GEOL388
6-16-08

Galápagos Botany and Ecological Zones



John Merck

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:

  • "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 reproduce in perpetually moist environments. We'll see some of these on the tour.

  • Epiphytic moss on Santa Cruz

  • Tracheophyta - "Vascular plants".

    Vascular plants possess

    Primitive vascular plants include: They retain:
    • Lack of seeds or flowers.
    • Reproduction by means of spores.

    Soft tree fern with sporangia

    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:
    Female cycad with seeds

    Anthophytes:
    • Became common 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.
    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.

    Traditional Linnean taxonomy breaks Angiosperms up into Monocotyledonae (aka "monocots") and Dicotylodonae ("dicots"). It turns out that only some of the dicots (termed "Eudicotyledonae ") form a natural group of closely related organisms. Better to think on the non-Eudicot dicots as "primitive anthophytes." So, we have


    Galápagos shore petunia Exedeconus miersii

    In the Galápagos, we will see many eudicots, a few monocots, and, in some environments, a surprising diversity and number of ferns and other primitive vascular plants. We now turn to the subject of the environments in which they occur.

    Galápagos Ecology

    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:

    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:

    The Galápagos 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 the air and the climate 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.

  • Coastal (littoral) zone:

    The narrow zone fringing the shore in which groundwater is salt water that has infiltrated from the ocean. This is the home of a special community of plants that can tolerate sea water. Usually, they eliminate salt by means of special glands. Sea water is nasty, but for those who can use it, there is plenty to go around. As a result the boundary between the littoral and arid zones is a marked contrast in greenness, even though both zones get the same rainfall. Some typical plants:

    • Saltbush: A common shoreline shrub with fleshy, waxy leaves.
    • Thorn shrub: A conspicuous hazard.

    Mangroves: A special assembleage of littoral zone plants are the mangroves "mangroves": Salt tolerant shrubs and trees. Mangroves represent a case of convergent evolution and are not one another's closes relatives:

    Than they are to one another.

    Most have adaptations for the disperasal of their offspring by ocean currents and rooting in shore environments. None are endemic to the Galápagos.
    Red mangroves are capable of drastically modifying their environment by sinking their roots directly in to the water. This enables them to trap sediments and form a distinctive underwater environment.


  • Littoral zone on Rábida

  • 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 groundwater. On low-lying islands, the arid zone may go all the way to the highest elevations. 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.

  • Arid zone on Isabela. Ecuador and Wolf Volcanoes in distance.

  • 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.

  • Transition zone on Santa Cruz.
    Galapagos Photos by Dr. Anna E. Ross

  • 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 Asteraceae - the composite-flowered 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!

  • Scalesia zone on Santa Cruz.

  • 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:


  • Brown zone pocket zone on Santa Cruz.

  • Micronia zone:

    A dense but mostly treeless heath of the shrub Micronia robinsoniana.


  • 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.


  • Pampa zone

    Pretty neat system. Here's why it doesn't work consistently:

    The net effect is that you can expect to see pockets of natural upland communities, interspersed with introduced plants like quinine, balsa, and Spanish cedar trees. Indeed, on Santiago, the Scalesia forest has been destroyed by feral goats.

    Ecological succession

    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:

    This ecological succession doesn't happen automatically. Rather, it represents the equilibrium of opposing forces among the plant communities:

    Thus, the first plants to colonize a disturbed area are usually small herbaceous ones that can grow quickly because they have full access to the sulight and aren't wasting metabolic energy on woody tissues. Eventually, however, they are overtopped and shaded out by larger woody shrubs. These, in turn, are eventually shaded out by trees. A climax forest consists mostly of a:
    • Canopy of trees
    • Understory of shrubby shade-resistant small trees and shrubs.
    • Sparse population of forest floor herbaceous plants.
    In the absence of significant limits, this setup achieves its ultimate expression in tropical rain forests.

    Tropical rain forest

    So why do we still have herbaceous pioneer plants? Because there are many limiting factors to forest growth, including:
    • Seasons
    • Rainfall
    • Soil quality
    • Limits to tree growth.
    Indeed, in even the lushest tropical forest, old canopy trees die and collapse, making clearings in which pioneeers can flourish.

    In an our local area we have:

    Here, the limiting factor is winter, and the stages of ecological succession are replayed every year in microcosm, with herbaceous plants blooming and setting seed before the understory which does it before the canopy plants have leafed out and taken their sunlight.

    Mid-Atlantic understory in Spring

    In the Galápagos, the limiting factors are:
    • Aridity
    • Poverty of soils.
    Thus, in the arid zone, only a few small trees with special adaptations to low rainfall ever rise above the pioneers. Because many of them lose their leaves during the dry season, only a few can shade out their competition. Only in the Scalesia zone will we see anything resembling a climax forest.

    Arid zone "climax forest"on Floreana

    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 endemic forms could evolve.

    The last ecological topic, energy flux. we leave for tomorrow.