Population Ecology

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Chapter 52 Population Ecology
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Population Ecology

• Study of the factors that affect population size
  and composition.

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Population

• Individuals of a single species that occupy the
  same area.

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Important Characteristics

• 1. Density
• 2. Dispersion

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Density

• Number of individuals per unit area or volume.
• Ex
• Diatoms - 5 million/m3
• Trees - 5,000/km2
• Deer - 4/km2

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Dispersion

• Pattern of spacing among individuals.
• Types
• 1. Clumped
• 2. Uniform
• 3. Random

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Clumped Dispersion

• May result form a patchy environment.
• May increase chances for survival.
• Ex
• Schooling behavior
• Flocks of birds

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Uniform Dispersion

• Often the result of antagonistic interactions
  between individuals.
• Ex
• Territories
• Spacing between desert plants

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Random Dispersion

• Often the result of the absence of strong
  attractions or repulsions between individuals.
• Not a common pattern.

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Demography

• The study of the vital statistics that affect
  population size.
• Ex Birth and Death rates

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Demographic Factors

• Age structure of the population.
• Birth and Death rates.
• Generation time.
• Sex ratio and reproductive behavior.

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Life Tables

• Mortality summary for a cohort of individuals.
• First developed from life insurance studies.

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Life Tables Show

• Mortality rate per year.
• Life span of the organism.
• Fecundity (birth rate).

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Survivorship Curve

• Plot of the numbers of a cohort still alive over
  time.
• Curve Types
• Type I
• Type II
• Type III

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Type I

• Low early deaths.
• High late deaths.
• Ex
• Humans
• Other large mammals

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Type II

• Constant death rate.
• Ex
• Annual plants
• Many invertebrates

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Type III

• High early deaths.
• Low late deaths.
• Ex
• Trees
• Oysters

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Comment

• Curve type may change between young and adults.
• Ex Nestlings - Type III Adult Birds-
  Type II

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Life History Strategies

• 1. "r" or Opportunistic species
• 2. "k" or Equilibrial species

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"r" Species

• Increase fitness by producing as many offspring
  as possible.
• Do this by
• Early maturation
• Many reproductive events
• Many offspring

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Result

• Maximize reproduction so that at least a few
  offspring survive to the next generation.
• Most offspring die (Type III curve).

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"k" Species

• Increase fitness by having most offspring
  survive.
• Do this by
• High parental care
• Late maturation
• Few reproduction events
• Few offspring.

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Result

• Maximize survivorship of each offspring.
• Few offspring, but most survive (Type I curve).

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What is the strategy

• For a weed?
• For an endangered species?
• For Garden Pests?

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Population Growth

• DN/Dt b - d
• Where
• N population size
• t time
• b birth rate
• d death rate

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Rate of Increase

• r difference between birth
  rate and death rate.
• r b - d

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Equation

• DN/Dt rN
• N population size
• t time
• r rate of increase

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From Calculus

• The equation DN/Dt rN
  becomes
• dN/dt rmax N
• rmax intrinsic rate of increase

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Exponential Growth

• dN/dt rmax N
• Characteristic of "r" species.
• Produces a J-shaped growth curve.
• Only holds for ideal conditions and unlimited
  resources.

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Logistic Growth

• dN/dt rmax N K-N K
• K carrying capacity

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Result

• S-shaped growth curve.
• Characteristic of k" species.
• Common when resources are limited.

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Comment

• K is not a constant value.
• Populations often oscillate around K as the
  environment changes.

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Additional Comments

• Populations often overshoot K, then drop back
  to or below K.
• AP Exam rarely asks you to work the equations,
  but you should be able to give them.

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Regulation of Population Size

• 1. Density- Dependent Factors
• 2. Density- Independent Factors

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Density-Dependent

• Affect is related to N.
• As N increases, mortality increases.
• Ex Food, nesting space, disease

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Density-Independent

• Affect is not related to N.
• Mortality not related to population size.
• Ex Weather and climate

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Population Cycles

• Cyclic changes in N over time.
• Often seen in predator/prey cycles.
• Ex Snowshoe Hare - Lynx

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Causes

• Density dependent factors.
• Chemical cycles.
• Saturation strategy to confuse predators.

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Age Structure Diagrams

• Show the percent of a population in different age
  categories .
• Method to get data similar to a Life Table, but
  at one point in time.

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Importances

• Can be used to predict future population growth
  trends, especially for long lived species.

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Exponential Growth

• Produces age structures that are a triangle or
  pyramid shape.

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Logistic Growth

• Produces age structures that have even sizes
  between most age categories.

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Declining Populations

• Produce age structures with a narrow base and
  wider middles.

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Summary

• Know density and dispersion patterns.
• Know Life Tables and survivorship curves.
• Be able to contrast and compare r and k
  strategies.

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Summary

• Know exponential and logistic growth curves and
  equations.
• Know density and density-independent growth
  factors.