Population Growth
Chapter 11
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1. Draw a graph and define Exponential Growth?
Draw examples from animal populations and
explain the growth.
• Continuous population growth in an unlimited
environment can be modeled exponentially.
dN / dt = rmax N(k-N/K)
As population size (N) increases, rate increases
Exp or log growth due to rate increases
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Ex1. Whooping crane population growth.
• Endangered species, human aided growth,
slow lag period, log growth starts 1970.
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Ex 2. paramecium
• Close to max growth, short lag period,
expontential growth starting day 5. 4
2. Draw a graph showing Exponential
Growth in collared doves. Why didn’t the
population grow as expected?
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3. What is Logistic Population Growth?
Define S curve & carrying capacity.
Sigmoid (S-shaped) population growth
curve.
As resources are depleted, population
growth rate slows and eventually stops:
Carrying capacity (K) is the number of
individuals of a population the environment
can support.
Finite amount of resources can only
support a finite number of individuals.
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Example of S curve or population growth
curve
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4. What influences Logistic Population
Growth? Know equation from ch 10 and
how to work problems.
dN/dt = rmaxN(1-N/K)
• rmax = Maximum per capita rate of increase under ideal
conditions.
• dN/dt = increase in pop size over time.
• N = number of individuals in starting population
• 1-N/K = impact of carrying capacity vs env. resistance
• When N nears K, the right side of the equation nears zero.
Highest when N=K/2.
N/K = Environmental resistance.
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5. Discuss Limits to Population Growth.
What are DD & DI factors?
• Environment limits population
growth by altering birth and
death rates.
Density-dependent factors
Disease, Resource
competition, predation
Density-independent factors
Natural disasters, floods,
earthquakes
Ex. Crane pop declined in
2005 due to hurricane
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6. What factors lead to Declining
populations? Draw a graph showing this.
• DD & DI factors
• Allee effect: small pop size = reduce mating potential (can’t find mates,
sterility, inbreeding)
• Ex. Spotted owl 10
Death portion of pop curve
• Declining populations lead to:
• Threatened - low numbers lead to scientist’
designation to “save” species. If trend continues
then labelled…
• Endangered – in danger of becoming extinct.
Increased protection of species, enhance natural
resources to increase numbers
• Allee effect – fewer than 600 individuals – rapid
decline leading to extinction.
• Extinction – loss of species- env resistance
overcomes ability of pop to reproduce.
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7. Draw a graph showing Galapagos Finch
Population Growth from 1974 to 1984. How
did rainfall influence pop growth?
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7. Con’t
• Compare no egg clutches low vs high rainfall
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7. Con’t Cactus Finches and Cactus
Reproduction
• Grant and Grant documented several ways
finches utilized cacti:
Open flower buds in dry season to eat
pollen
Consume nectar and pollen from mature
flowers
Eat seed coating (aril)
Eat seeds
Eat insects from rotting cactus pads
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Cactus Finches and Cactus Reproduction
• Finches tend to destroy stigmas, thus
flowers cannot be fertilized.
Wet season activity may reduce seeds
available to finches during the dry season.
Opuntia helleri main source for cactus
finches.
Negatively impacted by El Nino (1983).
Stigma snapping delayed recovery.
– Interplay of biotic and abiotic
factors.
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8. What is the relationship between Intrinsic
growth rate and body size? Draw a graph
showing this.
• On average, small organisms have higher rates of
per capita increase than large organisms.
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9. Compare Population Growth of Small
Marine Invertebrates and whales.
• Populations of marine pelagic tunicate (Thalia
democratica) grow at exponential rates in response
to phytoplankton plumes (both species = high r). 17
Growth of a Whale Population
• Pacific gray whale (Eschrichtius robustus)
divided into Western and Eastern Pacific
subpopulations.
Rice and Wolman estimated average
annual mortality rate of 0.089 and
calculated annual birth rate of 0.13.
0.13 - 0.089 = 0.041
Gray Whale population growing at 4.1%
per yr.
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Growth of a Whale Population
• Reilly et.al. used annual migration counts
from 1967-1980 to obtain 2.5% growth rate.
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