The first settlers in the Galapagos that arrived to the islands played an intense game of fate and luck. Surviving under the hot equatorial sun without food or fresh water for up to weeks at a time was certainly not for everyone. However, when they got to the islands, neither was there a five-star hotel waiting. The introduction of life to these barren volcanic islands was a slow and extremely complex process, as it didn’t just have to arrive, but also find the appropriate habitat to survive and reproduce in a land made from molten lava and ash.
The arrivals of first settlers in the Galapagos
Today, the Galapagos Islands have a multitude of ecosystems that contain some of the highest levels of endemism in the world; however this was not always the case. The Archipelago originally formed from the build-up of molten lava pouring out of the earth, thus it contained nothing more than volcanic rocks and was entirely void of living organisms.
Bacteria and small plant spores were most likely the first settlers in the Galapagos to arrive to the islands and actually survive. From there, simple plants known as pioneer plants (algae, lichen and ferns) that arrived to the islands via birds or wind currents were able to take hold of the minimal nutrients present, slowly giving way to a richer habitat. Pioneer plants are typically the first plants to grow in a new habitat, as they are able to produce their own food through photosynthesis and require only limited amounts of soil (or none at all). More importantly, pioneer plants leave a richer environment in their place by increasing soil fertility through their organic matter and survival processes. An excellent example is lichen, which can grow on rocks, weakening the rock and eventually breaking it down into soil. On the other hand, some plants introduce nitrogen to the surrounding soil through nitrogen fixation. The lava cactus is one of the most common pioneer plants in the archipelago and is endemic to the Galapagos.
Over time, which may be decades or even centuries, the environment becomes richer and a more diverse array of plants begins to take seed on the islands, further increasing the fertility of the soil and forming distinct habitat zones. Slowly herbivores such as tortoises and iguanas begin to arrive to the islands, followed by the most demanding organism of all, the carnivore.
Adaptive Radiation and Diversification
Despite the relatively young age of the Galapagos Islands, the imbalance of fauna (ex. the number of mammals vs. reptiles) and its isolation from the mainland, the archipelago is certainly not short on species. Furthermore, there is a high rate of endemism among every species in the islands, even lichens. Just to put this into perspective, 20 of the 22* reptiles, 20% of inshore marine species, 80% of land-based animals and one third of the approximately 600 land plants are endemic, including the giant Galapagos tortoise, the scalesia tree and the flightless cormorant.
The extensive speciation processes that have occurred (and still occur) in the Galapagos Islands are a result of adaptive radiation, natural selection, and the archipelago’s unique location along the equator and at the conflux of three major ocean currents. The distinct ocean currents bring varying water temperatures and nutrients to the islands creating a diverse array of habitat zones, from terrestrial to marine, humid to arid. These habitat zones are able to divide a population as it grows and spreads, in turn provoking evolutionary processes.
If a population is split into different niches as a result of population growth or some other factor, it is able to evolve over just a few generations through a process known as adaptive radiation. (This is most famously displayed through Darwin finches, in which the original finch evolved into 13 or 14 species). Random variations occur in individuals of a population and those traits that are most efficiently adapted to the niche will have the greatest reproductive success, and the trait will proliferate throughout the population over time through natural selection. In this way, a vast multitude of species has formed from the precious few that arrived to the islands.
This process is not unique to the Galapagos Islands, but because of the limited number of predators and external influences (before the arrival of humans), and the generous amount of space in the archipelago, each organism was free to evolve to each niche as appropriate. Furthermore, the islands are relatively young and some species are still in the initial phases of speciation, making the Galapagos a remarkable laboratory for studying the fascinating processes of evolution.