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The Hawaiian Archipelago -- An Ecosystem in Crisis

[Hawaiian Islands] [The Origin of Hawaiian Life] [Paradise Lost]
[Mosquitos and Malaria] [The Biocomplexity Project]

The Hawaiian Islands

Shivering in the cold atop one of Hawaii’s thirteen-thousand-foot volcanoes, one can look down upon miles of alpine desert and sub-alpine woodland, across an expanse of tropical rainforest and dry forest, out over moon-like lava flows toward a ring of coastal cliffs, beaches, and coral reefs. All of these environments team with life, much of which is found nowhere else on earth. But until relatively recently, nothing existed here but a broad expanse of ocean. The current “high” islands of Hawaii originated as a chain of fiery volcanoes that began to emerge from the depths of the central Pacific Ocean about 6-7 million years ago.

Arial photo of the Hawaiian Islands

The Source of Hawaii's Diversity

The process of "Adaptive radiation" allowed a small number of immigrants to evolve into hundreds of new species, found only in Hawaii.

Flowering Plants
270 Species
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960 Species
Ferns and Fern Allies
135 Species
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168 Species
Insects and Allied Forms
300 to 400 Species
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6,000-10,000 Species
Land Snails
22-24 Species*
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1,200 Species
Land Birds
20 Species
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115 Species**
*
**
Probably carried by birds.
Some of these have since gone extinct, and we only have their fossils.

These mountains of volcanic rock, isolated by almost three thousand miles of ocean in every direction, were completely separated from the rest of the world. As they emerged, these islands began to be settled by the few plants and animals hardy enough to survive the journey. Seabirds and shorebirds may have been among the first vertebrate arrivals, perhaps bringing with them and depositing the seeds of recently eaten plants. The seeds of some plants attached themselves to the birds plumage and “hitchhiked” to the islands, whereas other plants have seeds so small that they could easily be blown across the ocean by the wind. Many invertebrates, such as spiders and insects, likely arrived in the islands by the wind as well. Over time, some of those animals and plants that survived the journey evolved into many new species through a process known as “adaptive radiation”.

The Origin of Hawaiian Life

The story of the Hawaiian honeycreepers is an outstanding example of how the Hawaiian islands became home to countless forms of unique life. About 3-4 million years ago, a small flock of cardueline finches arrived in Hawaii, probably blown in on a storm from North America. With few competitors and countless environments with many different types of food, Hawaii was ripe for colonization. As the new colonists spread out over the islands, individual populations became isolated from one another and began to evolve in different ways. Some evolved long, curved bills to extract nectar from flowers, others evolved short, stout bills for crushing hard seeds. By the time Polynesians arrived about 2000 years ago, they found over 50 unique species and subspecies of honeycreepers - all descended from that one original group.

A Diagram of Various Honey Creepers
Original art © H. D. Pratt, Louisiana State University Museum of Zoology

Paradise Lost

Although the Hawaiian Islands rested alone in the Pacific for millions of years, the arrival of mankind permanently broke Hawaii’s isolation. Starting around 400 AD when Polynesian sailors first colonized the islands, humans began to carry foreign species across the ocean. Some of the early arrivals were dogs, pigs, jungle fowl, rats, and various reptiles and insects. Many types of plants were introduced to support early Hawaiian agriculture, textiles, and musical instrument construction. Hawaiians also began to clear lowland forests for farming, hunt native birds for food and plumage, and raid offshore sea-bird nests for eggs. All of these practices began to have an impact on Hawaii’s ecosystem. Some estimates say that over half of Hawaii’s unique bird species had gone extinct before Europeans arrived to record them.
Captain Cook’s arrival in 1778 only accelerated the process of introducing species to Hawaii. As European sailing ships started a constant traffic to and from the islands, stowaway animals and deliberate introductions carried a constant stream of invasive species. Several new species of rats were introduced, which immediately began to eat plants and sometimes even bird eggs. The mongoose was introduced to combat rats, but immediately preferred to prey on native birds instead. An estimated 900 different species of plants were introduced--Hawaii had no conifers (pine trees, fir trees, etc) until they were introduced by Europeans. All of these new species immediately began to compete with native species, and the native species almost always were the losing end of the battle. For honeycreepers, however, one of the most destructive new inhabitants was the mosquito.

Mosquito and Malaria

Before Europeans arrived, Hawaii had no mosquitoes or other bloodsucking flies. For humans and animals alike, this meant no annoying bites and more importantly, no mosquito-borne diseases. However, the first mosquito species was introduced in 1826, and over the next hundred and fifty years, at least four more species arrived. It is thought that sometime in the 1800’s, the first of two mosquito-transmitted avian diseases was introduced to the islands--avian pox virus. This pathogen causes large tumor-like swellings on the feet and legs and around the eyes and beak of infected birds and is transmitted on the mouthparts of mosquitoes, in effect making them “flying needles”. Early naturalists in the islands reported the first large scale epidemics in native bird populations in the late 1800’s, but it wasn’t until the early part of the 20th century that we think that the second pathogen, a species of avian malaria (Plasmodium relictum) reached Hawaii. Similar to human malaria in many aspects of its life cycle, avian malaria probably reached the islands in imported cage birds that were released to replace declining native birds. With mosquito vectors already in place, it is likely that avian malaria spread quickly into native birds. With no prior exposure or natural immunity to this disease in native species, we think that it probably swept through remaining lowland native bird populations, further hastening the extinction of several species and decline of many more. Only high on the volcanoes, where colder-temperatures limit the spread of mosquitoes, do native forest bird communities remain relatively intact.

The Biocomplexity Project

The Biocomplexity Project is attempting to unravel the complexities and impacts of introduced pox virus and avian malaria in native forest bird populations with the hope of developing long-term strategies for preventing further extinctions of threatened and endangered native birds and restoring them to habitats where they were formerly common. The story of native birds being pushed into the highest reaches of the mountains by mosquitoes is just one example of the dynamics we are only just beginning to understand--feral pigs are digging up forests and creating breeding sites for mosquitoes, suburban development is eliminating large areas of lowland native forest, and the pathogens and hosts themselves may be undergoing natural selection and coevolutionary changes. At the same time, we hope that getting a clearer picture of a single environment like Hawaii where so much environmental and climatic diversity is squeezed into a small geographic area will give us knowledge that we can apply to other environments. Possibly we can learn enough about malaria to help control its spread elsewhere; possibly we can learn enough about the Hawaiian forests to help protect forests elsewhere. Hawaii is an environment that we hope will teach us more about not only Hawaiian ecosystems, but about the world in general.

 

This material is based upon work supported by the National Science Foundation under Grant No. 0083944 and by the U.S. Geological Survey, Invasive species and wildlife programs. Any opinions, findings and conclusions expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation or the U.S. Geological Survey.

The Biocomplexity Project thanks the following organizations for their support:
United States Geological Survey, University of Hawaii at Manoa, University of Hawaii at Hilo, Smithsonian Institution, Princeton University, and many other collaborators
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