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)( )