Print this chapterPrint this chapter

Module 8

1. Module 8

1.35. Page 2

Lesson 9: Page 2

Module 8—Populations, Individuals, and Gene Pools

 

Explore

 

How do field biologists count organisms that run, swim, burrow, or fly away? What about organisms that are too dangerous to get close to or are too small to see? One of the biggest problems that field biologists face is how to get accurate population counts. Counting exact numbers is often difficult, so estimations of population size have to be made. In this lesson, you will learn some of the sampling methods used in field studies and how to do some of the simple calculations that give these population numbers meaning for wildlife managers who analyze the data.

 

Read

density (Dp): the number of individuals in a given unit of area (land) or volume (air or water)



growth rate (gr): the change in the number of individuals in a unit of time

 

 

gr will be positive if the population size is increasing, and negative if it is decreasing.

 

per capita growth rate: the rate of change per individual

 

 

Cgr can also be positive or negative depending on whether the initial N is.

 

For an introduction to the methods and calculations involved in studying population growth, read from the introduction on page 704 to the beginning of “Factors That Affect Distribution Patterns” on page 705. Then read “The Rate of Population Growth” from page 708 to page 711. 

 

Pay close attention to the important concepts of density (Dp), growth rate (gr), and per capita growth rate (cgr) as different ways of expressing how the population has changed. Take note of the reasons for using each of the three expressions. You need to be comfortable calculating each of these variables: Dp, gr, and cgr. Create a clear and easily accessible table in your course folder for the formulas and their definitions. Examples are a helpful study tool.

 

Remember that in the Diploma Exam you will encounter many of these problem-solving situations. Whenever possible, practise solving problems from the text or from this lesson. If you are having any difficulty, consult your teacher

 

Read

 

clumped distribution: organisms grouped together; occurs in highly competitive environments

 

random distribution: no pattern exists in organism distribution; occurs in environments with little competition

 

uniform distribution: organisms are equally spaced apart; occurs in artificial environments (e.g., agricultural crops)

It is tempting to assume that the density of a species in a particular area might represent that individuals are spread out evenly to fairly distribute the resources available. However, some species live clumped together in tightly knit cooperative groups. Some organisms are loners that require large territories that separate them from others of their kind. Some organisms rely on the forces of nature such as wind or water to disperse them, and so are randomly distributed in their environment. Uniform distribution, where individuals are evenly spaced in an area, is rarely observed in nature.

 

Read “Factors That Affect distribution Patterns” on pages 705 to 707 of the textbook to learn more about the concept of distribution.

 

Make careful notes on the significance of each distribution pattern and what conditions lead to each pattern. Diagrams are also a good way to store this information in your course folder.

 

Self-Check

 

SC 1. The population size of microscopic Paramecia in a 1-L hay infusion needed to be determined. By using a depression slide (a microscope slide with a well in the center) at medium power and averaging the results of three 1-mL samples, it was found that there were, on average, four Paramecia/mL of hay infusion. What was the population (N) in the entire 1-L infusion?

 

SC 2. Refer back to “Get Focused” at the start of this lesson. Will the lack of competition and natural selection in white-tailed deer populations increase or decrease the genetic diversity of the gene pool? Explain your answer.  

 

SC 3. There is a lovely white daisy with fern-like leaves that adds brightness and cheer to the roadside ditches of rural Alberta. Unfortunately, it is an inadvertently introduced species and is very invasive, competing vigorously with indigenous wild species. Your job is to provide data on the spread of the species, beginning with a population count of a 1-km2 parcel of land.

  1. How would you count the daisies? (Refer to samples on pages 704 and 705 in your textbook.)

  2. Would you do one sampling or several? Why?

  3. Would it be more informative to express the count as N (population size) or as density? Why ?
Check your work.
Self-Check Answers

 

SC 1. N = 4 Paramecia/mL X 1000 mL = 4000 Paramecia

 

SC 2. It depends. If conditions have been highly competitive up until recently, then the allele frequencies of poorer alleles will now increase in the gene pool. However, if no particular phenotype has been selected for in some time, all genotypes will have equal reproductive fitness. Allele frequencies may not change significantly.

 

SC 3.

  1. Create a small quadrat sample of 1 m2 and count the number of daisies within the quadrat. Randomly choose the location of several quadrats (ten) and average the results. Extrapolate that number to the number of 1-m2 parcels in the count area. Example: the average of ten quadrats was 4 plants/m2. 4/m2 x 1 000 000 m2/km2 = 4 000 000 plants. Therefore N = 4 000 000 plants.

  2. Plants do not show uniform distribution, so averaging the results of randomly selected samples improves the statistical validity.

  3. Density gives a visual idea of what the area looks like. N = 4 000 000 plants doesn't mean much to an observer, but 4 plants/m2 is much easier to visualize.