Island

• 1. Introduction

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  In 1995, fifteen iguanas washed ashore on Anguilla in the Caribbean, riding a tangle of uprooted trees across 300 kilometers of open sea. No iguana had ever lived there before. They survived a storm-driven journey that reads like an accident, and yet it is exactly the kind of accident that fills islands with life. An island is a piece of land, distinct from a continent, completely surrounded by water. That simple definition hides an enormous range. Some islands split from continents through plate tectonics. Others, born from volcanoes or coral, were never part of any continent at all. How does life cross an ocean to reach a place no creature has touched? Why do animals there grow strange, becoming giants or dwarfs? And why are some of these places, after thousands of years of human settlement, now at risk of vanishing under the sea?

• 2. Born From Fire And Water

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  The Hawaiian Islands trail across the Pacific in a line, the oldest of them 25 million years old, the youngest, Hawaii, still an active volcano. They sit above a hotspot, a place where the mantle runs hotter than the rock around it, feeding volcanoes whose lava builds new land. As tectonic plates drift over a stationary hotspot, islands form one after another, the distant ones older and more eroded until they slip beneath the sea. The Line Islands tell a different story. All of them are estimated at 8 million years old rather than a graded sequence of ages. That uniformity suggests they formed at once, through fractures in the tectonic plates themselves. Coral builds islands too, growing atop volcanic islands that have sunk below the surface. When a reef encircles a central lagoon, the result is an atoll. A tectonic lift of as little as one meter can let sediment gather and a new island rise. Barrier islands take shape from waves alone, long sandy bars deposited along shorelines. They shift with wind and water, and they shield the coast by absorbing the energy of large waves. Other islands are the wreckage of lost continents. New Zealand sits on Zealandia, a continent-like slab of crust estimated to have had 93 percent of its original surface submerged.

• 3. Crossing The Water

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  Freshwater fish on an ocean-bound island are a clue, a sign that the land once touched a continent, because such fish cannot cross the sea on their own. This is the logic of dispersal. Oceanic islands, never connected to any shore, can only be populated by life able to make the crossing. Birds and bats fly. Other creatures are carried, or drift in sea currents in what scientists call a rafting event. Tortoises can endure weeks without food or water, floating on debris. Plants travel far as well. New Zealand and Australia share 200 native plant species despite a 1500 kilometer gap between them. The southern beech grows in Australia, New Zealand, parts of South America, and New Guinea, lands now scattered across the globe. One explanation is Gondwana, the ancient continent these landmasses once formed together before tectonic drift pulled them apart. Competing theories hold that the tree simply crossed the oceans through dispersal. Tropical cyclones can fling species across great distances, washing plants far inland and seeding new habitats.

• 4. Giants, Dwarfs, And Finches

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  Darwin's finches number up to fifteen tanager species endemic to the Galapagos Islands, each shaped by what it eats. The large ground finch wields a heavy bill to crack seeds and eat fruit. The Genovesa cactus finch favors cacti, its beak built to pull pulp and flowers free. The green warbler-finch, behaving like a true warbler, hunts spiders and insects on plants. This is adaptive radiation, the rapid splintering of a single arriving species into many. A colonist that meets little competition, or misses the resources it once relied on, branches into new survival strategies. The pattern repeats in Hawaii and Madagascar. Island species also follow Foster's rule, also called the island rule. Small mammals such as rodents tend to grow larger, a trend known as island gigantism, seen in the giant tortoise of the Seychelles. Larger animals such as the hippopotamus shrink, as with the pygmy hippopotamus, a process called insular dwarfism. Fewer predators may release small animals from the pressure to stay large. Bigger animals, meanwhile, can strip food supplies fast, starving their young and favoring smaller bodies that need less. Islands carry the highest rates of endemism on Earth, yet their total species richness stays lower than the mainland's, scaling with the island's area in a pattern called the species-area relationship.

• 5. Darwin's Beagle

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  In 1835, Charles Darwin reached the Galapagos Islands as a naturalist aboard HMS Beagle, partway through a five-year circumnavigation of Earth. He wrote that "the different islands to a considerable extent are inhabited by a different set of beings". The tanager birds and other creatures he found there pushed him toward a single idea. Organisms survive by changing to fit their habitat. The field he helped open, insular biogeography, studies how evolution, extinction, and species richness play out on islands, treating each one as an isolated model of natural selection. Island ecology has fed insights to its parent field since Darwin's time. It would be over twenty years before he published his theories in On the Origin of Species.

• 6. Seafarers And Settlers

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  Reaching Flores and Timor in the Paleolithic era, 100,000 to 200,000 years ago, meant crossing at least 29 kilometers of water. That is the first probable evidence of humans colonizing islands. Some islands, like Honshu, were likely reached over a land bridge before the sea cut them off. The Polynesians were the first to settle distant oceanic islands. While earlier voyages often spanned less than 100 kilometers, Polynesians may have crossed 2000 to 3200 kilometers to reach places such as Tahiti. They sent navigators out without instruments to find new land. Between 1100 and 800 BC they sailed east from New Guinea and the Solomon Islands and reached what is now Fiji and Samoa. Their migration stretched to Easter Island in the east and New Zealand in the south, where the first settlements date between 1250 and 1300. Not every island culture kept its boats. The Canary Islands were occupied from the first century until the Spanish Empire conquered them in 1496, yet the inhabitants had lost their seafaring ability, with little incentive to trade and little contact with the mainland. The Polynesian diet drew most of its protein from fishing, close to shore and in deep water, with the Rapa Nui reported to fish as far as 500 kilometers out at coral reefs.

• 7. Empire, Atolls, And Aid

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  Pohnpei was colonized by Spain as early as 1526, then passed from Germany to Japan to the United States before joining the Federated States of Micronesia in 1982. Beginning in the 16th century, European states placed most of Oceania under colonial administration. Christian missionaries pressed into these islands and met resistance, but gained ground when local chiefs used European backing to centralize their power. Guam stayed a Spanish territory until 1898 and is now an unincorporated U.S. territory. Decolonization brought independence or self-government to many, though the wounds ran deep. Nuclear weapons testing on the Marshall Islands left atolls destroyed or uninhabitable, forcing people from their home islands and driving up cancer rates from radiation. In Hawaii, Native Hawaiians are now a minority, and traditional practices have declined. Some Pacific nations still bind themselves to larger powers. Palau, the Federated States of Micronesia, and the Marshall Islands hold a Compact of Free Association with the United States, covering defense, aid, and immigration. French Polynesia draws substantial military spending and aid from France.

• 8. Vanishing Ground

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  In 2017, Hurricane Maria stripped away almost all the infrastructure of Dominica, a glimpse of what warming seas and fiercer storms can do to an island. Sea level rise can shrink freshwater reserves and bring drought, eroding the very habitability of small islands. The losses extend beyond people. An estimated 50 percent of land species threatened with extinction live on islands. A 2017 review of 1,288 islands found them home to 1,189 highly threatened vertebrate species, 41 percent of the global total. Tuvalu drew media attention with a press conference on its own submerging, then signed an agreement with Australia to send 280 of its citizens annually into permanent residency there. The Marshall Islands, a country of 1,156 islands, faces the same existential threat from rising seas. Stronger storms also ferry invasive species farther and more often, washing plants inland into new habitats. The apple snail, first brought to the United States by aquarium owners, has since spread by hurricane across the Gulf Coast and nearby islands, crowding out native life. Humans build islands too. The people of the Solomon Islands once piled coral and rock to make eighty islands in the Lau Lagoon, and the first permanent artificial island, Al-Sayah in Bahrain, was created at least 1,200 years ago. Modern projects pour millions of tons of sand into the sea, as with Pearl Island in Qatar and the Palm Islands in Dubai. When China covered former low-tide elevations in concrete in the South China Sea, the United Nations Convention on the Law of the Sea held that such islands may not carry the legal rights of a naturally occurring one.