The document discusses population biology and the factors that influence population growth patterns. It describes concepts like biotic potential, exponential and logistic growth models, and carrying capacity. Density-dependent and density-independent factors are explained as regulators of population size. Specific topics covered include r/K selection strategies, life history traits, survivorship curves, and case studies of predator-prey cycles and locust outbreaks. The summary concludes with discussions of minimum viable population sizes, conservation genetics challenges, and metapopulation dynamics.

1. Population
Biology:
IB TOPIC 5.3
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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
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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.
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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.
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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
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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
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9. Sigmoid Growth Curve or S Curve
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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.
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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.
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12. Reproductive Strategies
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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.
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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.
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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.
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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
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17. Survivorship Curves
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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.
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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
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20. Density Dependent Factors
• Reduce population size by decreasing natality or
increasing mortality.
• Interspecific Interactions (between species)
- Predator-Prey oscillations
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21. Predator-Prey oscillations
• Canada lynx vs. snowshoe hare
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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.
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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
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24. Density Dependent Factors: A Case Study
• Desert Locust (Schistocerca gregarius)
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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
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26. Density Dependent Factors: A Case Study
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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.
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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.
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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.
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31. Genetic Drift
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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.
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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.
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35. Metapopulation
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