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Population Characteristics. A population is a group of individuals of the same species living in the same geographical area, and usually able to breed together. The study of human populations is called demography . The study of the other organisms populations is population ecology .

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population characteristics
Population Characteristics
  • A population is a group of individuals of the same species living in the same geographical area, and usually able to breed together.
  • The study of human populations is called demography.
  • The study of the other organisms populations is population ecology.
  • Population information about plants and animals is used in park planning, setting hunting & fishing limits and identifying endangered species.

The fur seal population of St. Paul Island, off the coast of Alaska, has experienced dramatic fluctuations.

What are some challenges in counting this population?


It is difficult to count the seal population because :

    • the animals are always moving
    • the large colonies are randomly dispersed in the habit
    • the habitat is partly terrestrial and partly aquatic.
population variables
Population Variables
  • The population is represented with the variable N.
  • Densityis the number of individuals per unit area or volume represented by DP .
  • Dispersion is the pattern of spacing among individuals within the boundaries of the population.
density a dynamic perspective
Density: A Dynamic Perspective
  • Determining the density of natural populations is difficult.
  • In most cases, it is impractical or impossible to count all individuals in a population.
  • Density is the result of an interplay between processes that add individuals to a population and those that remove individuals.



Individuals are added through births and immigration.

Individuals are removed through death and emigration.





patterns of dispersion
Patterns of Dispersion
  • Environmental and social factors influence spacing of individuals in a population.
  • There are three major population dispersion patterns.


1 uniform
1. Uniform
  • A uniform dispersion is one in which individuals are evenly distributed.
  • It may be influenced by social interactions such as territoriality.
2 random
2. Random
  • In a random dispersion, the position of each individual is independent of other individuals.
3 clumped
3. Clumped
  • In a clumped dispersion, individuals aggregate in patches.
  • A clumped dispersion may be influenced by resource availability and behavior.
population sampling techniques
Population Sampling Techniques
  • There are a number of techniques used to estimate a population and calculate a population density including:
      • Transect sampling
      • Quadrat sampling
      • Mark-recapture sampling
  • All these require the use of random sampling in which all individuals in the population have an equal chance to be represented.
a transect sampling
A. Transect Sampling
  • This is best used for plants, which are sessile (stationary) organisms.
  • Researchers choose a specific length line and a random starting point in the study area.
  • All organisms within a certain distance of the transect line (ex. 1 m) are counted.
  • If mobile organisms are counted then a wider transect is used, ex. within 25 m of the transect line.

Transects are useful when the terrain is varied.


b quadrat sampling
B. Quadrat Sampling
  • Several quadrats are randomly chosen.
  • All of the target organisms in the quadrat are counted.
  • Also used for stationary organisms like plants.
  • The area to sample is divided into blocks or quadrats.


calculating population
Calculating Population
  • To calculate population for transects and quadrats use the following formula.
  • To determine population density use the formula :
    • DP = NSampled



  • Now the total population of the study area can be calculated by extrapolation.
    • NTotal= DP x AreaTotal
c mark and recapture
C. Mark and Recapture
  • This technique is better for organisms that move around.
  • The animals are trapped and marked with a band or tag attached to their body.
  • Awhile later, animals are trapped again in the same location.
  • Some of the animals will have tags and some will not the second time.
This is a Spotless Crake, a New Zealand bird that lives in wetlands.
  • What sampling method is best to estimate this population?
  • Explain why.
These are periwinkle snails, a small, edible species that lives in an intertidal zone.
  • What sampling method is best to estimate this population?
  • Explain why.