GALAPAGOS - Colonisation and Evolution

PART 3: COLONISATION

Traveling around the Galapagos and seeing the great spectrum of wildlife that exists within the archipelago, one begins to ask the question ‘where did it all come from?’

Even if one takes 600 miles as the minimum distance to travel from the mainland, it is still a great expanse of open water to cross. When the peaks of the volcanoes first reared their heads above the water they were initially devoid of life. Oceanic Islands can only obtain their biota by long distance dispersal, thus it is a means of transportation that is the main factor influencing the number of species. Discounting man’s influence, species can only arrive by water or air. Some animals arrived by means of their own propulsion i.e. swimming or flying, others may have been blown by strong winds, or were carried across the sea on rafts of vegetation – Tierras Caidas, such as we saw when approaching Panama (Photo in Journal June 1). Plant seeds or spores can be carried by both air and ocean currents, although most were probably brought by birds. The Galapagos Islands are well placed for receiving long-range immigrants – both ocean and air currents (Northeast and Southeast trade winds) move out into the Pacific from the South American mainland, thus the potential for carrying organisms is increased.

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Red Mangrove shoot growing on lava flow

Some of the first species to arrive may have been bacteria and the minute spores of plants such as algae, fungi, ferns, mosses and lichens. These plants not only have spores that are easily dispersed by the wind, they are also have the ability to grow without a lot of organic material. Such plants are known as ‘pioneers’ or ‘colonizers’ they not only survive, but also thrive is hostile conditions and eventually provide organic matter, or humus for higher plants. The seeds of larger plants are more likely to have arrived within the digestive systems of birds, or attached as part of natural rafts. Some seeds also stick to the feathers and wings of seabirds, and of course most coastal plants such as Mangroves, Salt Bush, Galapagos Cotton etc. have seeds that float and are salt tolerant.

Winged insects, and also small flightless insects, spiders and tiny land snails can be transported by winds forming a sort of ‘aerial plankton’, which eventually gets blown, on to land.

Seabirds would have had no problem reaching the Galapagos islands, since most are excellent long distance fliers, land birds and bats however, which are weaker fliers, were probably blown to the islands by strong winds. This is probably why land birds are so poorly represented on the islands in comparison to the mainland.

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Lava Cactus a pioneer plant endemic to Galapagos 
grows on young lava flows
.

Marine animals such as sea lions, fur seals, sea turtles and penguins are all good swimmers and probably made their way to the islands assisted by favorable currents. Also Giant Tortoises can float for a considerable length of time, and may likewise have been swept across the sea by ocean currents. Other land animals could only have arrived on natural rafts. Natural rafts may be anything from a single tree trunk to huge floating islands of vegetation broken loose from riverbanks and swept out to sea during floods.

Again the animals ability to survive a long sea passage, under the baking sun, without food or water would be the limiting factor in determining which species arrived alive.

For example, the islands have a large number of reptiles, which can fast for long periods, but no amphibians, which could certainly not have survived a long hot journey without fresh water. Likewise there are very few mammals that could survive such a long rigorous journey.

Having made the long journey across the open ocean, the species must become established. This is it, must find a suitable habitat where it can survive and flourish. The plant types most likely to survive are those with a high environmental tolerance, requiring little soil or moisture. Pollination is also critical for plants. Those plants with complex pollination mechanisms requiring a particular type of insect, for example, are missing from the Galapagos Islands, since it is unlikely that both plant and animal would be transported at the same time. Galapagos therefore has very few large flowering plants and orchids in comparison to the mainland.

Seabirds that nest on the ground and require very little vegetation for nest building are ideal candidates for colonizing oceanic islands. Once a species has found a suitable niche it must reproduce and create a stable population before it can be considered established. Arriving species obviously have greater chances of survival if their niche is not already occupied when it arrives. For species that reproduces sexually, both sexes must arrive within each others reproductive lifespan for successful breeding to occur. The chances are of course are a million to one, but given enough time, enough species will arrive, find their niche and become established.

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Blue-footed Boobies nest on the bare ground

 

PART 4: EVOLUTION

One can hardly visit the Galapagos Islands without touching on the subject of evolution. The Galapagos Islands have been made famous through the visit of a 26 year old university drop-out. When the young Charles Darwin arrived at the Galapagos islands in September of 1835 he had lead a less than auspicious career, dropping out of medical school at Edinburgh University and failing as a student of Theology. However his detailed and perceptive accounts of the natural world made during his voyage on the HMS Beagle lead us to believe that up until then he had not really found his ‘niche’.

Arriving at the Galapagos Islands Darwin immediately noticed that the island species had many similarities with those he had seen on the mainland, but that there were also marked differences. He realized that at some stage, species had traveled across between 600 and 1000miles of ocean, colonized the islands, and that the decedents of these species had somehow become modified to fit their own particular niche.

Natural Selection

The impressions he had gained matured over many years, eventually resulting in his seminal work ‘The Origin of Species’ published in 1859. In this Darwin put forward his theory of evolution by natural selection, or survival of the fittest. That is that members of a particular species with positive physical or behavioural traits would be more likely to survive and reproduce than others. Thus the positive characteristics would be passed on. The most important traits would be for example, strength, aggressiveness, fertility, pigmentation and intelligence etc. Species therefore become modified by the gradual accumulation of new or changed characteristics. These changes are now known to be the result of genetic inheritance and mutations.

On Galapagos changes probably occurred very quickly. The initial population of arrives would only have been very small, this immediately increases the chances of the population being not being representative to start with. Breeding enhances the traits within a small population, producing an immediate differentiation from the mainland population. Also any genetic mutations occurring would have a larger impact on a small population. This causes further differentiation, again passed on by breeding. Some animals have differentiated enough from their mainland counterparts to be classified as subspecies for example; Galapagos sea lions (a subspecies of the Californian Sea Lion) The Brown Pelican Pelicanus Occidentalis urinator (a subspecies of the Brown Pelican). These subspecies may be endemic (occurring only on within the Galapagos archipelago) such as the Galapagos Sea Lions, or indigenous such as the Brown Pelican ‘urinator’ (occurring on the Galapagos, but also found on the mainland, or other pacific islands).

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Blue Boobies mating, genetic traits and 
mutations are passed on by breeding

If isolation continues the population may differentiate, through natural selection, such that an entirely new endemic species is created, for example, the Galapagos Penguin, the Galapagos Fur Seal and the Flightless Cormorant, the Marine Iguana and Darwin’s Finches. Species that have become so specified that they are found nowhere else in the world.

Adaptive Radiation

One of the things that Darwin also noticed about the Galapagos Islands was the similarity of some species, that suggested that they obviously belonged to the same parent population, but that there were morphological distinctions that were obviously the result of the species adapting to the particular vegetation of an island for example. Darwin noticed this most with the Finches on Galapagos; in particular he noticed the ‘perfect gradation in the size of the beaks in the different species’, suggesting that ‘one species had been modified for different ends’ (Darwin, 1845). This process of modification is known as Adaptive Radiation. Darwin’s Finches are not the only example of this process the Giant Tortoises has also evolved 14 subspecies, mostly with differences in the shape of the shell. Saddle-back tortoises for example have shells that are elevated at the front end and a long neck, they can also stand on their hind legs allowing them to reach branches and cactus pads on arid islands with no grasses.

Endemism

Due to the problems of transportation and survival on the Galapagos Islands, there are relatively few species when compared to the mainland, however the combined affects of natural selection and adaptive radiation have resulted in a large percentage of animal and plant species found nowhere else in the world. These species are termed ‘endemic’. Of the land based animals 80% are endemic to the islands, of the 600 plants one third are endemic, and of the fish, about 20% of the inshore species are endemic.

The following is a list of species we have observed during our trip around the Galapagos Islands, showing whether they are endemic or indigenous.

Species Table Chart

References/Acknowledgements

Galapagos - Key Environments (1984) ( Contribution no. 355 of the Charles Darwin Foundation for the Galapagos Islands) R. Perry (ed.) Pergamon Press.
Humann, P (1993) Reef Fish Identification – Galapagos (Ned Deloach, Ed.) New World Publications Ltd.
Jackson MH (1993) Galapagos Natural History. University of Calgary Press.

Many thanks to Veronica Toral-Granda (M.Sc.); Marine Biologist at the Darwin Center for discussing many aspects of speciation with me, and for reviewing this text.

Dr Janet Sumner-Fromeyer