Above: Darwin's finches or Galapagos finches. Drawing by John Gould published in Darwin's Journal of researches into the natural history and geology of the countries visited during the voyage of H.M.S. Beagle round the world, under the Command of Capt. Fitz Roy, R.N. (1885).
When I visited New Zealand a few years ago, I had to make sure I didn’t bring any food or seeds into the country. I also had to declare that the shoes I was wearing hadn’t been on any forest hikes recently. Why were the Kiwis so interested in how I spent my free time? It makes sense when you remember that New Zealand is an island. And islands usually have very unique ecosystems!
Continental and oceanic islands
Did you know? Island species often exhibit gigantism or dwarfism. Species that are usually smaller on the mainland, such as komodo dragons and giant tortoises, evolved to become larger. The reverse is true for species like the extinct pygmy mammoth. There are two types of islands. Continental islands, such as the main islands of New Zealand, were previously attached to the mainland, but are no longer connected due to rising sea levels, tectonic activity, or erosion. Oceanic islands, on the other hand, are typically a result of volcanic activity and arose in the deeper ocean far away from any mainland.
Oceanic islands like Hawaii or Mauritius are home to fewer species than continental islands, but many of the species that do live there are endemic. That means they can’t be found anywhere else in the world. In fact, almost a third of all biodiversity hotspots—areas with an extremely high number of different species—are islands! For instance, Madagascar is home to more than 8000 endemic species and 90% of all Hawaiian species are endemic.
When continental islands get cut off from the mainland, some species stay behind on them. But when oceanic islands emerge, they are newly colonized by species that arrive through long-distance dispersal by air or water. Some animals will actively fly or swim to their new habitat, while some plant seeds or even small insects might be passively blown by the wind. Another option for colonizers such as mammals is to “hitch a ride” on natural rafts, such as floating branches. Furthermore, some seeds can be carried over long distances in birds’ stomachs and guts, while others arrive in birds' the feathers or in the mud on their feet. The further the island is away from the mainland and the smaller its size, the fewer species it can attract and accommodate.
Adapting to a new ecosystem
Did you know? Because there are often fewer predators and stronger winds, many birds and insects living on islands evolved to become flightless, such as the dodo on Mauritius.When a new species arrives on an island, it may be faced with a very different ecosystem compared to where it came from. This can result in some species thriving particularly well in an island ecosystem. For example, some animals may find they don’t have any predators on the island and some plants may no longer have herbivores to keep their growth in check.
Colonizing species may gain access to empty niches—food sources or habitats—that were not previously used by any other animals or plants, meaning they can occupy those niches without any competition. Occupying different niches can also lead to so-called adaptive radiation. For example, some individuals of a newly-arrived bird species might feed on seeds, and others on insects. Over time, the seed eaters evolve to have a shorter beak, whereas the insect eaters’ beaks get sharper and thinner.
Over thousands or millions of years this specialisation can lead to the evolution of new species. This is exactly what happened to the famous Darwin’s finches on the Galapagos Islands: one group of birds fed on seeds, another on insects, and another on cacti. As a result, their beaks evolved to have different shapes better adapted to what each of them was eating. Since these new species evolved on an island, it is clear why they are endemic to that ecosystem.
Did you know? Golden lancehead vipers have evolved to become one of the most venomous snake species in the world. They live on an isolated island off the coast of Brazil, where they need to be able to kill birds very quickly. While biodiversity hotspots are wonderful, many of them are also threatened. Islands are usually small and the species that live on them tend to have a low genetic variability. This makes islands species particularly vulnerable to extinction. The main threats to island ecosystems are habitat destruction, rising sea levels and extreme temperatures due to global warming, and invasive species.
Island species are not always adapted to combat predators or aggressive competitors that arrive from elsewhere. In this way, new species that colonize fragile island ecosystem threaten native species either by preying on them, competing with them, or introducing diseases. For example, the brown tree snake, which was accidentally brought to the Pacific island of Guam around 1950, ended up eating most of island’s native birds.
No wonder New Zealand checks visitors so thoroughly before they set foot in the country’s delicate island ecosystem. In fact, prior to human colonization, this island country was only inhabited by three species of land mammal (all bats)! When people started to arrive, they brought animals such as rats, cats, and foxes that were responsible for the extinction of many bird species. Some fast-growing weeds such as a freshwater plant called Lagarosiphon also caused problems by replacing native vegetation and preventing some species of fish from reaching their spawning areas.
Considering that nowadays species are going extinct at a faster rate than the dinosaurs did, it is very important to conserve islands as biodiversity hotspots. So be sure to clean your soil- and spore-covered shoes before leaving on a vacation!