Elemental Galapagos: Birth of an Archipelago

Hidden beneath the waves, some 250km north of Puerto Ayora and at a depth of around 2,500m, lies the Galapagos Rift, an east-west cleft between the Cocos Plate to the north and the Nazca Plate to the south. As these two tectonic plates move apart, molten rock rises up from the Earth’s mantle to fill the space. Also out of sight, currently lurking beneath the Galapagos island of Fernandina, is a hotspot, a region of mantle so energetic that it can break through a tectonic plate.

It is thought that the Galapagos Rift and the hotspot may once have been much closer to each other and the complex interplay between these two geological forces built a broad submarine platform that rises well above the surrounding seafloor and less than 1,000m below the sea surface. Occasionally the bubbling, effusion and explosions of magma were so great that they rose above the waves, giving birth to the Galapagos Islands that we know and love.

The first lifeforms to take a hold on these freshly spewn landscapes of lava were, by necessity, some of the toughest organisms there are. Lichens are one of the best-studied of these so-called ‘pioneer species’, associations between fungi and photosynthetic microorganisms that are dispersed as tiny spores, can anchor themselves to bare rock, tolerate high temperatures and survive prolonged drought. While the photosynthetic microbes get on with capturing sunlight energy and making food for both partners, the fungi release acids onto the rock that make minerals available for their growth. This physical and chemical dissolution of the rock plays a vital ecological role, speeding up erosion and the formation of soils in which a plant might take root.

One of Charles Darwin’s many conundrums after his return from the Beagle voyage was to figure out how the first plants might have reached isolated barren volcanoes like Galapagos. At his home in Kent, he carried out experiments on almost 100 species to see if their seeds could survive in salt water. On potting them out into soil, almost all of them germinated, even after weeks in these hostile conditions. With the currents from the South American continent to Galapagos often running at 100km a day, it would take a floating seed or tangled mass of vegetation around one week to reach the Islands. Darwin also imagined an even quicker mode of travel, with birds transporting seeds in mud caked between their toes or secreted safe inside their digestive tracts.

Of course, if a seed that washes up on a Galapagos beach is to settle for good, it must be able to survive long periods of drought, which accounts for the kinds of plant that inhabit the arid lowlands of Galapagos. For instance, it’s not that surprising that the Opuntia cactus should have found a footing in this habitat; these species have a long tap root to reach deep down for water, fleshy pads in which to store water and modified leaves (we call them spines) to minimise evaporation of precious water back to the atmosphere. It also pays to be wind pollinated, which is the case for the vast majority of flowering plants in Galapagos. Insect-pollinated species have presumably reached the Islands and may even have germinated, but relying on some special insect to ferry pollen from one flower to the next is not a great strategy in a world with so few insects to assist. More often than not, these would-be colonisers left no descendants.

There are even more obvious absences in the makeup of the Galapagos ecosystem. There are no amphibians, for instance: for species with a fondness for freshwater, 1,000km of open ocean is just too big a leap. Most mammals were not equipped for the journey either: some ratty ancestors seem to have made it, giving us the rare Galapagos rice rat, and there are bats that have been blown across from the continent, but these are exceptional cases.

Those species that had a means to reach Galapagos and the fortuity to survive and then reproduce began to splinter. The many different habitats on each island and the different islands themselves gave plenty of opportunity for one population to become isolated from the next. Natural selection did the rest, steering secluded pockets of organisms in different directions until they became distinct species. For example, the first finches to have flown to Galapagos and survive long enough to reproduce did so around two million years ago, but these settlers have since diverged into the 17 different species we find today. The story is the same for most lineages that have been studied and explains why so many Galapagos species are unique to these islands. These so-called endemic species were, quite simply, made in Galapagos.

As species have come and gone and the make-up of Galapagos residents has evolved, so the Islands themselves have been changing. The Nazca Plate on which the Islands sit is drifting east at a speed of around 4cm a year and taking the Archipelago with it. In one million years, Galapagos is likely to have moved some 40km towards the continent and further from the hotspot that played such a key role in shaping the Islands. Away from the hotspot, the islands cool and contract, and erosion will eventually weather them down beneath the waves once more. This may not be the end of Galapagos, of course, for the hotspot may yet throw up more islands in their wake, steaming towers of undressed rock that will act as the stage for the origin of yet more brilliant species.