Module 8
1. Module 8
1.41. Page 4
Module 8—Populations, Individuals, and Gene Pools
Reflect and Connect
Based on the concepts of this lesson, reflect on whether the distinction between logistic and exponential growth is always clear.
Mice, as discussed in the beginning of this lesson, have amazingly high, near-exponential reproductive rates. When graphed, their growth will show a very steep, almost J, curve. But mice too will eventually respond to famine and environmental resistance and come to a carrying capacity with their environment.
Mosquitoes might be considered an exponentially reproducing species. As long as there are pools of water in ditches and ponds and temperatures stay below freezing, mosquitoes will lay thousands of eggs, reproducing exponentially at biotic potential. But even mosquitoes’ reproductive rates are limited by whether or not they can obtain a blood meal from warm-blooded sources (such as humans, elk, or deer) to incubate the eggs they carry within them.
Consider humans. Human reproductive rates respond to density-limiting factors such as lack of food. All mammals, including humans, will produce fewer eggs and sperm when body fat content becomes low, sometimes preventing reproduction entirely. In Unit B you discovered why many anorexic women or female elite athletes may not have a menstrual cycle. The human growth curve should, therefore, have the S shape of a logistic growth pattern. However, the human growth curve is a distinct J. Environmental resistance does not appear to have any significant effect. This is largely due to human's ability to use technology to produce more food and combat disease. It remains to be seen whether the curve will flatten into an S shape as the carrying capacity is determined or whether it will continue through to the usual conclusion of J curves—a population crash. Fortunately, the most recent data shows that the human growth curve is getting somewhat flatter—perhaps more a result of birth control and higher education levels than the effects of environmental resistance.
Discuss
You have studied the graph on page 733 of your textbook, which shows the human growth curve. Can human population growth continue like this indefinitely? Does the word carrying capacity even apply to Homo sapiens? Has human society seen evidence of environmental resistance? Are density-dependent and density-independent factors at work now? What are the consequences to Earth? Develop, present, and defend your position on Earth’s carrying capacity of Homo sapiens on the discussion board. Work with other students and your teacher in developing more ideas. File you work in your course folder.
Module 8: Lesson 10 Assignment
Retrieve the Module 8: Lesson 10 Assignment you downloaded earlier. Complete all three parts (Part A, B, and C) and save a copy of the completed assignment to your course folder. Submit the completed assignment to your teacher for assessment.
Self-Check
SC 6. Which of the following would not be an example of environmental resistance?
- drought
- increase in predators
- disease due to waste accumulation
- lack of food
- lack of nesting sites
- organisms with a high biotic potential
- organisms with a low biotic potential
- organisms reproducing at their biotic potential
- organisms that have many offspring at a time
- stationary phase: high b, low d
- lag phase: low b, high d
- exponential phase: high b, low d
- lag phase: b and d are equal
- exponential growth and population crash
- logistic growth and J curve
- exponential growth and carrying capacity
- exponential growth and S curve
- Density of hares was highest in the control group.
- Removing predators doubled the population density.
- When population density rises, environmental resistance offered by lack of food and increased predators are equally important in controlling density.
- The combined effect of removing both food and predators was greater than the sum of both effects.
Self-Check Answers
SC 6. E
SC 7. C
SC 8. C
SC 8. A
SC 9. D