The goal of this lab is to explore global population growth using STELLA. Historically human population has grown very slowly. However, this pattern has been disrupted within the last two centuries by exponential human population growth rates.
World population now stands at more than 6 billion people and is projected to continue growing through this century. Since the world population is estimated to have been 1.6 billion people at the beginning of the 20th century, this means that global population increased by four fold in only 100 years. This unprecedented increase in global population is due to a number of factors that decreased mortality rates worldwide. Factors that increased life expectancy include, better sanitation, increased agriculture, and antibiotic, vaccine, and pesticide use (Environmental Literacy Council, 2005).
The Global human population is expected to continue to rise over the next twenty years or more due to a large number of women reaching child bearing age. Recent estimates predict that the population will continue to increase by ~1.3 percent per year, adding about 78 million people each year (ELC, 2005).
Although the global population growth rate has slowed in recent years, the population is estimated to continue to grow over the next twenty years or more. Most of the increase will occur in nations that have the lowest income levels, depend heavily on natural resources and in areas of rich biological diversity where deforestation for fuel wood and cropland is a serious concern (ELC, 2005).
This rapid population growth has been associated with global environmental changes including:
To understand this overall pattern of population growth, it is useful to review a basic condition of demographic history, known as the demographic transition: the change of a population from high birth and death rates to low birth and death rates. The demographic transition generally occurs in four stages.
The following two charts illustrate this change.
Past and present demographic
It is important to note that this same transition took place
in every industrialized country in the world (all of Europe and
In developed countries, the decline in death rates was due to three major factors: the trade revolution, the agricultural revolution and the industrial revolution. All of these changes were gradual, and increased the general standard of living for the population, without major medical breakthroughs.
Today, the same demographic transition is occurring throughout the world’s less developed countries, though the chart shows some dramatic differences with the past transition.
The worldwide total fertility rate was estimated to be 5 births per woman when total fertility rates peaked during the period from 1965 to 1970; it is now estimated at 2.7 births.
Additional demographic trends are emerging: the aging of the population. People are living longer and having fewer children. As a result, the average age of the population is increasing, with a larger percentage of the population aged 65 years or older. The aging of populations will mean that larger numbers will require medical and other social services, services that will be provided by fewer numbers of young, productive workers.
Explore population growth and the past and present demographic transition by developing models of the dynamics characterizing:
Use Figure 6.a.3 to create your first model.
Human population growth 500,000 years before present to present
Start up STELLA and click on the globe icon so that it changes to a X2 (Chi-square) symbol to begin modeling.
Model the last 2000 years of human population growth. Global Population will be your stock . Use Figure 6.a.3 at 2000 years BP to determine an appropriate initial stock population.
Recall that changes in stocks are modeled in STELLA with the flow tool: . Changes in population are caused by births and deaths.
The final pieces needed for a complete model are birth rate and death rate. Add this using the converter tool: .
Use the connector tool: to complete the connections in your population model.
Based on the introductory reading, use 2.7 for your average birth rate. Define births with the following equation:
Add death rate to your model. What would your equation be for deaths? What would be an appropriate death rate? Determine this by changing the death rate to end your model with human population rate at roughly 6 billion people.
Specify an amount of time for the model under Length of Simulation:
From = 0, to = 2000, DT = 10, and Unit of Time = years.
Set up the viewing graph with the graph icon . Define your graph with the Population stock.
Add an end value to your model using the rectangle icon. Define it with your population stock. This will give you the ending value of your population at the end of the model run.
Run your model.
What happens when you run your model to 2150? How many people will be living on the planet according to your model?
The model we have created works well, however all populations have limitations to their growth. How can we fix the model to give a more realistic result? We can show this effect in our model by adding a carrying capacity variable and changing the equation for Births.
Add a converter to the model, below and to the right of the flow icon, and name it Carrying Capacity. Connect it to Births.
Change your equation to include the carrying capacity component of your model.
Births = Birth Rate*Population*(1 - (Population/Carrying Capacity))
What do you think is an appropriate carrying capacity for Earth? Why? Add this value to your model and run it. What happens to your population?
Now that you have a grasp on basic human population dynamics, we will explore the past and present demographic transition using STELLA. Open a new STELLA modeling page, copy and paste the model you have created that includes the carrying capacity. You will then alter the model to characterize the past demographic transition. Once the paste demographic transition has been characterized you will copy and paste this model, then alter it to characterize the present demographic transition. Recall that:
Compare the global population rates at the end of your simulations. What do you see?
Explain the difference between J and S shaped curves. How does carrying capacity of a population influence these relationships?
Does our environment today limit us? What may influence the carrying capacity of our environment (the Earth)?
What adverse effects might the human population experience as we get closer to our carrying capacity? In what forms (biological, social, political) would these adverse effects occur?
What other factors influence population growth? What other converter could be added to your model? How would it affect population dynamics?