Historical Background and Location

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The Galapagos Islands lie on the equator just off the coast of Ecuador in South America. Comprised of 13 main islands and 48 smaller islands of more than 5 million years old, these land masses have been recognized as a site of great biodiversity and endemism. Known to be the place where Darwin created his famed theory of evolution and process of natural selection, these islands possess much ecological and historical value and have remained to be an environmental treasure to this day.                  

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The galapagos islands and its many ecosystems are unique in their geographic setting and ecological background. Historically these islands formed via volcanic activity that rose from the sea floor. These eruptions built underwater mountains that continued to grow until it formed the modern archipelago we see today. Each island is a single shield volcano, except Isabela, which is comprised of a total of six shield volcanoes. Today, the islands remain to be one of the most volcanically active sites on Earth. Since their formation, however, many of the islands have experienced great amounts of erosion, especially in places near the sea. In the arid lowlands in particular, however, the process is slow, because there is little soil present in those areas. The soil tends to remain amidst the volcanic cracks in the earth and is a reddish tint, in opposition to the darker soil found at higher altitudes. Historically, the climate of this ecosystem has remained fairly dry, due to the melting of the glacial ice sheets of North America that occurred nearly 2 million years ago. The dry, yet cool temperature has both had strong effects on the types of vegetation permitted to grow there, as well as the species that have been able to thrive.

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Due to the dry climate, the main vegetation of the arid lowland ecosystem in the galapagos are cacti. These plants are able to survive in severe weather conditions by storing water in their leaves and stems. The most common cactus present is the Prickly Pear Cactus (Optuntia Cactaceae), which produces yellow flowers and grows like a shrub. It is the main food source of the Galapagos Land Iquanas (Conolophus subcristatus) , a fauna that is also commonly seen in this ecosystem. Other types of flora in the region include the Lava Cactus (Brachycereus Nesioticus) which are small and grow in clumps, producing a white flower, and the Candelabra Cactus (Jasminocerus Thouarii) which have tube-shaped pads. 

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References: 

"Galapagos Cactus". Galapagos Online. 23 Nov. 2012.     <http://www.galapagosonline.com/>.

Jackson, Michael Hume. Galapagos: A Natural History Guide. University of Calgary Press, 1993. Google Books. 23 Nov. 2012.
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Neall, E. Vincent and Steven A Trewick; "The Age and Origin of the Pacific Islands: a geological   overview. vol. 363 no. 1508. Royal Society Publishing, 27 Oct. 2008. Web. 23 Nov. 2012. <http://rstb.royalsocietypublishing.org/>.