Lesson 11 - The Biosphere
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To do:
- Check the schedule for this week's reading & upcoming assignments
- Read the lecture and assigned reading in the text
- Take the Week 11 Quiz
- Participate in discussions & post questions about the final exam review sheet
By the end of this lesson you should be able to:
- Discuss the five key factors in soil formation
- Illustrate specific examples of limiting factors in ecosystems
- Review the process of bioaccumulation and discuss its effects on different levels of the food pyramid
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Introduction to the Biosphere
The biosphere is the final of the four earth spheres that we will examine in this course. The biosphere is a complex, interactive system of abiotic (nonliving) and biotic (living) components that interact with the atmosphere, hydrosphere and lithosphere. Our first topic of discussion, soils, is the link between the lithosphere and the plants that cover our earth and interact with the atmosphere and the hydrosphere. Plants play an essential role in the development of soils, and they are the foundation of all earth ecosystems. Large, terrestrial ecosystems that are characterized by specific plant communities and formations are termed biomes. These biomes are the foundation of the biosphere and a key to understanding the vitality of all earth systems.
Soils
Soils cover the majority of the earth's land surface. Soil has an enormous effect on the ability of land to sustain vegetation (or crops for human consumption), and therefore is primarily responsible for the carrying capacity of land. Carrying capacity is the maximum number of organisms that an area can support without experiencing degradation. In other words, the quality of soil in an area determines the type and quality of agriculture. The better the soil in a region, the more people it can feed. It is therefore very important to understand the components that make up 'good' soil, and those which make up 'bad' soil, and we can use this knowledge to better understand the distribution and economic development of human populations.
The science of studying soils is called pedology. Soil scientists study soils in units called pedons. A pedon is a hexagonal column measuring 1 to 10 square meters in top surface area. Many pedons together make up a polypedon. A pedon is the basic sampling unit in a soil survey and a polypedon is the soil unit used in preparing soil maps.
The next time you walk by a construction site where the crews have dug a deep pit to lay a buildings foundation, take a look at the profile of the soil that has been exposed. Soils are made up of layers called horizons, which are roughly parallel to the surface of the ground. The top layer of the soil profile is labeled as O or organic. The O layer of soil is made up of humus, a complex mixture of decomposed organic material, microorganisms, and sediment. The bottom of a soil profile is called the R or rock horizon, made up of bedrock.
The soil horizons between the O and the R horizon are labeled alphabetically: A, B, C. Sometimes, after the A horizon, there is a layer of coarse sand, silt and minerals resistant to dissolving by water. This process, by which water percolating through the soil removes fine particles and minerals leaving behind more coarse material, is called eluviation. Therefore, this horizon directly after the A horizon is termed the E horizon, for the eluviation that takes place. Typically, plant roots interact with the O, A and E horizons.
The B horizon is below the A and E horizons, and the fine particles and minerals that are dissolved from the E horizon are generally deposited in the B horizon. The B horizon is typified by reddish or yellowish hues because of the minerals and organic oxides that are deposited. The C horizon is comprised of weathered bedrock or regoliths. The C horizon is outside of the zone of biological influence and therefore has little organic matter included in it. This deposition process is termed illuviation. So a typical soil profile might be labeled as: O, A, E, B, C, R.
Soils are described by their color, structure, texture, porosity, moisture and chemistry. We will discuss the most important of these factors from an agricultural standpoint: texture and structure. There are five key factors in soil formation:
- Type of parent material
- Climate
- Overlying vegetation
- Topography or slope
- Time
Type of parent material influences the soil pH, structure, color, etc., in a profound way. High-rainfall climates tend to have less-fertile soils, due to rainwater's effect in leaching nutrients down to lower levels of the soil profile, and have more acidic soils. Low-rainfall climates tend to accumulate salts near the surface and have generally higher soil pH. Soils that form under coniferous forests tend to be more acidic than those under deciduous forests, and root action is also critical in soil formation. Soils generally have a harder time forming on steep slopes, due to runoff of soil particles during rain events. The longer a soil has to form, the deeper its profile is going to be.
Now that you have the basics of soil science under your belt, check out your state soil (you probably didn't know that you had a state soil!). Compare the state soils of several different regions of the USA (note that the links below are PDF documents):
Ecosystem Essentials
An Ecosystem is a self-sustaining association of living plants and animals. The basis of an ecosystem is the plants which harvest energy from the sun and convert it into sugars and starches through a process known as photosynthesis. Plants also depend on soils to harvest nutrients. Animals then consume the plants and use the stored energy from the structure of the plants. When the plants and the animals die, they are recycled by decomposers, such as microbes and fungi that convert the tissue from the plants or animals into organic matter that becomes a part of the soil. The cycle begins again when a new plant begins growing in the soil enriched by the decomposition of prior organisms. These interrelated groups of organisms are called communities. Within a community two concepts are important:
A habitat is the type of environment in which an organism resides or is biologically adapted to live.
A niche is the function of a life form within a given community. Every organism occupies a niche within its habitat.
Ecosystems are affected by temperature and precipitation. Recall from Lecture 6 that a major effect on temperature of a region is its latitude and elevation.
The limiting factor in an ecosystem is the one physical or chemical component that most inhibits biotic function. Look up limiting factors on the internet, a text, or your library, and write down at least five limiting factors:
1. __________________________________
2. __________________________________
3. __________________________________
4. __________________________________
5. __________________________________
As discussed above, ecosystems are comprised of communities that depend on each other. The first level of these communities are called producers. Producers are those organisms (plants) that harvest the sun's energy and convert it to sugars and starches. Producers are eaten by primary consumers, creatures that eat plants. For example, a primary consumer might be a grasshopper. Primary consumers are eaten by secondary consumers (such as a chicken). The secondary consumers are then eaten by a tertiary consumer (such as a human).
As you consider the food pyramids, consider what the effects would be if a producer took in one unit of a pesticide, and that pesticide was magnified in each level of the food chain. For example, DDT is an insecticide which is harmless in small doses. However, DDT is stored in the fats of insects and animals, and can be passed on to offspring. This process is called bioaccumulation. Read more about bioaccumulation at a site hosted by the Michigan State University Institute for Environmental Toxicology.
We will focus on the bioaccumulation of DDT, a pesticide which was widely used because it could be safely handled by humans. It was extensively used shortly after its discovery just before WW II. During the war, it was used to reduce mosquito populations and thus control malaria in areas where US troops were fighting (particularly in the tropics). It was also used on civilian populations in Europe, to prevent the spread of lice and the diseases they carried. Refugee populations and those living in destroyed cities would have otherwise faced epidemics of louse-born diseases. After the war, DDT became popular not only to protect humans from insect-borne diseases, but to protect crops as well.
By the 1960's, global problems with DDT and other pesticides were becoming so pervasive that they began to attract much attention. Credit for sounding the warning about DDT and biomagnification usually goes to the scientist Rachel Carson who wrote the influential book Silent Spring (1962). The silent spring alluded to in the title describes a world in which all the songbirds have been poisoned.
DDT stands for dichloro, diphenyl trichloroethane. It is a chlorinated hydrocarbon, a class of chemicals which often fit the characteristics necessary for biomagnification. DDT has a half-life of 15 years, which means if you use 100 kg of DDT, it will break down as follows:
Year |
Amount Remaining |
0 |
100 kg |
15 |
50 kg |
30 |
25 kg |
45 |
12.5 kg |
60 |
6.25 kg |
75 |
3.13 kg |
90 |
1.56 kg |
105 |
0.78 kg |
120 |
0.39 kg |
This means that after 100 years, there will still be over a pound of DDT in the environment. If it does bioaccumulate and biomagnify, much of the DDT will be in the bodies of organisms. DDT actually has rather low toxicity to humans (but high toxicity to insects, hence its use as an insecticide).
As the first of the modern pesticides, DDT was overused, and soon led to the discovery of the phenomena of insect resistance to pesticides and bioaccumulation and biomagnification. One of the most bizarre events to accompany this early use of DDT occurred when it became necessary to parachute cats into remote jungle villages in what was then Burma. The following has taken on urban legend proportions. I have not been able to verify its truth. Nonetheless, it is a good illustration of bioaccumulation:
Operation Cat Drop: In the early 1950s, the Dayak people in Borneo suffered from malaria. The World Health Organization had a solution: they sprayed large amounts of DDT to kill the mosquitoes which carried the malaria. The mosquitoes died, the malaria declined; so far, so good. But there were side-effects. Among the first was that the roofs of people's houses began to fall down on their heads. It seemed that the DDT was also killing a parasitic wasp which had previously controlled thatch-eating caterpillars. Worse, the DDT-poisoned insects were eaten by geckoes, which were eaten by cats. The cats started to die, the rats flourished, and the people were threatened by outbreaks of sylvatic plague and typhus. To cope with these problems, which it had itself created, the World Health Organization was obliged to parachute live cats into Borneo.
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