1. AP Environmental Chapter 6
Population Biology
AP Environmental Chapter 6

2. Dynamics of Population Growth
A population is defined as the number of an individual species at a given time in a defined area
Populations are dynamic meaning that a number of factors cause the numbers in a populations to be in constant flux
Population numbers can also influence the environment they inhabit through the use of resources both biotic and abiotic

3. Describing Population Growth
Population growth patterns can be described using factors and basic equations
N = population size
r = growth rate
t = time
Ignoring factors such as predation and resource limitations population change can be calculated using a starting population size and the rate of reproduction which should remain constant

4. Describing Population Growth
Time Interval
Starting Population (N)
Rate (r)
r x N 1 2 10 20
200 3
2,000 4
20,000
This constant growth rate can also be simplified as:
Nt = N0rt

5. Exponential Growth
Growth that occurs when resources are unlimited and reproduction is rapid
Expressed in a J shaped curve
Exhibited by species that have rapid growth patterns also called r-selected species

6. Exponential Growth
Because exponential growth can show continuous change instead of intervals of change the equation can be reworked to express this:
dN/dt = rN
This shows that a change in the population over time equals a continuous rate of increase

7. Doubling Time
An easy expression of population growth is to calculate the amount of time that it will take for the population to double
A shortcut for determining doubling time is the “Rule of 70” which states that if you divide 70 by the rate of growth percentage you get the approximate doubling time in years
Doubling time = 70/r

8. Exponential Growth Limitations
Since all environments have a limited ability to provide food and other resources to a species exponential growth inevitably leads to a population crash
This is most often depicted as a “boom and bust” cycle of population increase followed by rapid dieback

9. Exponential Growth Limitations
The point at which the ecosystem can no longer support the size of the population is the carrying capacity
Initially the population may overshoot this capacity but limited resources force a population crash that lowers the population until resources balance out and the population can recover

10. Logistic Growth
Population growth that is influenced by resource availability and therefore slows as it reaches carrying capacity is considered logistic growth
In this pattern the rate of growth slows as the population approaches the carrying capacity represented by the letter K
dN/dt = rN (1-N/K)

11. Logistic Growth
Logistic growth curves may initially appear exponential but then level off around the carrying capacity to give them an S shaped appearance
Since the growth rate depends on the size of the population as it nears the carrying capacity of the environment logistic growth is considered density- dependent
Organisms that exhibit this type of growth are called K- selected

12. Logistic Growth

13. Reproductive Strategies

14. Maximum Sustainable Yield
If a growth rate can be predicted for a population then population biologists can determine acceptable levels of harvest for a population to maintain population size and ecosystem resources
Harvest levels take into account reproductive rates, historic population sizes and carrying capacities for the ecosystem to determine the maximum sustainable yield

15. Factors That Regulate Population Growth
Four factors contribute to the rate of growth (r) in populations:
Births
Immigration
Deaths
Emigration
Rate of growth = (Births + Immigration) – (Deaths + Emigration)
Mortality refers to death rate while natality refers to birth rate

16. Survivorship Curves
The rate of growth, maturity and survival over time of a species can be visualized in a survivorship curve that accounts for the likelihood of survival at different stages in the life of an organism

17. Survivorship Curves
Type I : Species with low reproductive rates but high juvenile survival (elephants)
Type II: Species with steady survival rates regardless of age (seagulls)
Type III: Species with high reproductive rates but low juvenile survival (redwood trees)

18. Intrinsic and Extrinsic Factors
Intrinsic factors are influences to population growth that come from within individuals or a population while extrinsic factors come from outside of the population

19. Abiotic and Biotic Factors
Abiotic factors are non-living factors that influence population growth while biotic factors come from interactions with living things
Generally biotic factors are density- dependent while abiotic factors are density-independent

20. Interspecific Interactions
Interactions between different species that influence population growth
Most common interactions are predator-prey relationships that change in relation to one another in cycles

21. Intraspecific Interactions
Interactions such as competition can put stress on the individuals in a population, influence natural selection, and determine population size based on resources available
Organisms in a population more successful at intraspecies competition are more likely to survive and pass those traits on to subsequent generations

22. Population Size and Species Conservation
In order for a species to persist the population must have growth rates that will support healthy reproductive numbers and survival rates
Small and isolated populations are more prone to extinction than large populations with better resource and individual movement
Small populations below a critical size may not be viable in just a few generations

23. Island Biogeography
A 1967 study by R.H. MacArthur and E.O. Wilson describes the influence of population size and isolation on the long term success of a population
Island biogeography states that diversity of an ecosystem depends on rates of colonization and extinction
Colonization chances decrease with distance from a large landmass
Extinction rates increase with a smaller island size

24. Island Biogeography

25. Genetic Diversity and Population Survival
The more genetically diverse a population, the higher the chance of species survival
Genetic diversity increases the chance that if a population is impacted by a major disturbance some within the population will survive
After a disturbance in a population if a small population with lower genetic diversity survives the event is called a demographic bottleneck with those repopulating the species considered founders

26. Genetic Diversity and Population Survival
Small changes in the genetic frequencies of a population is called genetic drift
Genetic drift is influenced by random mutations, selective advantages due to environmental changes and increased diversity due to sexual reproduction
Evolution and natural selection can hone a species leaving behind only those genetic traits that are most fit for survival over time if conditions remain the same

27. Population Viability
Predicting species survival must factor in all of the influences discussed in the chapter and the unknown influences due to constantly changing interactions between populations and the environment
Minimum viable population size describes the minimum size of a population possible to sustain the population long-term
Increased human influence and habitat destruction are the two most common threats to species survival

28. Population Viability
Human influence in the form of habitat fragmentation has created higher incidences of metapopulations that demonstrate gene flow even across separate habitat spaces
Some habitat spaces in a metapopulation may be better than others so population must look at all habitat areas as a whole to determine the overall health of the population
Humans can help gene flow in metapopulations through the creation of natural corridors