Planet Earth (which is estimated to be 4.5 billion years old) is one thousand times older than Samoa. Yet, the five million or so years since these islands broke the surface of the Pacific have been plenty of time for myriads of new species to arrive and evolve into new forms. But how did animals and plants get here and how did new forms evolve here? This is the focus of this chapter.

When the Samoan islands first appeared, they would have been barren and inhospitable places of active volcanoes spewing lava. Far too hot and barren for life. However, when volcanic activity subsided and lava cooled and hardened, species would have started to arrive and some would have survived. Plants and animals arrived in a number of ways, including floating here on rafts of vegetation or in the water column, swimming here, flying here, being blown here during storms, even arriving in the stomachs of bats or birds or on the feet or in the fur or feathers of larger animals. Charles Darwin did an interesting experiment once: he scraped the soil off seabird legs and found seeds germinated from this soil!

Arriving in Samoa was not enough of course: species needed to find themselves in a suitable habitat for life and if they were not pregnant females, had to find a mate to reproduce with. Exceptions are species that can breed without mating, such as the “parthenogenic” geckos and many plants. Undoubtedly many plants and animals died and perhaps there were multiple attempts before some species were able to establish themselves. But 5 million years is plenty of time for multiple arrival events to result in success eventually!

The origins of most of Samoa’s biodiversity is in Southeast Asia and New Guinea (see Figure below). There is a general reduction in the number of marine and terrestrial species from west to east across the South Pacific. There are for example no native amphibians east of Fiji and no native terrestrial mammals east of the Cook Islands. The eastward reduction of biodiversity reflects several interacting factors. First of all, the ocean would be expected to filter out species that are not adept at crossing ocean gaps, such as amphibians (e.g. frogs) which cannot tolerate salt water.

Other key factors dictating the number of species on an isolated island group such as Samoa, is the distance from the major centres of evolution and distribution, such as Southeast Asia, island size and complexity, and age of the islands. Larger, more physically complex islands will have more niches or habitats for colonisation. Samoa has relatively large and rugged islands so has higher species diversity than many smaller, lower islands.

The closer an island is to a centre of evolution, the greater the opportunity that species from that area will have been able to colonise it. Samoa is only 1,100km east of the much older Fijian islands, therefore many of our species may have come from Fiji or from the Melanesian islands further west, through a process called “island hopping”. Such island hopping may have been much easier during ice ages when sea levels were 120 m lower than now, and many islands were joined, making ocean gaps much shorter.  Since most currents flow and winds blow from east to west throughout much of the year in the tropical South Pacific, the movement of species into the Pacific from the other direction may seem surprising, until you consider how few islands there are east of Easter island and how many there are to the west.

Once established and again relatively isolated, many of our species evolved into new forms. The degree of “endemism”, or the number of new species evolving in a place, is a product of isolation, local conditions and time. Samoa’s rugged topography with many unique micro-environments allows for isolation and the possibility of new forms being developed that are separated from, and can no longer breed with, other members of the original species. Given sufficient time, under this scenario new species can evolve which are, by definition “endemic” to the new island.  Samoa in turn is older than many island groups further east such as the Cook Islands and French Polynesia: so there has been more time for colonisation events to occur and species to evolve.