Galápagos Botany and Ecological Zones
Prickly-pear trees, daisy trees, and tree-ferns (Oh my)
Plant evolution (in two minutes):
Land plants were definitely present by about 440 million years ago. We trace their evolution in a series of evolutionary stages:
Vascular plants possess
- "Bryophyte" grade - the ancestral condition for land plants, included plants like mosses and liverworts which lacked the ability to move water around actively in their tissues. Thus, they could only survive and reporduce in perpetually moist environments. We'll see some of these on the tour.
- Tracheophyta - "Vascular plants".
- Vascular tissue of tubes to allow water and nutrients to move where it is needed in the plant.
- Waxy surface to prevent dessication. Air is allowed in and out of tissues by means of specialized openings called
Primitive vascular plants include:
- Lack of seeds or flowers.
- Reproduction by means of spores.
Seed plants: More derived vascular plants, seed plants, are characterized by seeds (duh) in which the embryonic plant is provided with a food source and protected. They can also possess woody tissues that enable them to achieve considerable size on land. They include:
Thus, angiosperms "pay" animals to help them reproduce. The flower pays pollinators with nectar. The fruit pays animals to disperse seeds and deposit them in a blob of fertilizer.
- Appear during the Cretaceous (roughly 100 million years ago.)
- Seed is sheathed in a protective layer that can develop into fruit.
- Pollination is facilitated by means of a flower that attracts pollinators.
Traditional Linnean taxonomy breaks Angiosperms up into Monocotyledonae (aka "monocots") and Dicotylodonae ("dicots"). It turns out that only some of the dicots from a natural group of closely related organisms. Better to think on the non-Eudicotyledonae dicots as "primitive angiosperms." So, we have
Ecology: The scientific study of the interactions of organisms with one another and their physical environment. In the Galápagos, we will see manifestations of four major ecological concepts:
- Ecological communities
- Ecological succession
- Environmental energy flow
Niche partitioning: Within a given physical environment, species of organisms tend not to be in direct competition with one another. This may be because:
Ecological Communities: Over time, organisms that inhabit the same habitat non-competitively come to form recognizible communities. These may reflect:
- Native species evolve adaptations that limit competition. Obviously, within a population, those individuals whose behavior brings them into less frequent competition with others have an advantage in passing on their genes. E.G.: Marine and land iguanas, although descended from a recent common ancestor, exploit different food resources, habitats, and nesting sites.
- New immigrants are unlikely to become established in an environment already inhabited by creatures that use the same resources.
- The coevolution of species, E.G. the mutualism of East African acacia trees and the hoofed mammals that feed on their fruit and distribute their seeds)
- An epiphenomenon of the life histories of the individual species.
The Galápagos are have a "classic" set of ecological communities that are readily distinguished by their flora. Ultimately these exist because of a climatic phenomenon: The rain-shadow effect. Prevailing winds blowing across the ocean contain some moisture. When they encounter an island with high elevations, like Isabela or Santa Cruz, they are forced to rise. At higher elevations, they become chilled and their water condenses as rain or mist, which precipitates out at high elevation on the windward side of the island. On the leeward side, there is very little moisture left in theair and the climeate is, if anything, more arid than normal. Thus, the larger island have a distinct zonation of moisture that forms the basis for its ecological communities.
The communities, from bottom to top: For each, a typical plant or two are given. Remember these plants and be prepared to learn to identify them in the field.
Pretty neat system. Here's why it doesn't work consistently:
- Coastal (littoral) zone: The narrow zone fringing the shore in which groundwater is salty. This is the home of a special community of plants that can get their moisture from sea water. Usually, they eliminate salt by means of special glands. Some typical plants:
- "mangroves": Salt tolerant shrubs and trees. Mangroves represent a case of convergent evolution. They are not one another's closes relatives. Most have adaptations for the disperasal of their oppspring by ocean currents. None are endemic to the Galápagos.
E.G.: Red mangrove: Sink their roots directly in to the water, forming a distinctive underwater environment.
- Saltbush: A common shoreline shrub with fleshy, waxy leaves.
- Arid zone: A few meters from the shore, one passes into the typical Galápagos desert. This community is omnipresent at low elevations, once one leaves the zone of salt ground-water. On low-lying islands, the arid zone may go all the way to the top. Even on tall islands, it extends far up the leeward slopes.
Arid zone plants depend on fresh water but obviously don't need much of it. Plants in this zone show considerable diversity and endemism. A small selection:
- Palo Santo: These trees shed their leaves qucikly after the rainy season. Their bark is covered with a white lichen, giving them the appearance of dead white trees.
- Prickly Pear aka Opuntia: The Galápagos boast a major 14-species endemic adaptive radiation of these common New World cacti. Species tend to reflect adaptations to local herbivores and pollinators. For example, island with giant tortoises and land iguanas tend to have tree-sized prickly-pears like Opuntia megasperma, whereas on herbivore-free islands, the prickly pears are of normal proportions, like Opuntia helleri. This prickly-pear, btw, has reduced thorns to facilitate pollination by Galápagos doves.
- Gray matplant: In places where nothing else grows, you can see these little herbaceous plants.
- Transition zone: Characterized by increasing moisture. Since our highlands tour will take us from the coastal straight to the Scalesia zone, we are unlikely to see the transition zone up close. Some less drought-resistant forms start to appear, including:
- Pega-pega: A deciduous endemic tree with round leaves and sticky fruit.
- Scalesia zone: A humid dense forest fed by the garua. It's typical plant is also one of the islands' most interesting:
- Scalesia pedunculata: Besides the prickly-pears, the adaptive radiation of Scalesia, an endemic genus, is the Galápagos' most interesting. Scalesia is a member of Compositae - the daisies. In most places, these are little herbaceous plants. Members of Scalesia , however, are all woody shrubs with one exception, Scalesia pedunculata, which is a large tree. These form the backbone of the Galápagos' upland forests. Daisy trees. Wow!
- Brown zone: As we ascend, we reach the zone in which the land sometimes sticks up out of the garua and in which the ground doesn't retain moisture as well. Thus, the plant biomass starts to diminish. The zone above the Scalesia zone is an open forest named for the brown lichens and liverworts that cover the trunks of trees, however the common tree is:
- Micronia zone: A dense heath of the shrub Micronia robinsoniana.
- Pampa zone: Above it all is the coolest of all Galápagos zones. The pampa zone is a sedge of ferns (with a few grasses.) In it, one can see a rare example of the kind of environment that was quite common before the evolution of grasses about 50 million years ago. There is no one dominant species, but once can hope to see:
- Galápagos tree fern: As the name implies, a tree-sized fern. Again, this is a reminder of a bygone age. This time, of the coal-swamps of the late Paleozoic, before the proliferation of seed plants.
The net effect is that you can expect to see pockets of natural upland communities, interspersed with introduced plants like quinine and balsa trees.
- Based on Santa Cruz and really only ever described it perfectly. Islands without sufficient elevation to have extensive Garua are strictly arid (E.G. Española) Thus, only Santa Cruz, San Cristóbal, and southern Isabela.
- The moist upland zones happen to be the only places on the Galápagos that were friendly to agriculture, so they have been heavily eroded by human activity. Agriculture and introduced species have fragmented and erased natural zonations.
- Introduced mammals (especially goats) have destroyed Scalesia zone on Santiago and damaged it elsewhere.
One other reason that communities are not nicely delineated is that they change over time, as well as with elevation, in the process of ecological succession. To take a simple example, consider a new ly soldidfied glassy lava flow. Not an inviting environment for most plants, that need to sink their roots in soil. Nevertheless, some pioneers like the lava cactus can root themselves in its cracks. As their action weathers the flow, and they die, adding their bodies to the mix, a thin soil develops that can sustain fussier plants. Over hundreds to thousands of years, this process reiterates until a stable climax forest is present. That's the theory. In fact, few places on Earth have a climax forest because terrestrial environments haven't yet reequilibrated from the deglaciation of 11,000 years ago.
Climate change issues: This brings us to one last headache. While endemism is high in the arid - Scalesia zones, it is very low in the higher-elevation ones. Why? Well, not only do communities change over time, so do climates. Ancient pollen deposits tell us that during the last ice age, the Galápagos were drier than they are now. Thus, the arid zone would have extended much higher up the slopes of the large islands, obliterating the Micronia and pampa zones. Thus, today's high elevation zones are probably no older than the last deglaciation (11,000 years) whereas the lower zones have had millions of years in which to spawn endemic forms.
The last ecological topic, energy flux. we leave for tomorrow.