Plantes et roches : un lien spécial
Les Galápagos sont des îles volcaniques ; mais comment se sont-elles formées exactement et pourquoi la vie y est-elle foisonnante ? Découvrez comment des organismes végétaux ont contribué à transformer des îles rocheuses arides en paradis écologiques.
To get an understanding of this we need to know about plate tectonics and ‘hot spots’. There are seven major tectonic plates making up the ‘jigsaw’ of the Earth’s crust and upper mantle, with the Galápagos lying right on top of the Nazca Plate. This plate is moving towards the South American continent at a rate of 2.7 inches a year, carrying the islands along with it.
Underneath certain plates, intense heat from within the Earth is transferred to the surface. Here, magma bubbles up through cracks and crevices, solidifying until it eventually breaches the surface of the ocean. Sometimes this can be accompanied by truly violent eruptions, lifting up whole sections of the seabed and blasting rocks and lava into the sky. Volcanic islands then appear in the middle of the ocean, creating familiar landmasses such as Hawaii, the Azores … and Galápagos
These processes can take millions of years, and as the plates keep moving the hot spot remains anchored in the same place, creating new volcanic islands. Chains of these islands are formed in much the same way a confectioner squeezes out dollops of icing through a nozzle. In Galápagos, the islands in the east are far older than those in the west – and this implies that the hot spot is currently below the islands of Isabela and Fernandina, the westernmost outcrops, while the islands in the east are becoming eroded and sinking back into the ocean.
When the plants arrived
Much like the animal species that first arrived here, the first plant species to colonise the Galápagos had to be hardy pioneers. It’s likely that spores and seeds were carried on the wind or tides and ended up landing on the barren shores of the newly formed islands. Others could have arrived in the guano of migratory birds, and the only way for them to germinate and grow would be to fuse with the rocks and survive on a bare minimum of water and nutrients.
Nevertheless, a few hardy species did manage to – literally – put down roots, thus starting the process of building up topsoil and making it possible for other life forms to survive. Today, many of the trees we find growing on the islands trace their lineage back to humble ‘weeds’ such as daisies and dandelions – the true masters of colonisation. Without these initial plants the islands would have remained barren volcanic wastelands, with little to no terrestrial life.
Know your vegetation zones
Because of the differing ages of the islands, along with other factors including altitude, oceanic currents and rainfall patterns, distinct vegetation zones form. In general, life becomes more fecund and lusher as we progress westwards through the chain, and altitude also makes a big difference. For example, the highlands of SantaCruz are green and moist enough to support a relatively large population of giant tortoises, while the spiny and brittle landscape of Española support much less terrestrial life.
The state of the island vegetation and life in the surrounding seas varies depending on the El Niño and La Niña cycles. In El Niño years, ocean productivity fares poorly and marine life has a difficult time, while the increased rainfall means vegetation and land animals thrive. The opposite is the case during La Niña years, when the dry weather disrupts land-based life, but oceanic life thrives.
On some islands we can find a Humid Zone, where epiphytes such as orchids, mosses, ferns and lichens are able to thrive due to the presence of constant moisture and heat. Typical of this zone are trees and shrubs with colourful flowers and foliage, and the high degree of humidity supports scalesias – a type of tree that’s actually a descendent of the humble daisy. Scalesias for dense forests filled with numerous species of birds and reptiles.
Areas close to the tidal zone on islands are ideal for plants that can tolerate saltwater, such as the red mangrove with its distinctive aerial roots. Mangroves are important for fish species as nurseries, but due to the limited availability of nutrients in the shallow waters these semi-aquatic trees grow very slowly. Interestingly the roots of red mangroves are raised up in the air where they’re able to absorb oxygen more easily than if they were under water.
This is mostly cactus territory, but other plants can and do survive here including various shrubs. There are several different types of cacti but the opuntia – or prickly pear – is the most readily observed. Its succulent pads have a high moisture content, and on some islands it’s the main source of food and water for reptiles. Birds help to disperse its seeds, which are contained in the fruit.
Croisière d’expédition, La faune des îles Galápagos
16 oct. 23, 30 oct. 23 + 10 dates de départ
11 160 € TTC
MS Santa Cruz II
Croisière d’expédition en Amérique du Sud | Machu Picchu et les Galápagos
14 oct. 23, 28 oct. 23 + 10 dates de départ
13 380 € TTC
MS Santa Cruz II