BIOLOGY 403: PRINCIPLES OF ECOLOGY (Populations)

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BIOLOGY 403 PRINCIPLES OF ECOLOGY
(Populations)
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POPULATIONS

• What is a population?
• all the individuals of a certain species in a
  particular area
• adjacent populations of the same species usually
  have some degree of interaction (immigration,
  emigration, gene flow, resource exchange, etc.)
  thus forming a METAPOPULATION

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EMERGENT PROPERTIES

• Each level of organization has certain
  properties.
• As we go from one level to the next (e.g. from
  atoms to molecules or individuals to populations)
  we see that the higher level has many of the
  properties of the lower level(s) that make it up.
• HOWEVER, we also see properties or attributes
  emerging in the whole which were not evident in
  the parts that make it up.
• In other words, the whole is more than the sum of
  its parts.

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Some Important Population Factors / Attributes

• Natality (crude, age or sex-specific) (0 or )
• Mortality (crude, age or sex-specific) (0 or )
• Growth Rate ( or 0 or -)
• Carrying Capacity (K)
• Density
• Density Dependent Factors
• Density Independent Factors
• Age Distribution
• Dispersion
• Survivability
• Competition (intra species)
• Evolution

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POPULATION GROWTH I

• The rate at which a population grows depends on
• Natality
• Mortality
• Immigration
• Emigration
• Of course, each of the above factors is itself
  affected by other factors.

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POPULATION GROWTH II

• All rates (growth, birth , death) can be
  expressed by the general formula ?N / ?T
• r biotic potential (the ability of a population
  to increase)
• r depends on
• Survival to reproductive age
• Age of 1st breeding
• Duration of reproductive portion of the life span
• Number of offspring per reproductive episode

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FACTORS AFFECTING POPULATION SIZE
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SURVIVORSHIP

• Mortality slows population growth
• Concentrate on those that live rather than those
  that die
• The reciprocal of mortality is survivability
• Rate at which organisms die often is not uniform
  during the lifespan of a species

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GENERALIZED SURVIVORSHIP CURVES
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SURVIVORSHIP REVISITED

• Not all organisms fit neatly into the generalized
  categories
• Herring gulls have Type III early in life and
  Type II later
• Oysters Salmon have Type III
• Hydra, many annual plants and some reptiles show
  Type II
• Humans?????

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TYPES OF GROWTH (I)

• Linear Growth
• a quantity increases by a constant amount per
  unit of time additive
• produces a straight line when graphed
• Exponential Growth
• a quantity increases by a fixed percentage of
  the whole per unit of time same phenomenon as
  compound interest
• produces a curve when graphed

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TYPES OF GROWTH (II)
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POPULATION GROWTH PATTERNS

• Exponential (J-shaped curve)
• largely density independent
• Logistic (s-shaped or sigmoid)
• more density dependent
• factors such as territoriality, aggression,
  inter- and intraspecific competition, predation
  and disease are of major importance here

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POPULATION GROWTH CURVES J vs. S (I)
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POPULATION GROWTH CURVES J vs. S (II)
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POPULATION GROWTH CURVES J vs. S (III)

• Populations which follow a J-shaped curve usually
  lack control by density dependent factors.
• Populations which follow an S-shaped curve have
  one or more DENSITY DEPENDENT factors controlling
  their growth (e.g., territoriality, aggression,
  inter- or intraspecific competition, predation,
  disease).
• All populations can be affected by DENSITY
  INDEPENDENT factors such as catastrophic weather,
  earthquakes, volcanic activity, etc.

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FORMULAE FOR J S CURVES

• J Curve Growth r N
• S Curve Growth r N 1 (N / K)
• When N is small, (N / K) is small, and
  thus1 (N / K) is relatively large and growth
  is rapid
• When N is larger, (N / K) is larger and thus
  1 (N / K) is smaller and growth slows
• When N K then (N /K) 1 and 1 (N / K)
  0 and growth stops

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HUMAN POPULATION GROWTH

• What type of Growth pattern? J or S?
• Why?

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HUMAN POPULATION GROWTH

• EXPONENTIAL ?????
• Until rather recently in our evolutionary history
  human numbers were held in check by famine,
  disease, war, lack of technology, etc. (
  Environmental Resistance)
• Recently these factors have been greatly
  minimized ( less environmental resistance).
• Population is growing rapidly due to good death
  control but poor birth control.

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SOME ADDITIONAL POPULATION GROWTH ITEMS

• Replacement Level Fertility
• Zero Population Growth
• Doubling Time For A Population
• The Rule of 70
• 70 / growth rate doubling time
• e.g. 70 / 1.6 43.75 years
• 70 / 5 14 years
• 70 / 2 35 years

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DENSITY I

• Population size (numbers or biomass) per unit of
  area or volume
• Absolute Density (actual count of numbers or
  biomass)
• Relative Density (some type of sampling)
• Number or biomass per unit of time
• Abundance (rare, common, etc.)
• Frequency of encounters in sampled plots or in
  time intervals
• Counts in random selected plots

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DENSITY II

• Population size can be controlled by Density
  Independent Factors and Density Dependent Factors
• Density Independent (usually abiotic)weather,
  earthquakes, landslides, volcanic activity
• Density Dependent (usually biotic)predation,
  parasitism, stress, territoriality and other
  behaviors

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DENSITY III

• Density Independent factors can affect both J
  (exponential) populations and S (logistic)
  populations
• Organisms with logistic growth (S) have one or
  more internal or external Density Dependent
  factors regulating their population size

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POPULATION AGE STRUCTURE I

• The proportion of various age groups in a
  population can have a profound effect on growth
  of a population.
• It depends on a number of factors
• Age group studies in a population often bring to
  light interesting / important aspects of the
  species
• Three Ecological Ages
• Pre-reproductive
• Reproductive
• Post Reproductive

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POPULATION AGE STRUCTURE II

• of life span spent in various stages varies
  considerably
• Mayflies mostly pre- (a year), 2-3 days repro,
  no real post-
• Cicadas mostly pre- (years), a month or two
  repro-, maybe a little post-
• Trees years/decades pre-, decades/centuries
  repro-, maybe no post-
• Humans at one time no post- ???? Now pre- is
  the shortest, repro- long and post- longer??

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POPULATION AGE STRUCTURE III

• Humans (and some other organisms as well) have
  two types of pre-reproductive ages.
• Absolute ( biological)
• 12 to 15 years ?????
• Dropping a bit over the last 50 years ?????
• Socially Acceptable
• varies with the culture

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AGE CLASSES

• Chronological classes
• Do not have to be equal
• 0-1 year, 1-5, 5-10, 10-20.70-75
• Age class data can be presented as a table, bar
  graph or Age Polygon

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POPULATION AGE STRUCTURE (I)
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POPULATION AGE STRUCTURE (II)
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POPULATION AGE STRUCTURE (III)
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POPULATION AGE STRUCTURE (IV)
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DISPERSION I

• Dispersion refers to the pattern of the organisms
  on the landscape (or in a 3-dimensional system)
• Dispersion types
• Random
• Non-Random
• Uniform
• Clumped (aggregated)
• Random c, Uniform c, Aggregated c

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DISPERSION II

• Causes of the dispersions
• Response to local habitat conditions
• Indigenous
• Produced by the organism (allelopathy, etc.)
• Response to daily / seasonal weather changes
• Reproductive processes
• or - social interactions
• Extrinsic biological factors (predation, etc.)

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RANDOM DISPERSION

• There is no pattern in the distribution of
  individuals in the population.
• No factors are working on this population to
  influence the association of individuals.
• THEREFORE, where an organism is found is due to
  chance.
• RARE!

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UNIFORM DISPERSION

• Spacing is fairly regular. Generally each
  individual has its own area.
• This can be due to competition, Allelopathy or
  other antagonistic behaviors.

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CLUMPED DISPERSION I

• Organisms are more likely to be found associated
  with others
• Can result from
• Asexual reproduction (especially in plants)
• Heavy fruits or seeds in plants
• Social interactions in animals (these often allow
  for protection, learning, division of labor)
• Habitat irregularities

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CLUMPED DISPERSION II

• The aggregates of organisms can then be
  distributed
• Randomly --- rare
• Uniformly --- bunchgrass in arid regions
• Clumped --- due to habitat irregularities so that
  certain critical resources are located only in a
  few areas social interactions

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DETERMINING DISPERSION PATTERNS I

• Poisson Distribution Test a test for randomness
• Dependent on count data
• Divide area in subplots (? how small ?)
• Use Poisson Formula to determine the number of
  plots expected to contain 0, 1, 2, 3, etc.
  organisms
• Count the number of plots ACTUALLY having 0, 1,
  2, 3, etc. organisms

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DETERMINING DISPERSION PATTERNS II

• Use Chi2 to determine if the deviations between
  the observed and predicted numbers are likely due
  to chance
• IF Chi2 is significant then most likely it is a
  non-random distribution
• If more plots than expected with one organism,
  then probably uniform
• If more plots with 2 and higher and fewer with 0
  or 1, then probably clumped

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COMPETITION

• COMPETITION --- a striving for something that
  (usually) is in short supply
• INTRAspecies competition
• A negative interaction
• Usually gets more intense as density increases
• At the population level this usually means that
  one or more of the following will be inhibited to
  some degree
• Density, Organism size, Population energy flow

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EVOLUTION

• Competition and Evolution go hand in hand
• Who survives competition?
• REMEMBER FITNESS is really an average arrived
  at from the interaction of many factors

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DISPERSAL

• How a species moves
• When it moves
• Types of disseminules
• Migration patterns (diurnal, seasonal, once
  during the life cycle or many times)