The formation of a sliver of land between two massive continents sounds small in the context of the Earth’s entire geological record. But the emergence of the Isthmus of Panama three million years ago had dramatic effects on life on the planet. The land strip, or bridge, brought the previously isolated North and South American land faunas together, while separating the waters of the Atlantic and Pacific Oceans. The result was major changes in biodiversity, ocean circulation, and climate.

For tens of millions of years, North America and South America remained separated by thousands of miles of ocean. Without the existence of Central America to join the two continents, the flowing waters of the Atlantic and Pacific Oceans kept the continents, and their terrestrial creatures, apart.

Scientists believe that over fifty million years ago geological processes taking place under the oceans’ surfaces changed all of this. It was at this time that two of the Earth’s oceanic plates, the Pacific Plate and the Caribbean Plate, slowly started to move together. The collision of the plates resulted in a lifting of the seafloor until eventually some parts of the seafloor rose above sea level. The buildup of pressure and heat from the collision caused the formation and continual eruptions of underwater volcanoes

Around 15 million years ago, the effects of the volcanic explosions and rising seafloor led to the emergence of the first islands between the two continents. The islands continued to rise to the ocean’s surface until, about nine million years ago, Central America started to resemble an archipelago, or a chain of islands. Paleontologists believe that the formation of the Central American island system allowed some of the larger mammals of the time to begin a species exchange between the two continents. Smaller mammals, called “island hoppers” had already made it to South America from Australia and Asia, during the 30 million years of the continent’s geographic isolation. Scientists believe it was perhaps their ability to swim that allowed them to migrate. These early explorers include the primates which gave rise to the New World monkeys, such as howler monkeys, marmosets, capuchins, woolly monkeys and spider monkeys and rodents such as the capybara, the agouti, the coypu, the cavy (Guinea pig) and the chinchilla.

The archipelago enabled these first explorers to walk most of the way and swim the short stretches of water in between. Gomphotheres, extinct relatives of the elephant, reached South America from North America 10 million years ago. Tapirs and camels made the journey a few million years later. South America’s giant ground sloths, or Megatherium, reached North America around 8 million years ago. The giant ground sloth, which reached up to 29 feet tall when standing upright on its hind legs, was the largest and heaviest of all land mammals but was also a good swimmer. Scientists estimate the creature swam about 40 miles to reach the new continent during its migration.

Over the next few million years, sediment from North and South America settled between the islands, filling in the gaps between the land masses with mud, sand and silt. When the gaps were filled, about 3 million years ago, the Isthmus of Panama was complete. This sliver of land would bring about monumental change, not only in the biodiversity of the flora and fauna of each continent, but by blocking the passage of water between the Atlantic and Pacific oceans, the bridge would help establish the global ocean circulation pattern and change the Earth’s climate.

The Isthmus became a land bridge that led the Great American Biotic Interchange, a mass migration of terrestrial species between the Americas. The species exchange worked in both directions. A few species that had evolved in South America migrated north. The anteater, porcupine, opossum and armadillo eventually became established in both North and South America.

The migration may have involved both continents, but the effects wouldn’t be felt evenly. Though some of the new arrivals to South America such as horses survived only for a brief period the majority of migrants from the north were quite successful. Deer, camels, raccoons, tapirs, mastodons, bears, peccaries, rabbits, shrews, cats, dogs, weasels and rodents, all invaders from the North, did well on the new continent, displacing many of the native South American species and driving many of these to extinction. In fact, the camel family has given rise to the very successful and widespread vicunas, guanacos, alpacas and llamas of South America. And some scientists suggest that climate change, rather than species shortcomings, may have been responsible for the eventual extinction of the horse in South America.

Current evolutionary theory has considered why the invasion of South America by North American animals was a disaster for many of the animals there. Scientists suggest that the mammals of South America had been evolving independently and with little competition for millions of years, in their relatively isolated continent. Paleontologists believe the North American species prevailed because they originated from a bigger landmass, once linked with Eurasia and Africa, and so many more evolutionary lines could be developed and tested in many more ecological niches. They suggest that South American animals were insular, and so, inferior survival-wise to their North American counterparts.

Though the formation of the Panamanian land bridge and the introduction of exotic species would drive some species to extinction, it would be humans who would ultimately exert the most disastrous effects on animals. When and how humans spread throughout the Americas is a hotly debated topic in paleoanthropology. There are many theories but some scientific findings suggest that a race of humans did come over the Siberian land bridge around 12,000 years ago and spread across North America and southward into South America via the Panamanian land bridge near 10,000 years ago. Once they arrived, humans hunted Ice Age megafauna such as giant sloths, short-faced bears, mammoths, and saber-toothed cats. And eventually, according to archeological and fossil evidence, these ancient hunters would be responsible for their extinction.

Western yellow jackets, an invasive species, sting the camera lens during filming

Surrounded by ocean and formed by volcanic hot spots, Hawaii’s isolation and geological activity shape the biodiversity of the islands. With over 25,000 unique species, Hawaii is one of the most biologically diverse regions on the planet. A large percentage of these species are only found in the islands of Hawaii. While the number of species is impressive, these birds, insects, mammals, and plants live in a delicate balance.

As an isolated oceanic archipelago, the Hawaiian Islands offer insight into evolutionary processes that are unmatched in their beauty and complexity. Left to evolve on their own in the middle of the Pacific Ocean, Hawaiian species developed their own unique evolutionary traits. Each species’ survival depends on a host of other species, and when one species is lost the entire framework of Hawaiian ecology is disturbed. Unfortunately, Hawaii has the highest rate of extinction per square mile on Earth, and many endemic Hawaiian species are currently threatened or endangered.

Much of this crisis is a direct result of human colonization of the islands. Originally there were no mosquitoes or ticks, reptiles or amphibians, and the only mammals were the Hawaiian hoary bat and the Hawaiian monk seal. When humans came, they brought pigs, mice, lice, cockroaches, and a whole host of invasive plants. These invasive species have wreaked havoc on the Hawaiian ecosystem. To reach Earth’s most isolated archipelago, new species had to arrive either by air or by sea. Before humans stepped onto their shores, it is estimated that only about every 100,000 years did a new species glide down to the islands or wash up on the beach. This gave Hawaii’s ecosystem a lot of time to acclimate to each new species and incorporate each addition into the Hawaiian biome.

Today, it is estimated that new species are being introduced to Hawaii at a rate that is 2 million times more rapid than the natural rate. With international travel and the importing of goods, it is easier than ever for new species to be introduced. It is also more crucial than ever that new invasive species be kept off the islands. With so many native Hawaiian species endangered, and millions of dollars worth of crops at risk, the threat from invasive species is just too high. Besides rigorous screening at airports, scientists are taking new measures to ensure the survival of Hawaii’s vulnerable biodiversity.

The western yellow jackets (Vespula pensylvanica) seen in Kilauea: Mountain of Fire are not the only species of wasp reshaping Hawaiian flora and fauna. Since 2005, tiny orange wasps called Erythrina gall wasps have been decimating the wiliwili tree population, an endemic Hawaiian deciduous tree that grows up to 30 feet tall. Native to East Africa, the tiny wasps, about one-third the size of a mosquito, inject their eggs into the leaves of the wiliwili tree. The egg mass grows into a small lump or gall, which interferes with the tree’s ability to harvest sunlight and water. As a result the tree withers and dies. All over the islands of Hawaii, massive wiliwili carcasses dot the forests as well as the residential and agricultural areas. Besides providing habitat for Hawaiian wildlife, wiliwili trees have long been used as windbreaks. Planted in a straight line along the edges of crop fields, the wiliwilis protect the crops from wind damage. If the wiliwili trees die, so do the crops. It is predicted that if the gall wasp epidemic is not stopped, massive portions of Hawaiian forest will turn into desert.

To eradicate the gall wasps, scientists have no choice but to introduce another species of wasp to the Hawaiian islands: the Eurytoma wasp. These larger wasps inject their eggs into the Erythrina egg galls, where they develop more quickly than the smaller gall wasp species and hatch first. Once the Eurytoma’s hatch they feed on the unhatched Erythrina larvae. This keeps the gall wasp population numbers low, and will hopefully save the wiliwilis.

The first batch of 500 Eurytoma wasps were harvested in Tanzania near Mt. Kilimanjaro, and released into Honolulu’s Liliuokalani Botanical Gardens in November 2008. Scientists will have to keep a close eye on these foreign settlers. Hawaii has a long history of good intentions gone bad when it comes to introduced biological-control agents. The worst example is the mongoose, which was brought to the islands in 1883 by sugar cane farmers to control rat populations. Sadly, the mongooses primarily preyed upon Hawaiian birds and eggs, driving several species to near extinction and posing a major threat to endangered animals.

Another invader, the coqui frog from Puerto Rico, arrived in shipments of plants in the late 1980s. Featured in NATURE’s Animals Behaving Worse, male coqui frogs sound a loud mating call that annoys some of Hawaii’s human residents. But aside from being a nuisance to humans, some people worry that as the frogs advance throughout the islands, they may be setting the stage for other invasions. For example, frog-eating snakes, like the brown tree snake, could find a plentiful food source in the coquis. In Guam, the brown tree snake has done major damage, wiping out bird populations.

No one knows what will happen to Hawaiian biodiversity in the future, but perhaps hints can be found in the fiery nature of Hawaii itself. Every day, as fresh lava spills into the ocean, new land is formed—land that will someday be new habitat for Hawaiian plants and animals, both native and invasive. Just as the geography of Hawaii is always changing, so is the shape of life on these islands.