Dynamics of Population Growth

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Chapter 6 Outline

• Dynamics of Population Growth
• Factors that Increase or Decrease Populations
• Factors that Regulate Population Growth
• Conservation Biology

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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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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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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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Carrying Capacity

• Carrying capacity - 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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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.
• S curve when the equation is graphed

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Logistic Growth Curve or S Curve
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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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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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Reproductive Strategies
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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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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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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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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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Survivorship Curves
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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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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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Density Dependent Factors

• Reduce population size by decreasing natality or
  increasing mortality.
• Interspecific Interactions (between species)
• Predator-Prey oscillations

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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.

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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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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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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 negative effect on
  the population.

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Genetic Drift
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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
  animals 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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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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Metapopulation