlers and business opportunists. The island's indigenous plant and animal species are thriving, and it was reported in 2010 that its coral reefs are among the most