Invaders in paradise

The 6,393 mi2 (16,558 km2) Hawaiian Islands chain, a collection of 132 islands, reefs, and small shoals, has only one native land mammal species, the Hawaiian hoary bat (Lasiu-rus cinereus semotus). Thus, the Hawaiian Islands are a good "laboratory" for the studying the ecological effects of introduced mammals.

Even such usually ubiquitous land organisms as ants and mosquitoes were absent from the Hawaiian Islands' native fauna. For much of its geological history the world's largest ocean, the Pacific Ocean, acted like a giant moat, keeping the Hawaiian Islands relatively isolated and deterring invading species. The winds, ocean currents, and migrating birds brought species to the Hawaiian Islands, but very few became established. On average over the last 70 million years, only one invading species per 35,000 years successfully established in the Hawaiian Islands.

However, a diverse topography, a warm tropical climate and an absence of predator species during most of the past 70 million years made the Hawaiian Islands a good evolutionary locale for new species formation via adaptive radiation. A spectacular example of new bird species formation by adaptive radiation in the Hawaiian Islands is the 54 species of Hawaiian honeycreepers (Drepanididae). Compared to the Galápagos Islands and its 14 Galápagos finch species that inspired Charles Darwin's theory of evolution, the Hawaiian Islands have more habitat diversity and a longer evolutionary history.

A mammalian invasion began transforming the Hawaiian Islands approximately A.D. 400, when the first Polynesian sailing canoes arrived. The Polynesian voyagers brought animals and

Raccoons (Procyon lotor) have adapted to urban living and thus become pests to humans. (Photo by Steve Maslowski/Photo Researchers, Inc. Reproduced by permission.)

plants with them, and changed the landscape with their agriculture. New species in the Hawaiian Islands increased to three to four per century after the Polynesians arrived, a huge increase from the pre-human one species per 35,000 year average.

Even before the Europeans arrived in the late eighteenth century, the Hawaiian Islands had a few hundred thousand people of Polynesian descent. The fauna introduced by humans included the Polynesian (or Pacific) rat (Rattus exulans), dogs, pigs, fowl, and reptiles. The Polynesian rat is a native of Southeast Asia that spread across the Pacific Ocean islands to the Hawaiian Islands with the Polynesians, but never reached the mainland of the United States.

Polynesian rats attract a lot of attention as pests of plantation agricultural crops like sugarcane and pineapple, though a broad range of crops are attacked. Polynesian rats are omnivorous, and studies show adverse impacts on coastal tree and lizard species in New Zealand and on seabirds on several Pacific islands. There are little data available on Polynesian rat ecological effects on now extinct Hawaiian Island birds.

In the Hawaiian Islands, about half the land bird species predating human arrival have vanished. Direct human impacts from hunting and gathering and indirect human impacts are strongly implicated in the decline or extinction of native species, particularly flightless birds and ground-nesting winged species. The magnitude of ancient human impacts on specific species is still the subject of vigorous debate and inquiry. However, there is little doubt that the rate of worldwide ecological change and species extinctions directly and indirectly attributable to human beings began increasing in recent centuries.

The European ships of the eighteenth and nineteenth centuries brought exotic mammals from around the world to the Hawaiian Islands, including new rat species, European pig genotypes, cattle, goats, sheep, the house mouse, and the mongoose. Goats and cattle trampled and grazed native plants and generally degraded habitats. Hawaii's native bird species suffered additionally when early nineteenth century whalers introduced the first mosquitoes and avian malaria. New diseases like smallpox and syphilis were transferred from the European arrivals to the Polynesian population. Clearly, as the top mammal species, human impacts increased as human populations increased and spread.

Globalization and the expansion of ship and airplane commerce in recent centuries accelerated the rate of new species

Invasive animals damage local ecosystems. 1. Wooded area; 2. Wild pigs root and wallow in the wooded area; 3. The pigs uproot plants and leave the area open to erosion; 4. New growth in the disturbed area includes invasive plants such as briars, burs, and other species carried as seeds in the pigs' manure. (Illustration by Wendy Baker)

Invasive animals damage local ecosystems. 1. Wooded area; 2. Wild pigs root and wallow in the wooded area; 3. The pigs uproot plants and leave the area open to erosion; 4. New growth in the disturbed area includes invasive plants such as briars, burs, and other species carried as seeds in the pigs' manure. (Illustration by Wendy Baker)

introductions. When all the newly introduced plant and animal species were added up, the rate of new species introductions into the Hawaiian Islands was estimated to have accelerated to several dozen species per year in the twentieth century. Ecological upsets and pest problems from introduced mammals became more noticeable in the Hawaiian Islands during the nineteenth and twentieth centuries.

Black rats, also known as roof rats (Rattus rattus), and Norway rats (Rattus norvegicus) disembarked on Pacific islands as stowaways aboard European sailing ships and spread rapidly in the nineteenth century. Norway rats and black rats are omnivorous, attacking agricultural crops and feasting on the young and eggs of seabirds like petrels, shearwaters, gulls, terns, and tropicbirds.

Birds on isolated islands like New Zealand and Hawaii evolved in pre-human times when there was no need for defenses against mammalian predators like rats. Bird depredations by rats are less common nearer the equator. One untested hypothesis is that birds nearer the equator developed better predator defenses useful against rats, because of land crabs preying on eggs and chicks.

Black rats are suspected in the demise of many native Hawaiian birds during the nineteenth century. But the bio logical documentation from that period is not considered conclusive by modern standards. Very recent technological advances like night vision videos provide more conclusive evidence. For example, night vision videos revealed beyond doubt that black rats were a major predator of New Zealand's endangered Rarotonga flycatcher (Pomarea dimidiata). Consequently, rat control became part of the program to save that endangered forest bird species.

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