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 dispersionUniform, 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/worldpop
• Current world r value = 1.2%birth rate – 19.95 births/1,000
peopledeath 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 peoplehttp://www.npr.org/2011/10/31/141816460/visualizing-how-a-population-grows-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 resourcescompetition 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 selectionR K
Unstable environment, density independent
Stable environment, density dependent
Small organisms Larger organismsEnergy used to make each individual low
Energy used to make each individual high
Early maturity Late maturityShort life expectancy Long life expectancyIndividual reproduces once Individuals reproduce
repeatedlyType III survivor curve Type I & II survivor curve
Density independent factors
Not affected by density of population
Natural disastersi.e. drought, temperature extremes, hurricanes
Density dependent factorsDependent 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 white-footed 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:
• Per capita rate of increase = (rmax)( )
• Population growth rate = (rmax)(N)( )
Practice problems• If carrying capacity = 500 individuals• Population size (N) = 400• Maximum rate of increase (rmax) = 1.0• Solve for:
• Per capita rate of increase = (rmax)( )
• Population growth rate = (rmax)(N)( )