TOEFL Academic Passage Practice 039Colonization and Biodiversity in Pacific Island Ecosystems

The Pacific Ocean spans roughly one-third of Earth’s surface and contains about half of the world’s total ocean area. Scattered across its vast expanse are approximately 25,000 islands, around 7,500 of which are considered oceanic because they are situated far from any mainland. Most of these islands originated as lifeless outcroppings of volcanic rock that emerged from the sea. A notable exception is New Zealand, which, along with Fiji, the Solomon Islands, and other islands in the western Pacific, is classified as continental due to its proximity to landmasses.

Life came to the majority of these islands either by accident or passive transport. Birds played a major role in introducing plants—roughly 40 percent of Hawaii’s early flora arrived as seeds that had passed through the digestive systems of birds. The first species to settle were those that could float across long ocean stretches or whose seeds could survive inside a bird’s gut. At times of lower sea level, land bridges linked, or nearly linked, many islands in the far western Pacific, so some species colonized these islands without being notably good floaters or stowaways. In contrast, the more remote eastern islands, such as Easter Island, were colonized only by the hardiest of species capable of long-distance travel. As a result, western islands host a greater number of species and exhibit significantly higher biodiversity than their eastern counterparts. For example, before European impact, the western island of Bougainville in the Solomons had several thousand plant species, while Easter Island had only 30. Mammals struggled to reach these remote islands; only species like bats and rats managed to establish populations beyond New Guinea. Most of the flora and fauna in the Pacific islands originated in Asia, as the Pacific was largely influenced by Asian biogeography, with very few species coming from the Americas. In general, islands farther from Indonesia supported fewer species and had ecosystems that were less stable and less resilient to change. This trend was strongest among land-dwelling species, somewhat weaker for marine life, and virtually absent among seabirds, although still noticeable for land birds.

The ecosystems of the Pacific islands developed in varying degrees of isolation from the biologically rich continental environments. This separation allowed many new species to evolve and fill ecological roles that were already occupied elsewhere. A classic illustration of this phenomenon is found in Darwin’s observations of finches in the Galápagos Islands, which had evolved into multiple species, each adapted to a specific niche. On islands lacking mammals, reptiles and birds sometimes evolved to fill similar roles. For instance, the Galápagos Islands became home to giant tortoises, and New Zealand once had giant birds that functioned much like grazing mammals. In many of these islands, the absence of herbivorous mammals meant plants had little need for natural defenses like spines, bitterness, or toxic chemicals. Isolation also led to extremely high levels of endemism—species found nowhere else. In Hawaii, for example, nearly 99 percent of native species were endemic. However, this evolutionary uniqueness also left many island species biologically fragile when forced to compete with species from more competitive mainland environments. This vulnerability tended to increase with geographic isolation, particularly in the more remote eastern islands.

A second and possibly more damaging threat to Pacific ecosystems came with the arrival of humans. Many of the island species had evolved without any exposure to large land predators or human activity. As a result, they lacked natural defenses or fear, making them particularly easy targets. In extreme cases, such as in the Galápagos, Darwin described birds as so tame they would allow humans to approach within an arm’s length. Pacific plant life had also evolved in areas where natural fires were extremely rare. Consequently, most of these plants were not fire-resistant and suffered significant damage when exposed to fire. This stands in contrast to continental plants, which had long coexisted with frequent fires and had evolved traits that allowed them to recover—or even benefit—from occasional burning. Ultimately, what made Pacific ecosystems so unique—geographical isolation—also made them especially sensitive to outside disturbances.