Understanding POPULATIONS

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Understanding POPULATIONS

• Chapter 20

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Definition

• A population is a group of organisms that belong
  to the same species live in a particular place
  at a particular time, using same resources.
• Examples
• All of the bass in a pond
• All of the blue jays in Washington County
• All of the red maples in Western PA

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Examples of populations

• All the bacteria of one All the gorillas of
  1
• kind on this agar plate species on this
  mountain

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Population ecology

• is concerned with fluctuations in population size
  and the factors that regulate populations.

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Properties of Populations
May differ from properties of the individual

• 1. Size
• 2. Density
• 3. Dispersion
• Population dynamics
• A. Growth rate
• B. Age structure
• C. Survivorship.

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1. Population Size

• Is the of individuals
• Can be difficult to measure directly
• How is counting a tree species different than
  counting a fish population?
• Ways to find size
• Counting individuals (might be impossible for
  some large, mobile, widespread populations)
• Estimate

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2. Population Density

• - Measures how crowded a population is.
• Expressed as number of individuals per unit or
  volume.
• Example
• pop. density of humans in USA is 30people/ sq km.

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Estimating Density

• 1. Count all individuals in a sample plot.
  (Estimates become more accurate as sample plots
  increase in size.)
• 2. Estimate by indirect indicators (such as
  number of nests or droppings or tracks.)
• 3. Mark-recapture method. (animals trapped in
  boundaries, marked later retrapped. )

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To Estimate Population Size

• Instead of laboriously counting everyone, just
  count organisms in 1 area multiply
• Example- If 25 oaks in 1 Km2, then estimate 250
  in 10 km2.
• (This assumes even distribution, or there will be
  an error in )

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3. Population Dispersion

• Clumped-most frequent pattern of distribution.
  Individuals clustered together in groups in
  response to uneven distribution of resources.
• Even - members maintain a minimum distance from
  one another, generally indicates strong
  competition
• Random - spacing is the least common pattern.

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4. Population Dynamics

• All populations change in size and composition
  over time.
• A. Growth Rate includes
• Birth rate births
• Death (mortality) rate deaths
• Life expectancy average life span

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B. Age Structure

• The distribution of ages in a population.
• Important to know ages of individuals.
• Example- older populations do not reproduce but a
  young population may have rapid growth.

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Age structure represented by graphs.
users.rcn.com/.../ P/Populations.html
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C. Patterns of Mortality

• Survivorship curves- Show the likelihood of
  survival at different ages for an organism.
• Data tends to fall into 3 types

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3 types of Survivorship curves

• Type I - likelihood of dying is small until old
  age (humans, elephants)
• Type II - likelihood of dying is even over
  lifetime. (birds)
• Type III - likelihood of surviving youth is
  small (many die young) but if live- then may live
  to be old (insects, fish)

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Charles Darwin
Calculated that a single pair of elephants could
increase to a population of 19 million
individuals in 750 years. Since the earth is NOT
overrun with elephants, this is evidence that
factors in the environment limit population growth
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Charles Darwin (1809-1882)

• His Life and Work
• Born to a wealthy family
• Studied medicine - quit.
• Then studied to enter the clergy.
• Spent a lot of time collecting biological
  specimens.
• Quit religion studies to become a NATURALIST.

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Voyage on the H.M.S. Beagle

• studied many things on the islands passed by
  boat.
• In part because he was so seasick he wanted to
  get off the boat.
• Basis for his life work.

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Darwins Evolutionary Theory

• (which he liked to call natural selection
• not evolution)
• States that
• -over many generations, natural selection causes
  characteristics of populations to change.
• -natural selection is responsible for evolution.

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What is survival of the fittest?

• It is NOT- King of the Jungle- with the
  meanest, biggest, most muscular animal killing
  all the smaller ones.
• Example- 2 dogs- One is the perfect specimen-
  shiny fur, wins shows, smart, etc. the other is
  small, ugly, mangy, living in the alley.
• The ugly dog visits every female dog in the
  county and reproduces many times, but the perfect
  dog never has puppies-
• which dog will pass on his traits?

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Measuring Populations

• 1. Growth rate
• 2. Exponential Model
• 3. Logistic Model
• 4. Population Regulation

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1. Growth Rate

• Definition
• The amount by which a populations size changes
  over time.

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• 4 Processes that affect growth
• Birth
• Death
• Immigration (individuals moving in)
• Emigration (individuals moving out)
• Usually assume Immigration Emigration are zero

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Growth Rate

• Describe graph growth rate mathematically
• Divide large populations into groups of 1000.
• Growth Birth rate Death rate

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Example population

• If 32 Births 10 Deaths per 1000 per year of
  individuals, total population is 40,000.
• Birth rate is 0.032
• Death rate is 0.010
• Growth rate is 0.022
• To find the of new individuals, multiply growth
  rate by total population
• 40,000 X 0.022 880 - increase in 1 year

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2. Exponential Model

• Describes a population that increases rapidly
  after only a few generations.
• The larger it gets, the faster it grows.
• Assumes birth/death rate are constant
• Assumes unlimited resources (like food).

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http//www.agen.ufl.edu/chyn/age2062/lect/lect_27
/39_05.GIF
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Limiting factors

• In reality, populations cannot grow indefinitely
  because
• Resources become depleted
• Waste accumulates
• As Competition increases
• Birth rate declines
• Mortality increases

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3. Logistic Model

• Builds on Exponential Model
• Accounts for limiting factors (environmental
  resistance)
• Carrying Capacity
• The number of individuals the environment can
  support over a long period of time.

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http//www.stockton.edu/cromartw/populationbio/lo
gistic.JPG
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Population Regulation

• Densityindependent factors
• Weather, fire, etc
• Reduces population regardless of size
• Densitydependent factors
• Resources shortages- like food, nesting sites-
  triggered by increase in population

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Populations may fluctuate in response to
predator- prey relationships
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Small Populations

• Decreased genetic variations due to small gene
  pool.
• Inbreeding (mating with close relatives)
• May reduce species ability to adapt to changes in
  the environment
• May be due to human activities
• Examples in wild
• Less than 200 Siberian tigers left
• Only 9 California condors in wild 1980s

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Human Population History

• For most of Homo sapiens existence, population
  grew very slowly
• Small Nomadic Hunter gather groups
• Agricultural Revolution- about 10,000 years ago-
  Human population grew moderately.
• Population Explosion Middle ages
• Times of decrease from disease, wars.
• Population grew from 3 to 5 billion people
  between 1960 1987.

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Growth today

• Developed Countries
• Leveling off or decreasing
• Developing Countries
• Still growing rapidly

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Demographic Transition Model
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Questions

• 1. Which of the following is a population?
• A. all the fish in a pond
• B. all the birds in New York City
• C. all the members of a family of humans
• D. all the fish of the same species in a lake
• 2. Which of the following refers to the
  population size that can be sustained by an
  environment over time?
• A. bell curve
• B. allele frequency
• C. carrying capacity
• D. exponential growth

d. c.
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More questions

• 3. Which of the following is true in the
  exponential model of population growth?
• a. Population growth continues indefinitely.
• b. Population growth stops at the carrying
  capacity.
• c. Population growth increases and then
  decreases.
• d. The immigration rate falls with increasing
  population size.
• 4. Which of the following is a density-dependent
  factor for a population of deer in a forest?
• a. a drought
• b. a landslide
• c. a period of freezing weather
• d. the number of cougars in the forest

a. d.