Hand-out 1: Ecology for Benildeans



Hand-out 6: Population Ecology

Lecturer: Mr. Jose Mari Balines Gonzales

M.Sc. in I.T. for Natural Resources Management

Objectives:

• To examine the positive and negative impacts of population growth to the natural environment and human society.

• To interpret population trends.

• To formulate strategies of regulating population growth.

Introduction

Examine the figure (Fig.6-1) below. Take time to determine significant lessons from it.

[pic]

Population

• Describes the number of individuals of a species occupying a defined area at a given time. (Bush, 1997)

o Dynamic – is an adjective that relates to the effects of forces in nature. It also relates to activity or things in movement. As a noun, it pertains to any driving force instrumental in growth or change (esp. social): a pattern of growth or change. (Chambers 20th Century Dictionary, 1983)

• Populations have traits of their own that differ from those of the individuals composing the populations.

• Populations are described in terms of density (population density).

Population Density

• Population density is expressed as the number of individuals of a species per unit area.

Example: 50 apple trees/hectare

3 black bears/km2

20 people/ km2

• It (population density) is of great importance because it is a limiting factor that affects living condition, mating activity and efficiency of a species.

• It is related to population regulation (Fig.6-2).

• It affects habitat distribution.

o Animals initially choose the most preferred sites. As they fill-up, they become less suitable because of the unfavorable effects of crowding. Later-arriving individuals go to habitats that have lower basic suitabilities but are not crowded.

• It determines the carrying capacity of an environment (Fig.6-3).

o Carrying capacity (k) – the population size that the area has the resources to support.

• It affects the rate of intraspecific competition.

Competition may be simply a matter of each individual taking as much as it can get of some resource, called exploitation.

Example: food or territory

Dispersion (Spacing)

• Refers to the position of members of a population relative to their neighbors.

• 3 basic kinds:

1. Random – no pattern to their spacing. Not very common.

2. Clumped – individuals tend to occur in bunches with empty spots between. Most common.

3. Even – individuals are spaced more regularly. Not very common.

• Close contact between two individuals is commonly due to mating and care of dependent young.

Survivorship Curve

• A visual aid to detect changes in survivorship (and mortality) by period of life.

• 3 general categories (Fig.6-4):

1. Convex curve (Type I) – indicates high survivorship of the population up to a certain age then drops at an age where most of the population dies. This happens when environmental factors are unimportant and when most of the organisms live out their full physiological longevity.

2. Diagonal curve (Type II) – shows that the same amount (in percentage) of the population is lost (dies) each time period (or age). The environment is an important factor here.

3. Concave curve (Type III) – indicates high death among juveniles and low mortality as the remaining ones succumb to environmental factors.

• Most frequent type of survivorship curve (Fig.6-5):

[pic]

Models of Population Growth

• These are models that show the different ways by which a population may grow.

• 2 models:

1. Exponential Population Growth:

• The curve of numbers of individuals versus time becomes steeper and steeper. An example of this model is human population growth (Fig.6-6).

• The growth rate is low when the population size is small, and becomes fast when population size becomes big.

• Two influencing factors: population size and the capacity of a population to increase.

• Capacity of a population to increase is brought about by:

a) Number of offspring per breeding season.

b) Survival up to and through reproductive age.

c) Age at which reproduction begins.

d) Length of the reproductive age.

2. Logistic Population Growth:

• At the beginning it grows exponentially (Fig.6-7). Then, when population size is medium, it begins to slow until it reaches zero population growth (BR=DR, refer to Fig.6-2).

• This kind of growth is also determined by the size of the population and by the capacity of the population to reproduce or increase as modified by the various effects of crowding.

Example: Mother’s poor nutrition

High death rates because of predators

Increased emigration

• Population growth is influenced by:

1. Sex Ratio

• Proportions of the two sexes (Fig.6-8).

• Expressed as the number of males/number of females.

• Sex ratio is measured at:

a) Conception – primary sex ratio.

b) Birth – secondary sex ratio.

c) Juveniles – tertiary sex ratio.

d) Adults – quartenary sex ratio.

2. Age Structure

• Proportions of individuals of various ages (Fig.6-9).

• Age of an individual is important in specifying its role in the population, life’s expectation and reproductive rate, guess social relationships and energetic requirements.

[pic]

Population Regulation

• Density-dependent populations are affected by:

1) Extrinsic factors – population’s interaction with the rest of the community.

Example: predation, parasitism, disease and interspecific competition.

2) Intrinsic factors – populations own response to density.

Example: intraspecific competition, immigration, emigration, and physiological and behavioral changes affecting reproduction and survival.

E N D

References:

Brewer, R. 1994. The Science of Ecology, 2nd Ed. Saunders College Publishing, USA.

Bush, M.B. 1997. Ecology of A Changing Planet. Prentice Hall, Upper Saddle River, New Jersey

Chambers 20th Century Dictionary. 1983. W & R Chambers Ltd. Edinburgh, Great Britain.

Enger, E. and Smith B. 2006. Environmental Science: A Study of Interrelationships, Tenth Edition. McGraw-Hill Companies, Inc. New York, USA.

URLs:

Essay of Thomas Malthus:



APPENDIX 6-1

What is an Insight Paper?

An Insight Paper does one of two things:

1) It shares some insight that you’ve gained from reading the text (or viewing a film).

2) It strives to achieve some insight from the reading you’ve done.

• Insight papers are not summaries of what you’ve read (or viewed).

• Your paper should reveal an intellectual engagement with the text (or film) and that engagement may take any number of forms: evaluation, analysis, critique, synthesis, etc.

• Note: insight refers to the act or result of apprehending the inner nature of things, so you’ll want to try to grasp the nature of the thing in front of you.

o You could analyze in some detail a particular knotty part of an argument.

o You could evaluate an argument or claim as either good or bad.

o You could contextualize the text you have read (or film you had viewed) across others you’ve read (or watched).

o You could explain why a particular author happens to be right or wrong. There are other possible reactions, too—those listed are just examples. But the bottom line is that your Insight Paper must demonstrate intellectual engagement with the text (or film).

Source:



APPENDIX 6 -2

How to Construct a Population Pyramid in Microsoft EXCEL

SOURCE:

1. Go to the Philippines National Statistics Office web site on projected populations (medium series) for 2000 - 2040: .

2. Examine the data for Philippines (covers projections for both sexes, males and females in specific age ranges) with columns projecting the population from 2000 until 2040 in five-year intervals.

3. Copy the data on Age, Male, and Female population (under the 2005 column) into Excel for the country of your choice.  To get the first column to record the age range correctly, choose the column, format cells, and choose TEXT.  Format the spreadsheet so it looks similar to the China example below.

 

4. In cell D4 enter the formula =B4+C4.

5. Copy Cell D4 down to the bottom of the column (cell D21 in example above)

6. Put the active cell in cell B22 above and click on the AutoSum and Enter. This will calcualte the total population of males.

7. Copy this cell across to row D (D22 in example above).

8. To calculate the percent males in cell E4, enter the formula =100*B4/$D$22 (notice the direct cell reference to the total population cell with the use of the $).  Some of the demographic tables do not include ages 80 and older, so the total population cell reference may be different than the example above (for example D21 instead of D22).

9. Copy the formula from cell E4 down to the bottom of the column, E22 in the example above.

10. Format cells E4 to E22 to two decimal places by blocking that range and choosing Format-->Cells, Number Tab, Number, 2-decimal places.

11. To claculate the percent females in cell F4, enter the formula =100*C4/$D$22

12. Copy the formula from cell F4 down to the bottom of the column, cell F22 in example above.

13. Format Cells F4 to F22 to two decimal places by blocking that range and choosing Format-->Cells, Number Tab, Number, 2-decimal places.

14. If you have not done so already, Autosum the percent male and female columns in the cells E22 and F22 in the example above and below.

15. We must fool Excel by making the % male column negative, to make the population pyramid.

16. Alter the formula in cell E4 to be = -1*100*B4/$D$22 and copy this formula down to cell E21.

17. The spreadsheet should now look like this:

Constructing the population pyramid:

1. To build the pyramid, we need to choose a horizontal bar chart with two series of data (% male, and % female) and the age labels in column A as the Category X-axis labels.  

2. Block out the range A3 to A21 , hold down the CTRL key and block out the range E3:F21

3. Click on the chart Wizard button, select the default of clustered BAR chart with horizontal bars.  Click Next.

4. Click Next again. In the Titles tab enter the title: Population Pyramid for Philippines, 2005, Data Source: Philippines Statistics office.

5. Enter Percent for the Y-Axis label, click next, and choose as an object in this sheet.

6. Press Finish.  You should have a graph that has two series of data.

7. To format the pyramid, grab a corner of the graph and expand it so you can see it clearly.

8. Put the tip of your mouse arrow on one of the bars on the left so it says "Category Axis" and double click.

9. Choose Patterns tab and set the major and minor tick marks to None, tick mark labels to Low, and from the Scale tab set Number of Categories between tick mark labels to 1, click OK.

10. Double click on any of the bars in your pyramid, and in the Options Tab area set the Overlap to 100 and GapWidth to 0. Click OK.

11. Double click the horizontal axis, in the Number Tab select Custom and for type enter 0;0 and click OK.  

12. Click on the scale tab and set the major unit to 2, click OK.

13. Your pyramid should look something close to:

You should notice the percentages of males and females within each age class are not equal.  Why might that be? Examine the population pyramid you have just constructed and draw insights from it.

[pic][pic][pic][pic]

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Homework 2:

The objective of this activity is to construct the age structure of the Philippine population. Through this, social and environmental impacts may be determined.

Instructions:

1. Work individually.

2. See and complete Appendix 6-2.

3. If you can creatively modify your population pyramid, the better.

4. Print your population pyramid.

5. Compare your output with your classmates.

6. Submit as scheduled by your lecturer.

7.

Class Activity 6:

Film Viewing: Population is a film that expounds on the various factors that affect population dynamics.

Instructions:

1. Work individually.

2. Write an insight paper on the film (see Appendix 6-1 of this handout).

Figure 6-6. Exponential population growth. Human population growth has drastically increased since the 1800s. What do you think is the implication of this rapid growth to both the biosphere and human society? (Figure by UNEP, downloaded from the Yahoo! search engine)

Figure 6-2. As long as birth rate (BR) exceeds death rate (DR) a population will grow (and if death rate exceeds birth rate, it will decline). The graph on the right shows, over the whole span of the graph population growth continues. From A to B, the per head population growth rate is the same. From B to C the rate drops, but even at C, population growth continues. If the lines cross (point at which DR=BR) will population growth stop.

Figure 6-1. Populations have different kinds of driving forces instrumental in growth or change (population dynamics). Can you explain why there is a need to study population dynamics?

Figure 6-9. Age Structure. A sample of age structure plotted against a graph for visual interpretation. Which country has the most young males and females? (Figure by Benjamin Cummings, downloaded from the Google search engine)

Figure 6-8. Sex Ratio. A sample of a plotted sex ratio for the Philippines, 1995. The first bar (blue) in a pair of bars represents male while the second bar (red) represents females. (Figure downloaded from the Google search engine)

Figure 6-5. The graph illustrates that the species have high early mortality; lower, nearly constant mortality in adult life; higher mortality among elderly.

Figure 6-3. If birth rate (+ immigration) meets death rate (+ emigration) it means the population had reached the carrying capacity of the environment.

Figure 6-4. The graph illustrates the three general categories of the survivorship curve. What do you think is the importance of determining the survivorship curve of a species?

Figure 6-7. Logistic population growth. If the human population growth eventually follows this trend, what factors do you think may have contributed to it?

Individual…

• Born once, dies once.

• Male or female.

• Young or old.

Population…

• Continues (size change depending on birth rate/death rate of populations).

• Has sex ratio.

• Has age structure.

We need to study population dynamics

• Resources are readily available and are sufficient.

• Species prevail over time.

• Unique ecosystem characteristics prevail.

• Become pests.

• Endangered of extinction.

Why do we need to know?

Because we do not want to end up poor, hungry, homeless and as refugees.

(Malthusian concept, 1798)

It helps us know what stabilizes

or

destabilizes

a population

➢

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NATSC13 De La Salle – College of Saint Benilde

Hand-out 6: Population Ecology

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