Chapter: 6 Population Dynamics

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Chapter 6 Population Dynamics

• To understand the factors regulating populations
  in the habitat, community, and ecosystem

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I. Dynamics of Population Growth

• A. Exponential Growth and Doubling Times
• Ideal environmental conditions can cause a
  population to grow exponentially
• Exponential growth is growth at a constant rate
  (per unit of time)
• Can be expressed as a constant fraction, or as an
  exponent, by which the original population is
  multiplied
• Usually yearly in macro-organisms
• Sometimes hourly or daily in micro-organisms

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I. Dynamics of Population Growth

• A. (cont)
• Ex. 22, where the 2 is the exponential growth
  rate
• 2, 4, 16, 64, etc.
• Also called Geometric growth
• the sequence of growth follows a geometric
  pattern of increase
• Graphically looks like a J
• Called a J-shaped curve
• Sometimes called unfettered growth

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I. Dynamics of Population Growth

• A. (cont)
• Population doubling times are necessary to
  predict the effectiveness of changes to the
  habitat
• Useful rule of thumb to find doubling rates of
  the population is the 70 rule
• Divide 70 by the annual percentage growth rate

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I. Dynamics of Population Growth

• A. (cont)
• Ex. A population has an annual growth rate of
  35, therefore, the doubling time will be 70 / 35
  2. Thus, the population will double every 2
  years
• Ex. The US population has a growth rate of 1.2,
  What is the doubling time? (initial pop is 300
  million)
• 58 years, 300 million to 600 million

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I. Dynamics of Population Growth

• B. Arithmetic Growth
• Less than ideal environmental conditions will
  produce a population growth rate that is a
  constant fraction that is added to the original
  population
• Called Arithmetic Growth
• Produces a straight line on a population graph

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I. Dynamics of Population Growth

• C. Biotic Potential
• Based on the ability of an organism to reproduce
• The maximum reproduction rate for an organism is
  its Biotic Potential

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I. Dynamics of Population Growth

• D. Population Oscillations and Irruptive Growth
• In Vitro, populations can have no limits
• In Situ, populations have limits
• Negative growth rates occur when the population
  exceeds the carrying capacity for the habitat
• The carrying capacity is the maximum number of
  organisms a habitat can have at any given time
  period

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I. Dynamics of Population Growth

• D. (cont)
• Negative growth rates are called Dieback
• The death rate is greater than the birth rate
• A small population growth above the carrying
  capacity is called overshoot
• A large population growth above the carrying
  capacity is called a population explosion
• A large negative population growth rate is called
  population crash

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I. Dynamics of Population Growth

• D. (cont)
• Malthusian Growth or Irruptive Growth is when
  there is a population explosion followed by a
  population crash
• Populations grow until they exhaust resources
  (typically food)
• May occur repeatedly
• Can occur irregularly
• Isle de Royal, Newfoundland Canada

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I. Dynamics of Population Growth

• E. Growth to a Stable Population
• Internal and external factors which regulate
  population growth
• Harmony with the environment
• May initially experience exponential growth, but
  slow as resources dry up
• Closer to carrying capacity
• Called Logistic Growth Model
• Add to growth rates, environmental resistance
• Looks like an S, graphically, then the tail moves
  above and below the carrying capacity line
• Also called a sigmoid curve

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I. Dynamics of Population Growth

• F. Chaotic and Catastrophic Population Dynamics
• Since many population growth curves dont follow
  linear growth curves, growth curves are called
  Chaotic
• Exhibit variability
• Non-random events
• Minute differences in conditions, change the
  populations dramatically
• Small events strung together form a large affect

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I. Dynamics of Population Growth

• F. (cont)
• Catastrophe theory is hypothetical
• Used by biologists to explain population dynamics
  showing abrupt discontinuities
• Catastrophic systems may jump from one state to
  another
• Chaotic systems can be predicted over a longer
  period of time, catastrophic can not

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I. Dynamics of Population Growth

• G. Population Growth Strategies
• Malthusian growth strategies are followed by most
  animals in the lower trophic levels
• Some are pioneers
• Most are generalists
• Use large numbers to offset predation
• Little investment to the individual
• Called Extrinsically (externally) controlled
  growth or, r-selected (strategies) controlled
  reproduction
• Most insects, rodents, marine invertebrates,
  parasites, crustaceans use this method

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I. Dynamics of Population Growth

• G. (cont)
• Logistic Strategies are followed by animals
  higher up the trophic levels
• Larger organisms
• Live longer
• Mature slowly
• Provides more care for offspring
• Called intrinsically (internally) controlled
  growth, or k-selected (strategies) controlled
  reproduction

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II. Factors that increase or decrease populations

• A. Natality, Fecundity, and Fertility
• Natality is the production of new individuals
• Tied to nutrition, climate, soil, water, and
  species interactions for success
• Fecundity is the physical ability to reproduce
• Does not mean they will mate
• Can have high fecundity without high Natality
• Fertility is the number of offspring produced

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II. Factors that increase or decrease populations

• B. Immigrations
• Introduced organisms into a new habitat or
  community
• Ex seeds, spores, boats, wind (floating)
• Increases population growth rates
• C. Mortality and Survivorship
• Mortality is the ability to die
• Called death rate
• Number of living divided by the number of deaths
  in a given amount of time

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II. Factors that increase or decrease populations

• C. (cont)
• Survivorship is more important to scientists
• The percent of the population/ that survives to
  the next year
• A cohort is all of the individuals that are born
  in a specific generation
• Life expectancy is the probable number of years
  an individual will survive
• Life span is the maximum number of years a person
  can survive
• Very different amongst organisms

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II. Factors that increase or decrease populations

• C. (cont)
• 4 Survivorship patterns
• Type A
• Tend to live full life expectancy
• Low death rate in pre-reproductive and
  reproductive years
• Higher death rate in post-productive years
• k-selected reproduction
• Ex bears, whales, humans, elephants

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II. Factors that increase or decrease populations

• C. (cont)
• 4 Survivorship patterns
• Type B
• Death rate is unrelated to age
• k-selected reproduction
• i.e. constant over the life span
• Ex. Seagulls

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II. Factors that increase or decrease populations

• C. (cont)
• 4 Survivorship patterns
• Type C
• Tend to have high mortality rate in the
  pre-reproduction period (juvenile), once they
  reach the reproduction stage, very high survival
  rate until post-reproduction stage
• r-selected reproduction
• Ex. Song birds, rabbits, deer, etc.

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II. Factors that increase or decrease populations

• C. (cont)
• 4 Survivorship patterns
• Type D
• Very high mortality rate in early life (most prey
  species), when they reach reproduction stage very
  low mortality rate, even through
  post-reproduction stage
• r-selected reproduction
• Ex. Crustaceans, fish, plants, insects

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II. Factors that increase or decrease populations

• D. Age Structure Diagrams
• Combine mortality and natality
• Proportions of individuals in various age classes
• Pre-reproduction
• Reproduction
• Post-reproduction
• Population momentum is dependant on the number of
  individuals in the pre-reproductive stage

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II. Factors that increase or decrease populations

• D. (cont)
• Very large number, compared to reproductive
  group, is increasing population growth
• Same size number, compared to reproductive group,
  is a stable population
• Very small number, compared to reproductive
  group, is a decreasing population

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II. Factors that increase or decrease populations

• E. Emigration
• The movement of organisms out of a population
  permanently
• Different from migration
• Migration is temporary and the organisms will
  return during the next cycle

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II. Factors that increase or decrease populations

• F. Population Growth Equation
• PG(R) (BR I) (DR E)
• PG Population Growth (rate)
• BR Birth Rate
• I Immigration
• DR Death Rate
• E Emigration
• Can be positive or negative
• Growing population is positive
• Decreasing population is negative

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III. Factors that Regulate Population Growth

• A. General Information
• Can be intrinsic
• Can be extrinsic
• Can be biotic and/or abiotic
• Can be density dependant
• Can be density independent
• Biotic regulators tend to be density dependant
• Abiotic regulators tend to be density independent