Population Growth Models and Methods

Mr. and Mrs. Rabbit … +

Exponential Population Growth ?

Growth cannot continue indefinitely All populations reach a maximum limit

Factors the affect population size Factors that increase population size: Natality Natality is recruitment to a population through reproduction Immigration Immigration from external populations e.g. Bird migration Factor reducing population size: Mortality Mortality which is the death rate from any source e.g. predation Emigration Emigration, where individuals leave the population for another habitat

The Logistic Model

Logistic Population Growth Model initial low resistance In this model the population is expanding into a habitat offering initial low resistance or low limiting factors Exponential Phase a) Exponential Phase of rapid population growth rate Transition stage b) Transition stage of slowing yet still high growth rate against time or generations Plateau stage c) Plateau stage in which the population remains stable with time. There maybe slight oscillations in the numbers

Exponential Phase low or reduced limiting factors With 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 increased competition As a population grows then there will be increased competition between the individuals of that population for the same resources. lower rate of population increase Overall the effect is to produce a lower rate of population increase that observed during the exponential phase. selection There will not just be a reduced rate of population growth but also a selection (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 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 Strategies Reproductive 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 care Can 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

Elephants First 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