Population Ecology

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Chapter 52

• Population Ecology

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Characteristics of Populations

• Population group of individuals of one species
  living in a defined region
• Density number of individuals per area
• Dispersion pattern of spacing among individuals
  in the area

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Measuring Density

• How do we count all members of a population in a
  geographic area?
• Count them?

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Counting Techniques

• Sample plots
• Take random samples and count individuals in that
  area
• Mark-recapture method
• N(marked in first capture X total in second
  capture)/ of recaptures in second catch
• Ex. 50 hares are captured and marked on day 1
• 100 hares are captured on day 15
• 10 hares have tags on day 15

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Patterns of Dispersion

• Clumped most common pattern
• Organisms live in microenvironments that meet
  their needs
• Needed for mating behavior
• Uniform evenly spaced
• Result of territorial behavior
• Plants shade one another
• Random rare
• Rain forest trees

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Demography

• Study of factors that affect growth and decline
  of populations
• Specifically looking at birth rates and death
  rates
• Life Tables age-specific summary of survival
  pattern

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Survivorship Curves

• Graphic representation of life tables

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Reproductive Rates

• Focus on females they are the reproductive
  units of populations
• Reproductive table age specific summary of
  reproductive rates in a population

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Life Histories

• Life history traits that affect an organisms
  schedule of reproduction and survival.
• Types of reproduction
• Semelparity (big bang reproduction)
• Organism reproduces once, but produces lots of
  offspring
• Iteroparity (repeated reproduction)

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Reproduction and Survival

• Natural selection would favor individuals that
  reproduce early, often, for long time
• Why dont organisms do that?
• LIMITED RESOURCES

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Population Growth

• Consider a single bacteria
• Divides every 20 minutes under ideal conditions
• So in 20 minutes 2 bacteria, 40 min. 4 bact, 80
  min. 8
• In 36 hours there would be a layer of bacteria 1
  foot deep over the entire Earth
• Consider Elephants
• Produce 6 young in 100 years
• 1 pair of elephants could produce 19 million in
  only 750 years
• Why arent there more Elephants?
• LIMITED RESOURCES

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Population Growth Models Exponential Growth

• Population under ideal conditions (no limited
  resources)
• What we want to see is how the population size
  changes from 1 year to the next.
• OR change in population size during some time
  interval Births during time interval Deaths
  during time interval
• Now mathematically
• N population size
• t time period
• B of births
• D of deaths

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Population Growth Models Exponential Growth

• We can rewrite the equation
• DN/Dt B-D
• Take B (average births) and convert it to the
  birth rate (avg. of offspring produced per
  individual in population)
• Example population of 1000 individuals has 34
  births in one year, then the birth rate is
• 0.034
• Do the same thing with D (average deaths)
• NOW b birth rate and ddeath rate

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Exponential Model

• BbN and DdN
• Example from before. The figure out how many
  births there are for a population lets take
  b0.034.
• Now how many births are there in a population of
  500 individuals.
• B17 per year
• Now our equation looks like this
• DN/Dt bN - dN

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Exponential Model

• Population ecologists are not concerned with how
  many births and how many deaths there are just
  how these factors affect overall population. SO
  the equation gets changed again.
• r b d or r birth rate death rate
• Example population 1000 individuals, 50 births,
  80 deaths.
• b0.05
• d0.08
• r -0.03
• Now our equation looks like this
• DN/Dt rN

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Exponential Model

• Finally most ecologist use calculus notation to
  illustrate instantaneous changes
• dN/dt rN
• Remember we are talking about a population that
  is living under ideal conditions (no limited
  resources)
• Intrinsic rate of increase a populations
  maximum rate of growth rmax
• FINAL EQUATION for exponential growth
• dN/dt rmaxN

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Population Growth Models Logistic Growth

• Now lets study a population that is NOT under
  ideal conditions (limited resources)
• There is never such a thing as unlimited
  resources so this model is closer to real life
  than exponential growth. Although some
  populations exhibit exponential growth in a novel
  environment early on.
• Carrying capacity maximum population size a
  given environment can hold
• Symbolized as K

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Logistic Model

• When a population is well below K it exhibits
  exponential growth, but as it approaches K
  population growth slows down.
• Maximum population size is K, then K-N tells us
  how many more individuals the environment can
  hold. And (K-N)/K tells us the fraction of K
  still available for population growth. SO
  therefore
• dN/dt rmaxN(K-N)/K
• Lets look at a sample population

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How well does Logistic Model fit real populations?

• Logistic model gives ecologists a start, but
  sometimes other factors not included in equation
  have affects
• Periods of severe weather
• Overshooting carrying capacity

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Logistic Model assumptions

• Assumes more individuals is negative
• Not true in some cases Allee effect
• Plant and wind affect
• Small populations and finding a mate

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Population-limiting Factors

• Density Dependent and Density Independent birth
  and death rates
• Example as population density increases so does
  death rates (Density dependent death rate)
• Example as population density increases birth
  rates decrease (Density dependent birth rate)
• Example as population density increases neither
  birth nor death rate change (Density Independent
  rates)

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Negative Feedback

• Prevents unlimited population growth
• Species produce fewer offspring when density
  increases

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Negative Feedback

• Some resources such as space become limited with
  increased population density
• Gannets and rocky cliff nests
• Population density affects health and survival of
  organisms

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Abiotic and Biotic Factors affecting Population
Growth

• Carrying capacities change over time due to
  weather, and other variables

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Abiotic and Biotic Factors

• Dungeness crab large adult males are harvested
  by humans, small juveniles are cannibalized, and
  ocean currents affect offspring

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Boom and Bust Cycles

• Some populations exhibit wild fluctuations in
  numbers over time
• Snow shoe hare and lynx populations
• Why do they fluctuate on a regular basis
• H1 food shortage
• H2 excessive predation
• H3 combination of both
• How could we test these hypothesis?

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Human Population Growth

• Population growing exponentially for three
  centuries
• Impacts of human population
• Use of resources
• Loss of habitat
• Loss of species

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Human Population Growth

• 1650 500 million people
• 1850 2 billion
• 1975 4 billion
• 2000 6 billion
• Every day 214,000 people are added to the Earth
• Like the size of Amarillo, TX
• Every 3 years it adds a new United States

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Replacement Level Fertility

• Demographic transition when zero population
  growth occurs
• Just as many babies are born as people who die
• USA RLF is 2.1 and that is equal to fertility
  rate of females
• Some countries are below RLF
• Means populations are declining (outside of
  immigration)
• Developing countries constitute most of worlds
  population boom

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Age Structure Diagrams

• Relative number of individuals in each age group

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Estimating Earths Carrying Capacity

• How many people can the Earth Hold?
• Answers vary from 10 billion to 1 trillion
• Why so varied?
• Promising study of Ecological footprint
• Determining amount of resources a country has to
  provide needs for its population

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