Learning Time- Week 2

Even a near-sighted poorly-educated chimp flying overhead would recognize the extreme differences between the ecosystems of West Texas and those of Malaysia. Here I will try to figure out what causes these differences and why the ecosystems have the exact characteristics that they do.

We often define ecosystems and biomes based on the characteristics of the dominant plants in that region (i.e., forests are dominated by trees and prairies are dominated by grasslands). Thus, if we want to understand the causes of the differences between different ecosystems, it might be helpful to start by studying how the abiotic (non-living) environment affects the types of plants that grow in a region.

Primary Productivity- Plants use the process of photosynthesis to convert electromagnetic energy in sunlight in to chemical energy (potential energy) stored in the chemical bonds of the sugar glucose. It is the energy in glucose that provides the energy for virtually all organisms on the planet. The rate at which sunlight is converted into sugar is known as primary productivity (usually measured as the mass of carbon produced/per area/per time).

The rate of photosynthesis is influenced by a number of factors including moisture availability and temperature. Not surprisingly, areas that are warmer and wetter (e.g., tropical rainforests) tend to have higher rates of primary productivity than areas that are cold and dry (e.g, arctic tundra). Interestingly, the types of plants that grow in a region depend on the amount of precipitation and the temperature as well.

What is going on in Texas? As you move from west to east across Texas, there is an increase in precipitation. Here is the pattern of vegetation that you would see-

- In southwestern Texas where precipitation is very low, you would find the Chihuahuan Desert. The dominant plants in desert ecosystem are "woody" plants like cacti and desert shrubs (e.g., mesquite and creosote).


- As you move further east, the precipitation increases. Deserts are replaced by prairies in which the dominant plant type are grasses. Grasses have a unique growth form that allows them to deal with abiotic factors (fire) and biotic factors (large grazers such as bison) that are characteristic of prairie ecosystems.

- In east Texas, where precipitation is much greater the prairies are replaced by forests, so of course the dominant plant growth form is trees.

What is going on in Malaysia? The precipitation in most of Malaysia is much greater than in any part of Texas. Not surprisingly, plant growth in Malaysia is not limited by the plants ability to take up water, but instead by their ability to get light. Because light always shines down, the tallest plant always wins when competing for light. Thus, tropical rainforests have very tall trees and epiphytes, small plants that grow on top of other plants.


The presentation "Deserts, Prairies, and Forests" attempt to explain why we see these patterns. http://www.slideshare.net/secret/aVrFdv9S7038HJ Enjoy!!