Understanding the environment: Flows and feedback
Understanding the environment: Flows and feedback

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Understanding the environment: Flows and feedback

Reading 4.2: Positive feedback and change

Simple positive feedback loops are easily illustrated since they are the mechanism through which anything changes rapidly. Take for example the explosion of water hyacinth when introduced into new environments:

Water hyacinth is a floating plant that has spread from South America to waterways around the world. It can cover the water so completely that it obstructs the movement of boats. Imagine a lake that is 10 kilometres in diameter. It takes 8 billion hyacinth plants to cover a lake of this size completely. To start with, our lake has no water hyacinth. Then we introduce one hyacinth plant on to the lake. After one month, this plant forms two plants. After another month the two plants have multiplied to four, and the doubling continues month after month. Two years pass, and the hyacinths have multiplied to 17 million plants. Nobody pays attention to them because 17 million plants cover only 0.2 per cent of the lake. Six months later, 30 months after we put the single plant on the lake, there are one billion hyacinth plants, which cover about 13 per cent of the lake. Now people notice the hyacinths. Although there are not enough hyacinth plants to be a problem for the movement of boats, some people are worried. Other people say, ‘Don’t worry. It took a long time to get this many hyacinths. It will be a long time before there are enough to cause a problem.’ Which people are right? Is the problem a long time in the future, or will there be a problem soon? In fact, with hyacinth doubling every month, the lake will be completely covered after only three more months.

(Marten, 2003, pp. 15–16)

The exponential growth of the water hyacinth population is a positive feedback relationship between the population number and the amount of births (see Figure 4.2). The greater the population, the greater the amount of births, and the more births there are, the bigger the population gets and the faster it does so.

Figure 4.2
Figure 4.2 Positive feedback diagram of births and population

Gerald Marten’s example is actually a very real occurrence in many lakes and rivers in Africa, North America, Australia and Asia. The water hyacinth prevents navigation and blocks sunlight from reaching the water, while contributing to the water’s dead biomass, further starving the water of oxygen. A large-scale death of fish results and fishermen can't even get to the few fish that are left. A basic understanding of positive feedback may have allowed fishermen to destroy the few water hyacinths at the beginning of the invasion.

This is an example, however, of positive feedback involving the flow of matter and energy within wetland systems. Positive feedback also works with flows of information in living systems. Consider a model of the interaction between teacher and pupil. Here I can model the variables ‘teacher attention’, the amount of time the teacher is prepared to give to a particular student, and ‘student’s performance’, the marks they achieve in examinations and tests. The model implies that teacher attention drives an increase in student performance, which in turn drives an increase in teacher attention, thus creating a positive feedback loop. As in the case of the water hyacinth, the positive feedback model indicates that any increase in the attention that the teacher pays to the student, if sustained, will result in ever increasing performance. At the start the change will be slow, and perhaps not very noticeable, but with time the rate of change will dramatically increase. Modelling change in this way indicates that in order to create change you must put in place a positive feedback loop.

Take, for example, the sale of fair trade and organic products. In order to increase sales of ethical and organic products with limited funding for marketing, companies have to ensure that people who buy the product will tell their families, and friends what a good product they have purchased. That has implications for the quality, price, environmental impact, health benefits and ethics of the product. There will be no word of mouth recommendations if the product does not address most if not all of the previous criteria. But if it does, then increased sales will automatically imply increased word of mouth recommendations, which will further imply increased sales – the positive feedback loop that is required for positive change in society.

The above examples are actually not the full story. All positive feedback must eventually come to an end for, sooner or later, the ‘resources’ on which the rapid change is based will also come to an end. But at the end of this process, there is no guarantee that the situation will revert back to its original state. The lakes in Africa are still smothered in water hyacinth, and the high performing student may go on to a successful and fulfilling career. Securing and sustaining positive change in society is a much greater challenge, with many examples of failure as well as success.


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