The extensive biodiversity of the Galapagos Islands is in part due to the natural physical barriers that exist between the islands; however, it is also a result of the wide variety of habitat zones that exist in the islands, particularly the larger ones. While the shores of the islands are typically rocky and barren, the highlands are extremely luscious and moist, and separating the two is a variety of transition habitat zones.
The islands of the Galapagos Archipelago are located at the conflux of three distinct ocean currents that reach a varying range of altitudes. These two variables result in several ecosystems within the archipelago because, in accordance with the elevation and/or temperature of the surrounding water, so varies the climate and consequently the plant and animal life. These different habitat zones can be difficult to delineate due to their variety and the transition vegetation that exists in between; nonetheless, at least three central habitat zones are typically defined: shore, arid and humid.
The most prominent habitat zone in the Galapagos is the stretch of salt-ridden land that borders each island, known as the shore or littoral zone. This zone can range from beautiful white-sand beaches to rocky brown cliffs, but is generally characterized by a lack of fresh water and an abundance of salt, due its proximity to the ocean. High salt levels can make life particularly difficult for plants, so shore vegetation must be very hardy. Not much more than shrubs, herbaceous plants and mangroves are found in this zone. Nevertheless, life always finds a way to survive, and the shore is typically full of geckos, lava lizards and sally lightfoot crabs. The marine iguana also frequents the shores of the Galapagos Islands, along with the sea lion, which are so numerous visitors have to be careful not to stumble across them. A particular attraction that this zone holds is the occasional saltwater lagoon. For example, on Santa Cruz and Rabida islands, a wide variety of birds, such as the Galapagos Flamingo, can be observed wading in these briny pools and feeding on the nutrients they contain.
What most visitors to the islands are particularly surprised by is the arid landscape that pervades much of the islands. The arid zone is the most extensive of any other habitat zone in the archipelago and is generally located at elevations of between 80 and 200 meters above sea level; therefore, this zone is located on all of the main islands. Furthermore, on some of the smaller islands, such as Pinzon, in which the volcanoes do not reach high enough to break the cloud cover, the arid zone covers almost the entire island.
Similar to the shore zone, life in the arid zone has adapted to survive with minimal fresh water. Vegetation is characterized by cacti, herbs, small deciduous trees, shrubs and xerophytic species such as Palo Santo and Palo Verde. The latter species are particularly well-adapted to the dry conditions. In fact, they are actually able to shut down their metabolic system in order to conserve water and energy. On the other hand, several types of vegetation have small leaves and deep roots to acquire and conserve water sources. Additionally, they are typically well spaced out in order to avoid competition for fresh water.
Visitors to the islands particularly enjoy the wide variety of birds that can be seen in this zone, especially during nesting season. A number of land birds live in the arid zone and several species of marine birds use this zone as a nesting site, including blue-footed boobies and storm petrals. Thus, small fluffy chicks are often seen wandering around, not too far from their nests.
However, for those that arrive to the tropics in search of, well, the tropics, do not despair. Once you get beyond 300 meters above sea level, the rocks and cacti begin to turn into bright green ferns and luscious forests, known as the humid zone. As the peaks of the volcanoes reach into the low cloud clover, moisture condenses resulting in rich vegetation and bird life year round. Moisture levels are maintained even during the dry season when the infamous Galapagos garua fog hovers over the island.
The first 300-600 meters AMSL of this zone are dominated by Scalesia forests, which can reach up to 20 meters (60 feet) high. Beyond this is the Miconia zone, where shrubs replace the tall trees (only San Cristobal and Santa Cruz islands reach elevations high enough to host this). Above this (900+ m), is the rarest and wettest zone in the archipelago, known as the pampa. Decorated with beautiful ferns, grasses and mosses, this zone is particularly famed for the spectacular 11 species of orchid and the endemic tree ferns, which reach up to three meters (9 feet) high.