The Galapagos Islands
Arrival & Establishment
"[…] we can see why oceanic islands should be inhabited by few species, but of these many should be peculiar. We can see clearly why those animals which cannot cross wide spaces of ocean, as frogs and terrestrial mammals, should not inhabit oceanic islands; and why, on the other hand, new and peculiar species of bats, which can traverse the ocean, should so often be found on islands far distant from any continent. Such facts as the presence of peculiar species of bats, and the absence of all other mammals, on oceanic islands, are utterly inexplicable on the theory of independent acts of creation".
Charles Darwin,
Voyage of the Beagle, 1835.
Arrival to the Galapagos is no small feat whatever the mechanism; to then be able to survive and flourish meant many species had to adapt to cope with the conditions in which they found themselves. In order to establish themselves as a viable species many had to evolve.
Darwin wrote, "The archipelago is a little world within itself, or rather a satellite attached to America, whence it has derived a few stray colonists, and has received the general character of its indigenous productions. Considering the small size of these islands we feel the more astonished at the number of their aboriginal beings, and at their confined range".
The flora and fauna of the Galapagos Islands are therefore considered a disharmonic population. In simple terms this means that the numbers and kinds of species of plants and animals found on the islands are not representative of those found on the nearest land mass, that of mainland South America. The reasons for this difference are twofold.
Firstly one has to take into account the distance that separates the islands from the mainland and the huge barrier that 600 miles, 1000 km. of open ocean represents to any species attempting to cross it. Reaching the islands may be termed arrival.
Secondly, one has to consider the conditions that face each new species upon their arrival to these distant shores. Frequently conditions are very different to those from whence they came, and therefore present a whole series of new challenges and problems to that species, if it is to survive. Overcoming these problems, and breeding successfully in this new environment is termed establishment.
There exist two ways of arriving in the Galapagos Islands, active and passive. A bird that flies to the islands, or a fish that swims there is arriving actively. A seed that is blown by the wind, floats upon ocean currents, or is carried in the intestines of a bird that flies from the mainland, is arriving passively. All plants in the Galapagos arrived by passive means. Some animals arrived by passive means but others by active ones.
Many species of plants and animals cannot withstand being immersed in seawater for long periods of time. For this reason, there are very few native or endemic mammals in the Galapagos. (By endemic, we refer to those plants or animals that are only found or only reproduce in the Islands and nowhere else). In fact with the exception of the bats and rice rats all of the land mammals in the islands, such as cats, dogs, goats, pigs, donkeys, cattle and horses have been introduced by man. For this reason they are referred to as introduced animals.
Reptiles, on the other hand, such as the giant tortoises, iguanas, snakes, lizards, and geckos are able to withstand the difficult conditions that such a sea crossing involves. Their ability to survive for long periods of time, weeks or months, without fresh water means that not only could they make the crossing but could then continue to survive on land. For those species landing on an arid island, this would mean the difference between life and death. Whereas giant tortoises may have arrived by floating on ocean currents the smaller reptiles probably "hitched" a ride on rafts of floating vegetation, that were washed down river and into the open ocean in floods during the rainy season on the mainland. Other species face different obstacles. Parrots although common on mainland South America, are not found in the Galapagos. Even if they could fly to the islands it is unlikely that they could survive due to the lack of fruiting trees found there. The same goes for the hummingbirds. The flowers they depend upon for their nectar do not occur and therefore they would find no food. Nor could these flowering plants survive as many of them depend in turn upon the hummingbird for their pollination.
In general terms the more specific a plant or animal is, in other words the less adaptable to brusque changes a species is, the less likely it is to survive in a remote ocean island ecosystem such as the Galapagos.
Many species of plants and animals that managed to reach the islands were unable to make these changes to adapt to the islands’ environment and consequently perished. Those that did survive however have since evolved into the unique forms of flora and fauna that we see today in the Galapagos Archipelago.
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