Galápagos Islands xeric scrub

• Category: Marine and Oceanic Biomes
• Geographic Location: Pacific Ocean
• Summary: The Galápagos archipelago is a veritable laboratory for species diversity and evolution, with habitat niches that support a complex web of distinct life forms.

Although it straddles the equator, the Galápagos archipelago is not a typical tropical island paradise. Life there is molded by the availability of freshwater and dictated by two very different seasonal weather patterns and one climatic anomaly. Though the archipelago has remained relatively pristine due to its recent age and isolation, threats include introduced species, loss of native biodiversity, effects from humans, and the effects of climate change.

The Galápagos Islands are famous for their endemic species (found nowhere else on Earth). Over 97 percent of the reptiles and 80 percent of the land birds are endemic. Because many of the island’s mammals and birds have adapted to the harsh and specific conditions of the Galápagos, any changes in climate affecting the terrestrial and marine life greatly impact these species. Changes in weather patterns, for example, the El Niño/Southern Oscillation (ENSO) climatic anomaly, benefit land creatures thanks to abundant rainfall, but make life difficult for seabirds and marine life due to warmer waters. Climactic changes favoring warmer El Niño years could be disastrous for the reproductive cycles of such animals.

Climate

The volcanic islands of the Galápagos archipelago lie at the intersection of several major oceanic currents, the seasonal dominance of which influence weather patterns at any one time. From January to June, weather patterns from the north bathe the area in warm winds and currents, like the Humboldt and Cromwell Currents, creating clear skies with occasional heavy rain showers. These showers provide the only major source of precipitation to the islands' lowland regions.

From July through December, cold currents from the south create a cool, moist season (the garúa) characterized by strong upwelling of nutrient-rich waters at the coast and the formation of thick fog at upper elevations. While this fog provides continuous moisture to the highlands, the lowlands receive virtually no precipitation during this season, perpetuating their harsh, desertlike conditions. Transition periods between the two seasons are temporally and spatially variable and highly unpredictable.

Coastal or Littoral Zone

The coastal or littoral zone supports a variety of salt-tolerant plants that are mainly sea-dispersed, and thus often found elsewhere in the Pacific. Mangroves are common in sheltered coves, while creeping vines, grasses, and succulent shrubs grow in sandy areas.

Seabirds are prevalent in this zone. Approximately 90 percent of the entire population of Galápagos penguins (Spheniscus mendiculus) lives in the western region of the archipelago on the islands of Isabella and Fernandina. Other endemic bird species include the waved albatross (Diomedea irrorata), the flightless cormorant (Nannopterum harrisi), the swallow-tail gull (Creagrus furcatus), and the rare lava gull (Larus fuliginosus). Some birds, such as the island's largest bird, the waved albatross, may spend most of their time out at sea, while others are well adapted to blending in with the lava-formed landscape.

The Galápagos sea lion (Zalophus wollebaeki) lives exclusively along the coastal areas of the Galápagos and Isla de la Plata, Ecuador, as does the Galápagos fur seal (Arctocephalus galapagoensis). The Galápagos sea lion and Galápagos fur seal are two of only a few mammalian species found on the islands. The others include seven species of mice and rats and two species of bats, the hoary bat and the red bat. Around twenty species of cetaceans (dolphins, whales, and porpoises) live in the waters around the islands, while others use the archipelago as part of their migration paths.

Arid Zone

The arid zone is the most extensive and unique zone, covering most of the low islands and a good portion of the high islands. Species in this area are drought-tolerant and adapted to extreme conditions; many are endemic. The Galápagos's arid zone supports several cactus species, including the giant prickly pear cactus (Opuntia spp.), an excellent example of adaptive radiation. In this process, one ancestral prickly pear-like colonist is believed to have evolved over time and across the different islands into six species and fourteen varieties, radiating out from the original.

Prickly pear cactus pads, fruits, flowers, and seeds are important resources for many Galápagos animals, including a variety of insects, the Galápagos dove (Zenaida galapagoensis), the four species of Galápagos mockingbirds (Nesomimus spp.), two species of Darwin's finches (Geospiza scandens and G. conirostris), land iguanas (Conolophus subcristatus), and the island-specific varieties of the Galápagos giant tortoise (Chelonoidis niger). Indeed, Opuntia cacti on islands with land iguanas and tortoises are often taller than varieties found on other islands, presumably an adaptation to reduce predation by these herbivorous reptiles. Other species, such as the Palo Santo, meaning "sacred wood," tree (Bursera graveolens), cope with drought by remaining dormant most of the year, putting out a new flush of leaves when the rains come. These trees are often regularly spaced across the landscape, an arrangement that minimizes competition for water.

Transition Zone

Above the arid zone lies the transition zone, characterized by an ever-decreasing abundance of arid-zone plants amid a dense and diverse community of tangled shrubs, perennial herbs, and epiphytes. The mainly deciduous, lichen-festooned forest here is dominated by two endemic trees: the broad Pega Pega tree (Pisonia floribunda), which produces sticky fruit that is widely distributed by birds, and two varieties of guayabillo or Galápagos guava (Psidium oligospermum) that produce small, apple-like fruit. The matazarno (Piscidia carthagenensis) is the tallest tree in the transition zone, possessing an extremely durable and valuable wood. Lichens typify this zone, as they are drought-tolerant but able to take advantage of occasional garúa mists that penetrate down into the region.

Moist Uplands Zone

Above the arid and transition zones lies the moist uplands zone, characterized by stands of the softwood genus Scalesia; the prickly-ash (Zanthoxylum spp.); the flowering fruit source Miconia spp.; and coverings of pampas grass, ferns, and sedge. These stands are supported by the garúa fog, which supplies a relatively consistent source of moisture, often in the form of droplets that condense on the leaves.

Endemic to the Galápagos, Scalesia, like the Opuntia cactus, is a fine example of adaptive radiation. The fifteen species and six subspecies of Scalesia shrubs and trees, members of the sunflower family, have adapted widely to different zones and different islands. The majority of species are found in the transition zone, but the most dominant and easily identified species, Scalesia pedunculata, a tree that can reach forty-nine feet (fifteen meters) or more, is found in the moist uplands, which indeed are sometimes called the Scalesia zone.

The prickly-ash stands are often seen with a dry-season drapery of mosses, liverworts, and ferns festooning an open forest of spiny evergreen cat's claw (Zanthoxylum fagara) and other xerophytic shrubs.

The Miconia version of the moist uplands zone is found only on the islands of Santa Cruz and San Cristobal and is named for its dense, almost impenetrable stands of Miconia robinsoniana, a shiny-leaved shrub up to sixteen feet (five meters) in height. In this zone, ground burrows or cavities provide a nesting habitat for the critically endangered dark-rumped petrel (Pterodroma phaeopygia).

At the highest elevations of the tallest islands, often on the very rims of the volcanoes, lies the zone of pampas or mixed fern-sedge. During the garúa season, this zone can receive up to 98 inches (2,500 millimeters) of rain in certain years, supporting a lush profusion of grasses, sedges, and ferns, its tallest being the Galápagos tree fern (Cyathea weatherbyana), which can reach heights of up to ten feet (three meters.)

Among the animals living in the humid zone are the famous Darwin's finches, also known as Geospizinae or Galápagos finches. They live on many of the islands at the higher elevations. Seventeen species of Darwin's finches are found among the Galápagos Islands. Two live in the arid zone, while the other species prefer the higher elevations of the moist uplands zone.

Preservation

Because of their isolation and relatively recent discovery, the Galápagos have escaped the destruction so often experienced by other oceanic islands. The biggest threats to the archipelago include introduced species, loss of native biodiversity, and human visitors. For this reason, the Ecuadorian government declared all the islands, except areas already colonized by humans, to be a national park in 1959, managed in conjunction with the non-profit Charles Darwin Foundation and Research Station.

While this measure greatly protected the islands, conservation efforts to protect flora and fauna in the region became increasingly important with the rise of climate change. Protecting the Galápagos xeric scrub in the twenty-first century focuses on eliminating invasive non-native species. Conservationists continually remove established plants that rob the environment of the resources necessary for xeric scrub to thrive. Technology like drones, machine learning, and genetic studies help identify and eliminate invasive species. The Galápagos Conservation Trust and the Galápagos Conservancy's projects include giant tortoise restoration, marine protection, and iguana restoration, safeguarding Galápagos penguins from climate change, monitoring climate change's impact on sea lions and fur seals, and eliminating invasive species.

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