Introduction to agricultural systems


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Discuss course objectives and outline, grading system, etc.
1.1 What is a system?

  1. Abehaviour as a whole in response to stimuli to any part@ (Spedding, 1979, p. 15)

  2. Aanything that is not in chaos, any structure that exhibits order and pattern@(Boulding, 1985, p. 9).

A change to one part of a system affects all other parts of the system. For example, in dry southwestern Saskatchewan where moisture is limiting, grass grows if it rains, and if grass grows the grasshopper and deer populations increase, the burrowing owl population increases, the coyote population increases, and so on. It is not possible to change only one part of a system.

Most of our direct experience with systems is with engineered systems such as telephone, transportation, and computer systems. As in all systems, more is better (Kelly, 1995). One telephone is not a telephone system and would be useless. Even two telephones would not be very valuable because you could only call one other person. Telephone systems are only valuable communication devices when most people have one, and you can contact by telephone anyone anywhere in the world. The same reasoning applies to transportation systems. In Canada, most people have a vehicle because we have a very extensive road system. If the only road in the country ran from Vancouver to Calgary to Winnipeg to Thunder Bay to Toronto, it would not be worth owning a vehicle in Saskatoon because we would not have access to the road. However, since there are roads connecting almost every place where people live with every where else, vehicle transportation is a very effective means of getting around and almost everyone has a car. The internet would not be a very important computer system if only five people had access, but when millions of people have access it is a true information system.

1.2 Major characteristics of biological systems

There are five main characteristics that describe all systems. They are discussed more fully in Wilson and Morren (1990, p. 69-73).

Holism. Holism means that the whole is more than the sum of its parts. Each system (plant or animal organism , population, community, ecosystem, or human community) is a whole with internal organization and some self-regulation.

An animal easy to see as a system, because it has structure, it behaves as a whole in response to stimuli and where it begins and ends is clear (Spedding, 1979). A study of the limbs or the organs of the animal separately would never lead to an understanding of how the animal functions as a whole. A study of the legs alone does not lead to an understanding of balance, or running, or feeling, properties that are all associated with the legs. A study of the legs alone provides no information about how they are integrated with the circulation, nervous and bone systems of the body and function to make the animal capable of movement.

Systems such as ecosystems (grasslands, boreal forest) or human activity systems (governments, education system) are analytic constructs and their defined beginnings and ends (boundaries) may not be as clearly defined in a concrete organism, like an animal.

Transformations. Systems transform themselves continuously. If they fail to transform themselves, or if they are seriously disrupted by external forces, they cease to exist. The extinction of the dinosaurs shows the effect of severe external factors and a failure to adapt.

One type of transformation involves the conversion of system inputs into outputs. For example, solar energy is transformed into plant carbon (C) through the process of photosynthesis.

One rule of systems is that their own outputs does not directly affect their function (Figure 1-1, Spedding, 1979, pp. 16-17). One hen could not heat up the atmosphere of its entire environment, but a hen in a sealed box could cause a temperature increase within its environment, in which case the system is a hen+box. This is a closed system in which the hen is a large component capable of influencing its relatively small environment. Consider outputs from human systems - do we directly affect our environmental systems.

Control. Systems have the capacity to maintain key components within an appropriate range of values in the face of external disturbance. The mechanisms used to maintain control are feedback, equilibration, adaptation, and self-regulation.

In Figure 1-1, negative feedback from an increase in the temperature inside the box results in decreased consumption of food by the chicken. Negative feedback occurs when an increase in one variable, (the transmitter), in this case temperature in the box, results in a decrease in another variable (the receptor), in this case food consumption.

Another common example is the thermostat. The receptor is the thermometer and when it registers a temperature above that of the thermostat setting (the ideal), the transmitter relays the information to the furnace, which shuts off so that the system will cool down to the ideal temperature. If the temperature is below the ideal, the transmitter will relay information to turn on the furnace, which will warm the system up to the ideal temperature. This is negative feedback because the transmitter and the furnace operate to reverse processes away from equilibrium in either direction (Boulding, 1985, p. 19).

Other examples of negative feedback are opening and closing of stomata cells in a plant leaf in response to turgor pressure, sweating in animals in response to heat, and grain price subsidies that mask market signals so farmers fail to adapt to changing market conditions.

Feedback is any mechanism by which a system responds to changing input; negative feedback reduces deviation within a system, positive feedback acts to amplify deviation (Vasey, 1992, p.6).

Hierarchy. Hierarchy describes how subsystems are >nested= within larger systems (Figures 1-2 and 1-3). Hierarchical systems are pyramidal, with one or few systems at the top, which are subdivided into more subsystems at each successively lower level in the hierarchy.

Because agricultural systems are too complex to understand as a whole, we usually study subsystems of agriculture. These can be parts of the system (i.e., the soil , the crop, or the microclimate subsystems of the whole environmental system) or subprocesses, such as nutrient transport processes, photosynthesis, and water uptake as part of the process of plant growth. The objective is to study and understand the subsystems, and to eventually interconnect those results to gain and understanding of the larger parts of the system (Leffelaar, 1990, p. 57). Plant growth could not be understood from a study of nutrient transport, photosynthesis or water uptake, each taken alone. Conversely, it is not possible to understand plant growth without understanding the details of photosynthesis or nutrient transport. Understanding of the process requires both an understanding of the subprocesses, and an understanding of how they function together in plant growth.

Behaviour of higher systems in a hierarchy is not readily discovered from a study of lower systems, and behaviour of lower systems in a hierarchy is not readily discovered from a study of higher systems. For example, the study of the arms and legs of human beings could not lead to an understanding of how the whole body works, or even how humans move (Conway, 1987, p. 99). An understanding of how limbs function in conjunction with other subsystems of the body, such as the nervous system, balance controlled in the inner ear, and the circulation system are also required.

There is a biological hierarchy within the biosphere, with ecosystems subdivided into communities, communities divided into local species populations, species populations divided into individual organisms, and so on down to cells. Figure 3 shows the Northern Great Plains (Anderson et al., 1993) region of North America, and how it can be subdivided into sixteen communities.

Political systems with national government, provincial government, municipal government, boards such as health boards, school boards etc., are also hierarchical. There is one national government in Canada, ten provincial governments, hundreds of municipal level governments, etc.

Emergent Properties. New properties of a system emerge as you move from one level to another. A crystal is capable of bending light to create a prism, and this ability derives from the arrangement of atoms within the crystal material. A physicist could study the chemical composition of the crystal, its physical properties, its mineralogy, but if he/she did not understand the crystal structure of the material, its ability to bend light and create a prism could not be predicted. In other words, the study of individual atoms could not possibly indicate how the crystal will react with light. It is only at a higher level of understanding - how the atoms are crystalized - that it is possible to predict that a crystal can bend light. The light-bending characteristic is a property of the prism=s internal organization, not of the atoms themselves.

2.1 Natural (Environmental) Systems
2.1.1 The Earth

If you start with view of earth from space, it is clear that the whole planet is one system nested within the solar system which is nested within the galaxy, etc. The only major input is solar energy and only major output is heat. The view from space would show that the earth consists of subsystems, such as land, oceans, and climate.

Solar energy and climate: Sunlight does not strike the earth uniformly, but strikes more directly on equatorial than polar regions. Thus, equatorial regions are heated more than polar ones. Heated air at the equator rises until it reaches the atmosphere, where temperature no longer decreases with altitude, and is blocked by the stratosphere from any further rise. With more air rising, it is forced to spread out north and south toward the poles. As the air masses approach the poles, they cool, become heavier, and sink over the arctic regions. This heavier cold air then flows toward the equator, displacing the warm air rising over the tropics (Smith, 1974).

Solar energy, climate and winds: Because the earth spins, and because land and ocean masses heat differently (land heats and cools more rapidly than the oceans), the flow of air from the poles to the equator is deflected and causes winds (Smith, 1974). Figure 1-4 (Strahler, 1973, p. 87) shows the Earth=s prevailing wind patterns, such as westerlies, trade winds, etc.

Solar energy, climate patterns, winds and ocean currents: The turning of the earth, solar energy and the winds produce ocean currents. As with air masses, the rotation of the earth deflects the currents of the ocean, which are further affected by the land masses. The mixing of warm and cold water by ocean currents feeds back to influence climate and winds (Smith, 1974).

This view of Earth, and the effect of solar energy is not the whole picture, because at this level we cannot see plants and animals - the life on Earth. To look at life and its influence on Earth cycles, we must move closer to the Earth's surface and view the world at the level of the biosphere.
2.1.2 The Biosphere (Myers, 1984)

The biosphere is the interface of land, atmosphere and water which support the living material of the planet and includes plants and animals. At this level, we can see how life forms modify the Earth=s environment, making it a life-support system based on energy from the sun. Algae and plants, the Earth's green cover, convert solar energy (sunlight) through photosynthesis into the chemical energy that animals need to survive.

Life probably began with photosynthesizing algae in the oceans. They released free oxygen - a pre-condition for present day existence into the atmosphere. Ocean microflora still supply 70% of Earth's oxygen, and this maintains the protective ozone layer in the upper atmosphere. Oceans are also a major sink for atmospheric CO2. Plants provide the basis of all food chains, mediate water cycles, stabilize microclimate, and act with the soil and its microorganisms to recycle decaying matter back into the nutrient system (p. 12, Myers, 1984).

Temperature feedback: Surface temperatures have remained stable for several aeons, despite variations in incoming solar energy because feedback mechanisms mediated by plant life, maintain temperature within a range suitable for life. The major temperature control mechanisms are based on the levels of carbon dioxide and water vapour in the air, both of which are affected by plant cover. Plants influence temperature in two ways. First, they produce CO2, which is a greenhouse gas that acts as an insulator ans warms the Earth. Second, they produce water vapour, which influences cloud cover and cools the Earth. The cooling occurs because clouds increase the Earth=s albedo, along with ice cover and oceans, increasing the amount of energy reflected from the surface. Microflora in oceans and plants on land darken or lighten these areas, thus altering their albedo. Atmospheric pollution raises carbon dioxide levels, whole forest clearance lowers albedo (p. 13, Myers, 1984).

Increasing plant cover and albedo tend to cool the Earth, decreasing plant cover and albedo tend to warm the Earth.

The concept of Gaia Proponents of the Gaia concept contend that the Earth acts like a huge organic system. It has a self-regulating capacity that operates through natural feedback mechanisms, i.e., autotrophs (plants) and climate are a self-regulating system, maintained in a state of disequilibrium. Because of the capacity for self-regulation, the Earth can withstand major changes, such as an ice age.

The view of the Earth at the level of the biosphere also does not provide the whole picture. We know that plant and animal life are not the same around the world. Plant and animal types vary around the world, changing as the environment changes, especially climate. To understand how environment and life forms are related, we must move our down to the level of the ecosystem.

2.1.3 Ecosystems

All ecosystems (terrestrial or aquatic) have four basic components:

- the abiotic (physical) environment,

- the producers (plants),

- the consumers (animals) and

- the decomposers (microorganisms).

Ecosystems exist because the producers (plants or autotrophs) can convert sunlight into food (fixed carbon) using simple inorganic substances (C, H, O, N, etc). This food, or organic carbon, can be used by animals (herbivores) to supply their energy needs (live, grow, reproduce). Herbivores, in turn, are eaten by carnivores to supply their energy needs. This concept is illustrated in Figure 1-5 which shows a schematic ecosystem diagram.

Animals are often referred to as heterotrophs, whether they are herbivores or carnivores, to indicate that they cannot photosynthesize, but must rely on food that originates as plant material. As the autotrophs (plants) and heterotrophs (animals) die, the carbon in their bodies is used by microorganisms for their energy supply. In the process, called decomposition, some of the fixed C is incorporated into the microbial biomass, some is released as CO2 to the atmosphere, and some becomes soil organic matter carbon. The other nutrients tied up in the plant or animal material are recycled back to the environment for uptake again by plants.

All of the organisms within an ecosystem depend, directly or indirectly on each other for existence, as shown in food web figure. Energy flow diagrams show that at the higher levels, solar energy is the input (Figure 1-5) and everything else is recycled. There is no waste in ecosystems.

Ecosystem feedback mechanisms maintain environmental stability. Prey/predator are often used to illustrate ecosystem feedback relationships. The predator population will increase as the prey population increases until there are more predators than prey. At that point the predator population starves and decreases, allowing the prey population to rebuild. The decreasing prey population is a negative feedback signal that the predator population is too large. The feedback signals keep the predator and the prey populations from fluctuating too wildly out of control, but rather within normal ranges of variation.

A study of the world at the level of the ecosystem gives us a better picture of how solar energy, which controls climate, determines the type of ecosystem that will evolve. It also shows that life modifies the environment through feedback mechanisms that keep conditions within the normal range for life. However, the view at this level does not tell us very much about how solar energy is fixed by plants, how that energy is used by animals or how fixed C is eventually returned to the atmosphere. To understand function at that level, we can look at the plant-soil system.

2.1.4 Plant-soil system

The soil-plant system is the basis of the C cycle (Figure 1-6, Brady, 1984, p. 262). Plants are made of about 70% water, ~27% organic material (C,H,O), and 3% minerals (N, P, K, S, Ca, Mg, Na, Si, Cu, Fe, Mn, Zn, Mo, B, Co). Of the nutrient elements, carbon makes up the majority of dry weight of plants at ~ 40%.

Where does the carbon in plants come from? It comes from the atmosphere and is fixed in plants through photosynthesis. CO2 in the air, plus solar energy, plus water are converted to plant carbon (carbohydrate):

CO2 + H2O + solar energy ---> C6H12O6 + O2

Carbon is the backbone atom of all of the structural and metabolic molecules in biological systems (life). Because plants regulate CO2 and O2 levels in the atmosphere through photosynthesis, they are referred to as the lungs of the earth (CO2 in, O2 out).

Why has CO2 not been completely removed from the atmosphere and replaced by O2? The answer is related to the C and O cycle, which is called respiration. Microorganisms and animals (heterotrophs) use plant C as their source of energy. When herbivores eat plants, or carnivores eat an animal, or when plant or animal material is decomposed (eaten) by soil microorganisms, the fixed C (carbohydrate) is used for the energy needed to grow and reproduce. In the process, carbon dioxide and water are released:

C6H12O6 + O2 ---> CO2 + H2O + energy

The release of carbon dioxide in returns the carbon locked up in plant and animal tissue the atmosphere. Through a series of feedback mechanisms, respiration helps maintain the level of CO2 in the Earth's atmosphere within the normal levels required for life. If the carbon dioxide level of the atmosphere increases, perhaps due to volcanic activity, plant production increases because their source of C, CO2, is less limiting. Increased plant production results in greater carbon storage in plant biomass. Much of the plant material is added to the soil as plant residue when the plant dies. Soil microorganisms decompose the plant residue, some of which is eventually converted to soil organic matter and stored in the soil for up to thousands of years. Some of the biomass becomes converted to soil organic carbon by the soil microorganisms and is stored in the soil as soil organic matter. Thus, increased atmospheric CO2 is converted to more plant biomass, more plant residue is added to the soil and more soil organic matter is formed, resulting in a net decrease in atmospheric CO2 and a net increase in plant and soil carbon. If CO2 levels decline, there is less plant growth, less plant residues added to the soil, which means less food for the soil microorganisms, so they must use stored soil organic matter for food, resulting in the decomposition of soil organic matter, and the release of that stored C as CO2 to the atmosphere. Atmospheric levels of C are kept in balance.
2.1.5 Characteristics of natural systems

Holism. At each level, the system acts a whole, but the information we can gain from a study of only one level is limited. For example, a study of an individual plant growing in soil provides information about photosynthesis, decomposition and respiration, but provides no information about why not all plant/animal/soil communities are the same. A study of ecosystems provides some of that information, a view that local and regional environmental differences affect the type of plant/animal/soil communities that form an ecosystem, but does not provide information about how these communities collectively control the levels of CO2 and O2 in the atmosphere, or about global atmospheric patterns. The view, at the biosphere level, provides a general picture of the influence of all life forms on the basic climate patterns of the Earth. Putting all of the information together results in a very complex picture that could not have been gained by studying any one level alone. The study of an individual plant provides no information about how an ecosystem functions, and the study of one ecosystem provides little information about global climate patterns - the whole is greater than the sum of its parts. Conversely, the study of global climate patterns could not be understood without knowledge of photosynthesis and respiration at the individual plant level, and tells us nothing about those processes and how they work.

It is always necessary to understand the scale at which we are viewing a problem. It is also necessary to define the boundaries of the system or subsystem of study. Since systems interact with the larger environment, it is not possible to study them in isolation. Therefore, it is necessary to identify system boundaries in order to quantify the flows that result from the interactions or feedbacks between the system and its environment by taking measurements at the boundary (Leffelaar, 1990, p. 61).

Transformations. Systems transform themselves constantly. At each level, systems have the ability to change in response to external pressure, they can adapt to new conditions. They do this by transforming an input, such as solar energy, to an output, or organic carbon compound. They have the ability to adjust the output as the input quality or quantity changes. For example, although there are relatively large fluxes in the solar energy received at the surface of the Earth, feedback mechanisms at each level act to dampen the effect of those fluxes.

Control. Systems use feedback, adaptation and self-regulation to maintain normal conditions. For example, slight temperature increases generally result in increased plant growth which causes cooling in two ways. First, more CO2 is fixed in plants, reducing atmospheric CO2 , and second, more plants means an increased rate of transpiration (a process which cycles water from the soil, through the plant and into the atmosphere) which results in increased moisture in the atmosphere, increased cloud formation and precipitation, and cooling. Cloud cover increases the albedo, further cooling the Earth. Temperature thus drops slightly, and is maintained within its normal range. This control mechanism occurs at all levels - plant/soil, ecosystem, biosphere and Earth (Myers, 1984; Smith, 1974).

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