Population Dynamics

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Population Dynamics

• Unit II

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Characteristics of Populations

• Three important characteristics of a population
  are
• Its geographic distribution (range)
• Describes the area inhabited by a population
  can vary in size
• Population Density
• The number of individuals per unit area
• Growth rate
• The number of births minus the number of deaths

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Patterns of Dispersion

• Environmental and social factors influence the
  spacing of individuals in a population.
• 3 Patterns
• Clumped
• Uniform
• random

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Survivorship Curves

• A life table is an age-specific summary of the
  survival pattern of a population
• Data in a life table can be represented
  graphically by a survival curve.
• Curve usually based on a standardized population
  of 1000 individuals and the X-axis scale is
  logarithmic.
• Type 1
• Type 2
• Type 3

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Survivorship Curves

• Survivorship curves can be classified into three
  general types
• Type I, Type II, and Type III

TYPE 1 high survivorship until old age. TYPE 2
constant proportion of individuals die at each
age. TYPE 3 experience high mortality as larvae
but decreased mortality later in life.
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Type I curve

• Type I curve typical of animals that produce few
  young but care for them well
• (e.g. humans, elephants)
• Death rate low until late in life where rate
  increases sharply as a result of old age (wear
  and tear, accumulation of cellular damage,
  cancer).

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Type II curve

• Type II curve has fairly steady death rate
  throughout life (e.g. rodents).
• Death is usually a result of chance processes
  over which the organism has little control (e.g.
  predation)

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Type III curve

• Type III curve typical of species that produce
  large numbers of young which receive little or no
  care (e.g. Oyster).
• Survival of young is dependent on luck. Larvae
  released into sea have only a small chance of
  settling on a suitable substrate.
• Once settled however, prospects of survival are
  much better and a long life is possible.

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Population Growth

• Three factors that can affect population size
• Number of births
• Number of deaths
• Number of individuals that enter or leave the
  population (immigration emigration)
• A population will increase or decrease in size
  depending on how many individuals are added to it
  or removed from it

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Exponential Population Growth (EPG)

• Describes population growth in an idealized,
  unlimited environment.
• During EPG the rate of reproduction is at its
  maximum.

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Exponential Growth
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Exponential Growth Trends

• If a population has abundant space and food, and
  is protected from predators and disease, then
  organisms in that population will multiply and
  the population size will increase.
• The J shaped curve indicates that the population
  is undergoing exponential growth. This occurs
  when the individuals in a population reproduce at
  a constant rate.
• Under ideal conditions with unlimited resources,
  a population will grow exponentially and reach
  BIOTIC POTENTIAL.

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Carrying Capacity

• Exponential growth cannot be sustained for long
  in any population.
• A more realistic population model limits growth
  by incorporating carrying capacity.
• Carrying Capacity (K) is the maximum population
  size the environment can support.

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The Logistic Growth Model

• In the logistic population growth model the rate
  of increase declines as carrying capacity is
  approached.

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Logistic Growth Trends

• As resources become less available, the growth of
  a population slows or stops. The general S
  shaped curve of this growth pattern is called
  logistic growth.
• Logistic growth occurs when a populations growth
  slows or stops following a period of exponential
  growth.
• Can slow when the birthrate decreases, the death
  rate increases, or when both occur at the same
  rate
• Can slow when the rate of immigration decreases,
  the rate of emigration increases, or both
• Can slow as the population encounters a limiting
  factor
• The point at which carrying capacity line
  intercepts the y axis tells you the size of the
  population when the average growth rate is zero.
• That number represents the largest number of
  individuals that a given environment can support
  (carrying capacity).

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Phases of Logistic Growth Curve
3
2
1
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Logistic model produces a sigmoid (S-shaped)
population growth curve.
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K vs. R Reproduction Strategies

• K-selection, or density-dependent selection
• Selects for life history traits that are
  sensitive to population density.
• Produce relative FEW offspring that have a GOOD
  chance of survival.
• r-selection, or density-independent selection
• Selects for life history traits that maximize
  reproduction.
• High reproductive rate is the chief determinant
  of life history.
• The concepts of K-selection and r-selection have
  been criticized by ecologists as
  oversimplifications.
• Most organisms exhibit intermediate traits or can
  adjust their behavior to different conditions.

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Limits to Growth

• Limiting Factors- any factor that causes
  population growth to decrease

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Limits to Growth

• In the context of populations, a limiting factor
  is a factor that causes population growth to
  decrease
• Competition
• Predation
• Parasitism
• Drought other climate extremes
• Human disturbances

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Density Growth Regulation

• Density Dependent factors include disease,
  competition, parasites and food. These have an
  increasing effect as the population increases.
• Density Independent factors affect all
  populations regardless of their density (numbers)
  Most are abiotic factors such as temperature,
  storms, floods, droughts and habitat destruction.

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Population Regulation

• Populations are regulated by a complex
  interaction of biotic and abiotic influences
• In density-independent populations birth rate and
  death rate do not change with population density.
• For example, in dune fescue grass environmental
  conditions kill a similar proportion of
  individuals regardless of density.
• In contrast in density-dependent populations
  birth rates fall and death rates rise with
  population density.
• Density-dependent population regulation much more
  common than density- independent

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Density-Dependent Factors

• A limiting factor that depends on population size
  is called a density-dependent limiting factor
• These factors become limiting only when the
  number of organisms per unit area reaches a
  certain level
• Competition
• Predation
• Parasitism
• Disease

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Competition

• When populations become crowded, organisms
  compete, or struggle, with one another for food,
  water, space, sunlight, and other essential
  elements of life
• The more individuals that live in an area, the
  sooner they will use up an available resource
• Interspecific competition occurs between members
  of two different species.
• Intraspecific competition occurs between members
  of the same species.

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Predation

• Populations in nature are often controlled by
  predation
• The regulation of a population by predation takes
  place within the predator-prey relationship (one
  of the best known mechanisms of population
  control)

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Predation Can Affect Population Sizes
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Parasitism Disease

• Parasites can limit the growth of a population
  because they take nourishment at the expense of
  their hosts
• This often weakens the host and can lead to
  disease or death

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Density-Independent Factors

• Density-independent limiting factors affect all
  populations in similar ways, regardless of the
  population size
• Weather
• Natural disasters
• Seasonal cycles
• Human activities
• Damming rivers, cutting forests

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Human Population Growth

• Like the populations of many other living
  organisms, the size of the human population tends
  to increase with time
• Began growing more rapidly 500 years ago because
  agriculture and industry made life easier
• Improved sanitation, medicine and health care
• Butearths resources are limited!

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Human Population Growth
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Patterns of Population Growth

• The scientific study of human populations is
  called demography
• Examines the characteristics of human populations
  and attempts to explain how those populations
  will change over time
• Birthrates, death rates, and the age structure of
  a population help predict why some countries have
  high growth rates while other countries grow more
  slowly

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The Demographic Transition
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The Demographic Transition

• Over the past century, population growth in the
  US, Japan, and much of Europe has slowed
  dramatically. One hypothesis as to why is that
  these countries have completed the demographic
  transition (a dramatic change in birth and death
  rates).
• As countries modernize, advances in nutrition,
  medicine, and sanitation result in more children
  surviving into adulthood and more adults living
  to old age. These changes lower the death rate
  and begin the demographic transition.

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Age Structure

• Population growth depends in large part on how
  many people of different ages make up a given
  population
• We can predict future growth using models called
  age-structure diagrams
• Graph the numbers of people in different age
  groups in the population

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Age Structure Diagrams

• Age structure diagram- a population profile,
  graphs the numbers of people in different age
  groups in the population