Population size and density Population size Population size Determined by: Determined by: actual count actual count Sampling Sampling Population Density Population Density Expressed as: Expressed as: Number of Individuals per unit area. Number of Individuals per unit area. How many populations are shown here? If this was a km 2 of land how dense would the zebra population be?

Population Dynamics Dynamics means changing over time. Dynamics means changing over time. Change is due to: Change is due to: Birth rate. Birth rate. Death (mortality) rate. Death (mortality) rate. Life expectancy. Life expectancy.

Survivorship curves Type I – Few offspring, low youth mortality, long life span Type I – Few offspring, low youth mortality, long life span Type II – moderate number of offspring with moderate mortality at of different age categories Type II – moderate number of offspring with moderate mortality at of different age categories Type III – many offspring, high youth mortality, a few adults surviving a long time. Type III – many offspring, high youth mortality, a few adults surviving a long time. Can you think of another example of an organism that fits each one of the above?

Age structures Populations can have different age structures For example sea turtles have many young and few long lived adults In humans age structures have a large impact on society How many children to care for, how many working adults Take a moment to compare the age structures in these two nations. In 20 years how would each change?

Population Growth Population growth rate is size change over time. Population growth rate is size change over time. Growth can be negative or positive. Growth can be negative or positive. Depends on any of four factors: Depends on any of four factors: Birth rate & death rate Birth rate & death rate Emigration (leaving ) & immigration (arriving) Emigration (leaving ) & immigration (arriving) What would happen if the death rate was higher than the birth rate? What would happen if the death rate was higher than the birth rate?

Exponential Growth Curve Populations grow more rapidly as they increase in size. Populations grow more rapidly as they increase in size. “J” curve. “J” curve. Demonstrates species biotic potential. Demonstrates species biotic potential. Unlimited resources. Unlimited resources. Only occurs in rare cases. Only occurs in rare cases. Can this growth curve occur for long periods of time? Why?

Logistic Growth Curve Same as exponential for short time. Same as exponential for short time. Growth slows and finally levels out. Growth slows and finally levels out. Known as “S” curve. Known as “S” curve. Carrying capacity has been reached. Carrying capacity has been reached. Carrying capacity is the number of individuals the environment can support Carrying capacity is the number of individuals the environment can support What resource most likely causes this curve to flatten out?

Factors that Limit Population Growth Causes growth to be something less than biotic potential. (if all offspring survive to adulthood) Causes growth to be something less than biotic potential. (if all offspring survive to adulthood) Referred to as limiting factors Referred to as limiting factors Density dependent: limited resources control growth Density dependent: limited resources control growth (the larger the population the more will die) (the larger the population the more will die) Limits on resources. Limits on resources. Disease Disease Density independent: events that reduce population size Density independent: events that reduce population size (regardless of the size of the population) (regardless of the size of the population) Weather Weather Fire Fire Why so many rabbits in Australia? They are an introduces species with no natural predators.

Factors that Limit Population Growth Predator/ prey or producer/ herbivore relationships control each other’s growth E.g. more hares for the lynx to eat then the more lynx will survive, the more lynx preying on the hares the more hares die… and the cycle continues

Dangers of Small Populations Lack of genetic variation. Lack of genetic variation. Inbreeding. Inbreeding. Lower birth rates. Lower birth rates. Susceptible to extinction. Susceptible to extinction.

Summary review 1. What is the difference between population size and density? 2. What factors influence population size? 3. What are the 2 growth models (graphs)? 4. What are limiting factors? 5. Compare/ contrast density dependent and independent factors. 6. ( extra challenge question) Cheetahs have experienced a genetic bottleneck and are endanger of extinction. Explain what this statement means.