Population Dynamics

1. Population Dynamics April 28, 2004

2. Population (def.) – a group of actively interbreeding individuals Therefore, they’re the same species Individuals are in the same place at the same time Groups of the same species can be separated They become separate populations Review

3. Factors influencing growth rates of populations • Population density • Population size • Population dispersion • Age structure • Sex ratio • Environmental factors

4. Population Density • Def. – the number of individuals/area • Eg. #prairie dogs/acre • Larger density means that population growth increases • But as growth increases, certain factors begin to limit growth = limiting factors • Eventually, population growth decreases

5. 2,000 1,500 Number of reindeer 1,000 500 1910 1920 1930 1940 1950 Fig. 9.6, p. 201 Year

6. Limiting Factors • Fewer resources • Parasites and disease • Predation • Physiological and sociological changes

7. Population Size • The number of individuals in a population at a given time • Small populations • Less genetically diverse • Less resistant to disease • Large populations • More genetically diverse • More resistant to disease • Optimum size

8. Cheetah

9. Population Dispersion • The spatial pattern in which the individuals of a population are distributed • 3 patterns • Clumping • Uniformly dispersed • Randomly dispersed • Patterns vary in response to limiting factors

10. Population Dispersion Clumped (elephants) Uniform (creosote bush) Random (dandelions) Fig. 9.2, p. 199

11. American Dipper

12. Spadefoot Toad

13. Age Structure • The proportion of individuals in each age group in a population • 3 groups • Prereproductive • Reproductive • Postreproductive • Each age group has a characteristic birth rate and death rate • Birth rate = the number of offspring produced during a certain amount of time • Death rate = The number of individuals who die during a certain amount of time

14. Generation Time • The average life span between the birth of individuals and the birth of their offspring • Related to body size • A shorter generation time will result in faster population growth

15. Sex Ratio • The proportion of individuals of each sex • The number of females is usually directly related to the number of births that can be expected

16. Environmental Factors Affecting Population Growth • Abiotic • Biotic • Ability to adapt to environmental change • Natural selection • Evolution

17. Growth Factors POPULATION SIZE Growth factors (biotic potential) Decrease factors (environmental resistance) Abiotic Abiotic Too much or too little light Temperature too high or too low Unfavorable chemical environment (too much or too little of critical nutrients) Favorable light Favorable temperature Favorable chemical environment (optimal level of critical nutrients) Biotic Biotic High reproductive rate Generalized niche Adequate food supply Suitable habitat Ability to compete for resources Ability to hide from or defend against predators Ability to resist diseases and parasites Ability to migrate and live in other habitats Ability to adapt to environmental change Low reproductive rate Specialized niche Inadequate food supply Unsuitable or destroyed habitat Too many competitors Insufficient ability to hide from or defend against predators Inability to resist diseases and parasites Inability to migrate and live in other habitats Inability to adapt to environmental change Fig. 9.3, p. 200

18. Growth Curves • Demography = the study of factors that affect the growth and decline of populations • Growth = (birth + immigration) – (death + emigration) • Populations grow exponentially • Yields a J-curve • Plotted in terms of doubling time

19. Doubling Paper Thickness

20. Factors Affecting Growth • Resources • Fertility • Clutch size • Frequency of births

21. Limitations Affecting Growth • Environmental Resistance • Determines the carrying capacity (K) • S-curve – population fluctuates around K. • K-selected species • Boom and bust curve – species grow exponentially and use up their resources • R-selected species

22. K-Selected Species elephant saguaro Fewer, larger offspring High parental care and protection of offspring Later reproductive age Most offspring survive to reproductive age Larger adults Adapted to stable climate and environmental conditions Lower population growth rate (r) Population size fairly stable and usually close to carrying capacity (K) Specialist niche High ability to compete Late successional species Fig. 9.10b, p. 205

23. Sheep in Tasmania 2.0 1.5 Number of sheep (millions) 1.0 .5 Fig. 9.5, p. 201 1800 1825 1850 1875 1900 1925 Year

24. r-Selected Species cockroach dandelion Many small offspring Little or no parental care and protection of offspring Early reproductive age Most offspring die before reaching reproductive age Small adults Adapted to unstable climate and environmental conditions High population growth rate (r) Population size fluctuates wildly above and below carrying capacity (K) Generalist niche Low ability to compete Early successional species Fig. 9.10a, p. 205

25. Snowshoe Hare and Lynx Population 160 Hare 140 120 100 Lynx 80 Population size (thousands) 60 40 20 0 1845 1855 1865 1875 1885 1895 1905 1915 1925 1935 Year Fig. 9.8, p. 203