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The Galapagos Islands

ENVIRONMENTAL SETTING

  • The Galapagos Archipelago has 13 large islands (greater than 10 km2), 6 smaller islands, and over 40 islets.
  • The Archipelago is well isolated from other land masses (South America, 960 km, Cocos Island 720 km, Central America).
  • The total land area is within a marine reserve of 45,000 square kilometers.

GEOLOGY

The islands are located on the NAZCA Plate, close to its junction with the Cocos Plate. As a result of the spreading of the sea floor (the movement of plates in relation to each other) along the Galapagos Rift and the East Pacific Rise, the islands are moving southest at a rate of more than 7 cm/yr., which may not seem fast but would, over a million years or so, amount to 70 km of movement!

The evidence that the plate on which the islands sit is moving eastward is that the oldest islands are in the eastern part of the archipelago. The islands furthnest west is where the most recent volcanic activity has ocurred. The “Hot Spot Theory” states that in certain places around the earth, there are more or less stationary areas of intense heat in the mantle. These hot spots cause the crust to melt in certain places and give rise to volcanoes.

The Galapagos and Hawaiian Islands have mild volcanic eruptions where volcanic material comes out gently to form large lava flows.

The result is that the major Galapagos volcanoes tend to have a smooth shield –shaped outlines with rounded tops, rather than cones – like Mt. Fuji in Japan, which was founded by explosive eruptions.

MARINE CURRENTS AND WEATHER

Despite their tropical location, cold waters from the southeast, are carried by the Humboldt Current around the islands. The Galapagos has two main seasons, each of which has an effect on the flora and fauna; the warm and wet season from January to May and the cool and dry season (garua), from June to December.

January to May: The southeast trade wind diminishes in strength and warmer water from the Panama Basin flow through the islands. The average sea temperature rises to 25 °C (77 °F). Warmer water causes the cool season inversion layer to break up, and Galapagos experience a tropical climate with blue skies and occasionally heavy showers.

In some years, the flow of warm water is much greater than normal, and we have the presence of “El Niño”.

Surface water temperatures are higher and rainfall increases greatly.

On land life blossoms, but seabirds and sea life, may experience dramatic breeding failures because they depend on the more productive cooler waters.

June to December: During the garua season, cooler water from the Humboldt Current is driven to the Galapagos by the southeast trade wind, with an average sea temperature of 14 °C (71 °F). As a result, there is warm tropical air passing over cool water. The moisture evaporating from the sea is concentrated in an inversion layer (300 to 600 m above sea level), and the higher parts of the islands, which intercept this layer, receive precipitation in the form of garua (mist rain), while the lowland areas remain dry and cool.

ARRIVAL AND ESTABLISHMENT

When the tips of the Galapagos volcanoes first appeared above the sea’s surface some three to five million years ago they were devoid of life. The ancestors of every plant and animal species native on the islands must have arrived there from some other part of the world. We will never know exactly how colonization occurred, such events do not leave records for us, but we may guess about what probably happened.

One thousand kilometers of ocean separate the Galapagos from the mainland of Ecuador. Despite this barrier, a large number of species arrived to the islands. Oceanic volcanic islands such as the Galapagos, differ from continental islands in that they never had contact with continental landmasses. Any plant or animal now native on the Galapagos must have originally dispersed through the islands. If organisms survived the hazardous journey and were able to survive in the unfamiliar environment, and there were enough to reproduce successfully, a colonizing population would exist.

The question that once perplexed biologists was how it was possible for so many venturesome vagabond species to survive such a long journey to the islands, when many others would surely have perished at the touch of seawater.

Exceptional hardships must have been overcome. Nonetheless, close scrutiny of the original flora and fauna of remote islands suggests that they were indeed derived by chance from weedy colonists from the mainland, which has been called “sweepstakes dispersal”.

Flotation rafts made of vegetation or debris, and even winds and jet streams could be some mechanisms of transport for living organisms or seeds to the newly formed islands. Birds displaced from their traditional migratory routes, or seeds and invertebrates hitchhiking on the feathers and feet of aquatic and semi aquatic birds could be named as the process of colonization. Of course, species are present in proportion not only to their capacity to disperse, whether actively or passively, but also to their ability to establish themselves after arrival. The need for an appropriate mate in sexually reproducing animals, or a compatible pollinator in out-crossing plants, poses a formidable challenge to long-term establishment.

The idea that specific groups of organisms have different hurdle values which determinate the limits of their dispersal, is fundamental to the concept of disharmony in the living organisms of oceanic islands. Disharmonic floras and faunas are characterized by the absence of conventional groups such as large carnivorous and heave mammals, amphibians, freshwater fishes and large-seeded forest trees.

EVOLUTION

The Galapagos Islands have often been called “a laboratory of evolution”. There are few places in the world where it is possible to find such a variety of species, both plant and animal, which shows so many degrees of evolutionary changes, in such a restricted area. Once organisms reach oceanic islands they are essentially isolated from other landmasses.

If the islands are distant enough from a source to make colonization, then they may be thought of as almost independent biological units. Oceanic islands can have species, which though related to mainland forms, have evolved in different ways from their mainland relatives as a result of their isolation in a different environment. This is a key factor in result of their isolation in a different environment, and in island evolution. It is not surprising that Charles Darwin was so struck by the life he found on these islands. Formulated by Darwin, Natural Selection is the process by which propagation becomes to change, and species diverge from one another.

A classic example of adaptive radiation in birds, which has served generations of evolutionary biologists, is Darwin’s finches. A total of 13 species evolved within the Galapagos’ archipelago from a common ancestor whose founding type and source from the American continent have not been identified yet. A single fourteenth species occurs on Cocos Island off of Costa Rica, about five hundred miles northeast of the Galapagos.

That all the finches are closely related, and presumably evolved from the same progenitor stock, is indicated by a complement of characteristic common to all. The word endemic refers to organisms, which are found nowhere else in the world due to the fact that they evolved and remained isolated on a given area and therefore developed unique characteristics. In the Galapagos you will find several species that fall into this classification.

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