Of all of the native species of Galapagos, giant tortoises were the most devastated after the endemic rice rats, with the majority of rice rat species now extinct. The front of the carapace angles upward, allowing the tortoise to extend its head higher to reach the higher vegetation, such as cactus pads. Saddle-backed shells evolved on the arid islands in response to the lack of available food during drought. Domed tortoises tend to be much larger in size and do not have the upward thrust to the front of their carapace they live on the larger, higher islands with humid highlands where forage is generally abundant and easily available. Colonizing the eastern-most islands of Española and San Cristóbal first, they then dispersed throughout the archipelago, eventually establishing at least 15 separate populations on ten of the largest Galapagos Islands.Īlthough there is a great amount of variation in size and shape among Galapagos tortoises, two main morphological forms exist - the domed carapace (similar to their ancestral form) and the saddle-backed carapace. They were already large animals before arriving in Galapagos. Scientists believe the first tortoises arrived to Galapagos 2–3 million years ago by drifting 600 miles from the South American coast on vegetation rafts or on their own. The closest living relative of the Galapagos giant tortoise is the small Chaco tortoise from South America, although it is not a direct ancestor. The Galapagos Islands were named for their giant tortoises the old Spanish word galapago meant saddle, a term early explorers used for the tortoises due to the shape of their shells. While giant tortoises once thrived on most of the continents of the world, the Galapagos tortoises now represent one of the remaining two groups of giant tortoises in the entire world - the other group living on Aldabra Atoll in the Indian Ocean. The giant tortoises of Galapagos are among the most famous of the unique fauna of the Islands. The majority of islands in the archipelago do not rise in elevation above the arid zone. The humid zone is only found on the larger, higher islands. In portions of this zone, Scalesia trees form a very dense forest in the humid zone, with their branches adorned with mosses, liverworts, and epiphytes - non-parasitic plants that use larger trees only for support. Located above the dry zones are the very lush and green, humid zones.The dry area is the most extensive zone in Galapagos and is comprised of plant species that are highly adapted to drought-like conditions, such as succulent cacti and leafless shrubs that flower and grow leaves only in the brief rainy season.They also provide much needed shade regions for iguanas and sea lions, as well as refuges for sea turtles. Mangrove trees are one of the most common plants found in this zone, and they serve an important role as the breeding sites for many birds, such as pelicans and frigatebirds. Coastal plants are found in the narrow zone near the shore and are distinctive because of their tolerance to salty conditions.The flora of Galapagos can be grouped into three major vegetation zones: the coastal zone, the arid zone, and the humid highlands. It is worthy of note that more than 30% of native plant species found in Galapagos are endemic (not found anywhere else on earth). The discrepancy between species number on the Islands and the mainland highlights the fact that the Galapagos Islands are separated from the continent by a hostile saltwater barrier reducing the potential for arrival and, once a plant has arrived, establishment is difficult due to the harsh environment. Mainland Ecuador, on the other hand, has about 20,000 species. Three introduced plant species have been eradicated. More than 100 of the introduced species have become established in the wild, with many of them extremely invasive and of major concern. To date, the Islands are believed to be home to more than 600 native species of vascular plants and approximately 825 introduced species, the majority introduced by humans. Geologically speaking, the islands are young, and much of the island’s plant life reflects this many species seem to be in the midst of the evolutionary process, which makes classifying them a difficult task. The Galapagos Islands are situated in the Pacific Dry Belt, and in average years only the highest altitudes of the larger islands receive enough rainfall to support tropical plant life. In fact, the majority of the archipelago’s land area is covered by the brown and grey vegetation often found in deserts. Many visitors to Galapagos are surprised to be greeted by desert-like vegetation-most are expecting a continuation of the lush greenery they witnessed on mainland Ecuador.
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