Activity 5E: Developing your own system dynamics diagram
In this activity the aim is to practice system dynamics diagramming as a means to visualise the interaction between stocks and rates as part of a systemic understanding of a complex situation.
For this activity, I would like you to focus on a few components which you will need to choose from the sign graph you developed in Activity 4A. This can be an entirely paper-based exercise – you do not have to use NetLogo (or a graphics software package) to draw your system dynamics diagram. To construct the diagram proceed as follows:
- i. Sketch out a system dynamics diagram with one of your chosen components as the ‘central’ stock.
- ii. Name the rates that cause an increase and a decrease in the stock level.
- iii. Now, identify at least two other stocks which influence the input rate and/or the output rate.
- iv. Finally, identify (by drawing links) and describe (verbally and/or mathematically) the relationships between these stocks. Are there any positive and negative feedback loops?
Now for the reality check. For at least the central stock, collect trend data. This does not necessarily have to be numerical – graphs will be fine. There are several excellent sources of global and national trend data online, including the. You may encounter significant challenges for this part of the activity – the measurement of trend data is highly dependent on the definition attributed to a particular stock or rate. For example, some of the data you will come across will only represent a subset of what you are looking for – limited to a particular geographical region or sub-topic of your particular area of interest. You will therefore need to develop more precise definitions for your stocks and/or accept less accurate data.
You should choose at least a 50-year time span for your stock(s). Obviously, the greater the time span, the more you can infer about the relationship between the stocks. If you cannot find all the required data you can ‘estimate’ some values, as long as the variations over time make sense. Use this data to explore the dynamic changes in the stock(s) over the time period you have selected by roughly sketching one or more graphs. Now answer the following questions:
- What trend do you notice in your stock(s)?
- Is there constant or exponential growth or decrease at some stage in the dynamics?
- Is there a levelling off occurring?
- Can you detect oscillations in the stock(s)?
- What does this tell you about the feedback loops in operation?
This activity is very much a comprehensive visual, verbal (and mathematical) modelling exercise of the behaviour of a complex dynamic system over time. The system dynamics diagram, the graphs, verbal and/or mathematical descriptions of the stocks, rates and their relationships, and how these change over time all contribute towards a systemic understanding of the complex situation of your choice.
Since I have already explored a complex situation – the interplay between human population and biocapacity – by developing a system dynamics diagram and associated verbal and mathematical descriptions in the previous activity, I thought it would be interesting to find trend data to support my simulation. There is a wealth of information on human population dynamics on the internet (e.g. http://www.eoearth.org/ article/ human_population_explosion ) and all the trends support the very first part of my simulation output i.e. an exponential growth in human population. However, data to support the subsequent simulation behaviour is more difficult to find. I therefore chose to focus on one of the few historical accounts of population collapse which has been directly correlated to biocapacity overshoot: Easter Island. The following is an account compiled from a variety of sources.
When Captain Cook landed on Easter Island in 1774, during his exploration of the south Pacific, he found a tribe living in desperate conditions surrounded by great monuments; evidence of a once prosperous and technically advanced civilisation. When these Polynesian people first arrived on Easter Island, in the 5th century, they found a land covered in lush forests and fertile valleys. Over 1000 years these resources fuelled a great cultural flourishing, resulting in the population growing to an estimated 7000 people and the construction of prodigious monuments.
Recent archaeological evidence shows that the last trees were harvested around 1550. The sudden disappearance of their primary resource resulted in a dramatic implosion of the Easter Island civilisation with population numbers crashing to as low as 3000 in just a few decades. Without the trees the islanders could no longer cook efficiently, build houses, and protect their fragile topsoil. In his diaries, Captain Cook recorded that the now primitive population lived in makeshift reed huts or caves, and were engaged in cannibalism in order to supplement their limited food supplies.
Some researchers (Rojstaczer et al., 2001) have estimated that humans are currently appropriating between 10% and 55% of the products of terrestrial photosynthesis, the fundamental basis for life on earth. The problem now is that humanity may be reaching the global limit of such exploitation. Although population growth is predicted to stabilise during this century, there is no sign of an equivalent stabilisation in the per capita consumption of biocapacity. Can we rein back from the current overshoot to avoid collapse? This is the question which you will be exploring in the next section.