Population Ecology

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Population Ecology
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Objectives

• 5.3.1 Outline how population size is affected
  by natality, immigration, mortality, and
  emigration.
• 5.3.2 Draw and label a graph showing a sigmoid
  (S- shaped) population growth curve.
• G.1.3 Describe one method of random sampling
  based on quadrat methods that is used to
  compare the population size of two plant or two
  animal species.
• G.1.4 Outline the use of a transect to
  correlate the distribution of plant or animal
  species with an abiotic variable.

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Characteristics of Populations

• A population is a group of individuals of a
  single species that simultaneously occupy the
  same general area. Characteristics of populations
  are shaped by the interactions between
  individuals and their environment.
• Populations have size and
• geographic boundaries.
• Density is the number of
• individuals per unit area.
• Dispersion is the pattern
• of spacing among
• individuals within
• geographic boundaries.

Darker color greater density
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Patterns of dispersion

• Dispersion patterns depend on variable population
  densities. Dispersion and density both depend on
  resource distribution.
• Random dispersion, individual positions are
  independent.
• Uniform dispersion individuals are evenly
  spaced.
• Clumped dispersion individuals aggregate in
  patches.

Random
Uniform Clumped
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Random sampling

• Measuring population density is a difficult task.
  Do you
• Count individuals - often impractical if
  individuals do not want to cooperate, or
• Use sampling techniques to estimate numbers.
• The mark-recapture method Individuals are
  trapped,
  marked with a tag, recorded, then
  released and captured a second time. The
  percentage recaptured gives an estimate of the
  true population.
• Quadrat methods count all individuals within a
  pre- cisely outlined area, then extrapolate to
  the larger environment. ? an estimate.

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Quadrat sampling techniques

• A quadrat is a square of a convenient size,
  perhaps 1 m x 1 m, in which species of interest
  are counted.
• Later relate totals to the area of interest
  perhaps an entire forest.

Counting rule Omit objects on the left
and bottom edges.
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Quadrat sampling techniques
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Demography

• Demography is the study of factors that affect
  the growth and decline of populations.
• Additions occur through
• birth (natality) and im-
• migration subtractions
• occur through death
• (mortality) emigration.

Population Growth by County April, 2000, to
July, 2003
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Demography

• Represent a life table
  data with a
  survivorship
  curve which plots
  the
  number of indi-
  viduals in a cohort
  
  still alive at each age.
• Type I curve shows
  a low death
  rate early in life (humans).
• Type II curve shows constant mortality
  (squirrels).
• Type III curve shows high death rate early in
  life (oyster).

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Population Growth (ideal conditions)

• Exponential growth increases faster and faster
  un-checked, like compound interest, because
  resources are unlimited. Growth becomes faster
  and faster.
• Change in population size Births -
  Deaths
• This model is best for single-celled life
  (bacteria).

a J-shaped curve
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Population Growth (ideal conditions)

• No population can in-
  crease forever! Why?
• It eventually runs out of
  resources
• too little food,
• too little space,
• too many waste products,
• too many predators,
• too much disease.

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Population Growth (typical conditions)

• The logistic growth model shows a leveling-off
  because of environmental feedback. The carrying
  capacity (K) is the maximum population size that
  can be supported by available resources.
• Typically, unlimited resources
  are rare, so this graph is
  S-shaped.

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Population Growth

• Stages of growth
• a) Lag phase growth is slow as 1?2, then 2?4,
  4?8, etc.
• b) Exponential phase eventually growth speeds
  up as 1 million ? 2 million, etc.
• c) Transitional phase at some point the
  food begins to run out, and
  the growth slows down.
• d) Plateau phase carrying capacity has
  been reached, and births equal
  deaths.

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Population Growth

• The logistic growth model does not fit all
  situations!
• Resource availability depends on the situation.
• Natural selection can vary with density
  environment.
• There are lag times that alter the S-curve.
  Populations can grow large, then many may starve
  (birds, below).

Microbes in the lab fit the model well.
Higher organisms in nature are influenced by
many factors.
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Population-limiting Factors

• Why do all populations eventually stop growing?
• What environmental factors stop a population from
  growing?
• Examine effects of increased population density.

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Population-limiting Factors

• Density-independent factors act the same
  regard-less of population size.
• Natural disasters such as hurricanes, tsunamis,
  forest fires destroy all but the lucky
  individuals in large or small populations.

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Population-limiting Factors

• Density-dependent factors (negative feedback)
  increase their effect on populations as
  population density increases.
• Negative feedback prevents unlim-
  ited population
  growth.
• Intra-specific competition for food, shelter,
  or mates can cause density-dependent
  behavior of populations.
• Resource limitation in crowded populations
  can stop growth population by reducing
  reproduction.

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Population-limiting Factors

• Negative feedback prevents unlimited population
  growth.
• Territoriality (defense of a space) may set a
  limit on density.
• Predation may cause mortality of prey species.
• Waste accumulation can regulate population size.
• Ex yeast make alco- hol during
  ferment- ation of grape juice to wine,
  but yeast begin to die as alcohol
  content approaches 13.
• Disease spreads much
• more rapidly in dense
• populations.

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Population-limiting factors

• Some populations have regular boom-and-bust
  cycles.
• Lynx hare populations in Alaska cycle on a
  10-year basis.
• More hares allow more baby lynxes to survive
  reproduce
• More lynxes eat more hares, then lynx population
  crashes.
• Fewer lynxes allow more hares to survive - cycle
  repeats.

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Over-population

• The human population has been growing almost
  exponen-tially for three centuries but cannot do
  so indefinitely.
• What might put a stop to this
  growth?

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An apocalypse?
Conquest, War, Famine, Disease