Population Ecology

A population is a group of individuals of the same species living in a particular area. The population ecologist studies the dynamics of populations as they interact with their environment.

Today's no-brainer

  • If population gain (births, immigration) equals population loss (deaths, emigration), then population size will not change.
  • If population gain is greater than loss, population size will increase.
  • If population loss is greater than gain, population size will increase. Population Numbers and Distribution
  • density - the number of individuals per unit area
  • dispersion - the pattern of spacing among individuals

  • clumped dispersion may indicate
  • uniform distribution may indicate intraspecific interactions
  • random distribution - individuals' positions are independent of other individuals' positions


    Demographics Demography is the study of changes in the vital statistics of populations over time. (An actuary is the guy who determines how likely you are to be dead in the next year, and adjusts your insurance rate accordingly.)

    A life table is a representation of age-specific survival in a population. Click on the cute squirrels to see how they live and die.

    The life table is constructed by following a cohort (individuals of the same age) from birth to death. Life table data can also be represented as a survivorship curve. Over time, demographers have come to recognize three types of idealized curves.

  • Type I - low risk of death in early life, increasing with advanced age
  • Type II - constant risk of death throughout life
  • Type III - high risk of death in early life, but long life expectancy once maturity is reached


    Population Growth Mathematical models can be used to describe population growth.

  • arithmetic growth - population increases by the same amount over each time interval

  • exponential (=geometric) growth - increase rapid, reflecting the maximum intrinsic rate of growth.

    This is described by the equation:

    dN/dt = rmaxN
    in which...
  • dN = the change in population size (in small increments)
  • dt = the time interval (change in time)
  • rmax = maximum population growth rate (intrinsinc rate of increase, equal to per capita birth rate minus per capita death rate; (remember what is implied by the term RATE!))
  • N = population size But how long can this last? At some point, a population will reach carrying capacity, and growth will level off.

    At this point, the population exhibits logistic growth.

  • logistic population growth - exponential growth with environmental resistance (carrying capacity of the environment = K) incorporated into the equation:

    dN/dt = [rmaxN][K-N/K]
    in which...
  • dN = the change in population size (in small increments)
  • dt = the time interval (change in time)
  • rmax = maximum population growth rate (intrinsinc rate of increase, also known as biological potential)
  • N = population size
  • K = carrying capacity (maximum number of individuals the environment can sustain indefinitely)


    Most natural populations exhibit logistic growth.
    Life History Strategies The logistic growth model predicts population growth at both very high and very low population densities. Consider this in real populations...


  • What reproductive strategies would be advantageous at low population densities (i.e., population is close to rmax)?


  • What reproductive strategies would be advantageous at high population densities (i.e., at or close to K)?


    Population-limiting Factors

    Common results of high population density include

    What are the actual factors that cause these phenomena?

    A limiting factor is any environmental condition that stops a population from unlimited growth at its intrinsic rate of increase.
    Limiting factors that increase in intensity as population density increases are known as density dependent factors. Example: Any limited resource.

    Limiting factors that do not increase in intensity with population density increase are known as density independent factors. Example: sunlight, rainfall, temperature, etc.; any factor that affects all individuals, no matter what the population size.

    In most natural populations, both density-dependent and density-independent factors play a role in controlling population size.

    Some populations undergo regular "boom and bust" cycles. But there are some unfortunate myths out there.

    Human Populations

    The human population has been exhibiting exponential growth in the last 150 years.

    Though we do seem to be slowing down.

    Age structure pyramids can graphically represent population dynamics at a given time, and allow some predictions.

    Birth and Death

  • Infant mortality is the number of infant deaths per 1000 live births.
  • Life expectancy is the predicted average length of life.

    These vary widely across human populations, as does energy use.

    Where will we go from here? It is your (collective) decision to make.