Let’s Make a Deal
Are you still on your “mind trip?” Next think about why this environment looks like it does. That depends, in large part, on the abiotic (non-living) factors in the area. What is the average temperature? How much rain falls? How much of that actually percolates (sinks) into the soil? Are winds or fires significant factors for the survival of living things? Climate is the term for trends in weather over long periods of time. In most areas, it can be neatly described by graphing temperature and precipitation. Look at the graph—a climatogram. It represents the climate in eastern Illinois—an area that would be deciduous forest if it were undisturbed.
Climate sets the rules. Organisms play the game. Imagine a large, open field that had been cleared in Illinois. (The farmer magically disappears once his plowing is done!) Hundreds of species of plants and thousands of animals move into the field. They compete for space, for nutrients, for water. Some may be unsuited for the climate, but others thrive. First come ferns and the quick-growing plants we call “weeds.” (Biologists actually call the way they grow “weedy.”) Sumac soon follows. The environment selects the plants, and the plants alter the environment around them. Some organisms are “more fit” and get an advantage. Some hold their position for a few years, then get edged out by others because of changes that they caused by just growing there.
The seeds of a few trees manage to germinate. Aspen usually grows quickly in the first few years. Slowly but surely they grow taller—a few centimeters a year, since they are perennials. Soon the trees begin shading the weeds, and the microenvironment begins to change. But aspen are small (as trees go) and don’t provide a lot of shade. Maple and oak trees grow more slowly, but when they do they provide more shade and out-compete the others. Weeds disappear. So do ferns, sumac and eventually the aspen. The few plants that survive in the forest floor do most of their growing during the early spring before the trees’ leaves are fully out. Eventually, the process stops. A balance has been achieved. Those organisms that are most fit for the ultimate abiotic conditions have “won” in the ecological sense. They form what’s called a climax community. In that stable system, there’s a balance among producers and consumers. If the ecosystem is not disturbed, the climax community may last for a very long time. In eastern Illinois, it’s an oak-maple forest. Farther west it would be prairie. Around the world, each climate is characterized by a specific climax community. You may not find it everywhere in that area, but it’s the inevitable result of the process we call succession. What results is a biome--desert, prairie, taiga, chaparral…or rainforest.
Climate Deter,omes Biomes
It’s hard to imagine the world without human intervention. But from the climate data, biologists can predict the climax communities that would exist around the world absent interference. These broad areas, called biomes, fall into a surprisingly few categories.
While this outline is certainly simplistic, it illustrates the basic structure of biomes:
þ Tundra—where limiting factors are temperature, permafrost and low (percolating) precipitation, the climax community consists of mosses, grasses and dwarfed tree species.
þ Taiga—where limiting factors are temperature and wind, the climax community is conifers like pine, spruce and fir.
þ Deciduous forest—where limiting factors include light and cold winters, the climax community is often oaks and maples.
þ Prairie—where dry, hot summers encourage periodic fires, the climax community is comprised of grasses.
þ Desert—where water is the limiting factor, the climax community consists of succulents like cactus.
þ Chaparral—where dry seasons and rocky terrain limit growth, resistant plants that bloom quickly like Indian Paint Brush, Coyote Brush and Yerba Santa dominate the landscape.
þ Tropical Rainforest—where ample light and water encourages rapid growth and quick competition, the climax community is most diverse, consisting of dozens of kinds of trees, epiphytes and lianas.
Image source: www.USDS.gov