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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
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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
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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).