practical 5 understanding population trends (asha n siti)

8/12/2019 Practical 5 Understanding Population Trends (Asha n Siti)

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Practical 5 Understanding Population Trends

Introduction

Population growth is the change in population size (N) over time, and this is

determined by births, deaths, immigration and emigration, Population growth can be

modelled by a geometric curve (that is, a non-linear curve). This will be either an S-

shaped (sigmoidal) curve or an exponential curve. If the population size doubles

between each time interval then this form of geometric growth is called exponential

growth. Studies of both short-lived and long-lived organisms have indicated that

population growth, and therefore the form and shape of population growth curves, is

ultimately controlled by three factors:

1. The initial population size (N0)

2. A population growth factor (R) which measures the rate at which a population

would grow if it had unlimited resources

3. The carrying capacity (K), which is determined by environmental factors.

Population are dynamic and change in response to environmental stress or

change in environmental conditions. They change in size, density, dispersion, and age

distribution. These changes occur in response to environmental stress and change in

environmental conditions.

The limits of population growth are births, deaths, immigrations, and emigration.

The population change [population change = (births + immigration) – (deaths +

emigration)] Population vary their capacity for growth, also known as biotic potential of

the population. Biotic potential is represented the letter r when used in mathematical

equations.

The biotic potential is an ability of populations of a given species to increase in

size. The factors contributing the biotic factors are the reproductive rate, generalized

niche, ability to migrate or disperse adequate defense mechanisms and ability to cope



with adverse conditions. Characteristics of individuals in population with a high intrinsic

rate of increase: reproduce early in life, have short generation times, can reproduce

many times, and have many offspring g each time they reproduce. The intrinsic rate of

increase of many species depends on having a certain minimum population size.

The environmental resistance is an ability of populations of a given species to

increase in size. This consists of all the factors acting jointly to limit the growth of a

population. Together the biotic potential and environmental resistance determine the

carrying capacity (K), the number of individuals of given species that can be sustained

indefinitely in a given space. The factors contributing the biotic are low reproductive

rate, specialized niche, inability to migrate or disperse inadequate defense mechanisms

and inability to cope with adverse conditions.



Task 1

Task 1

What type of population growth does Rhinoceros have, and why are the final population sizes

different?

Population estimates for two populations of rhinoceros in the same environment, over 5 time

intervals (5 generations).

Table 1.0

Comparison of population size of Rhinoceros A and B.

Time interval (t) =

generation

Rhinoceros Population A Rhinoceros Population B

Pop size (N) Growth factor (R) Pop size (N) Growth factor (R)

0 N0 = 30 N0 = 20

1 58 1.9 39 2.0

2 123 2.1 82 2.1

3 236 1.9 161 2.0

4 484 2.1 318 2.0

5 955 2.0 643 2.0

Geometric Mean =2 =2



Graph1.0

Shows the population growth curves for both populations on the same axis.

Question:

Check the three factors that influence population growth. Why is there a difference in population

size between these two populations at time interval 5 ? Is it due to differences in initial population

size, the growth factor, or the environment ?

Answer:

The factor that influence population growth are the factor of initial population size of each

rhinoceros type. The rhinoceros population A have the more initial population size compare to the

rhinoceros population B. As shown in Table 1.0, we can see that the growth factor for both

3058

123

236

484

955

2039

82

161

318

643

0

200

400

600

800

1000

1200

0 1 2 3 4 5 6

P o p u l a t i o n

S i z e

( N )

Time Intervals (t)

The Rhinoceros population growth

Rhinoceros

Population A

Rhinoceros

Population B

Geometri

c Mean =



Rhinoceros populations are quite constant and they also have the same geometric mean which is

2 and even the rate of the breeding of the two Rhinoceros populations are the same. Population

A has more initial population size which will affect the population A different more from population

size B. Hence, both of the Rhinoceros populations are living in the same area so we can say the

environment factor should be the same and this also would not affect much on their growth for

both populations.

Task 2

Task 2: What type of population growth does the Koala have, and why are the final population

sizes different?

Population estimates for two koala populations in the same environment over 5 time intervals

(5 generations)

Time interval (t) =

generation

Koala Population A Koala Population B


0 N0 = 20 N0 = 20

1 36 1.8 31 1.6

2 63 1.8 47 1.5

3 118 1.9 66 1.4

4 212 1.8 103 1.6

5 375 1.8 150 1.5

Geometric Mean =1.8 =1.5

Table 2.0 Comparison of population size of Koala A and B.



Graph 2.0 the population growth curves for both Koala populations on the same axis.

20

36

63

118

212

375

20

31

47

66

103

150

0

50

100

150

200

250

300

350

400

0 1 2 3 4 5 6

P o p u l a t i o n

S i z e

( N )

Time Intervals (t)

The Koala population growth

Koala Population A

Koala Population B

Geometric

mean= 1.8

Geometric

mean= 1.5



Question:

What type of growth does each of the koala populations’ show: exponential or sigmoid?

Answer:

The koala populations show exponential curve.

Question:

For the two koala populations, why is there a difference in population size at time interval 5?Is it

due to differences in initial population size, the growth factor, or the environment?

Answer:

The differences in population size between the two Koala populations at time interval 5 are dueto the second factors, the growth factor. Population A has higher growth factor than Population

B where the geometric means of them are 1.8 and 1.5 respectively. Hence Population A has

higher population growth.

As shown in Table 2.0, we noted that the initial population size for both Koala Population A and

B are the same that are 20, and so it is not the main factor which will affect the further

population growth. Beside that, both of the Koala populations are living in the same area and sothe environment factor should be the same and this also would not give much influenced to their

growth.

Question: In Table 2, if the data for koala population A were for the same, but under plentiful

rainfall conditions, and the data for koala population B were for drought conditions, why might

the populations show a change in the growth factor (R) ? Remember that the growth factor R =

Births – Deaths.

Answer:

Population A birth rate is higher than population B but the overall both birth rate must be higher

than death rate.However, Population B could not adapt themselves well in the drought

conditions and so the death rate is higher than the birth rate, so the R is low.



Task 3

Task 3

What type of population growth does Zebra have, and how does

carrying capacity affect the final population size?

Population estimates for two Zebra populations in different environments over ten timeintervals (generations).

Time interval (t)

= generation

Zebra Population A Zebra Population B


0 N0 = 20 N0 = 20

1 80 4.0 50 2.5

2 230 2.9 100 2.0

3 400 1.7 220 2.2

4 500 1.3 360 1.6

5 550 1.1 460 1.3

6 580 1.1 520 1.1

7 590 1.0 550 1.1

8 595 1.0 570 1.1

9 600 1.0 580 1.0

10 600 1.0 600 1.0


Table 3.0

Comparison of population size of Zebra A and B



Graph 3.0 Shows the population growth curves for both Zebra populations on the same

axis.

Question:

What types of growth do each of the populations show: exponential or sigmoidal?

20

80

230

400

500

550

580590 595 600 600

20

50

100

220

360

460

520

550

570580

600

0

100

200

300

400

500

600

700

0 2 4 6 8 10 12

P o p u l a t i o n

S i z e

( N )

Time Intervals (t)

The Zebra population growth

Zebra

Population A

Zebra

Population B



Answer:

Both of the zebra populations show sigmoidal curve.

Question: Which population, A or B, has the highest growth factor?

Answer: Both population A and population B have the same growth factor. Furthermore both of

this population also have the same geometric mean.

Question: Is the growth factor (R) constant ? Describe any changes that occur in R.

Answer: Population A and population B have no major different on their growth factor one to

another. The differences of R can we see from the beginning population A have a big number of

the population growth which is 4.0 compare to the population B 2.5. But this value finally same

at the ten intervals which is 1.0.

Question: What is the carrying capacity (K or the asymptote of the curve) for each population?

Answer: The carrying capacity is the maximum capacity that the environment can bear the

population. From this graph, the carrying capacity will be happen when it reach the constant

value on this sigmoid graph which approximately population at 600.

The definition of K is the limiting value of the population that can be supported in a particular

environment is called its carrying capacity and is designated K. The K is t intervals, so the

carrying capacity for population A is K=9th interval and population B is K=10th interval.



Task 4

Task 4

How does competition between two populations of Zebra affect the carrying capacity?

Population estimates for two populations in the same location over ten time intervals

(generations).

Time interval (t) =

generation

Population A Population B

Pop size (N) Growth factor

(R)

Pop size (N) Growth factor

(R)

0 N0 = 20 N0 = 20

1 90 4.5 50 2.5

2 220 2.4 120 2.5

3 340 1.5 210 1.8

4 410 1.2 260 1.2

5 440 1.1 260 1.0

6 450 1.0 230 0.9

7 450 1.0 200 0.9

8 440 1.0 170 0.9

9 460 1.0 150 0.9

10 450 1.0 150 1.0


Table 4.0

Comparison of population size of Zebra A and B.



Graph 4.0

The population growth curves for both Koala populations on the same axis.

Geometric mean = 1.2

20

90

220

340

410

440450 450

440

460450

20

50

120

210

260 260

230

200

170

150 150

0

50

100

150

200

250

300

350

400

450

500

0 2 4 6 8 10 12

P o p u l a t i o n

S i z e

( N )

Time Intervals (t)

The Zebra population growth

Zebra

Population A

Zebra

Population B

Geometric mean =1.4



Question:

What type of growth do each of the Zebra populations show: exponential or sigmoidal?

Answer:

The Zebra population show sigmoidal.

Question: Which population, A or B, has the highest growth factor?

Answer: Population A has the highest growth factor.

Question: Is the growth factor (R) constant ? Describe any changes that occur in R.

Answer: From the beginning the growth factor (R) did not constant. Population A have constant

value of R when it reach the six intervals. Population B reach the constant R start in interval sixbut in the intervals ten the R value add up 0.1 to 1.0.

Therefore, the both population have the different growth factor. Both of the population did not

have the same R for the very beginning. But population A start constant at 7th interval and

population B at 9th interval.

The R for both populations was keep decreasing from higher values to the lower one until it

became 1 then it went constant.

Question: Has the carrying capacity (K or the asymptote of the curve) for population A

changed? If so, in what way?

Answer: The definition of K is the limiting value of the population that can be supported in a

particular environment is called its carrying capacity and is designated K.

Yes, the population A carrying capacity has changed because the competition between both

population effects the carrying capacity.

The K is t intervals, so the carrying capacity for population A is K=9th interval and population Bis K=10th interval.

Question: Both zebra populations had the same initial population size. Why has one population

out-competed the other? Check the three factors that control population growth.



Answer: The growth factor of Population A is higher than Population which its geometric mean

is 1.4 which is 0.2 higher than Population B. Hence, Population A has higher rate in breeding.

Question: If the data in Table 3 are for the two populations living separately, and in Table 4 for

the same populations living together, what has been the effect of competition on R for both

zebra populations?

Answer: The effect of competition on R for both zebra populations is both of the populations will

reach to the value of R which is quite closer to each other.

Report

1. What is the difference between exponential and and logistic (sigmoidal) growth?

Exponential is the graph that is no limit and the sigmoidal growth have the limiting factor.

2. How do populations gain and lose individuals?

The population gain and lose individuals by birth, death and immigration. Animal

populations change over time due to births, deaths, and the dispersal of individuals

between separate populations. When resources are plentiful and environmental

conditions appropriate, populations can increase rapidly.

3. What are the characteristics of a population which has a high intrinsic rate of

increase?

The characteristic is the population can adapt very fast and well to the environment

changed. Mostly, this kind of populations are small in their body shape and so they will

only have to take less time to adapt themselves to live in the new environment. Besides

that, such parents have as many offspring as possible by starting early and quickly

repeating breeding. Then the young needs less parental care and has short life span.

Another characteristic are high immigration and low emigration of the population that

may cause the intrinsic rate increase.



4. What are the environmental factors which act to (1) increase population growth

and (2) decrease population growth ?

The environmental factors are supplier of foods, temperature, humidity and also space

or territory of the population in their own habitat. All of these environmental factors

actually also contribute to the decreases of the population growth if there are lack some

of the factors. The destruction of the habitat will also causes the decrease population

growth. Beside that if the population breeding is not succeed because of the

unsupported environmental condition, it also will effect the population growth later.

Some environmental and biological factors can influence a population differently

depending on its density. If population density is high, such factors become increasingly

limiting on the success of the population. For example, if individuals are cramped in a

small area, disease may spread faster than it would if population density were low.

Factors that are affected by population density are referred to as density-dependent

factors.

5. What happens if the population size exceeds the carrying capacity?

If population size exceed carrying capacity K- population growth decline, population size

will drop or decrease.

6. Carrying capacity (K) can vary from year to year (see Miller 2001 figure 7.5). In

some years K is high and in some years it is low due to unpredictable changes in

environmental conditions. Commercial exploitation of populations, such as

harvesting fish or deer, relies on the harvest of a known proportion of the

population. For example, commercial fishermen are licensed to take a certain

weight of fish (their “quota”) each year. The quota is determined by dividing the

total biomass or weight of the population that is available for harvest, by the

number of fishermen (or licenses). Given this, why is a random change in K from

year to year a problem for management of commercially exploited wild

populations?



When random change in carrying capacity K, management cannot predict how weight of

population that still available for harvest in that particular year. So there are probability

that the management had already give over licensed to the commercial fisherman.

7. Your Minisrty sells 10 licenses for deer harvesting each year, and commercial

harvesting of deer is based on 20% of the population at carrying capacity (that is

each quota is 2% of the population). Now that you understand the concept of

carrying capacity and population growth, what will you do if there is a good

season and deer populations for the next season are predicted to overshoot

carrying by 20 % ?

In my opinion, if there is a good season and deer populations for the next season are

predicted to overshoot carrying by 20%, I will not give more licenses for the deer

commercial harvesting because I think the 20% of the populations that will be kill before

is more than enough. We should not be to greedy to take the advantage because maybe

the couple year later there are no breeding of the deer offspring and the population

decreases. It will cause problems to us to settle it when we have to face the mother

nature that unpredictable.

References:

1. Raven, Peter H. (2002). Biology. McGraw Hill Higher Education, New York.

2. Cambell, Reece & Mitchell. (2003).Biology Concepts and Connections (fourth edition)

.Benjamin Cumming, San Francisco.

3. http://en.wikipedia.org/wiki/Carrying_capacity

4. http://www.gdrc.org/uem/footprints/carrying-capacity.html

5. http://en.wikipedia.org/wiki/Population_growth

6. http://animals.about.com/cs/zoology/a/zoo101ae.htm

http://en.wikipedia.org/wiki/Carrying_capacity


http://www.gdrc.org/uem/footprints/carrying-capacity.html


http://en.wikipedia.org/wiki/Population_growth


http://animals.about.com/cs/zoology/a/zoo101ae.htm






practical 5 understanding population trends (asha n siti)

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