SE3 Analyze population dynamics within an ecosystem

Suggested time: 4-6 hours

Learning Objectives

1. Explain various ways in which natural populations maintain equilibrium and relate this equilibrium to the resource limits of an ecosystem.
   
2. Construct and/or interpret graphs of population dynamics. (NUM)
   
3. Explore the technologies used to study biotic and abiotic components of ecosystems. (TL)
   
4. Discuss the ethics of studying biotic components of ecosystems. (CCT, COM)
   

Key Questions

1. What natural factors keep populations of organisms from growing forever?
   
2. What natural biotic and abiotic factors influence populations within an ecosystem?
   
3. How do scientists document population dynamics?
   

Key Concepts

• A population is all the members of a species that are living in the same habitat at a particular time.
  
• A community is all of the organisms in an ecosystem.
  
• Biotic potential is the maximum number of offspring that a species could produce if resources were unlimited.
  
• Carrying capacity is the maximum number of individuals of a species that an ecosystem can support.
  
• Natality is the birth rate in a population.
  
• Mortality is the death rate in a population.
  
• Immigration is the movement of members into a population.
  
• Emigration is the movement of members from a population.
  
• A closed system is one in which substances do not enter or leave.
  
• Scientists classify limiting factors that regulate populations using multiple criteria: intrinsic or extrinsic, biotic or abiotic, or density-dependent or density-independent.
  
• The effects of density-dependent factors increase in significance as a population grows (e.g., disease due to overcrowding).
  
• The effect of density-independent factors does not depend on the population size (e.g., fire).
  
• Science searches for cause-effect relationships that enable predictions to be made.
  

Pre-Instructional Questions

1. Are students aware that some populations fluctuate dramatically in size while others remain relatively stable?
   
2. Do students understand the natural factors that cause populations to change?
   
3. Do students understand how humans' actions can influence populations within ecosystems?
   

Suggested Teaching Strategies and Activities

1. Students should develop a brief case study of specific plant or animal populations to show how the carrying capacity of an ecosystem depends on the available resources in the environment.
   
   
2. Students should identify the major causes of change in the population of species in general (e.g., natality, mortality, immigration, emigration, predator-prey relationships, disease, competition, and resource limits) and describe those characteristics for a specific plant or animal population. Students should classify these factors as density-dependent or density-independent, and explain the differences in effects between these two categories of factors. Students could share their findings in order to search for patterns of factors that exist among similar types of species or species in similar locations.
   
   
3. Students should obtain population data or graphs for a specific population in an ecosystem. These are typically available in print format or on-line through governmental agencies or environmental groups. Students should analyze the data or graphs and provide explanations for the shapes of the population graphs and the resulting changes in populations. Students should identify characteristics that indicate growing, stable, or declining populations. Students might also analyze population graphs that indicate changes due to predator-prey relationships. (NUM)
   
   
4. Students should participate in a population simulation that will enable them to recognize the role of limiting factors and carrying capacity of a population in an ecosystem. An example of such an activity is "Oh Deer!" from Project Wild .
   
   
5. Students could research and describe the techniques and technologies (e.g., quadrat, mark-recapture method, radio collars, and GIS) that scientists use to determine characteristics of populations. Students should be able to explain why any specific technology or technique is more effective for certain populations. Students should also discuss the ethics of studying animals which may involve tranquilizing or immobilizing animals to attach radio collars, etc. (IL, PSD, CD 6.3)
   
   
6. Students could write a story from the perspective of a plant or animal within an ecosystem about how changes in other biotic or abiotic factors affect that particular plant or animal. Students might consider scenarios such as how might a wolf's life be different if the deer population were drastically reduced or how cutting a road through a stand of aspen might affect the community. Alternatively, students might create a dance, drama, or music piece to represent this scenario. (COM)
   
   
7. Students could discuss why scientists believe in cause and effect relationships, and why many scientists continue to look for these relationships in nature. For example, the construction of logging roads has led to increased access to wilderness areas, thereby leading to an increase in the number of hunters which in turn has led to a decrease in the elk population. Students could also discuss how other worldviews provide alternative explanations of relationships in nature. (PSD)