1. “Nature teaches more than she preaches.”
Population Biology
“Nature teaches more than she preaches.”
~ John Burroughs

2. Learning Outcomes You should be able to…
1. Explain exponential growth and logistic growth.
2. Compare and contrast the factors that regulate population growth.
3. Summarize the characteristics of r-selected species and K-selected species.
4. Understand the difference between density dependent and density independent factors.

3. Population  group of individuals of same species in same general area
 rely on same resources
 interact
 interbreed

4. Births, Immigration, Deaths and Emigration.
Changes to population size
 Adding & removing individuals from a population
 birth
 death
 immigration
 emigration
Four factors affect growth rate:
Births, Immigration, Deaths and Emigration.
(r=B+I-D-E)

5. Biotic Potential
Biotic potential refers to unrestrained biological reproduction.
Limited by Constraints:
Scarcity of resources
Competition
Predation
Disease
Accident

6. Population growth change in population = births – deaths
N = # of individuals
r = rate of growth
t = time period
maximum rate =
intrinsic rate of increase

7. Exponential growth rate
 Characteristic of populations introduced to a new environment or rebounding from a catastrophe.
Whooping crane
coming back from near extinction
African elephant
protected from hunting

8. Exponential Growth Always Has Limits
Exponential growth only can be maintained by a population as long as nothing limits its growth.
In the real world there are limits to growth that each population will encounter.
Eventually, shortages of food or other resources lead to a reduction in the population size.

9. Carrying Capacity Carrying Capacity (K)  Can populations continue
grow exponentially?
 Of course NOT!
What sets limit?
Carrying Capacity (K)
maximum population size that environment can support with no degradation of habitat.
 not fixed; varies with changes in resources

10. Population Terminology
Overshoot – when a population exceeds the carrying capacity of the environment and deaths result from a scarcity of resources.
Population crash – a rapid dieback in the population to a level below the carrying capacity.
Boom and bust – when a population undergoes repeated cycles of overshoots followed by crashes.

12. Resource Scarcity Slows Exponential Growth
Sometimes population growth slows down as resources become scarce and a population nears its carrying capacity.
Such growth is also sometimes referred to as logistic growth

13. Logistic model of growth

14. Factors Affecting Population Growth
Logistic Growth is density-dependent which means that the growth rate depends on the population density.
Many density-dependent factors can influence a population including: disease, physiological stress and predation.
Density-dependent factors intensify as population size increases.
Density independent factors may also affect populations. These may include drought, fire, or other habitat destruction that affects an ecosystem. 14

15. 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. For example: a clam can release a million eggs in a lifetime, with few surviving to maturity.
K-selected species have few offspring, slower growth as they near carrying capacity and exercise more parental care. For example: an elephant produces one offspring every 4 or 5 years, but nurturing by a herd increases the likelihood of it surviving to maturity. 15

16. Life Span Vary by Species
Maximum Life span - the longest period of life reached by a given type of organism
Bristlecone pines can live up to 4,600 years.
Humans may live up to 120 years.
Microbes may live only a few hours.
Differences in relative longevity among species are shown as survivorship curves.

17. Survivorship Curves Vary by Species
There are three general patterns:
I. Full physiological life span if organism survives childhood
Examples: elephants bears, humans
II. Probability of death unrelated to age
Examples: gulls, mice, hydra
III. Mortality peaks early in life.
Examples: trees, fish, oysters 25
1000
100
Human
(type I)
Hydra
(type II)
Oyster
(type III) 10 1 50
Percent of maximum life span 75
Survival per thousand

18. Survivorship Curves Elephants (tendency to survive to old age)
Sea gulls (die randomly, fairly constant mortality at every age)
Red wood trees (long adult lifespan if reached)

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 (competition, disease, space).
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 19

20. Conservation Biology and Conservation Genetics
Conservation Biology: Critical question in conservation biology is the minimum population size of a rare species required for long term viability.
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.
In small populations small events can have large effects on the gene pool.
Genetic Drift which is a change in gene frequency due to a random event. 20

21. Genetic Drift
Founder Effect occurs when a few individuals start a new population.
Demographic bottleneck occurs when just a few members of a species survive a catastrophic event such as a natural disaster
Founder effects and demographic bottlenecks reduce genetic diversity.
Each may also result in inbreeding due to small population size. 21

22. 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 today. This species range is just 1% of what is once was. It is fragmented into 6 separate populations.
Biologists need to know how small the bear populations can be and still be viable in order to save the grizzly. 22

23. Metapopulation are connected populations.
A metapopulation is composed of several local populations linked by regular (solid arrows) or occasional (dashed lines) gene flows. Source habitats (dark) provide excess individuals, which emigrate to and colonize sink habitats (light).

24. Introduction of Yellowstone Wolves
Earth: A New Wild Episode 1 – Chimpanzee Habitat start at 7:37 (