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5.3 Populations

5.3 Populations. 5.3.1 Outline how population size is affected by natality , immigration, mortality and emigration. Factors that increase population size : Natality is recruitment to a population through reproduction Immigration from external populations e.g. Bird migration

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5.3 Populations

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  1. 5.3 Populations

  2. 5.3.1Outline how population size is affected by natality, immigration, mortality and emigration • Factors that increase population size: • Natality is recruitment to a population through reproduction • Immigration from external populations e.g. Bird migration • Factor reducing population size: • Mortality which is the death rate from any source e.g. predation • Emigration, where individuals leave the population for another habitat

  3. Population Growth…. • In 500 years 2 elephants could breed and produce 15 000 000 elephants • In 6 months, 2 mice could breed and produce 10 000 000 mice • In one week one bacteria could reproduce enough to cover the entire earth to the depth of one metre

  4. 5.3.2 Draw and label a graph showing a sigmoid (S-shaped) population growth curve. This model shows the changes in the population size against time. In this model the population is expanding into a habitat offering initial low resistance or low limiting factors plateau phase: population now constrained by resource availability (resources of the environment cannot support any more individuals ). Natality equals mortality. This is also known as the carrying capacity of the environment transitional phase: developing shortage of resources and increase competition between members of the population exponential growth phase:rapid population growth because of abundant resources lag phase: the initial stage in which population growth rates are slow as a result of a small population size

  5. 5.3.3 Explain the reasons for the exponential growth phase, the plateau phase and the transitional phase between these two phases. exponential phase : • This would be typical of a population of germinating annual plants in a new season or a bacterial population during the initial phases of an infection. • population doubles per unit time producing exponential/geometric growth • no limiting factors: nutrients, oxygen, space in ample supply transitional phase : • As a population grows then there will be increased competition between individuals for resources. • Some will obtain resources and survive and others will not. • Overall effect - population continues to increase, but at a lower rate • limiting factors : nutrients, oxygen, space

  6. 5.3.3 Explain the reasons for the exponential growth phase, the plateau phase and the transitional phase between these two phases. We can start thinking about evolutionary theory,… ''........can we doubt (remembering that many more individuals are born than can possible survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and procreating their kind? On the other hand, we may feel sure that any variation in the least injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection'Darwin C. (1859)The Origin of Species plateau phase : • population growth slows to zero: population becomes stable so that Natality + Immigration = Mortality + Emigration • The population size is determined by the carrying capacity of the habitat at that point in time • limiting factors inhibit growth: nutrients, oxygen, space

  7. 5.3.4 List three factors that set limits to population increase.

  8. 5.3.4List three factors that set limits to population increase • With unlimited resources the population would increase exponentially. However, environmental resistance limits the population growth and determines the carrying capacity of the habitat: • density-dependent factors (these are dependent on the population/area) • shortage of food or prey • accumulation of waste • Shortage of space or territory • Disease • density-independent factors (these are independent of population density) • predation or parasitism

  9. At what stage is human population growth?

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