Galápagos Islands xeric scrub
The Galapagos archipelago is best known for its amazing array of endemic species. These plants, invertebrates and vertebrate species have evolved over time due to isolated island life. Endemism is high in all living organisms on these islands from lichens, flowering plants, insects and land snails to finches, giant tortoises, iguanas and rats. Darwin first studied the wildlife of the islands taking note of the evolution of the landbirds such as finches and mockingbirds that adapted to a specific niche in the available habitat. These islands are a natural, living laboratory with minimal human disturbance until recently. Only five of the islands are inhabited. These islands are some of the largest and thus contain some of the highest diversity. The variation in elevation, size, and distance between the islands has contributed greatly towards the species diversity and level of endemism on each island.
Location and General Description
The Galapagos Islands are part of Ecuador although they lie in the Pacific Ocean about 960 kilometers (km) to the west of mainland South America. This unique ecoregion encompasses the entire Galapagos archipelago including 13 islands larger than 10 square kilometers (km2), 19 larger than 1 km2, 42 islets smaller than 1 km2 and at least 26 emergent rocks. There are approximately 128 named islands in the archipelago with Isabela as the largest with 4,588 km2 reaching an elevation of 1,707 meters (m) and Darwin as the smallest with only 1.1 km2 and an elevation of 168 m. The total land area of the ecoregion sums to 7,882 km2, with 1336 km of coastline.
This archipelago is continually changing due to its volcanic past and present. Most of the islands represent the summit of a volcano, some of which are anchored at more than 3,000 m below on the Pacific Ocean floor. The western part of the archipelago contains the youngest islands, and experiences the most intense seismic activity. Eruptions and earthquakes are common, as are other characteristics such as crater lakes and parasitic vents flanked by lava flows marking the summits. Other landscape features include fumoroles, lava tubes, sulfur fields and a variety of pumice, ash and tuff ejected from the volcanoes. To geologists these islands are considered quite young, dating back 3-6 million years for the oldest islands in the eastern part of the archipelago and less than 1 million years for the younger islands in the west. Soils are also young leaving mainly basaltic remnants, as erosion and other natural processes break down lava rock.
The climate of this ecoregion varies slightly within the 430-km area encompassing the islands. The climate also varies as a result of the multiple water currents intersecting, which shift accentuating the two main seasons and small transition periods where either season can prevail. The Garúa (misty) or dry season lasting from about May to December results from the cool Humboldt Current. This season is characterized by cool temperatures, mist and drizzling rain, also known as "gaura" that accumulates around the highlands of larger islands. The hot and wet season prevails from about December to May, as a result of the influence of the warm Panama Current shifting to surround the islands. This season is characterized by heavier rainfall and warmer temperatures with March reaching as high as 30 °C with as much as 80 millimeters (mm) of rainfall. The coldest and driest month is August with temperatures as low as 18 °C and as little as 5 mm of rainfall. Annual rainfall statistics vary according to temperature, which declines 0.9 °C for every 100-m elevation, resulting in an average rainfall for the archipelago of 356 mm at sea level and 1092 mm at 200 m above sea level.
Every seven years, due to the interference of El Niño currents the cold water upwelling is altered. The upwelling of the cold, Humbolt Current provides nutrient-rich waters with plankton that are the basis for the Galapagos marine food chain. Birds, reptiles and mammals that rely on the marine environment for their sustenance suffer immensely. El Niño currents are much stronger on average every seven years over powering the cold upwelling. This causes the surface waters surrounding the Galapagos ecoregion to be warmer and much less productive than normal. The marine-dependent creatures suffer losses while the land populations increase, as well as the vegetation and rainfall. The upwellings of the cold currents are important to the sea-dependent species such as Galapagos penguins and flightless cormorant. These birds do not breed until the nutrient-rich currents come in, and on El Niño years many will not breed at all. On a good year however, when the rich current is providing plenty, they may have more than one chick.
Four of the islands are inhabited: Isabela, Santa Cruz, San Cristóbal and Floreana. A defense base is also located on Baltra Island resulting in a combined resident human population of about 17,000. Fresh water is a limiting factor for humans. Only one island, San Cristobal has a perennial water supply that can adequately sustain humans. Seasonal springs occur on Santa Cruz and Floreana. Brackish water is available on all the inhabited islands.
The Galapagos ecoregion is home to about 500 species of native vascular plants, more than 180 are endemic. The actual number of plant species is continually challenged due to the young geologic age of the islands, many plants seem in the process of evolving. This makes their taxonomic classification difficult for botanists. Lying in the Pacific dry belt most of the land in the archipelago is covered by arid, semi-desert xerophytic vegetation. Only a small percentage of the islands reach the higher elevations where more rainfall is concentrated, yielding a more lush, tropical vegetation types. The vegetation of this ecoregion breaks into gradients from sea level to the highest elevation with the progression from littoral zone, to arid zone, to humid zone. A transition zone exists between the arid and humid zones.
The littoral zone is a narrow stretch of arid lowland near the shore, which leaves only the possibility of growth by salt- and drought-tolerant vegetation in this area. Mangrove species such red mangrove (Rhizophora mangle), black mangrove (Avicennia germinans), white mangrove (Laguncularia racemosa) and button mangrove (Conocarpus erectus) grow along the calm lagoons. Common shrubs include Maytenus octogona and saltbush (Cryptocarpus pyriformis). Herbaceous plants that have also developed salt and drought tolerance growing near the shores are Sesuvium portulacastrum, the endemic S. edmonstonei, and beach morning glory (Ipomoea pes-caprae), which is very important as a dune stabilizer.
The arid zone, inland of the littoral zone, occupies the most land in the Galapagos ecoregion extending over some entire small islands possessing the most diversity. This zone is composed of drought-tolerant, xerophytic species of cacti, trees, shrubs and herbaceous plants. The average range of this zone extends from 80 to 200 meters (m) in elevation or more. The most well known tree is the palo santo (Bursera graveolons), however a smaller species B. malacophylla occurs only on Seymour, Baltra and Daphne islands. Other trees in this zone include the endemic guayabillo (Psidium galapageium) and paga paga (Pisonia floribunda). Cacti have adapted water storage capacities in order to survive in these arid zones. The cactus genera Jasminocereus and Brachycereus are endemic as well as all Galapagos' species of the genus Opuntia. Examples of these endemic cacti are the lava cactus (Brachycereus nesioticus) and prickly pear cacti (Opuntia echios and O. helleri). Woody shrubs compose the lower levels of the vegetative layers in this zone such as the endemic Castela galapageia, Lecocarpus pinnatifidus and other species of the genera Lecocarpus. Herbaceous plants have also adapted to this arid zone including several endemic varieties of Galapagos tomatoes (Lycopersicon cheesmanii), passion flower (Passiflora foetida), and species of the endemic gerneras Tiquilia and Chamaesyce.
The humid zone follows the arid zone on islands that reach elevations above 300 m, with a transition zone between the two. This transition zone exhibits a combination of vegetation types and species slowing the gradient from arid to humid species including Pisonis floribunda, Psidium galapageium and P. galapageium with Scalesia trees. Garua fogs maintain this humid zone during the dry season when they form during the night and last well into the next day depositing moisture and acting as a shield in these areas of high elevation. Within this zone separate phases of vegetation have developed. These are very small layers within the humid zone yet they are lush and tropical as a result of the higher moisture received. From 300-500 m elevation, the Scalesia spp. dominated layer becomes apparent, emerging from the transition zone, with trees reaching 5-15 m in height. There are twenty different forms of this genus, the plant kingdom's version of Darwin's finches, which are scattered across the islands that reach these higher elevations. Cat's claw (Zanthoxylum fagara) is also most abundant, mainly above the Scalesia but below the Micronia acting partially as a transition within the humid zone species. Trees are replaced by shrubs on Santa Cruz and San Cristóbal islands. Miconia robinsoniana standing between 3-4 m tall at 600-700 m above sea level dominates the landscape creating a separate layer. This endemic shrub was the most extensive on Santa Cruz Island but is now almost completely altered by agriculture and cattle grazing.
The highest elevations, above 900 m, are the pampa layers within the humid zone. Known for the ferns, sedges, grasses, Sphagnum mosses and 11 species of native orchids including Purthieva maculata. The pampa holds the most ferns species, although 90 different species grow throughout the Galapagos archipelago. Endemic tree ferns such as Cyanthea weatherbyana grow in collapsed lava tubes, potholes and other cracks and crevices left by the tectonic activities.
The Galapagos ecoregion holds many native endemic snails, plants, insects, reptiles, birds and mammals. Due to its young age and level of isolation associated with being an island archipelago, the ecoregion has low number of species. The ecoregion does however remain high on the list of areas for conservation due to high endemism and low disturbance. Most species here are endemic which is unusual even for the most species-rich ecoregions. The endemic concentration is elevated when subspecies are brought into consideration. Most endemic species in the Galapagos archipelago have at least two subspecies due to the divergence of the species to better adapt to each island.
Twenty of the 22 species of reptiles in the Galapagos are endemic. The giant tortoise (Geochelone elephantopus) is a good example of how the variety of endemic individuals rises greatly when incorporating subspecies, since it has 11 subspecies remaining of the original 14. These land tortoises have developed slightly different attributes due to their locations. Isabela Island holds five of these subspecies, one on each of its main volcanoes. The other remaining subspecies are on James, Santa Cruz, San Cristobal, Pinzon, Espanola and Pinta Islands. There are 3 species of snakes that are all endemic and each has 2-3 subspecies including Philodryas biserialis, Alsophis dorsalia and A. slevini. There are also 5 endemic species of geckos belonging to the genus Phyllodactylus, 7 endemic species of lava lizards belonging to the genus Tropidurus, 2 species of land iguanas include Conolophus palidus on Santa Fe Island only and C. subcristatus, and finally the marine iguana (Amblyrhnchus cristatus).
There are many bird species native to the Galapagos ecoregion; the two major groups include seabirds and landbirds. Seabirds still use the land for breeding, resting and raising their chicks, however they obtain their food from the sea. Therefore they usually live near the shore on the beaches or cliffs of the littoral zone. There are 19 resident species of which 5 are endemic. One endemic species is the Galapagos penguin (Spheniscus mendiculus) and the only species of penguin to exist north of the equator and nest in the tropics. 90% of the population concentrates on the western most islands Fernandina and Isabela which coincide with the Cromwell Current that cools surface temperatures and increases marine productivity. Many other endemic birds also depend on this current as a food source. For example, the waved albatross (Diomedea irrorata), which is considered endemic to Espanola Island spends long periods of time at sea; the flightless cormorant (Nannopterum harrisi) is the only flightless species from its genus; the swallow-tailed gull (Creagrus furcatus); and lava gull (Larus fuliginosus) considered one of the rarest gulls in the world. Some of the non-endemic birds include the boobies, storm petrels, frigatebirds, and terns. Shore birds are also dependent on the rich upwelling current as abundant residents and migrant visitors of this ecoregion. There is only one specially adapted endemic, Butorides sundevalli, which blends with the lava landscape increasing its success at catching prey.
Landbirds are not as numerous as sea- and shore birds due to their relative immobilization to and from these islands but out of the 29 species, 24 are endemic. It seems that one of the most notable attributes of these birds is their relative lack of fear towards humans. This fearlessness was even noted by Darwin in his notes from visits to these islands in 1845. Some of these fearless, endemic predators are the Galapagos hawk (Buteo galapagoenis), the subspecies Galapagos barn owl (Tyto punctissima) and short-eared owl (Asio flammeus). Other landbirds endemic to the Galapagos ecoregion are the Galapagos dove (Zenaida galapagoensis), Galapagos rail (Lateralus spilonotus), Galapagos martin (Progne modesta), and four species of mockingbirds from the genus Nesomimus that are endemic to specific islands within the Galapagos ecoregion.
Much like the divergence of species seen in the land tortoises, Darwin's finches form their own family, with only one species outside of the Galapagos on Cocos Island, Costa Rica. There are 13 finches from the same family (Fringillidae); the most remarkable divergence of these species is their beak structure and specialized feeding capabilities. For example, on Wolf Island sharp-billed ground finches have been nicknamed vampire finches because they peck at the skin of masked and red footed boobies until they draw blood which they drink. On other islands small, medium and sharp-billed ground finches eat ticks and mites that they remove from iguanas and giant land tortoises. Woodpecker and mangrove finches (Camarhynchus pallidus, C. heliobates) also have notable feeding behaviors; as they feed on insects and larvae if they can not reach one they will find a tool to help remove the desired morsel. All Darwin's finches are specialized to their habitats. Some are not as pronounced as the ones previously mentioned, however their adaptations are most noticeable during times of reduced food availability including the warbler finch (Certhidea olivacea), small, medium and large tree finches (Camarhynchus parvulus, C. pauper, C. psttacula), vegetarian finch (Platyspiza crassirostris), and the large, cactus and large cactus ground finches (Geospiza magnirostris, G. scandens, G. conirostris).
Mammals are not very numerous due to their inability to survive such a long journey at sea from the mainland to the islands, except for the 6 mammals considered native to this ecoregion. The endemic subspecies Galapagos sea lion (Zalophus californianus wollebacki) and species Galapagos fur seal (Arctocephalus galagapoensis) most likely arrived by swimming to the islands. While endemic species of rice rats (Oryzomys bauri and Nesotyzomys narboroughii) most likely floated on raft like objects to the islands. Two bat species Lasiurus brachyotis and L. cinereus also have made it to the islands, and they are not endemic.
The Galapagos ecoregion is in almost its entirety designated a National Park. In 1968 97% of the land in the Galapagos archipelago was included in the park which was recognized as a World Heritage Site in 1978 and a Biosphere Reserve in 1984. The current human population is restricted to five islands, numbering about 17,000 individuals their waste and garbage disposal methods are a concern as well as agricultural practices and other resource uses.
The World Wildlife Fund and Charles Darwin Research Center are working together to produce an ecoregion assessment and biodiversity vision for the conservation and rehabilitation of this ecoregion. This is a five-year plan will designate necessary steps for lowering the influx of introduced species and the removal of established introduced species from the islands. Some of these actions include reducing the port of entry to one, completely human-free islands, population reduction on inhabited islands and active removal, and monitoring of introduced species.
The state of the ecoregion has been deteriorating since humans first began visiting the islands in the 1800's. These visitors and inhabitants introduced species which damage and out compete native species, causing population reduction and in some cases extinction. Pigs, cats, dogs, horses, burros, cattle, goats, rats, sheep and mice were all introduced in the past and are still a problem as conservation scientists struggle to remove them from the natural habitats. Introduced plant species, now outnumbering native species, represent a major problem as they compete with native vegetation, taking over habitats more quickly than the native vegetation can adapt to these assailants. Conservationists however are actively eradicating introduced plants and animals from the islands. They are especially concentrated on getting control of the expanding problems caused by fast growing plants such as guava (Psidiun guajava), quinine (Chinchona succirubra), lantana shrub (Lantana camara) and elephant grass (Chinchona succirubra). As of the year 2000, Genovesa and Fernandina are free of introduced organisms because of the efforts of the National Park Service and the Charles Darwin Research Station. This research station has been an active conservation movement in this ecoregion since 1964. One of the many important operations at the research station is its role as the nursery of these islands. Giant tortoises and iguanas have been bred and raised in the research station to such success that populations have been saved from extinction and released individuals have gone on to successfully breed in the wild.
Tourism is a huge income for the people of these islands. When organized tourism began in 1969 the National Park Service enacted strict rules to prevent the over 40,000 visitors a year from damaging the ecosystems and wildlife.
Types and Severity of Threats
The majority of the human population lives in the Galapagos archipelago. This archipelago encompasses three percent of this ecoregion that is not part of the National Park. The population is however out growing these small areas and is saying they need more space. Conservationists however are suggesting that reducing the number of human residence would greatly help the islands. In 1998 Bensted-Smith indicated that the island population was growing at 8% a year, mainly due to immigration from mainland Ecuador. These islands have become more appealing due to the possible income made fishing or by tourism. The lack of jobs and high amount of poverty in mainland Ecuador is triggering migration to the islands.
The main threat to the well being of all species of organisms within the Galapagos ecoregion are invasive, exotic and introduced species. The introduced herbivores such as cattle, goats and sheep are damaging entire ecosystems. They overgraze areas allowing severe erosion resulting in further degradation of the habitat. They are also removing food and water supplies from the native wildlife such as land iguanas and giant tortoises. The seriousness of this problem is very high. With more and more tourists and imported goods acting as vectors that increasing the number of introduced species admitted. Recently a tree frog was found. There are not any native amphibians in this ecoregion due to their intolerance to salt, until now. The implications of this introduced species are not yet known but it cannot be good. A common problem the introduced organisms have the upper hand in their assault, leaving the native organisms victim to its inability to adapt and evolve resistance before it is eradicated.
Introduced plants are a major threat to the native vegetation especially on islands where certain trophic levels may not be occupied leaving an opening, which can be readily occupied by an introduced species. Most of the current and predicted extent of invasion by introduced plants results from invasion of disturbed habitats. Plant introduction continues to be a concern due to the ease at which they are transported by local people between islands and the mainland by boat.
Climate change poses high consequences such as raised water temperatures much like those during El Niño years that resulted in a 75% die off of marine-dependent species such as Galapagos penguins and marine iguanas. Global warming could cause an over all temperature increase; lowering the productivity of the whole marine-based food chain for mammals, birds and reptiles. Global warming will also increase the growth and adaptability of introduced plants damaging native plants and the animals, which live off these resources.
Justification of Ecoregion Delineation
The Galapagos Islands were lumped under the broad classification of xeric scrub to better represent the broad-scale classifications of ecoregions, and we recognize the unique and diverse species associations that stray from this classification in the montane regions. These islands have been a laboratory for species endemism and diversity and need no formal justification. Classifications follows CLIRSEN/DINAF.
Additional information on this ecoregion
- For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion.
- To see the species that live in this ecoregion, including images and threat levels, see the WWF Wildfinder description of this ecoregion.
- World Wildlife Fund Homepage
- Acharya, Keya. 2000. Paradise in Peril. Life & Nature. Oct, 31. 27-31.
- Bensted-Smith, Robert (1998). The war against aliens in Galapagos. World Conservation.4/97-1/98:40-41.
- Charles Darwin Foundation/World Wildlife Fund. In prep. Galapagos biodiversity vision. World Wildlife Fund-US, Washington, DC, USA.
- CLIRSEN/DINAF. 1990. Mapa forestal de la Republica de Ecuador. Map 1:1,000,000. Quito, Ecuador.
- Jackson, Michael H. 1999. Galapagos: A Natural History. University of Calgary Press, Alberta, Canada.
- Kramer, P. (1984). Man and other introduced organisms. Biological Journal of the Linnean Society, 21: 253-258.
- Lubin, Yael D. (1984). Changes in the native fauna of the Galapagos Islands following the invasion of the red fire ant, Wasmannia auropunctata. Biological Journal of the Linnean Society, 21: 229-242.
- Mills, Kyra L. &Vargas, Hermán (1997). Current Status, analysis of census methodology, and conservation of the Galapagos Penguin, Sphenscus mendiculus. 50: 8-14.
- Moll, Eugene J. 1998, A further report on the distribution of introduced plants on Santa Cruz Island, Galapagos, Department of Botany, University of Cape Town, Republic of South Africa.
- Perry, R., 1984. Key Environments: Galapagos. Pergamon Press Inc. Elmsford, New York.
- Schofield, Eileen K. (1989) Effects of introduced plants and animals on island vegetation:Examples from the Galapagos Archipelago, Conservation Biology 3:3: 227-238.
- Swash, Andy & Still, Rob, 2000. Birds, Mammals & Reptiles of the Galapagos Islands. Pica Press. East Sussex, UK.
- Tye, Alan (1998). Alien plants and invertebrates: Turning the tide? World Conservation. 4/97-1/98:42.
- WCMC 1990. Protected Areas Datasheet on the Perque Nacioal Galapagos, Archipielago de Colon. Retrieved (2001) from http://www.unep-wcmc.org/protected_areas/data/sample/0069q.html
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