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Prac 5 Understanding population Trends

Task 1 Rhinoceros

Step 2 both populations show exponential growth

Step 4 Growth factor is approximately equal (~2) for both populations

Question: The difference in population size at time interval 5 is due to differences in

initial population size (ie. 30 vs 20). They both grow at the same rate.

Rhinoceros Populations

0

200

400

600

800

1000

1200

1 2 3 4 5 6

Time

PopulationS

ize

Rhinocerus population A

Rhinoceros Population B

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Task 2 Koala

Step 2 both populations show exponential growth

Step 4 Koala population A has the highest growth factor (approx 1.8) vs population B

(approx 1.5)

Question: The difference in population size at time interval 5 is due to differences in

growth factor. Both populations have the same initial population size (20) but population Agrows more quickly.

Task 2 Koalas

0

50

100

150

200

250

300

350

400

1 2 3 4 5 6

Time

Popul

ation

Size

Koala population A

Koala Population B

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Task 3 Zebra carrying capacity

Step 2 both populations show sigmoidal growth.

Step 4 Questions

o Growth factor is not constant for either zebra population.

o The growth factor is initially high in the first few time intervals, then

progressively decreases as the size of the population grows (and approaches carrying capacity).

o Zebra population A has the highest growth factor as its population size increases

more rapidly than zebra population B. They both have the same population size at time 0 (ie

N=20).

o The carrying capacity for zebra population A is 600 as the curve reaches an

asymptote. The carrying capacity for zebra population B is higher than 600 as the curve has not

reached an asymptote, and the growth factor is still >1.0 (ie. 1.03) at population 600. However,

its population size at K will not be much greater than 600.

Zebra Populations Task 3

0

100

200

300

400

500

600

700

1 2 3 4 5 6 7 8 9 10 11

Time

Population

Size

Zebra population A

Zebra Population B

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Task 4 Zebra competition

Step 2 both populations show sigmoidal growth.

Step 4 Questions

o Growth factor is not constant. Population A has the highest growth factor

o For zebra population A growth factor follows the expected pattern for sigmoidal

growth. It is high at first then decreases as population size approaches carrying capacity. Zebra

population B follows the same sigmoidal growth pattern (but with a smaller growth factor) untilit approaches carrying capacity, but it then becomes less than 1.0 and population size decreases

until it reaches a new carrying capacity.

o The carrying capacity for zebra population A is now approximately 450 compared

with 600 in Task 3. Carrying capacity for zebra population B is also lower than in Task 3 (now

approx 150 compared to approx. 600).o Zebra population A has out-competed population B (ie. it has a larger population

size). This can be attributed to a larger growth factor ie. population size grew more quickly.

o The effect of competition has been to decrease R for both species (ie. both species

are affected by competition).

Zebra Task 4

0

50

100

150

200

250

300

350

400

450

500

1 2 3 4 5 6 7 8 9 10 11

Time

PopulationS

iz

Zebra population A

Zebra Population B

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Other questions

Graphs and tables either from the prac or an Excel spreadsheet, but labeled with R.

the next four from the text book (Miller (2001) Ch 7).

Because the quota for commercial harvesting eg. fishing is determined by dividing total

biomass in the population by the number of fishers, random changes in K mean that the quotawill vary from year to year. This may be OK in good years, but in poor years when K is low, the

quota will be low. This means a reduced quantity of fish for sale and reduced incomes for the

fishers. Because people like to have a stable or growing income, there is pressure to maintain

quotas (or increase them) even though the carrying capacity may have declined. This then putseven more pressure on the population (possibly threatening its survival).

Do not carry out calculations for this last question. Basically either increase the number

of licenses, or increase the quota to absorb the predicted increase in biomass (population size).