Population Growth

Models and Methods

Mr. and Mrs. Rabbit

2 4 8 16

32 64 128 256

512 1024 2048 …

+

Exponential Population Growth

?

Growth cannot continue indefinitely

All populations

reach a maximum

limit

Factors the affect population sizeFactors that increase population size:

•NatalityNatality is recruitment to a population through reproduction•ImmigrationImmigration from external populations e.g. Bird migration

Factor reducing population size:•MortalityMortality which is the death rate from any source e.g. predation•EmigrationEmigration, where individuals leave the population for another habitat

The Logistic Model

Logistic Population Growth Model

In this model the population is expanding into a habitat offering initial low initial low resistanceresistance or low limiting factors

a) Exponential Phase Exponential Phase of rapid population growth rateb) Transition stage Transition stage of slowing yet still high growth rate against time or generationsc) Plateau stage Plateau stage in which the population remains stable with time. There maybe slight oscillations in the numbers

Exponential Phase • With low or reduced limiting factors low or reduced limiting factors the population

expands exponentially into the habitat.

• The rate of natality + immigration is greater than Mortality + emigration. Note that it does not mean that death/ emigration= 0

• This would be typical of a population of germinating annual plants in a new season.

• Another example would be a bacterial population during the initial phases of an infection.

Transition Stage• Resources are reduced and become limiting in the growth

of the population

• As a population grows then there will be increased increased competition competition between the individuals of that population for the same resources.

• Overall the effect is to produce a lower rate of population lower rate of population increase increase that observed during the exponential phase.

• There will not just be a reduced rate of population growth but also a selectionselection (survival and reproduction) of these individuals within the population best suited to using, accessing the resources.

Plateau Phase

• The Population plateau is where the population remains constant over time/ generations

• A constant population means that Natality + Immigration = Mortality + Emigration Natality + Immigration = Mortality + Emigration

• The population size is determined by the carrying capacity of the habitat at that point in time

Carrying Capacity

The population size a environment can sustain

Factors Limiting Population Growth

Aspects of the environment that influence rate of growth and carrying capacity

Include availability of…

– Food– Water– Mates– Space

Presence of…– Disease– Excessive waste– Predation

Food

Individuals must compete for food

When food becomes scarce the weakest starve

Survival of the fittest

Water

Water is a scarce resource in many parts of the world

The less water available the fewer organisms the region can support

Who will win the water wars?

Mates

We all need somebody to love.

Space

The available space has been maximized.

Reproductive StrategiesReproductive methods that species

have adapted to ensure survival of next generation

2 models: r and K

r K

r-strategists

• Found in rapidly changing environments

• Reproduce exponentially when conditions are suitable

• Population size drops quickly when conditions worsen

Phytoplankton - a classic r-strategist

Massive blooms in warm season

Followed by heavy predation

Large die off cause red tides

Dormant through winter

Characteristics of r-strategists

• Short life span• Reproduce early in life• Have many offspring each time they

reproduce• Offspring are small and mature

rapidly, with little or no parental careCan you name some other species that are r-strategists?

K-strategists

• Tend to live in stable environments

• Population density is usually near environments carrying capacity

• Populations grow slowly

Giant sequoia, Sequoiadendron giganteum

ElephantsFirst conception at 10 to 11 years; gestation 22 months

Interval between calves 4 to 9 years

Bulls of 25 and older begin competing reproductively

- a classic K-strategist

Characteristics of K-strategists

• Long life span• Few young• Slow maturing process• Reproduce late in life• Often provide extensive care for young

Can you name other examples of K-strategists?

r vs. K

Human Population

What is the Earth’s carrying capacity in regards to the human species?

Human Population Growth

What do scientists say?

• Lower limit• 2 billion• Assuming all

people have lifestyle similar to average American

• Upper limit• 40 billion• Assuming all

people have lifestyle similar to average third-world peasant

Current world population: 6.8 billion