As you know the Hawaiian Islands are volcanic in origin. They were completely built up from the ocean floor by erupting volcanoes over the past 44 million years and even today Hawaii, the youngest island, is still "under construction". Each island is made up of at least one primary volcano, although many islands are composites of more than one.

Hawaii is the most isolated island group in the world. The nearest continent, North America, is over 2500 miles (4000 km) away. All of Hawaii's many plants and animals are evolved from colonizers (seeds, plants, animals etc) that made it here either by air or by sea. But the extreme isolation made it difficult for plants and animals to colonize the islands. The odds of surviving the journey by air or sea is small; making it here and establishing a reproducing population is miraculous. On average, an invertebrate successfully colonized Hawaii once in every 70,000 years, a plant once in every 100,000 years, and a bird once in every million years. This is why a very distinct flora and fauna evolved in Hawaii over millions of years.

Many of the native plants and animals evolved with few diseases and no natural predators and therefore needed few natural defenses. Before humans arrived in Hawaii there were no large animals to eat plants. Mother Nature is very efficient. Defenses weren't needed and over time were lost. This paradise was harmed when Polynesian settlers found their way to the islands (about 1500 years ago) and brought with them other mammals like pigs, dogs and goats and other plants which literally took over and devastated many native ecosystems. Ever since then, people brought new species to Hawaii and every single one had an impact on the native plants and animals.

Until recently snakes had not found their way to the islands (it is illegal to bring them) but some reports indicate that some snakes have been seen, among them the brown tree snake.This is especially bad because snakes have no effective predators living on Hawaii and would quickly kill all the native birds and animals. A good example where this happened is the island Guam where snakes have virtually wiped out the native forest birds.

Please read the following answer for corrections to this answer.

Note from ScienceLine moderator:
The following corrections are from a scientist interested in improving the information on this answer, so the following corrections correspond to Answer 1 above.

1) Volcanoes have been erupting where Hawaii is located for 44 million years (It's actually been at least about 82 million years)

2) The nearest continent, North America, is over 2500 miles away (actually, San Francisco is 2319 miles from Hilo. That's the closest distance between Hawaii and the continent).

3) Polynesians settled Hawaii about 1500 years ago (It's been revised by archaeologists to about 1100 or 1000 years ago). 4) "(Polynesians) brought with them other mammals like pigs, dogs and goats and other plants which literally took over and devastated many native ecosystems." (Actually, they had no goats, but they did have rats. They also brought chickens, so 4 animals in total included pigs, dogs, rats, and chickens, along with geckos hitching a ride as stowaways. The "devasted many native ecosystems" is also debatable. Some native species were made extinct by this, but most whole ecosystems in Hawaii remained largely intact until the arrival of westerners).

5) "Snakes have no effective predators living on Hawaii" (not true, most Hawaiian islands have mongooses). Thanks,