1 1 Population Growth Chapter 11. 2 2 Outline Geometric Growth Exponential Growth Logistic Population Growth Limits to Population Growth Density Dependent.
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11Population GrowthChapter 11
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Outline
Geometric Growth Exponential Growth Logistic Population Growth Limits to Population Growth
Density Dependent Density Independent
Intrinsic Rates of Increase
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Geometric Growth- pulsed reproduction annual plant or insect When generations do not overlap, growth can be
modeled geometrically.
Nt = Noλt
Nt = Number of individuals at time t.
No = Initial number of individuals. λ = Geometric rate of increase (Constant ratio) t = Number of time intervals or generations.
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Fig. 11.2
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Fig. 11.3
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Exponential Growth Continuous population growth in an unlimited
environment can be modeled exponentially.
dN / dt = rmax N
Appropriate for populations with overlapping generations. As population size (N) increases, rate of population
increase (dN/dt) gets larger.
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Assumes a population is growing without limits at its maximal rate Rate is symbolized r and called the biotic potential
The Exponential Growth Model
Growth rate = dN/dt = riNNo. of individuals
in a population
Intrinsic rate of increase
Change over time
The actual rate of population increase is
r = (b – d) + (i – e)
Birthrate Deathrate Net immigration
Net emigration
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Exponential Growth
For an exponentially growing population, size at any time can be calculated as:
Nt = Noermaxt
Nt = Number individuals at time t. N0 = Initial number of individuals. e = Base of natural logarithms. rmax = Per capita rate of increase (constant) t = Number of time intervals.
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Exponential Population Growth
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Logistic Population Growth As resources are depleted, population growth
rate slows and eventually stops: logistic population growth. Sigmoid (S-shaped) population growth curve. 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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Logistic Population Growth
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Fig. 11.9
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Fig. 11.10
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Fig. 11.11
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Fig. 11.12
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dN/dt=rmax N
Add element that slows growth as pop size approaches K
dN/dt=rmax N(K-N) K
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Logistic Population GrowthdN/dt = rmaxN(1-N/K)
rmax = Maximum per capita rate of increase under ideal conditions.
When N nears K, the right side of the equation nears zero. As population size increases, logistic growth rate
becomes a small fraction of growth rate. Highest when N=K/2. N/K = Environmental resistance.
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Limits to Population Growth
Environment limits population growth by altering birth and death rates. Density-dependent factors
Disease, Resource competition Density-independent factors
Natural disasters
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Galapagos Finch Population Growth Boag and Grant - Geospiza fortis was
numerically dominant finch (1,200). After drought of 1977, population fell to (180).
Food plants failed to produce seed crop. 1983 - 10x normal rainfall caused population to grow
(1,100) due to abundance of seeds and caterpillars.
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Galapagos Finch Population Growth
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Fig. 11.19
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Fig. 11.18
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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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