Populations

Population Definition

• A quick review, what is the definition?

• A group of interbreeding organisms

Population growth

• The rate of growth of a population is most simplistically expressed as

r (for rate) = b (birth) – d (death)• The higher the r value, the faster the

population is growing.

How do populations increase in size?

• Work by very early biologists and mathematicians thought that populations increased by exponential growth

• Is this realistic?

How do populations increase in size?

• Since exponential growth was not realistic, the next model for population growth was that of logistic growth

• Is this realistic?

A closer look at logistic growth

Lag phase

Exponential phase

Stationary phase#

Logistic terminology

• Carrying capacity is that number of organisms the environment can support

• However, it is crucial to think about this – is the environment constant?

• No, therefore carrying capacity is not constant. Therefore the most realistic portrayal of population growth is….

Average carrying capacity

Time

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Population Properties

• Age Structure– Age classes are:

• Young• Adolescent• Sexually mature• Post-sexually mature

• Using age class structure, you can predict how fast a population will grow…

youngest

Expansive Population Growth

Constrictive Growth

Near StationaryGrowth

Morocco

United States Sweden

oldest

K - selected

K- and r-selection• The term r-selection comes from r = b-d and

denotes a population or organism that is fast growing, with lots of offspring and not very much maternal investment in each offspring.– Example: rabbits

• The term K-selection comes from K = carrying capacity and denotes a population or organism that is slow growing, with few offspring and a lot of maternal investment in each offspring.– Example: elephants

Immigration & Emigration

• Populations also change through immigration (organisms coming into the population from outside) and emigration (organisms leaving the population [not death]).

• Find that r-selected species are good colonizers

• K-selected species are not good colonizers

Human Population Growth

• How are we similar to and different from other animals?

• Population growth is subject not only to rules of biology, but also to societal issues.

• Look at the following transparencies….