These were the positive feedback loops that carried VHS to victory and caused Betamax to disappear. Populations and Feedback Systems 19 positive feedback Figure 2. A change in A causes an opposite change in B, and a change in B causes an opposite change in A. It keeps things the same. When part of a system changes too much from what it should be, other parts of the system change in a way that reverses the change in the first part.
The function of negative feedback is to keep the parts of a system within limits that are necessary for survival.
Negative feedback is a source of stability; it is a force against change. Homeostasis is an example of negative feedback in biological systems.
Homeostasis is control of an organisms internal physical and chemical condition within limits required for the organisms survival. If body temperature increases above 37 Celsius, negative feedback reduces the body temperature by: reducing metabolic heat generation; and increasing heat loss from the body more blood supply to the skin and more sweating.
If body temperature decreases below 37 Celsius, negative feedback increases the body temperature by: increasing heat generation shivering ; and decreasing heat loss less blood supply to the skin and less sweating. Keeping the body temperature close to 37 Celsius is essential for a persons survival. Negative feedback is common in social systems. For example, people use negative feedback to drive a car.
If the car starts to go off the road, you steer it in the opposite direction to bring it back 20 Human Ecology negative feedback Figure 2. Body response to reduce the difference Body temperature Compare with 37C Difference between body temperature and 37C onto the road. In other words, when the trajectory starts to change, your negative feedback as a driver reverses the change by bringing the car back. Engineers use negative feedback for machines. If an aeroplane starts to descend toward the ground when it should not, the automatic pilot in the aeroplane makes it go upward so it stays at the correct altitude.
Then one male and one female deer come to the forest. After one year they have two fawns. A year later the young deer are old enough to reproduce, and each pair produces two more fawns.
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The deer population continues to double each year, and after ten years there are deer. Deer need plenty of food to grow and produce offspring. However, there is not as much food as before, because the larger numbers of deer are eating so much of it. The deer are less healthy, more susceptible to disease and sometimes die at a young age because they do not have enough food. Moreover, a malnourished deer may produce only one fawn instead of two. This story tells us that deer are limited by their food supply: When population increases, the food supply decreases.
When population decreases, the food supply increases. When the food supply increases, births increase and deaths decrease. When the food supply decreases, births decrease and deaths increase. Therefore, when population increases, the food supply decreases, the birth rate births. When population decreases, the food supply increases, birth rate increases and death rate decreases. Population regulation and carrying capacity The story of the deer is the story of all plants and all animals, including people. Why do plants and animals have the abundance that they have?
Why arent there more? Or less? The explanation is Populations and Feedback Systems 21 population regulation population regulation. Population regulation uses negative feedback to keep plant and animal populations within the limits of the carrying capacity of their environment. Carrying capacity is the population that the food supply in the environment will support on a long-term sustainable basis.
Because the resources that sustain populations are limited, no population can exceed the carrying capacity of its environment for long.
With the positive feedback loop, an increase in the population leads to more births, which increases the population even more. With the negative feedback loop, an increase in the 22 Human Ecology population regulation Figure 2. Less food means more deaths and fewer births. If the number of plants or animals in a population is less than carrying capacity, births are greater than deaths and the population increases until it reaches carrying capacity. If the population is larger than carrying capacity, deaths are greater than births, and the population decreases until it reaches carrying capacity.
Once a population is close to carrying capacity, births are more or less equal to deaths, and the population does not change much. Returning to the story of the deer, Figure 2. The population growth curve is exponential because of positive feedback, but exponential growth cannot continue forever. What will happen during the next 20 years? An S-shaped sigmoid curve for population growth shown in Figure 2.
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The exponential growth in the first part of the sigmoid curve is followed by population regulation as the number of plants or animals approaches carrying capacity and negative feedback takes over. In many instances a population increases gradually and then fluctuates in the vicinity of carrying capacity the solid curve in Figure 2. It does not stay precisely at carrying capacity because: negative feedback is not highly precise; and other factors besides food supply can have an impact on births and deaths.
Populations and Feedback Systems 23 population regulation Figure 2. If a population overshoots, it usually depletes its food so severely that negative feedback in the form of more deaths and fewer births quickly reduces it below carrying capacity. Both kinds of feedback are essential for survival.
Negative feedback provides stability; it keeps important parts of the system within the limits required for proper functioning. Positive feedback provides the capacity to change drastically when necessary. The development and growth of all biological systems from cells and individual organisms to ecosystems and social systems is based on the interplay of positive and negative feedback. Ecosystems and social systems can stay more or less the same for long periods, but sometimes they change dramatically and rapidly.
They function best when they have an appropriate balance between the forces that promote change and the forces that provide stability. People constantly interact with these forces of change and stability.
People depend upon negative feedback to take care of things and keep everything functioning smoothly most of the time. When people try to improve their situation development or solving problems , they use positive feedback to help make the 24 Human Ecology practical significance Figure 2. However, in addition to working for people, positive and negative feedback can also work against them. Sometimes people try to improve things or solve a problem, and no matter what they do, there is no improvement because they are working against negative feedback that prevents the changes that they want.
Other times, people prefer things to stay the way they are, but positive feedback amplifies seemingly harmless actions into changes that they do not want. If we pay attention to the positive and negative feedbacks in our social systems and ecosystems, we can use the feedbacks to our advantage instead of struggling against them. In the case of ecosystems, this means fitting our activities with ecosystems to do things natures way, so nature does most of the work and keeps things going.
The concrete meaning of doing things natures way will become more apparent in subsequent chapters. Think of examples of positive feedback at different levels of social organization in your social system: family and friends, neighbourhood, city, national, and international.
Draw diagrams to show circular chains of effects ie, feedback loops. Do some of the feedback loops generate sudden changes? Think of examples of the replacement of one thing by another in your social system or ecosystem during recent years. Draw a diagram to show the chain of effects and feedback loops that generated the replacement.
Think of examples of negative feedback at different levels of social organization in your social system.
Draw diagrams to show the circular chains of effects. What kind of population change does this graph show during the first ten years? Does positive feedback or negative feedback dominate the form of the graph when the population is small? Will the same kind of population change continue forever? Draw the graph in Figure 2. Is negative feedback important when the deer population is small or when it is large?