Population Ecology
Ch 52
Population
• A group of individuals of a single species living in
the same geographic area
• How to describe populations:
• Density - # of individuals per unit area
• Dispersion – the spacing between individuals in
an area
• Demographics – study of vital statistics of
populations and how they change over time
Patterns of dispersion
Uniform, clumped, or random?
• Clumped
• – due to resources, mating, defense
• Uniform
• – often due to competition for resources by same
species
• Random
• – in absences of strong attractions or repulsions
between individuals
Demographics
• Birth rates & death rates
• Patterns of life expectancy
• Life tables – summaries of the survival pattern
of a population
Idealized survivorship curves
Type I – most individuals die late in life (humans, elephants)
Type II – constant death rate over lifespan (coral, birds)
Type III - large decline in young (plants, fish)
Exponential model
• Population growth in an ideal environment
– Abundant resources
– No external restrictions
• Density independent growth
• Change in population =
• Births + Immigrants – Deaths – Emigrants
• Ignore immigration, emigration
• Per capita birth rate – per capita death rate = per
capita rate of increase
rmax – maximum per capita rate for the species, under
ideal conditions
J shaped exponential growth curve
• Current world population: 7.3
billion
• http://www.ibiblio.org/lunarbin/w
orldpop
• Current world r value = 1.2%
birth rate – 19.95 births/1,000 people
death rate - 7.9 deaths/1,000 people
Doubling time = 70/% growth rate
At current r value – by 2050, population
will grow to 9.6 billion people
http://www.npr.org/2011/10/31/14181
6460/visualizing-how-a-populationgrows-to-7-billion
Which age structure diagram represents – Italy, Kenya, and
US?
• Darwin calculated that if you started with 2
elephants and exponential growth, after 700
years the world population of elephants
would be 19,000,000
• Why hasn’t this happened?
available resources
competition for these resources
• Resources are limited in the real world
Logistic Model
• K = carrying capacity
– The maximum size that a particular environment
can sustain
S-curve
Lab population of flour beetles
Life history
• What determines an organism’s reproduction &
survival
• 3 main variables:
– When reproduction begins
– How often the organism reproduces
– How many offspring produced per reproductive
episode
– There is a trade-off between present & future
reproduction
Evolution & life history
• Natural selection maximizes total lifetime
reproductive output
• Single, massive reproductive episode
• Don’t need resources for future survival & reproduction
• Repeated reproductive episodes
• Produce fewer but larger offspring each time, provide
more resources for offspring
“Big Bang” reproduction
• Semelparity
• Pacific Salmon – produces thousands of eggs in
single reproductive opportunity
• Annual plants, all grain crops
• Spiders
• Death may occur after single reproductive event
• Advantageous if adult survival rate is low
Repeated reproduction or “bet-hedge”
• Iteroparity
• Some Lizards - few large, nutrient containing
eggs each year
• Perennial plants
• Most mammals, all birds, most reptiles, most fish
• Advantageous in highly variable conditions that
affect juvenile survivorship
Factors for evolution
• Survival rate of offspring
• Likelihood that adult will survive to reproduce
again
• Low survival of offspring – highly variable
environment – big bang
• More dependable environment - repeated
Trade offs
• Trade offs due to limited resources
• between the number and size of the offspring
• Between reproduction & survival
K- selection
• Density dependent selection
• selection for traits that are sensitive to
population density, and favored at high densities
• Mature trees at old-growth forests
• In stable environments, organisms tend to make
fewer “expensive” offspring
r- selection
• Density independent selection
• selection for traits that maximize reproductive
success in low density (uncrowded)
environments
• Weeds
• In unstable environments, organisms tend to
make more “cheaper” offspring
Extreme r & K selection
R
K
Unstable environment, density
independent
Small organisms
Stable environment, density
dependent
Larger organisms
Energy used to make each
individual low
Energy used to make each
individual high
Early maturity
Short life expectancy
Individual reproduces once
Late maturity
Long life expectancy
Individuals reproduce
repeatedly
Type I & II survivor curve
Type III survivor curve
Density independent factors
Not affected by density of population
Natural disasters
i.e. drought, temperature extremes, hurricanes
Density dependent factors
Dependent on population density
- competition for resources
- predation
- toxic waste – i.e. ethanol produced by fermentation in
yeast
-intrinsic factors - i.e. hormonal changes that delay
sexual maturation & depress immune system in whitefooted mice
-territoriality
- disease
Population dynamics
• Fluctuations in populations
Practice problems
•
•
•
•
If carrying capacity = 500 individuals
Population size (N) = 300
Maximum rate of increase (rmax) = 1.0
Solve for:
K-N
K
• Per capita rate of increase = (rmax)(
K-N
K
)
• Population growth rate = (rmax)(N)( K - N )
K
Practice problems
•
•
•
•
If carrying capacity = 500 individuals
Population size (N) = 400
Maximum rate of increase (rmax) = 1.0
Solve for:
K-N
K
• Per capita rate of increase = (rmax)(
K-N
K
)
• Population growth rate = (rmax)(N)( K - N )
K