GEOL388
6-8-04

Galápagos Botany and Ecological Zones


Prickly-pear trees, daisy trees, and tree-ferns (Oh my)


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:

Vascular plants possess

Primitive vascular plants include:

They retain:

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:

Angiosperms: 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 from a natural group of closely related organisms. Better to think on the non-Eudicotyledonae dicots as "primitive angiosperms." So, we have

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 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: The net effect is that you can expect to see pockets of natural upland communities, interspersed with introduced plants like quinine and balsa trees.

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.