Chapter 9 Hands-On Problem Solving

Density-dependent and Density-independent Factors

Introduction

(This exercise is based on Karels, T. J. and R. Boonstra. 2000. Concurrent density dependence and independence in populations of arctic ground squirrels. Nature 408: 460–463.)

(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)

Tim Karels, Rudy Boonstra, and their colleagues at the University of Toronto at Scarborough have been engaged in a long-term study of the population ecology of arctic ground squirrels, Spermophilius parryii, that live in the boreal forest of southwestern Yukon. As part of this larger project, Karels and Boonstra monitored populations of these squirrels during the year 1996 for their population size, the proportion of adults that survived the following winter hibernation, and the proportion of females that successfully weaned their litters.

Figure 1  The proportion of females weaning their litters (top) and the proportion of adults that survived the following winter (bottom) as a function of population density for the year 1996.

Questions

Question 1
What can you infer about the actions of density-dependent and density-independent factors affecting overwinter survival and ability to wean litters?

Question 2
Two other populations were supplied with extra food. What would you expect if food limitation was an important density-dependent regulator of these populations? If food limitation was not an important density-dependent factor, what would you expect?

Figure 2

Question 3
Figure 2 shows the food-added populations (in squares). What inferences can be drawn?

Figure 3

Question 4
The squirrel populations were monitored the following year for the same traits (1997), and the results are shown in Figure 3. What can you infer about the actions of density-dependent and density-independent factors affecting overwinter survival and ability of females to wean litters during 1997?

Question 5
What are some possible density-independent factors that could have reduced weaning success in 1997?

Question 6
Given what you know about these squirrels, what relationship between density and population growth rate would you expect? Draw a graph of this expected relationship.

Question 7
What growth rates would you expect for the populations that had been supplied with extra food?