Comparing Exponential and Logistical Growth (4B)

 

1. Compare the tables for Addo Elephants in this exercise with the table at the beginning of the Introduction of Darwin's hypothetical elephants. Was Darwin as conservative in his calculations for reproductive rates as he claimed to be?
   • Using the data provided for year 40, Darwin was conservative in his calculations. He predicted the population to triple, and if you average out the predicted size from the exponential and logistic models, that is about what happens. However, the logistic model does predict a slower growth rate, so Darwin was not ridiculously conservative.
2. If the Addo elephants do eventually reach carrying capacity, what factors might limit further growth of the population? As a park manager trying to maintain biodiversity and ecosystem health, would you prefer to limit population growth at some point below carrying capacity? Explore some of the practical and ethical issues involved.
   • The most limiting factors, especially in a park, are space and resources. Elephants are large creatures who take up a large amount of space and who consume pounds and pounds of vegetation. They also have no natural predators, although hunting can be an issue. As a park manager, I may want to limit elephant populations to a level below carrying capacity in order to promote biodiversity, because elephants consume much of an area's natural resources. It may be impractical to cull elephant populations because the animals are so large. Ethical issues surrounding what to do with the removed elephants also arise. Killing animals is seen by many to be wrong, but if the extra elephants are not killed, where should they be relocated? Putting wild animals in zoos is also seen as unethical. There are also issues with translocating elephants because of the complex social structure; they may not embrace new individuals.
3. Since we really do not know what K is for Addo Elephant park, we presumed a carrying capacity of 500 for the purpose of this exercise. How might you determine a more realistic value for K in a field study of these animals?
   • In a field study of these animals, it would be necessary to determine the amount of space each elephant requires, and the amount of food and water in consumes in order to determine an accurate carrying capacity of the park. Water, food, and space are the most limiting factors, so elephant's use of these factors should be monitored in order to determine a more realistic carrying capacity.
   
    
4. Thomas Malthus was concerned that the global human population was growing exponentially, doubling every 50 years or less, and that we would eventually outgrow our food supply. What scientific advances since 1798 have altered r and increased K for human beings? Have these technologies actually solved the problems, or just postponed the famine Malthus was concerned about?
   • Advancements in agriculture, technology, and medicine have altered r and increased K for human beings. Agricultural advancements have made large-scale farms possible and have increased the availability of food. The discovery of electricity, the creation of the automobile, and the invention of modern appliances all worked to increase K by making it easier to get, store, and cook food. Medicinal advances, both those that fight illness as well as contraception, alter r by increasing life expectancy and allowing humans to change birth rates. These technologies have not solved our problems, they've just postponed Malthus' famine, by temporarily increasing the carrying capacity of Earth. We will soon run out of vital resources, causing a significant decline in population to a number that the planet can actually sustain.
5. The global human population is over 6 billion and still climbing. What do you think the global carrying capacity might be for humans? Consider agricultural production as a possible limiting resources, but also fresh water, regeneration of oxygen by plants, energy, minerals, and cultural factors. How might our lifestyle choices and diet affect the maximum number of people the earth can sustain?
   • Ignoring technological advances, I think the global carrying capacity for humans is around what the population was prior to the industrial revolution, around two or three billion people. Including technology, I would estimate it to be around four billion people. I present these numbers as sustainable populations sizes that can continue indefinitely without degrading human life, in a world where all people had equal impact on the earth. Many people in developed countries live in a manner that would require multiple Earths to support them; they can only continue in this manner because people in undeveloped countries have much smaller impacts on the planet. Our use of technology consumes absurd amounts of resources in order to feed us, and consumes many sources of energy in order to allow us to live our upper-middle class lifestyles. Large-scale agriculture consumes giant swaths of land and huge amounts of water, reducing fresh water and oxygen regeneration. If people lived simpler lives and ate locally, we could live on a healthy planet with vast natural areas. I would estimate the population size at which the earth can absolutely no longer support us to be around 10 billion.