![]() This ‘Biocapacity_Multiplier’ is simply introduced to the birth rate as a multiplier: This basically calculates the amount of biocapacity there is per individual: high levels of biocapacity, and low levels of consumption and population, will result in a high ‘Biocapacity_Multiplier’ value. The following is a simple equation that characterises the impact of population, biocapacity and consumption on birth and death rates:īiocapacity / (Consumption_per_Individual * Population) To address this a new ‘Biocapacity_Multiplier’ variable has been added which links the ‘Population’ and ‘Biocapacity’ stocks. But how do limitations in biocapacity affect the population birth and death rate? The above expression calculates how fast the ‘Biocapacity’ stock is drawn down. The equation in the ‘Consumption’ rate is as follows: This new variable has been linked to the ‘Consumption’ rate. Also, a new variable called ‘Consumption_per_Individual’ has been added and assigned the value of ‘1.8’ (every individual in the population requires the equivalent of 1.8 gh of biologically productive area per person per year, the value considered to be sustainable at current population levels as estimated by WWF for 2003). The ‘Consumption’ rate has to be divided by the number of individuals in the population, so a link has been inserted from the ‘Population’ stock to the ‘Consumption’ rate. Figure 5.14 System dynamics diagram of the NetLogo model, Activity_5D.nlogo, showing the interrelationship between population growth and. This figure shows a screen shot of the Activity_5D.nlogo system dynamics model which is an extension of the Activity_5C.nlogo model through the addition of the ‘Biocapacity’ stock and associated flows and variables. ‘Biocapacity’ is renewed by a flow of new resources into it, which I have called the ‘Regeneration’ rate and a flow out of it, which I have called the ‘Consumption’ rate. This biocapacity will represent the key limiting factor for the population in this new model. Here is where things get really exciting, in that the rather static ecological footprint calculations take on a new meaning through dynamic simulations. You may remember that you came across the term ‘global hectares’ in Section 3 as this was the unit used in measuring your personal ecological footprint. The World Wildlife Fund for Nature’s (WWF) 2006 Living Planet Report estimated that in 2003 we had just over 11 billion global hectares (gh) of biologically productive area ( Living Planet Report, 2006). ‘Biocapacity’ is the area of biologically productive land and water required to provide the resources we use and to absorb our waste. You will notice that this new model has an additional stock called ‘Biocapacity’ (Figure 5.14). Let’s explore the concept of overshoot and collapse by extending the population growth model. This occurs when, for example, populations use up resources faster than the resources can regenerate themselves – the population can still grow by drawing down on the resource, but once this resource is depleted and no alternative is available, collapse ensues. This is because there is a time lag between the carrying capacity threshold being crossed and negative feedback kicking in. However, this is not the typical behaviour of populations – it is often the case that the overall growth rate takes a population over the carrying capacity. This maintains the population below the carrying capacity threshold. The assumption in the model is that the death rate increases rapidly as the population nears its maximum carrying capacity. In the model used in Activity 5C the slightly more sophisticated ‘death loop’ results in a steady-state population. In this activity the aim is to understand the effects of time lags, overshoot and collapse. Activity 5D: Simulating overshoot, collapse and regeneration
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