Arid Zone Vegetation
July 6, 2004
"BLECHG!" This was the unmistakable sound of the sea lions in the Galápagos Islands. We heard this sound over and over again every day and will certainly not forget it. (How could we, when we imitated it every chance we got and made it into a new greeting amongst ourselves?) But nearly as common as this sound, was me asking our tour guide Luis, "What is this plant?" I asked this question to Luis repeatedly because of the abundance of plants in the Arid Vegetation Zone.
The Arid Zone is the most extensive vegetation zone in the Galápagos Islands (Jackson 63). As its name suggests, it is extremely dry, it has desert-like conditions, and it receives little rain. The vegetation in this zone has adapted to arid conditions and can survive with little fresh water. Futhermore, the vegetation in this zone has a large variety because it covers such a large land area. The arid zone extends just inland from the coast to higher elevations (Jackson 63).
Though the Arid Zone encompasses much of the Galápagos Islands and is most commonly seen by visitors, there are other vegetation zones in the Galápagos Islands. Along the coast, is the Coastal or Littoral Zone which consists of salt tolerant plants and mangroves. At elevations just above the Arid Zone, and mostly on the southern side of the islands is the transition zone, which experiences increasing levels of moisture because of the decrease in temperature. As the altitude increases there is a Scalesia Zone, which remains moist because of the garua mist and has dense forests. Even higher in altitude is the Brown Zone. At this zone, water retention is lower and plant biomass diminishes. The next zone is the Micronia Zone, which is dense in shrubs. The highest zone is the Pampa Zone, which is the coolest of all zones and contains sedges and trees of ferns. Each of these zones plays important functions in the Galápagos Islands. However, my travel log will mostly focus on the Arid Zone.
My introduction to the Arid Zone began on Thursday, June 17, 2004, as our flight (one of nine flights in ten days) landed in the town of Puerto Baquerezo Moreno in the island of San Cristóbal. Although we remained on the plane, I already began to get a good idea of what the Arid Vegetation Zone would be like in the Galápagos Islands. I was lucky enough to have a window seat on the plane, and as we landed, I could see all kinds of shrubs and bushes. I hadn't expected to see any flowers since I knew the arid zone was dry, but I did see some small yellow flowers. The two plants that were the most easily identified were the palo verde and the candelabra cactus. The palo verde (Parkinsonia aculeata) was easily identified because of its long green spines. Likewise, I could immediately identify the candelabra cactus (Jasminocereus thouarsii) because it took the shape of a candelabra.
Upon landing in Baltra, we saw the Prickly Pear Cactus (Opuntia) up close. As we finally got to exit the plane and waited in line to obtain our entrance tickets to the park, we had a chance to see some isolated Opuntia near the "Foreigner" sign (See Figure 1). This cactus was probably three to four feet in height. I was interested in the height of the cactus, because the height of the Opuntia varies across the archipelago. This occurs because of adaptations in response to the predators of the Opuntia. For instance, on islands with large populations of the giant tortoise, the Opuntia takes on the form of a tall tree to protect itself. In addition to the cactus we got our first glimpse of a lava lizard and some of the birds of the Galápagos.
Figure 1: Opuntia for foreigners
My first taste of Arid Zone vegetation at the Galápagos airports turned into complete immersion after we boarded the San Jose and took a dinghy ride to Bachas Beach on the northern shore of Santa Cruz. Arriving in the late afternoon, the landscape was quite striking and rather eerie, as it was covered by the palo santos and contained only speckles of the Opuntia and espino (See Figure 2). In Spanish, the name palo santo means "holy stick." It was given this name because of its white color. The palo santos (Bursera malacophyllia) is a small to medium sized tree with thin white branches. Since June marks the beginning of the dry season, these trees had no leaves or flowers on them, and appeared to be almost dead. Luis explained to us that this species of palo santos is endemic to (it occurs only on) Santa Cruz, Baltra, Daphne Major, and Daphne Minor. The other species of Palo Santos that occurs on the Galápagos Islands is the same species that occurs on the mainland of South America.
Figure 2: Palo santos and espino
In addition to the palo santos, we saw scattered Opuntia and espino. The Opuntia we saw in this location was bush size, like that in the airport, and its leaves grew in a downward direction (See Figure 3). The espino (Scutia pauciflora) was a shrubby bush that consisted of green spines or thorns (See Figure 4). I had expected to see a lot of this type of vegetation, but I did not expect it to be as dense as it was and to totally dominate the landscape. However, as we continued on with our walk along the beach, my attention to the vegetation turned to some of our first sightings of animals, as we encountered the Sally lightfoot crab, marine iguanas, the great blue heron, and a flamingo. Soon we began to click away with the cameras and would not focus on arid zone vegetation again until the next day.
Figure 3: Opuntia
Figure 4: Espino
After sleeping under the stars and the Milky Way on the sundeck Thursday night, I awoke Friday morning to a beautiful sunrise and a view of Bartolomé and Pinnacle Rock. After a breakfast of lots and lots of fruit, we took the dinghy to Bartolomé and began a hike up a cinder cone volcano. Like Thursday, we saw the Opuntia and the espino. The Opuntia was isolated and was bush sized. The side of the volcano was covered with the gray matplant (Tiquilia spp.). The gray matplant is a shrubby plant that grows in extremely dry areas such as the side of a cinder cone volcano. These plants collect water as soon as it hits the ground and therefore are able to survive in very dry environments (See Figure 5). As we hiked up to the top of the cinder cone volcano, we saw another variety of Tiquilia. This was probably when I started asking Luis what every single plant was that we encountered. This variety of Tiquilia was very light green in color, but was strikingly similar to the gray matplant.
Figure 5: Gray matplant
The next plant that Luis proceeded to describe to me was the Chamaesyce (See Figure 6). This was a small, green shrub with leaves and small white flowers. The leaves on the stem directly shaded each other. Because of this feature, the Chamaesyce could survive in arid conditions.
Figure 6: Chamaesyce
As we climbed higher, we had our first siting of the lava cactus (Brachycereus nesioticus). The lava cactus is a pioneer plant that usually occupies fresh lava flows. Because lava is exposed to the sun, it expands during the day and contracts at night. This causes the lava to fracture and allows material to enter into hollow areas. The lava cactus has the ability to grow and deposit seeds by itself, so it quickly inhabits these areas, and can take over young lava fields quickly, growing to congregations up to two meters wide. Each of the stems that we saw in this growth represented an individual plant (See Figure 7). We learned that the youngest growth was in the yellow areas of the cactus and that the yellow glow is a reflection of the sun that is hitting the plant. We also learned that the spines on the lava cactus function by collecting moisture when there is drizzle and fog. Because this congregation of lava cactus was located near the top of the cinder cone, it was quite photogenic and we began clicking away with our cameras.
Figure 7: Lava cactus
After a cold, but refreshing snorkel, a huge lunch, and our afternoon siesta, we visited a lava flow on Santiago. Though its outward appearance might make one believe that this area was barren of most forms of life, the vegetation on this island was quite interesting. Near the beach I was immediately able to site espino. As we traveled further across the lava flow, vegetation appeared to be sparse. However, we began to notice the appearance of a small orange plant with white flowers consisting of five petals close to the surface of the lava flow (See Figure 8). This plant was called mollugo, and was described by Luis as a lava carpet. Luis explained to us that this plant behaved similar to cactus because it could break down lava into soil and collect water efficiently in order to grow and form such "carpets."
Figure 8: Mollugo
However, the most interesting aspect of arid zone vegetation that we encountered that afternoon was the presence of "lava trees" (See Figure 9). Lava trees were imprints that had been formed in the lava when trees were cut down by the lava flow. The imprint of the bark of the tree appears in the lava in a brown shade because of the oxidation from the water that was present in the tree. The trees that we saw imprints of were identified by Luis as Maytenus octogona or Leather Leaf, a tree that is usually found in coastal areas. Though these imprints were not living forms of vegetation, they were interesting because they provided us with information about past conditions of that environment that could be contrasted to the present conditions, where life was just beginning to reoccupy that land area. These lava trees affirmed the definition of rocks that we learned in class-a rock is a record of the environment in which it was formed.
Figure 9: Lava tree
As we traveled further across the lava flow, you could see other forms of pioneer vegetation such as the lava cactus. In addition, the sides of the cinder cones were covered with espino and palo santos (See Figure 10). At the day's end I began to realize that the arid zone is an important ecological community within the Galápagos Archipelago. The vegetation in this zone is high in diversity and has adapted to severe conditions. In areas that seem lifeless, such as the lava flow, these forms of vegetations are the first to appear, and then can sustain other forms of life, such as the birds and the lava lizards.
Figure 10: The arid zone
On Saturday, during breakfast, we arrived in Elizabeth Bay in Isabela. As the boat was coming to a stop, Dr. Merck looked out the window, saw some small islands covered in a white substance and for a minute thought it was snow. However, he quickly realized this was impossible, and was amazed at the white cover on the islands courtesy of the palo santos and the bird droppings (See Figure 11). After breakfast, we went on a dinghy ride and had a chance to see a lot of the coastal zone vegetation. This was much different than the arid zone vegetation because it had adapted to salt water, contained waxy plants, contained more flowers, and was host to intertidal animals, rather than terrestrial animals. However, as we rode along the bay, the landscape was not void of arid zone vegetation. As usual there were the unmistakable palo santos and the Opuntia. The variety of Opuntia we saw in Elizabeth Bay was still bush sized, but it was different from other varieties we had seen in the past because the leaves were pointing sideways and up, rather than down. We also encountered a new plant called croton, which is endemic to the arid zones of the Galápagos, with alternating leaves on its branches. This vegetation was interesting because it inhabited these small, "white" inhabited islands, and was host to birds such as the penguins, the boobies, and the flightless cormorant.
Figure 11: Guano on the Marielas
In our afternoon visit to Urbina Bay, my understanding of the importance of the arid zone grew. It was at this stop that we encountered a new animal in our journey. It was a baby goat. Although it was incredibly cute, and it followed us around, we knew that it was going to die (See Figure 12). Goats are considered introduced species in the Galápagos Islands, as they were introduced by humans. They have caused a great deal of harm as they are predators to many of the native and endemic species, and as they have destroyed a lot of vegetation. Many animals, such as the land iguana and the giant tortoise depend on arid zone vegetation for food, and the destruction of this vegetation is a destruction of their habitat and food sources, and is a great threat to their livelihood. Luis pointed out to us just how much the goats could destroy the vegetation, and it was indeed extensive (See Figure 13). He also pointed out that vegetation has recovered in areas where goats have been eliminated. This example helped me to understand that the dense, shrubby arid zone vegetation has an important role in sustaining the livelihood of some of the archipelago's most treasured species, the giant tortoise and the land iguana.
Figure 12: Goat
Figure 13: Goat-dominated ecosystem
Our hike at Urbina Bay again introduced us to a mix of some familiar plant species, as well as several new plant species. Once again we saw the espino, the Opuntia, and the candelabra cactus. In addition to this, there were a lot of thick grasses that were about six inches tall. On this hike we encountered the poison apple (Hippomane mancinella) for the first time. This plant is extremely poisonous, and this was evident as there was no undergrowth in proximity to this plant. On this hike we also saw flowers in the arid zone that were much larger than ones we'd seen in other islands. For the first time we saw the Galápagos cotton (Gossypium barbadense). This plant is endemic to the Galápagos Islands. It has a pale yellow/white flower and has durable seeds that can last more than five weeks (See Figure 14). The other plant we saw with flowers was the Cordia lutea or muyoyo (See Figure 15). This plant has bright yellow flowers and little berries on it. We also saw waltheria for the first time on this hike. This plant was three to four feet tall, and contained light green, oval shaped leaves (See Figure 16). Another plant we saw for the first time was lantana. This plan is endemic to the Galápagos Islands, but was described by Luis as a pest. We also saw Pisonia for the first time, which is known as pega pega.
Figure 14: Galápagos cotton
Figure 15: Muyuyo
Figure 16: Waltheria
The abundance of arid zone plant species on Isabela was interesting because it gave me insight into island biogeography and the functioning of ecological communities. Isabela is the biggest island in the archipelago, so it made sense that there was a greater variety of plant species than there had been on previous islands we visited. In addition, as we went on our hike, we saw a number of terrestrial animals (land iguanas, lava lizards, and even one wild giant tortoise!) that clearly depended on this vegetation zone for habitation and food. This made me realize the importance of the arid zone vegetation. It was probably at this point in the trip that Laura told us that the surroundings reminded her of Jurasic Park, and we will forever connect the vast arid zone of the Galápagos with the Jurassic Park theme song.
Sunday morning began with a visit to Fernandina. Little did we know that this would become the most memorable day of our trip. Most of the vegetation on our morning hike was Coastal Zone vegetation, consisting of various mangroves. However, this was overshadowed by us witnessing the birth of a Sea Lion. In the afternoon we only had a chance to snorkel at Punta Vicente Roca, before we had to motor a great distance to our next destination. As such, I did not get to see much in the way of arid zone vegetation.
On Monday morning we began the day with a hike in Santiago. Once again we saw various grasses, Tiquilia, espino, and Opuntia. The Opuntia at this site was different from Opuntia we had seen in the past because it was slightly larger than that we had seen previously, and because it had soft spines. This adaptation enabled the Opuntia to avoid direct sunlight on its pads, but still gave it enough protection from predation. The softness of the spines was demonstrated by Luis hugging the cactus (See Figure 17). We also saw Cordia lutea with its bright yellow flower again. However, we continued to encounter forms of vegetation we had not seen before. These plants included the endemic bitter bush (Castela), Alternathera, Litopila, and Valecia. Luis told us about Tribulus, a plant that grows close to the ground and has little yellow flowers. This plant is important because it provides food for the finches, and is often the only food source available in the dry season. In addition, it is unique because it has a capsule with six seeds in it called goat's head. Again, I was amazed at the variety of vegetation, and the unique role that each plant played within the ecological community.
Figure 17: Luis with Opuntia
In the afternoon we visited the red beach of Rábida, and hiked through more arid zone vegetation. There was a lot of Opuntia present along this hike, and it was often in very picturesque locations (See Figure 18). This Opuntia was the same variety as we had seen earlier on Santiago, as it had soft spines. Luis explained to us that the goats in Rábida had been killed years ago, so vegetation such as the Opuntia has grown well on Rábida. In addition, the side of the volcano was covered with palo santos. Luis explained to us that palo santos have traditionally been used as incense. They have a very soft bark and a distinct scent that you could smell, just by rubbing the bark of the tree. There was concern about the palo santos in the 1980's because a lot of the sticks were being chewed off by introduced rats. However, the palo santos have rebounded, and this is important since they are important habitats and food sources for the cactus finches.
We also saw the presence of Rosella, a moss on the palo santos. Luis explained to us that mosses were important for making dyes. In addition, we saw croton again, and learned that the width of the leaves changes from island to island depending on the amount of precipitation. The leaves have a greater surface area on islands where there is more precipitation. We also got to see the Galápagos tomato for the first time (See Figure 19). There were no tomatoes on this plant at the time, but it was interesting to compare that plant to the tomato plants we are familiar with here. On this hike, we were also able to get a good idea of the transition in vegetation zones, as the vegetation transitioned from the waxy coastal plants, to the arid plants, to more greenery near the top of the cinder cone.
Figure 18: Rábida with palo santos and Opuntia
Figure 19: Galápagos tomato
On Tuesday we spent the day in Santa Cruz. We saw more vegetation on this island than we had on other islands. This was the case because the southeast trade winds bring more water to this island, allowing for a greater variety of plant life. At the Charles Darwin Research Station we had the opportunity to see the largest candelabra cactus and Opuntia we had seen yet (See Figure 20). These plants were the size of trees. These plants have grown taller because they have adapted along with tortoises. As the trees grow taller to avoid predation, the tortoises form a saddle-backed shell that allows them to reach higher, and this creates a form of evolutionary competition. We also saw palo verde for the first time since seeing it from the window of the airplane in Puerto Baquerezo Moreno. This plant has long stringy leaves that are well adapted to the arid zone since not much of its surface is exposed to light. While we were at the Charles Darwin Research Station, whose main attraction was the giant tortoise, we did learn that the tortoises grew so large since they were not in competition with many other herbaceous animals. This again reinforced the role that the arid zone played in the adaptation of other animals and the formation of ecological communities.
Figure 20: Opuntia gigantea
In the afternoon we visited the highlands of Santa Cruz. Although this visit introduced us to many forms of uplands vegetation, including the daisy tree (or Scalesia), we did get to see another Galápagos tomato plant. This plant actually had tomatoes on it, which several of us were able to pick off the plant and eat. The vegetation in the uplands was a stark contrast from the arid zone, because it was green, lush, and moist. It was in this zone that we saw several giant tortoises and several bird species. By the time we had reached the craters in the uplands, it felt like we were in a totally different place than we had started. I was amazed at how different the vegetation zones were, and how rapidly they transitioned.
We spent Wednesday in Española, which seemed to be a giant nesting ground for many of the archipelago's birds. There were various arid zone plants in Española, and they had the role as serving for nesting materials and habitats for birds such as the blue-footed booby and the albatross. These plants included the croton and various grasses. We thought it was odd that the leaves were broad on the croton, because it is usually dry on Española. There was little cacti on the island because goats have been introduced and have destroyed the cacti. Another oddity of the arid zone of this island was the presence of algae. Waves reached the arid zone in Española, so algae was present along with the arid zone plants. Española provided another example of how arid zone vegetation interacts with animal life to create such unique ecological communities.
Our last encounter with the arid zone occurred on Thursday morning before we flew back to Quito. At 6:15 AM we took the dinghies to North Seymour, where we once more were greeted by an array of palo santos, Opuntia, espino, and various shrubs. The Opuntia on North Seymour grew very close to the ground and was much more similar to the Opuntia you would see in North America. The Opuntia was so low because tortoises or land iguanas did not naturally occur on North Seymour. As we had the chance to see the displays of the frigate birds, we had one last glimpse of the arid zone vegetation.
As we left the islands barking like a bunch of sea lions, I walked away with the following thoughts about the arid zone. The vegetation in the arid zone has made a great deal of life possible in the Galápagos Islands. It has occupied severe terrain and made life possible for many animals by providing them with food and shelter. As the plants of the arid zone have adapted to the harsh conditions of the Galápagos Islands, the animals have too, creating unique ecological communities that cannot be found elsewhere. Both the interdependence between the plants and animals, as well as the competition, makes the arid zone a fascinating community. The Galápagos Islands are truly Las Islas Enchantadas, and this has been demonstrated by things such as the giant tortoises, the sea lions, the marine iguanas, the tree-like Opuntia, and even the dense forest of the palo santos.
Jackson, Michael. 1993. Galápagos: A Natural History. University of Calgary: Calgary.