4.2 Populations

1. 4.2 Populations Mrs. Jackie

2. Outline how population size can be affected by natality, immigration, mortality and emigration • Factors that affect population size • Natality ( birth rate) • Mortality (death rate) • Immigration (moving in) • Emigration (moving out)

3. Outline how population size can be affected by natality, immigration, mortality and emigration immigration + + - Population abundance natality mortality - emigration

4. Sigmoid shaped population growth curve • Exponential growth phase • All organism have the ability to increase their numbers rapidly if there are no factors which control them • A bacterium can undergo division every 20 min. In 2 hours the following happens

5. Exponential growth phase or geometric curve Example of the innate capacity for increase of a species where the increase in population size is as rapid as its members can reproduce

6. Carrying capacity P • Results will be: • Competition for food, shelter, nesting space etc. • This leads to differential mortality where the individuals which are best fitted to the environment survive and reproduce T E

7. The transitional and plateau phases • The exponential growth only occurs occasionally, for example when the environment suddenly changes and only a few organism survive. • The increase in number will not continue to be geometric and at certain point the increase will slow down until it is zero • The number of individuals in the population has then reached the maximum which can be supported by the environment. CARRYING CAPACITY

8. Factors that set limits to population increase • Divide into • density dependent factors- affect a larger proportion of the population as the density increases. • Examples- mortality due to predation and disease • Density independent factors- affect a proportion of the population regardless of its density • Examples- mortality due to weather conditions or earthquakes. • Tend to have a greater effect on a population when the density of the population is higher.

9. Another classification system • Extrinsic population-regulating mechanism- • Originate outside the population and include biotic as well as physical factors. • Food supplies, natural enemies, disease and weather • Intrinsic population-regulating mechanism- • Originate in an organism´s anatomy, physiology or behavior. • Some species like kangaroos and koalas can reabsorb the developing embryo when conditions are crowded and resources limited. • Lemmings will migrate when resources are depleted

10. Random sample • If we want to study some individuals to draw some conclusions about the entire population we want to take a random sample. • Random sample: a method to ensure that every individual in a population has an equal chance of being selected.

11. Technique used to estimate the population size Capture-mark-release-recapture method • You will capture a number of organisms (random sample) • Mark the organism (without harm them or changing their behavior). • Release them back into their original population. • The assumption is that they will mix with the unmarked individuals in a random way. • After a suitable time you will again capture a random sample. • A certain proportion of your second sample will be marked

12. Technique used to estimate the population size Capture-mark-release-recapture method • This is the same proportion as the original first marked sample. • This technique assumes that natality, mortality, immigration and emigration are zero.

13. Formula • IN every case • N1= number in the first sample captured and marked • N2= total number in the second sample. • N3= number of marked individuals in the second sample • Memorize this equation population size= n1Xn2/n3

14. Quadrat methods • Is a wire, shaped into a square of a known size for example 0.25m2 • Used when you want to know the population size of one or more plant species in an uniform area. • Throw the quadrat and investigate the area on the quadrat.

15. Calculate the mean of a set of values • Mean- average value obtained by dividing the total of a set of values by the number of values.

16. Standard deviation • State that the term standard deviation is used to summarized the spread of values around the mean and that 68% of the values fall +-1 standard deviation of the mean • Where __ X=mean x= class value f- frequency n=number of values

17. Standard deviation • First • Subtract the mean from each value • Then squared these numbers • Add them and divide them by the total number of values-1 • The number acquired is the variance • The Standard deviation is the squared root of the variance

18. Example • The number of eggs in the nests of a sample of a species of bird is shown below. Find the mean and sample standard deviation of these number of eggs • 3 5 3 4 2 0 2 1 2 Add the data=27 Find the mean= 27/10=2.7

19. Solution The sum of the third column gives =24.1 Divide between n-1=10-1=9 Squared root of 24.1/9)=1.636

20. Solution • Small standard deviation indicates that the data is clustered closer to the mean • In the example the sd is closer to 95% for the mean plus or minus 2 standard deviation

21. Explain why the SD is useful… • Useful for comparing the means and the spread of ecological data between two or more populations. • When comparing to samples from different populations, the closer the means and the sd, the more likely the samples are drawn from similar population

22. Classwork • IB Study Guide page 36 • Calculate the standard deviation for this set of values 27,33, 35,36,41,48,52.

23. Homework • Find the mean and standard deviations of the following data sets. Give your answers correct to one decimal place. • 51 57 57 58 57 • 55 55 61 52 55 • 56 61 52 60 56 55 53 54 58 56 60