Why the Galapagos Islands are Unique
Evolutionary Biologists are fascinated by island ecosystems and the clarity with which the species that inhabit them illustrate evolutionary processes. For this reason, as well as a world-changing historic visit from a man named Charles Darwin, the Galapagos Islands are quite arguably the most studied archipelago in the world.
The Galapagos Islands also have a unique set of environmental conditions that set them apart from all other island groups in the world. Their sunny equatorial position on the globe combined with their location amid the cool Humboldt and Cromwell ocean currents allows these special islands to display a strange mix of both tropical and temperate environments, which is reflected in the complex and unusual plants and animals that inhabit them.
Five to ten million years ago, the tops of underwater Galapagos volcanoes appeared above water for the first time about 600 km from mainland Ecuador in the middle of the Pacific Ocean. Those volcanic peaks were completely devoid of plant and animal life. All plants and animals that are now native to the islands must have arrived to the islands originally through some form of long-distance dispersal.
When considering the diversity of species that do inhabit the Galapagos Islands, it is important to note how “unbalanced,” in comparison to continental species diversity, the variety of Galapagos species are. For instance, there are many native reptile species, but no amphibians; there is an abundance of land and sea bird species, but very few mammals. When considering plants, those with large flowers and big seeds are absent while grasses and ferns abound.
There are two main ways for species to make their way to remote islands (aside from any methods involving humans). The first method is by air in the form of flying or being blown by wind, and the second method is by sea while swimming or floating, sometimes with the aid of rafts of tangled vegetation.
It is likely that the ancestors of present-day Galapagos animals that are good swimmers (sea lions, sea turtles, penguins) actually swam their way to the islands with the help of some swift ocean currents. On the other hand, it is believed that many of the reptiles and small mammals (rice rats) were carried to the islands from the South or Central American mainland on rafts of vegetation. The vast majority of such rafts would have sunk well before they ever reached Galapagos, but it would have only taken a handful of successful rafts to wash ashore to explain the present reptile diversity in Galapagos. This “raft” theory of arrival also explains why there are no native amphibians, few mammals, and many reptiles in the Galapagos Islands – reptiles are the best adapted to deal with the harsh salty and sunny conditions of weeks at sea.
Coastal plants, such as the mangroves and saltbushes of Galapagos, have seeds that are salt tolerant, and those seeds are, therefore, likely to have arrived by sea as well.
Wind is thought to have played a major role in transporting spores of the lower-form plants, such as ferns, mosses, and lichens, to the Galapagos Islands. Vascular plants with heavier seeds are quite scarce in Galapagos because those seeds would have had a more difficult time traveling by wind — with the exception of those plants with plumed seeds designed exactly for wind transport. This explains why members of the dandelion family (Compositae) are found throughout Galapagos.
Many small insects, and even tiny snails, could have easily been blown by the breeze. The weaker-flying land birds and bats (2 species) likely arrived with the help of the wind. However, land bird species in Galapagos represent only a tiny fraction of those living on the mainland, and this is because it would have been a very difficult journey for the few who did make it.
Sea birds, generally excellent fliers over long distances, simply flew their way to the islands. Birds likely brought with them hitch-hiking plant seeds or propagules that were attached to their feathers or feet, or even in their guts.
The mere arrival of an organism to the Galapagos Islands is just one piece of the early survival puzzle. Organisms also had to be able to establish themselves once there, and, most importantly, to go on to reproduce. Scientists can only guess that many plant seeds accidentally made their way to Galapagos, were deposited in an unfavorable area, and perished soon after arrival. Not surprisingly, those plant species that were most successful at colonizing the Galapagos Islands were those of the “weedy” variety with wide tolerances for varying environmental conditions.
One more problem facing new plant colonizers to the Galapagos Islands was pollination – many plants rely on insects or animals for pollination, and the chance of both a plant and its pollinator arriving to the islands together was unlikely. This can explain why there are so few showy flowering plants, which mostly require animal pollinators, but there are many wind-pollinated plants in the islands.
Quite simply, because animals are mobile, they have always had an advantage over plants in that they could move to more favorable areas on the islands, if such areas existed for them.
Arrival of Species to the Galapagos Islands TODAY
In the last few centuries, humans have taken the place of birds as the primary source of new introductions of plants and animals to the Galapagos Islands. Unfortunately, many of the human introductions have been detrimental to previously established native or endemic wildlife – for example, harmful species such as fire ants, goats, and blackberry have all caused great harm to one or more of Galapagos’ iconic long-established pioneering species.
(Note: Much of the information above was gathered from Galapagos: A Natural History by Michael H. Jackson.)
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