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Ecology Lecture 7 Ralph Kirby Population (Ecology) Group of individuals of the same species inhabiting the same area Interbreeding if sexual Limited in space Populations have Density Distribution in time Distribution in space These are defined by Rate of birth Rate of growth

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Lecture 7

Ralph Kirby

Population (Ecology)

Group of individuals of the same species inhabiting the same area

Interbreeding if sexual

Limited in space

Populations have


Distribution in time

Distribution in space

These are defined by

Rate of birth

Rate of growth

Rate of death

Resulting in age & sex structure

What is an individual

Easy for most large sexually reproducing animals

Not so easy for colonies of animals and many plants




Trees (Ramets)


    • Area over which a species occurs
  • Abundance
    • Numbers of that species present in the area
  • Both vary by available habitat
    • Temperature
      • Red Maple
        • -40oC
      • Carolina wren
        • Northwards -7oC
        • Westward>52mm
    • When all environmental factors within its range of tolerance, the organism can live in its habitat
      • Horned lark
        • Avoids forests
        • Available territory
Density can be a key factor
    • Number of individuals per unit space
    • Affects
      • growth rate
      • Resources
      • Mates
      • predation
  • Thus density controls in part
    • Birth rate
    • Growth rate
    • Death rate
  • Density is difficult to define or measure
    • What area do you use
    • What distribution has the species within the area
    • How do you measure the density
    • Ecological density
      • Some use a factor associated with the habitat
  • Distribution
    • Clumped
      • Normal
    • Uniform
      • High density and competition
    • Random
      • Social behavior
      • Available nesting sites
  • Metapopulation
    • Multi-Habitat based multiple populations
how do you count
How do you count
  • Direct Counts
    • Time consuming
    • Needs to be sure you can see all individuals in the area
  • Quadrats
    • Organisms must be static
    • Needs laying of quadrats to be random
    • Statistical analysis essential
  • Mark and recapture
    • Estimate
      • Assumes equal chance of capture for all
      • No deaths or births
      • Marked animals random among unmarked
      • No loss of marks
      • No emigration or immigration
      • Other factors include time of capture, stress of capture, sex, age, etc.
  • Relative abundance
    • Using a factor such as tracks, bird song, etc
    • Get a relative measure
Disperal – Active and Passive
  • Emigration – Escaping high density
  • Immigration – Moving into empty habitat
  • Migation – A round trip, perhaps involving mating
Age structure
    • Prereproductive
    • Reproductive
    • Postreproductive
    • Short life span
      • Rapid increase
    • Long Life span
      • Slow increase
  • Measuring Age structure
    • Mark and recapture
      • Accurate but hard to do
    • Special features
      • Teeth
      • Horn rings
      • Tree rings
  • Age Pyramids
    • Show status of population
Size/Age can affect the population structure
  • Sex ratios in populations shifts with age
    • Humans
      • More males
      • Males have shorter life span
        • War
        • High risk activities
      • Females need to survive giving birth to children
      • Therefore sex ratios have change a lot over the last two hundred years
        • Parturition fever in 19th Century
        • First World War in UK
        • Second World War in USSR
Plants are more difficult

Hard to detrmine age

Extremely high loss fo seedlings

Use of yield table

Probability of survival is age specific

See death table

Probability of death in next year

Life Table


Number in cohort


Probability of surviving from birth


Age specific mortality

    • Sex specific
    • Age specific
  • Net reproductive rate depends on
    • Fecundity
    • Survivorship
  • Given
    • Age specific mortality
    • Age specific birth rate
    • Project population growth/decline
  • Exponential Growth
    • Depends on rate of reproduction, λ
    • If λ less than 1, there is cline
  • Population growth is limited by the environment
    • Carrying Capacity
    • See reindeer

Change in age structure



Changes in birth and death rates

But stable once it reaches carrying capacity

Populations should work via positive and negative feedbacks

Does not work smoothly

Depends on environment such as overgrazing

Population cycles in simpler systems

Food availability

Can result in extinctions

Two types

Change in environment. Start locally

Mass extinctions. External event?

Three Models for regulation of population density
  • K represents equilibrium
    • Partial density regulation A
      • Birth Rate (B) independent of population density
      • Mortality Rate (M) increases with density
    • Partial density regulation B
      • Birth Rate (B) declines with population density
      • Mortality Rate (M) independent of population density
    • Full density dependent regulation
      • Birth Rate (B) increases with deceasing population density
      • Mortality Rate (M) increases with increasing population density
Carrying capacity
  • See effect of density for tadpoles and white clover
    • Maximum number of sustainable individuals
    • Results in intra-specific competition
    • Two types
      • Scramble competition
        • Growth and reproduction depressed for all individuals
          • Can result in local extinction or population crash
      • Contest competition
        • Some individual deny resources to other individuals resulting in some growing and reproducing better than others
          • Indirect via exploitation of resources
          • Direct via interference
Population crash caused by Rindepest
  • Note exponential increase from low base
  • Grass limited in dry season
  • Competition related to rainfall producing grass
  • Once in equilibrium, rainfall most important
Effect of repotting
  • Effects can differ
    • Bison is exponential curve
    • Grizzly bear is linear effect
  • Competition can affect reproduction
    • harp seals
      • Increased age of sexual maturity
    • maize
      • Linear
    • marsh herb
effects of high density on reproduction
Effects of high density on reproduction
  • Stress
  • Pheromones
  • Disease
  • Increased mortality of young
  • Results in dispersal
    • Sources Habitat
    • Sink Habitat
      • Can be dangerous
      • Lack protective cover
      • High number of predators
    • Move no further than necessary
    • Move in forays

Home Range

Can overlap


No overlap

Fight to protect

Dominant male gets

best locations

Males and female

ranges can differ

Male and female

ranges overlap

  • Social dominance can affect reproduction and dispersion
  • Fighting and social position. Both male and female hierarchies are possible
  • Results in limited mating
    • Alpha
    • Beta
    • Omega
Territory is protected
    • Food
    • Mating
    • Nesting site
    • Attraction of mates
    • Avoidance of suboptimal habitat
    • Needs energy
      • May not be optimal strategy when resources are low