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General Population Change
Chapter 8a
Raven and Berg
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Controls
A society which practices death control must at the
same time practice birth control.
John Rock (1963)
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I. Population-
Populations Change in size
• Rates of Change:
– Birth rate (b) – number of births per
1000 people per year
– Death rate (d) – number of deaths
per 1000 people per year
– Growth rate (r) – birth rate minus
death rate
–r=b–d
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Global Population Changes
How Do Populations Change in Size?
On a global scale (closed system):
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Local Population Changes
Immigration (i) – individuals move into a population
Emigration (e) – individuals exit a population
At a local level (e.g. national)
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Local rate of change
How Do Populations Change in Size?
Local scale: Death rate
Growth rate
Emigration rate
r = (b – d) + (i – e)
Birth
rate
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Immigration rate
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Examples of r
• Population of 10,000
• 200 births per year (or 20 births per 1000
people)
• 100 deaths per year (10 per 1000 people)
r = 20/1000 – 10/1000
r = 0.02 – 0.01 = 0.01
OR
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1% per year
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population growth controls
• Unchecked population growth
– Biotic potential – capacity for growth
– Intrinsic rate of increase – the rate at which a
population would grow if it had unlimited
resources
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Exponential growth
• Fast rate
• Optimal conditions
• J-curve
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Environmental Resistance
• Consists of all the factors acting jointly to limit
the growth of population
– Limited availability of essential resources
– Disease and predation
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Carrying capacity (K)
The largest number of individuals (N) of a
population that a given environment can
support
Causes leveling off of exponential growth
S-shaped curve of logistic population growth
Dynamic
Biotic potential and environmental resistance
determine carrying capacity (K)
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Logistic growth
• Number of individuals (N) is near the carrying capacity
(K) of the environment stabilizes
• S - curve
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Overshooting
Overshooting the carrying capacity can lead to a
population crash.
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Population Density
• Population density
– Number of individuals of species per unit of
area
– D=m/v
20 / 25 m2
20 / 100 m2
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Population Dispersion
Spatial pattern of distribution
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Variations
Boundaries must be defined
May vary in a single habitat, season to season,
or year to year
Changes over time
Environmental pressures
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Density-Dependent Factors
 As population density increases, densitydependent factors tend to slow population
growth
 Predation
 Disease
 Competition
 Easier to find mates and may offer protection
from predators
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Cycles
Density-Dependence and Boom-or-Bust Population Cycles
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Predatory Prey Dynamics on Isle Royale
Early 1900s, moose move across frozen Lake Superior to island.
Wolf and moose populations follow one another between increase and decrease.
Density dependent
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Density-Independent Factors
• Regardless of population density
• Usually abiotic
– Weather
– Fire
– Other natural disasters
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Case-in-point: Arctic mosquitoes
.
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When it is warm, a lot of mosquitoes.
When it gets cold, adult mosquitoes die.
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Reproductive strategies
• Influencing factors:
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Age reproduction begins
Length of time reproductive age lasts
Periods of reproduction per lifetime
Number of offspring per reproduction
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r - selected
K – selected
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Opportunists
Short life span
High intrinsic rate of increase
Lower evolutionary scale
Competitors
Long life span
Low intrinsic rate of increase
High evolutionary scale
Life history strategies
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r-selected species
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Early reproductive age
Many small offspring
Little/no parental care
Most offspring die before maturation
Early maturity
Small adults
Short life span
Examples: algae, bacteria, rodents, insects and
annual plants
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K-selected species
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Later reproductive age
Few, large offspring
High parental care
Most offspring reach reproductive age
Late maturity
Larger body size
Longer life span
Examples: large mammals (e.g. whales and
humans), birds of prey, large and long-lived plants
(e.g. saguaro cactus and redwood trees)
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Life expectancies
•Survivorship curve – the
number of survivors of each
age group for a particular
species
•3 types of curves
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