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# Population biology

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### Population biology

1. 1. Population Biology: IB TOPIC 5.3 1
2. 2. Biotic Potential• Biotic potential refers to unrestrained biological reproduction. Biological organisms can produce enormous numbers of offspring if their reproduction is unrestrained.• Constraints include:  Scarcity of resources  Competition  Predation  Disease 4
3. 3. Dynamics of Population Growth• Population - all the members of a single species living in a specific area at the same time• Exponential Growth - growth at a constant rate of increase per unit time (geometric) ; has no limit dN/dt = rN The change in the number of individuals (dN) per change in time (dt) equals the rate of growth (r) times the number of individuals in the population (N). r is often called the intrinsic capacity for increase. 6
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5. 5. Exponential Growth• Number of individuals added to a population at the beginning of exponential growth is relatively small. But numbers increase quickly because a % increase leads to a much larger increase as the population grows.• J curve when the equation• is graphed• Exponential growth is a simple, idealized model. In the real world there are limits to growth. 8
6. 6. Carrying Capacity• Carrying capacity (K) - limit of sustainability that an environment has in relation to the size of a species population• Overshoot - population exceeds the carrying capacity of the environment and death rates rise as resources become scarce• Population crash - growth becomes negative and the population decreases suddenly• Boom and bust - population undergoes repeated cycles of overshooting followed by crashing 9
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8. 8. Growth to a Stable Population• Logistic Growth - growth rates regulated by internal and external factors until coming into equilibrium with environmental resources  dN/dt = r N (1 - N/K)  Terms have the same definitions as previous slide, with K added to indicate carrying capacity.  Growth rate slows as population approaches carrying capacity.  Sigmoid (S-shaped) growth curve when the equation is graphed 11
9. 9. Sigmoid Growth Curve or S Curve 12
10. 10. Factors Affecting Population Growth• External factors include habitat quality, food availability and interaction with other organisms.• Internal factors include physiological stress due to overcrowding, maturity, body size, and hormonal status.• These factors are density-dependent, meaning as population size increases the effect intensifies.• Density independent effects (drought, an early frost, flooding, landslides, etc.) also may decrease population size. 14
11. 11. r and K Selected Species• r selected species rely upon a high reproductive rate to overcome the high mortality of offspring with little or no parental care. Example: A clam releases a million eggs in a lifetime.• K selected species have few offspring but more parental care. Example: An elephant reproduces every 4 or 5 years. 15
12. 12. Reproductive Strategies 16
13. 13. Factors that Increase Population• Natality - production of new individuals  Fecundity - physical ability to reproduce  Fertility - measure of actual number of offspring produced• Immigration - organisms introduced into new ecosystems  Dispersal of organisms by wind or water currents over long distances. Sometimes carried by animals or on rafts of drifting vegetation. 17
14. 14. Factors that Decrease Population• Mortality - death rate  Survivorship - percentage of cohort surviving to a certain age  Life expectancy - probable number of years of survival for an individual of a given age - Increases as humans age. By older age, most individuals destined to die early have already done so. - Has risen in nations/areas with good nutrition, sanitation and medical care - Women live longer than men. 18
15. 15. Life Span• Life span - longest period of life reached by a given type of organism  Bristlecone pine lives 4,600 years.  Human maximum lifespan is 120 years.  Microbes may live a few hours.• Differences in relative longevity among species are shown as survivorship curves. 19
16. 16. Survivorship Curves• Four general patterns:  Full physiological life span if organism survives childhood - Example: Humans in the U.S.  Probability of death unrelated to age - Example: Sea gull  Mortality peaks both early and late in life. - Example: Deer  Mortality peaks early in life. Example: Tree 20
17. 17. Survivorship Curves 21
18. 18. Factors that Decrease Population• The last factor in our list of factors that decrease population is emigration, the movement of members out of a population.  Many organisms have specific mechanisms to facilitate migration into new areas. 22
19. 19. Factors that Regulate Population Growth• Intrinsic factors - operate within or between individual organisms in the same species• Extrinsic factors - imposed from outside the population• Biotic factors - Caused by living organisms. Tend to be density dependent.• Abiotic factors - Caused by non-living environmental components. Tend to be density independent, and do not really regulate population although they may be important in increasing or decreasing numbers. Example: Rainfall, storms 23
20. 20. Density Dependent Factors• Reduce population size by decreasing natality or increasing mortality.• Interspecific Interactions (between species) - Predator-Prey oscillations 24
21. 21. Predator-Prey oscillations• Canada lynx vs. snowshoe hare 25
22. 22. Density Dependent Factors Continued• Intraspecific Interactions - competition for resources by individuals within a population  As population density approaches the carrying capacity, one or more resources becomes limiting.• Control of access to resources by territoriality; owners of territory defend it and its resources against rivals.• Stress-related diseases occur in some species when conditions become overcrowded.  become overcrowded. 26
23. 23. Density Dependent Factors: A Case Study• Desert Locust (Schistocerca gregarius) – Usually solitary creatures, like grasshoppers• Desert storms (~once every 10 years) cause vegetation to flourish – increases locust reproduction• Stress and high population causes physiological and behavioral changes  Stop reproducing  Grow longer wings  Group together in swarms 27
24. 24. Density Dependent Factors: A Case Study• Desert Locust (Schistocerca gregarius) 28
25. 25. Density Dependent Factors: A Case Study• Desert Locust (Schistocerca gregarius)  Clouds of locusts travel as much as 100km per day  A single locust can consume its own body weight in a day  Swarm can cover 1,200 km2 and contain 50-100 billion individuals - Strip pastures, denude trees, and destroy crops - Consume as much food in a day as 500,000 people need in a year 29
26. 26. Density Dependent Factors: A Case Study 30
27. 27. Conservation Biology• Critical question in conservation biology is the minimum population size of a species required for long term viability.• Special case of islands  Island biogeography - small islands far from a mainland have fewer terrestrial species than larger, closer islands  MacArthur and Wilson proposed that species diversity is a balance between colonization and extinction rates. 31
28. 28. Conservation Genetics• In a large population, genetic diversity tends to be preserved. A loss/gain of a few individuals has little effect on the total gene pool.• However, in small populations small events can have large effects on the gene pool.• Genetic Drift  Change in gene frequency due to a random event• Founder Effect  Few individuals start a new population. 32
29. 29. Conservation Genetics• Demographic bottleneck - just a few members of a species survive a catastrophic event such as a natural disaster• Founder effects and demographic bottlenecks reduce genetic diversity. There also may be inbreeding due to small population size. Inbreeding may lead to the expression of recessive genes that have a deleterious effect on the population. 33
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31. 31. Genetic Drift 35
32. 32. Population Viability Analysis• Minimum Viable Population is the minimum population size required for long-term survival of a species.  The number of grizzly bears in North America dropped from 100,000 in 1800 to 1,200 now. The animal’s range is just 1% of what is once was and the population is fragmented into 6 separate groups.  Biologists need to know how small the bear groups can be and still be viable in order to save the grizzly. 36
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34. 34. Metapopulations• Metapopulation - a collection of populations that have regular or intermittent gene flow between geographically separate units  Source habitat - Birth rates are higher than death rates. Surplus individuals can migrate to new locations.  Sink habitat - Birth rates are less than death rates and the species would disappear if not replenished from a source. 38
35. 35. Metapopulation 39