Population Growth

Unlimited Growth

• 1 bacterium dividing unchecked for 36 hours would result in the entire earth being covered 1 foot deep with bacteria

• 1 pair of elephants producing 16 offspring in a 100 year span would result in 19 million elephants in 750 years

Exponential Model of Population Growth

• Populations grow by births and immigration

• Populations diminish by deaths & emigration

• ΔN = B - D

Δt

N = population size; B = births, D = deaths

Exponential Model of Population Growth

• If b = average # of births & d = average # of deaths then

• ΔN = bN - dN Δt• r = b – d (r = overall change in population

size)• ΔN = rN Δt

Zero Population Growth

• Occurs when b = d; r would be 0

Exponential Population Growth

• Rapid population growth when ALL members have access to abundant food and are free to reproduce at their physiological capacity.

• ΔN = rmaxN

Δt

• rmax = intrinsic rate of increase

Exponential Population Growth

Logisitic Model of Population Growth

• Exponential growth cannot continue indefinitely; some resource will become limited

• Carrying capacity—the maximum potential size that a particular environment can support at a particular time with no degradation to the habitat

• Represented by letter K

Factors that influence carrying capacity

• Energy limitation

• Shelters

• Refuges from predators

• Soil nutrients

• Water

• Suitable nesting & roosting sites

• Anything that increases d or decreases b

Logistic Population Growth

Logistic Population Growth & Life Histories

• K-selection or K-strategies—At high population density, selection favors adaptations that enable organisms to survive & reproduce with few resources

• r-selection or r-strategies—In “empty” environments adaptations are favored that promote rapid reproduction

Population Limiting Factors• What regulates population size?• Resource Limitation• Territoriality• Safe nesting space• Health of organisms• Predation• Accumulation of toxic wastes• Impact of disease• Aggressive interactions• Stress syndrome

Boom-and-bust cycles

Human Population Growth

Human Population Growth

• In 1650—500 million people

• In 1850—1 billion people

• In 1930—2 billion people

• In 1975—4 billion people

• In 2004—over 6 billion people

• Increases 214,00 people per day

• Increases 80 million people per year

Human Population Growth

• In developed nations, human population growth is near equilibrium

• 80% of the earth’s population live in less developed countries with a growth of around 1.7% per year

Age Structures