The Biology of Populations

Population Characteristics Population Density: The number of organisms per unit area of land or volume of water Spatial Distribution: Dispersion: The pattern of spacing of a population within an area 3 main types of dispersion: Clumped Uniform Random The primary cause of dispersion is resource availability

Population Characteristics Population growth rate How fast a given population grows Factors that determine this are: Natality (birth rate) Mortality (death rate) Emigration (the number of individuals moving away from a population) Immigration (the number of individuals moving into a population)

Population Characteristics Actual population growth is a simple formula: NATALITY + IMMIGRATION = MORTALITY + EMIGRATION

Understanding Exponentials Put your pens down for a minute and think about this: An employer offers you two equal jobs for one hour each day for fourteen days. The first pays $10 an hour. The second pays only 1 cent the first day, but the rate doubles each day. Which job will you accept?

Understanding Exponentials 1->2->4->8->16 Now, how much would your employer owe you if you stayed at this job for another 2 weeks? Job 2 lags for a long time before exponential growth kicks in! What would happen if this type of growth took place within a population?

Population Growth Population growth models Exponential growth model First growth phase is slow and called the lag phase Second growth phase is rapid and called the exponential growth phase Unchecked, this produces a J curve Bacteria can grow at this rate, so why aren’t we up to our ears in bacterial cells?

Population Growth What population do you think this is?

Population Growth Why do you think this is happening? Be a Elephants in Kruger National Park, SA Why do you think this is happening? Be a conservation manager!

Population Growth with Limiting Factors Population growth models But usually there are limits to exponential growth Population Density – as the absolute number of individuals increases, so does the number of individuals per unit area Competition follows as nutrients and resources are used up, so population growth slows – this is the third or transitional phase Eventually the population size stabilises – the plateau phase This produces a sigmoid (S) curve for population growth The limit to population size that a particular environment can support is called carrying capacity (k)

Population Growth with Limiting Factors Population growth models Logistic Growth Model Often called the S-shaped or SIGMOID growth curve Occurs when a population’s growth slows or stops following exponential growth. Growth stops at the population’s carrying capacity Populations stop increasing when: Birth rate is less than death rate (Birth rate < Death rate) Emigration exceeds Immigration (Emigration > Immigration)

Population Growth with Limiting Factors The Sigmoid (S) Curve of Population growth

Population Growth Limiting Factors Density-independent factors Factors that limit population size, regardless of population density These are usually abiotic factors They include natural phenomena, such as weather events Drought, floods, extreme heat or cold, tornadoes, hurricanes, fires, etc.

Population Growth Limiting Factors Density-dependent factors Any factor in the environment that depends on the size of a population per unit area Usually biotic factors These include: Predation Disease Parasites Competition

Population Growth Limiting Factors

Population Growth Population growth models Populations don’t reach k as smoothly as in the logistic graph. Boom-and-Bust Cycles Predator-Prey Cycles

Population Growth Species that follow this pattern usually Have a Boom-and-Bust Life Cycle Have short life spans Produce many offspring Smaller organisms i.e., fruit flies, mice, locusts Don’t maintain a population near carrying capacity Controlled by density-independent factors They are called r-strategists (for rate of increase)

Population Growth Species that follow this pattern usually Have long life spans Produce few offspring that have a better chance of living to a reproductive age Are larger organisms i.e., elephants Maintain a population at or near k Controlled by density-dependent factors They are called k-strategists (for karrying kapacity)

Virtual Population Lab Now try this virtual lab. It adds the idea of competition to the population growth of two species. Virtual Population Lab

Reading And finally get into your text book (Biology for the IB Diploma, C.J. Clegg, Hodder Education, 2007), pages 154-156