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Models and Systems

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Paul Epps

on 25 September 2013

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Transcript of Models and Systems

Models and Systems
A theoretical Framework
An Overview of Systems
A system is defined as
an assemblage of parts and their relationship forming a functioning entirety or whole
. A system can be living or non-living and on any scale. A cell is a system as are you, a bicycle, a car, your home, a pond, an ocean, a computer, a farm and an iPod. We mostly consider ecosystems in this class and they can be on many scales from a drop of pond water to an ocean, a tree to a forest, a coral reef to an island continent. A biome can be seen as an ecosystem, though it helps if an ecosystem has clear boundries. The bioshphere is an ecosystem as well.
Equilibrium: Is the tendency of the system to return to an original state following disturbance; at equilibrium, a state of balance exists among the components of a system.
Key Points:
A system is an assemblage of parts, working together, forming a functioning whole.
Systems can be small or large.
Open, closed and isolated systems exist in theory though most living systems are open systems.
The first and second laws of thermodynamics and the concepts of positive and negative feedback mechanisms apply to both living and non-living systems.
Most living systems are in a steady state equilibrium, not a static one though they may be stable or unstable.
Material and energy undergo transfers and transformations in flowing from one storage to the next.
Models have their limitations but can be useful in helping us to understand systems.
Types of systems:
Entropy: What the heck?
Implications of the second law for environmental systems:
(Nope still not finished with entropy--this should be a clue)
Systems can be thought of as one of three types: open, closed and isolated.
An

open system

exchanges matter and energy

with its surroundings.
Most systems are open systems. All ecosystems are open systems exchanging matter and energy with their environment.
In forest ecosystems, plants fix energy from light entering the system during photosynthesis. Nitrogen from the air is fixed by soil bacteria. Herbivores that live within the forest may graze in adjacent ecosystems such as a grassland, but when they return they enrich the soil with faeces. After a forest fire topsoil may be removed by wind and rain. Mineral nutrients are leached out of the soil and transported in ground water to streams and rivers. water is lost through evaporation and transpiration from plants. Heat is exchanged with the surrounding environment across the boundaries of the forest.
EXAMPLE:
A
closed system

exchanges energy but not matter
with its environment.
Closed systems are extremely rare in nature. No natural closed systems exist on Earth but the planet itself can be thought of as an "almost" closed system. Most examples of closed systems are artificial, and are constructed for experimental purposes. An aquarium or terrarium may be sealed so that only energy in the form of light and heat but not matter can be exchanged.
EXAMPLE:
Light energy in large amounts enters the Earth's system and some is eventually returned to space as logwave radiation (heat). (Because a small amount of matter is exchanged between the Earth and space, is not truly a closed system.)
An
isolated system

exchanges neither matter nor energy
with its environment.
EXAMPLE:
Isolated systems do not exist naturally though it is possible to think of the entire universe as an isolated system because theoretically no matter or energy enters or exits the system.
Source: Directly taken from Environmental Systems and Societies, course Companion by Jill Rutherford Pages 71-73. Oxford University Press, (2009)
Source: Directly taken from Environmental Systems and Societies, course Companion by Jill Rutherford Page 69. Oxford University Press, (2009)
Energy in Systems
The
First Law of Thermodynamics:
energy is neither created nor destroyed.
What this really means is the total energy in any isolated system, such as the entire universe, is constant. All that can happen is that the form the energy takes can change. This first law is often called the law of conservation of energy.
EXAMPLE:
In a food chain the energy enters
the system as light energy; during photosynthesis it is converted to stored chemical energy that is passed from the flower to the caterpillar to the frog to the snake and then to the owl. No new energy is created; it is just passed along the food chain as food and transformed from one form to another.
The
Second Law of Thermodynamics:

states the entropy of an isolated system not in equilibrium will tend to increase over time.
What this means is that the energy conversions mentioned in the food chain above are never 100% efficient. When energy is transformed into work, some energy is always dissipated (lost to the environment) as waste heat. Entropy refers to the spreading out or dispersal of energy. as energy is dispersed to the environment, there will always be a reduction in the amount of energy passed on to the next trophic level.
Source: Directly taken from Environmental Systems and Societies, course Companion by Jill Rutherford Pages 71-73. Oxford University Press, (2009)
EXAMPLE:
If you exam the scene above in terms of the second law, when the lion chases the zebra, the zebra attempts to escape, chanig the stored chemical energy in its cells into useful work. But during its attempted escape some of the stored energy is converted to heat and lost from the food chain.
This process can be summarized by a simple diagram showing the energy input and outputs. The input is usually energy from the sun, the conversion process would be cellular respiration (in a food chain) Useful energy work includes activities like eating (or keeping from being eaten), breathing, reproducing, growing, escaping predators, attacking prey, in other words all of the activities an organism engages in. To summarize the second law of thermodynamics in a word equation:
energy = work + heat (and other wasted energy [maybe sound])
Energy spreads out. The useful energy consumed by one level is less than the total energy at the level below. Energy transfer is never 100% efficient. Depending on the type of plant, the efficiency at converting solar energy to stored sugars is around 1-2%. The Zebra shown earlier on average only assimilates (turn into animal matter) about 10% of the total plant energy [It is an herbivore]they consume. The rest is lost in metabolic processes. The lion's efficiency is also only around 10% [It is a carnivore].
So the lion's total efficiency in the chain is 0.02 (the plant's efficiency at converting the sun's energy into plant sugars) X 0.1 (the zebra's efficiency at converting the sugar in the plant) x0.1 (the lion's efficiency at converting the sugar in the zebra) = 0.0002%. This means the lion only uses 0.02% of incoming solar energy that went into the grass. The rest of the energy (99.98%) is dispersed into the surrounding environment.
Life is a battle against entropy and, without the constant replenishment of energy, it cannon exist. Consider this pictorial view of rowing upstream. Stop for a moment and you are swept bnack down stream by the current of entropy. In a simple example, a room tidied up is put in order relative to an untidy room. thus, the tidy room has order, while the untidy one has disorder. The tidy room has high order which equals low disorder. But it takes energy for the room to become tidy and it takes energy for it to stay tidy. If no one straightens up the room it will become untidy (low order= high disorder = high entropy). Life is the same way, if energy is not added to the system, life will dissolve into disorder ( disorder= death ).
solar energy posers photosynthesis; chemical energy, through respiration, powers all activities of life; electrical energy runs all home appliances; the potential energy of a waterfall turns a turbine to produce electricity, etc. These are all high quality forms of energy, because they power useful processes. they are all ordered forms of energy. Solar energy reaches us via photons in solar rays; chemical energy is stored in the bonds of macromolecules, like sugars; the potential energy of falling water is due to the specific position of water, namely that is high and falls. these ordered forms have low disorder, so low entropy.
On the contrary, heat may not power any process; it is a low-quality form of energy. Heat is simply dispersed in space, being capable only of warming it up. Heat dissipates to the environment without any order; it is disordered. In other words, heat is a form of energy characterized by high entropy.
1. The way we experience the second law in our everyday lives, namely the fact that entropy or disorder tends to increase spontaneously, is by realizing that all things, including living things, tend to change from order to disorder. In other words, the inescapable fate of all living creatures is death. Life is an anti-entropic system. Organisms manage to "survive" against the odds, that is, against the second law of thermodynamics, which dictates that they should die and their systems become more chaotic.
2. The way living creatures manage to maintain their order, that is, manage to stay alive, defying entropy, is by a continuous input of energy. as in the example of the room, the only way to keep the room tidy is to continuously clean it, that is, to expend energy. Similarly, organisms need to continuously obtain chemical energy from organic compounds via respiration to maintain their order. This is why energy is required even at rest, and not only when an organism is active. If any organism stops respiring, it dies.
3. actually, living activities conform very well to the second law, because they contribute to the increase of entropy in the universe, an isolated system. in any living process that maintains the low entropy of organisms, the entropy of the universe is increasing exactly like the law predicts. How does this happen? In any process, some of the useful energy turns into heat, namely a low-entropy (high quality) form degrades into high-entropy (low-quality) heat. Thus, while the entropy of the living system is maintained at a low level, the entropy of the environment is increasing. As energy moves through an ecosystem, solar energy is transformed into chemical energy, and chemical energy into mechanical energy. In both cases, a large amount of initial high-quality, low-entropy solar or chemical energy is "lost" as low-quality, high-entropy heat, increasing the entropy of the environment, in which heat dissipates.
4. As a consequence, no process can be 100% efficient.
Efficiency
is defined as
the useful energy, the work or output produced by a process divided by the amount of energy consumed
(the input to the process):
Efficiency = work or energy produced / energy consumed OR efficiency = useful output / input. Multiply by 100% if you want to express efficiency as a percentage.
5. A last philosophical implication is that, according to physics, the fate of all the energy that existsw today in the universe is to degrade into high-entropy heat. When all energy has turned into heat, the whole universe will have a balanced temperature, and no process will be possible any longer, since heat may not turn into something of higher entropy. this is refferd to as the thermal death of the universe.
There are two ways to think of equilibria,
stable or unstable
and
dynamic or static.

Open systems, which includes all the systems on Earth, tend to exist in a state of equilibrium. This doesn't mean that the system is frozen, most systems constantly change, but it means that the systems change within established limits. This continuous change in inputs and outputs is characteristic of
steady state equilibrium.
The whole system remains in a constant state, but there are small changes. An example would be your body. You may shiver or sweat depending on the surrounding temperature, but your body temperature stays relatively constant.
Other examples of
steady state equilibrium
:
A water tank, if it fills at the same rate that it empties, there is no net change but the water flows in and out. It is in a steady state.
In economics, a market may be stable but there are flows of capital in and out of the market.
In ecology, a population of ants or any organism may stay the same size but individual organisms are born and die. If these birth and death rates are equal, there is no net change in population size.
A mature ecosystem, like a forest, is in steady state equilibrium as there are no long term changes. It usually looks much the same for long periods of time, although all the trees and other organisms are growing, dying and being replaced by younger ones.
Static equilibrium
is a contrast to steady state equilibrium. Basically, there is no change over time. A pile of books which does not move unless some one moves it. When static equilibrium is disturbed a new equilibrium will develop. The pile of books is knocked over by a clumsy student, the books don't go back in the pile, instead they adopt a new equilibrium in a pile on the floor. Most non-living things like rocks or buildings are in a state of
static equilibrium.

Another way to look at equilibria is to ask, "does it return to the same equilibrium after it is disturbed?" If it does return to the same equilibrium (like in
steady state equilibrium
) then we say it is
stable.
If it does not return to the same equilibrium (like in
static equilibrium)
then we say it is
unstable.
EQUILIBRIUM:
Systems are continually affected by information from outside and inside the system. When the output of one system becomes an input for another (or the same) system that is feedback. You can imagine that changes in the inputs to a system would affect that systems equilibrium. Sometimes the inputs cause the equilibrium to remain where it is, other times the inputs cause the equilibrium to a new place. Feedback can be positive or negative.
Here are a few examples of feedback affecting a system's equilibrium:
If you start to feel cold you can either put on more clothes or turn the heating up. the sense of cold is the information, putting on clothes is the reaction, the equilibrium is that the clothes allow you to feel normal or comfortable again (keeping the equilibrium the same.
If you feel hungry, you have a choice of reactions as a result of processing this "information": eat food, or do not eat and feel more hungry. The first will return your equilibrium of not feeling hungry to its original equilibrium, the second will shift your equilibrium to being hungry.
If a teacher provides you with knowledge, knowledge is the information and "knowledge acquisition" is the reaction. The interaction may happen both ways, if you are encourage by the teacher and respond positively. The original equilibrium is ignorance and the new equilibrium is enlightenment.
If the students respond positively by learning and showing interest in the course., then the teacher realizes that the methodology is successful, and continues in the same way or improves it in response to constructive comments. In order words, the teaching process is reinforced and strengthened This is called Positive Feedback.
On the other hand, students might respond negatively showing indifference, distraction or even dissent. Such a "result" should clearly indicate to the teacher that the methodology is not appropriate, at least for the specific group of students, and the teacher should change the style of teaching back to the original style of teaching. This is negative feedback.

Negative Feedback:

Negative feedback tends to damp down, neutralize or counteract any deviation from an equilibrium, and it stabilizes systems or results in steady-state (dynamic) equilibrium. It results in self-regulation of a system.
Your
body temperature starts to rise
above 37 degrees Celsius because you are walking in the tropical sun and the air temperature is 45 degrees Celsius. The sensors in your skin detect that your surface temperature is rising so you start to sweat and go red as blood flow in the capillaries under your skin increases. Your body attempts to lose heat,
lowering your body temperature
back to 37 degrees.
Examples of Negative Feedback:
*Notice that the reaction at the first of the example (body temp rising) is opposite from the reaction of the body at the end of the example (body temp falling) That is a clue that it is a negative feedback loop!
A thermostat in a central heating system is a device that can sense the temperature. When the
temperature in a room drops
below a set point, it switches a heating system on
raising the temperature
in the room back to the preset temperature.
Predator-prey interactions in an ecosystem. When
prey populations (mice) increase
, there is more food for the predator (owl) so they eat more and breed more, resulting in
more predators
which eat more prey so the
prey numbers decrease.
If there are fewer prey, there is less food and the
predator numbers decrease.
The change in predator numbers lags behind the change in prey numbers. The snowshoe hare and the Canadian lynx is a well-documented example of this.
Some organisms have internal feedback systems, physiological changes occurring that
prevent breeding
when population densities are high,
promoting breeding
when they are low. It is negative feedback loops such as these that maintain "the balance of nature"
Positive Feedback Loops:
Positive feedback results in a further increase or decrease in the output, that is feedback enhances the change in the system and it is destabilized and pushed to a new state of equilibrium. The process may speed up, taking ever-increasing amounts of input until the system collapses. Alternatively, the process may be stopped abruptly by an external force or factor. Positive feedback results in a "vicious circle".
Examples of Positive Feedback Loops:
You are lost on a high snowy mountain. When your
body senses that it is cooling
below 37 degrees Celsius, various mechanisms such as shivering help to raise your body core temperature again. But if these are insufficient to restore normal body temperature, your metabolic processes start to slow down, because the enzymes that control them do not work so well at lower temperatures. As a result you become lethargic and sleepy and move around less and less, allowing your
body to cool
even further. Unless you are rescued at this point, your body will reach a new equilibrium: you will die of hypothermia.
* Notice that the reaction at the beginning of the example is the same as the reaction at the end of the example. That is a clue that it is a positive feedback loop!
In some developing countries poverty causes illness and contributes to poor standards of education. In the absence of knowledge of family planning methods and hygiene, this contributes to population growth and illness, adding further to the causes of poverty, so more people are impoverished.
Global warming--higher temperatures may cause more evaporation so lead to more water vapor in the atmosphere. Water vapor is a greenhouse gas so will trap more heat so the atmosphere will warm more.
Feedback:
Movement and Storage of Matter and Energy
Transfers and transformations:
Both matter (or material) and energy move or flow through ecosystems. A transfer happens when the flow does not invlove a change of form or state, An example would be water moving from a river to the sea, or chemical energy in the form of sugars moving from a herbivore to a carnivore. A transformation happens when a flow involves a change of form or state, e.g. liquid to gas, or light to chemical energy. Both types of flow reqire energy; transfers, being simpler, require less energy and are therefore more efficient than transformations.
Examples of Transfers:
the movement of material through living organisms (carnivores eating other animals)
the movement of material in a non-living process (water being carried by a stream)
the movement of energy (ocean currents transferring heat around the globe)
Examples of Transformations:
Matter to matter (glucose converted to starch in plants)
Energy to energy (light converted to heat by radiating surfaces)
Matter to energy (burning fossil fuels)
Energy to matter (photosynthesis--sunlight into glucose)
Flows and storages:
Both energy and matter flow (as inputs and outputs) through ecosystems but, at times, they are also stored (as storages or stock) within the ecosystem.

Energy flows from one compartment to another, e.g. in a food chain. But when one organism eats another organism, the energy that moves between them is in the form of stored chemical energy: the body of the prey organism.

Energy flows through an ecosystem in the form of carbon--carbon chemical bonds within the organic compounds. these bonds are broken during respiration when carbon joins with oxygen to produce carbon dioxide. Respiration releases energy that is either used by organisms (in life processes) or is lost as heat. The origin of all the energy in an ecosystem is the sun and the fate of the energy is eventually to be released as heat.

Unlike energy, matter cycles round the system as minerals. Plants absorb mineral nutrients from the soil. These nutrients are combined into cells. Consumers eat plants and other consumers, ingesting the minerals they contain and recombining them in cells. Eventually decomposers break down dead organic mater (DOM) and return the minerals to the soil. These minerals to the soil. These minerals may be taken out of the soil quickly by plants or can eventually, through geological processes, become locked within rocks until erosion eventually returns them to new soil.
Complexity and stability:

Most ecosystems are very complex. There are many feedback links, flows and storages. It is likely that a high level of complexity makes for a more stable system which can withstand stress and change better than a simple one can, as another pathway can take over if one is removed. Imagine a road system where one road is blocked by a broken-down truck; vehicles can find an alternative route on other roads If a community has a number of predators and one is wiped out by disease, the others will increase as there is more prey for them to eat and prey numbers will not increase. Tundra ecosystems are fairly simple and populations in them may fluctuate widely, e.g. lemming population numbers. Monocultures (farming systems in which there is only one major crop) are vulnerable to the devastating effect. The spread of potato blight through Ireland in 1945-48 provides an example; potato was the major crop grown over large areas of the island, and the biological, economic and political consequences were severe.
Models of Systems
Simplified models of a system can help predict changes in the system by modeling reality, as systems work in predictable ways, following rules. We just do not always know what these rules are. A model can take may forms. Such as: a physical model, e.g. a wind tunnel or river, a globe or model of the solar system, an aquarium or terrarium. A software model, e.g. of climate change or evolution. Mathematical equations or data flow diagrams.
An Example: Climate models:
Modeling climate change is a complex business requiring huge computing resources. Simple models of the climate system have been developed to predict changes with a range of emissions of greenhouse gases. The models solve complex equations but have to use approximations. They have improved over 30 years. The early ones included rain but not clouds. Now they have interactive clouds, rain, oceans, land and aerosols. the latest climate models predict similar possible global average temperature changes to those predicted by models five or ten years ago, with increases ranging from 1.6 to 4.3 degrees Celsius. Models have their limitations as well as strengths. While they may omit some of the complexities of the real system (through lack of knowledge or for simplicity), they allow us to look ahead and predict the effects of a change to an input to the system.
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