POPULATION ECOLOGY

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POPULATION ECOLOGY

• Chapter 53

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Density and Dispersion

• What is the density of a population?
• The number of individuals per unit area
• What are the three patterns of dispersion?

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Clumped

• Clumped dispersion has population occurring in
  patches
• Increased concentration of resources leads to
  clumping
• Defense against predators
• e.g. schools of fish, sea stars grouping together
  where food is abundant

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Uniform

• Even spacing of individuals within the population
• Diminishes competition
• Often seen with plants, they secrete chemicals
  that prevent germination and growth of
  individuals that may compete for resources.
• King penguins exhibit uniform spacing due to
  aggressive interactions between neighbors

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Random

• Spacing is varied and unpredictable
• Occurs in the absence of strong attraction
  (concentrated resources) or strong repulsion
  (avoidance of competition) between individuals.
• Dandelions grow where seeds randomly land

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Estimating Population Size

• Mark re-capture method.
• You capture 200 grasshoppers, mark them all and
  then release them back into the wild. A few days
  later you capture 100 grasshoppers and 50 of them
  were marked. How many grasshoppers do you
  estimate are in this population?
• N marked x total captured second time
• number marked recaptures
• 200 x 100/50 400

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Demography

• What is demography?
• Study of vital statistics that affects population
  size

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Survivorship Curves

• What is a survivorship curve?
• Plot of the numbers of a cohort that are alive at
  each age.
• Survivorship curves are generally classified into
  three types

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Characteristics of The Three Types Seen in
Survivorship Curves

• Type I Low death rate in early years.
• Small clutch sizes and good care for young
• Type II constant mortality throughout life
• Type III large clutch size, little care for
  young high mortality rate early on in life.

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Life Histories

• What is the life history of an organism?
• Traits that affect time of reproduction and
  death.
• 1. Clutch size
• 2. Number of reproductive episodes per
  lifetime
• 3. Age at first reproduction

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Birds with high probability of dying have larger
clutch sizes
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• Number of reproductive episodes
• Large clutch sizes mean one reproductive episode
  per life.
• Small clutch sizes means more than one
  reproductive episode
• Age at first reproduction is younger in large
  clutch size reproducers, they invest less energy
  in their own growth and development.

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Exponential Growth

• J shaped curve
• Unlimited population growth, due to unlimited
  resources.
• r selected populations favor the exponential
  growth curve. Seen in areas with little
  competition
• Opportunistic species display this pattern,
  grasses, insects
• But all populations eventually have a limit
  placed on their growth...

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Logistic Growth

• Limiting factors will affect the size of a
  population
• K carrying capacity, never changes

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Examples of logistic growth
K selected species - likely to be living at
carrying capacity, seen in areas with competition
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• r selected
• short maturation time
• short life-span
• high death rate
• many offspring/reproduction
• one reproductive event
• reproduces early in life
• small size to offspring
• no parental care

• K selected
• long maturation time
• long lifespan
• low death rate
• few offspring/reproduction
• several reproductive episodes
• reproduces later in life
• large sized offspring
• parental care for young

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Population Limiting Factors

• Density dependent factors
• Intensifies as population grows
• Density independent factors
• Affects same number of individuals regardless of
  population size.
• A predator is which sort of limiting factor?
• Density dependent
• A hurricane is which sort of limiting factor?
• Density independent

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What sort of limiting factor is seen here?
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Density dependent!
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What sort of limiting factor is seen here?
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Density independent
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Usually more than 1 limiting factor is at work
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• Fluctuations in sheep population due to weather
• Cold, wet winters weakens sheep and decreases
  food availability leading to a decrease in the
  size of the population

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• Moose population collapse coincided with a peak
  in the wolf population from 1975-1980
• Second moose population collapse in 1995
  coincided with harsh winter weather which
  increased the energy needs of the animals and
  made it harder for the moose to find food

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• Describe the population shifts of snowshoe hare
  and lynx

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Human Population

• Global population is still growing but the rate
  of growth is slowing down
• Age structures - relative numbers of individuals
  at each age. Used to tell if a population is
  increasing, decreasing or staying the same
• birthrate, death rate, generation time and sex
  ratios, all affect interpretation of age
  structures

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