From the Guide (IB ESS Guide 2015):

• Interactions should be understood in terms of the influences each species has on the population dynamics of others, and upon the carrying capacity of the others’ environment.

• A population is a group of organisms of the same species living in the same area at the same time, and which are capable of interbreeding.

• S and J population curves describe a generalized response of populations to a particular set of conditions (abiotic and biotic factors).

• Limiting factors will slow population growth as it approaches the carrying capacity of the system.

• Interpret graphical representations or models of factors that affect an organism’s niche. Examples include predator–prey relationships, competition, and organism abundance over time.(Applications and Skills). 

• Explain population growth curves in terms of numbers and rates. (Applications and Skills)

Student Task

Wolf Wars – Population Growth Curve 

When the new wolves in Yellowstone first came calling, the area’s elk and moose stood their ground as though they were still dealing with coyotes.  Bad idea.  Today, Yellowstone holds half the elk it did 15 years ago.  Yet by most measures, the population had swelled too high and their range was deteriorating.  Shortly after killing the last Yellowstone wolves in 1926, park officials were culling elk by the thousands.  The elk kept rebounding and overgrazing key habitats, creating a perpetually unnatural situation for a park intended to preserve nature. 

With a near-unlimited meat supply, Yellowstone’s new wolves rapidly multiplied.  But the count abruptly fell in 2005.  It increased again, reaching 171 in 2007, then sank to 124 by the end of 2008, a 27 percent drop this time.  Doug Smith, leader of the Yellowstone Wolf Project, recorded the fewest breeding pairs since 2000 in 2010.  “We have a declining wolf population,” he says.  “Numbers never got as high as we expected based on the availability of prey.  This suggests that once wolves reach a certain density, you start to get social regulation of their numbers.”

• National Geographic Magazine

Objective: Evaluate the population growth curve for wolves in Yellowstone post-reintroduction.

Procedures:

1. Since the reintroduction of wolves in 1995, the National Park Service has posted annual reports on the wolf populations in Yellowstone National Park. 
2. Review the Abstract/ Summary of each of the annual reports from 1995 to the most recent year.  
3. Within the abstract, identify the total number of wolves in Yellowstone National Park at the end of each year.  Begin with the first year – 1995.
4. Create a table and properly scaled graph to show the wolf population vs. time.  Be sure to title your table and graph and to label your axes. Use Microsoft Excel or Google Sheets to do this.
5. Label the following on your graph.  Create a key with definitions for each:
   1. Exponential Growth 
   2. Carrying Capacity (K)
   3. Overshoot​
   4. Dieback 

Analysis Questions:

1. Based on the 2014 annual report, how would you describe the population density of the wolf population?  (Random, Uniform, or Clumped) Why?  Is this what you would expect for wolves? 

2. Refer to your graph.  What is the relationship between the birth (or reproductive) rate and the death rate?
   1. Between the years 1995 and 1998
   2. Between the years 2002 to 2004

3. Refer to your graph and describe the effect of environmental resistance / limiting factors on the population growth at the time of wolf dieback.

4. Environmental resistance / Limiting factors can be classified in two broad categories – density-dependent factors and density-independent factors.  Based on your graph, explain which category you think is impacting the wolf population.

5. Describe what would happen to the elk population of YNP during the wolf population dieback. Sketch a graph of the interactions between elf and wolf from 1995 to the present.  This is just a sketch – I’m more interested in your knowledge of their interactions than specific numbers.

6. In recent years, there has been information documenting the wolves’ reintroduction as a success for the entire ecosystem. This phenomenon has been referred to as the cascade effect. What is the cascade effect and how has the return of wolves in Yellowstone benefited the entire ecosystem? Site specific examples.

7. What is the estimated carrying capacity of Yellowstone?  Understand that there are many factors that influence this number.  You may be able to decipher two different time periods where it appears that the wolf population has stabilized and reached a carrying capacity.  What do you think happened to cause a destabilization in the population size?  Site specific examples from the wolf reports.