Topic 5: Ecology & Evolution Miss Friedman 5.3: Populations Topic 5: Ecology & Evolution Miss Friedman

5.3.1: Factors affecting population size Four factors exist which influence the size of a population: Natality (birth rate) Mortality (death rate) Immigration (moving in) Emigration (moving out)

5.3.2: Population growth curve This is a graph known as a sigmoid (s-shaped) growth curve It shows changes in population size against time In this model, the population is expanding into a habitat offering initial low resistance or low limiting factors

5.3.3: Phases of the population growth curve Exponential Phase Rapid population growth rate b) Transition stage Slowing yet still high growth rate Plateau phase Population remains stable with time

Exponential Phase Unlimited growth due to absence of limiting factors Population may be increasing at 2n where n=number of generations Rate of natality + immigration is greater than mortality + emigration. (Does NOT mean that death/emigration = 0) This would be typical of a population of germinating annual plants in a new season or bacterial population during initial phases of infection

Transitional Phase Limiting factors start influencing the rate of increase of the population As population grows there will be increased competition between individuals of population for the same resources Competition means that some individuals will obtain resources and survive and others will not Overall the effect is to produce a lower rate of population increase

Notice the link between ecology and evolutionary theory! Natural selection (survival of the fittest) is taking place! 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 and accessing the resources

Population Plateau Population remains constant over time/generations Number of individuals of the population no longer increases, due to limiting factors Natality + immigration = mortality + emigration Population has reached carry capacity of the environment

Carry capacity Refers to the maximum number of individuals of a species that can be sustainability supported by the environment

5.3.4: Factors limiting population growth Limiting factors Those factors that control a process such as population growth

Examples of limiting factors for ANIMAL populations Amount of food available Presence of parasites and/or disease Amount of predation Available nesting sites Examples of limiting factors for PLANT populations Amount of light available Temperature Amount of carbon dioxide Amount of water available