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/worldpop 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/2011/10/31/141816460/visualizing-how-a-population-grows-to-7-billion

Which age structure diagram represents – Italy, Kenya, and US?

Why hasn’t this happened? 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 Unstable environment, density independent Stable environment, density dependent Small organisms Larger organisms Energy used to make each individual low Energy used to make each individual high Early maturity Late maturity Short life expectancy Long life expectancy Individual reproduces once Individuals reproduce repeatedly Type III survivor curve Type I & II 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 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)( )