Population Ecology Notes

Population Ecology

Introduction

• Population ecology is the study of the interactions or relationships between a population and its environment.

  • It examines how the environment affects population size, density, growth, and structure.

Measuring Population Density

• Population density is the number of individuals per unit area or volume.

• Methods to measure population density:

  • Count individuals and measure the area: Directly count all individuals in a defined area.

  • Count individuals in sample plots: Count individuals in representative sample plots and multiply by the total area to estimate the entire population size.

  • Mark and recapture method: A method used to estimate the size of a population by capturing, marking, and releasing individuals, then recapturing another sample to see how many are marked.

Population Growth

• Population growth refers to the rate of change in population size.

• Population growth models:

  • Exponential model

  • Logistic model

Exponential Growth

• Exponential growth occurs when a population grows at a rate proportional to its size.

Logistic Growth

• Logistic growth occurs when a population grows quickly at first, then more slowly as it reaches an upper limit, after which it levels off.

Exponential Growth Model ("J" Shaped Curve)

• The population grows in an exponential fashion, e.g., 2, 4, 8, 16, 32…

• A population that grows exponentially grows at a rate proportional to its size.

• Assumptions:

  • There is continuous reproduction in the population.

  • All organisms are reproducing.

  • The environment is constant.

• Real populations do not always meet these conditions.

• Exponential growth is generally short-lived and observed in organisms introduced to a new environment.

Definitions

• Population-limiting factors: Factors that restrict population growth (e.g., food, space).

• Carrying capacity: The maximum number of individuals an environment can sustain or support.

Logistic Growth Model (“S” Shaped Curve)

• Initially, the population grows exponentially.

• Then, it reaches a point where the environment is unable to support it.

• Population size levels off.

• Defined as growth where the population grows quickly at first, then more slowly as it reaches an upper limit after which it levels off.

• The logistic model predicts that:

  • When the population size is small, growth is exponential.

  • When the population size exceeds the carrying capacity of the environment, more deaths and less births occur, and population growth declines.

  • Eventually, the population stabilizes at or near its carrying capacity.

Regulation of Population Growth

• Density-dependent factors: Factors that are dependent on population density.

  • Examples: Limited food supply, build-up of poisonous wastes, competition, predation, parasitism, and disease.

  • Limited food supply causes a decrease in birth rate.

  • Lack of spaces to hide in kelp causes an increase in death rate because the perch are eaten by their predators.

  • Build-up of toxic wastes causes an increase in death rate.

• Density-independent factors: Factors that are independent of population density.

  • Examples: Weather-related events, natural disasters, and human activities.

Population Cycles

• A phenomenon where populations rise and fall over a predictable period of time.

• Example: Snowshoe hare and lynx populations.

Life Tables

• A listing of survivals and deaths in a population in a particular time period

• Example table from 2004 demonstrates calculating the chance of surviving an interval

Survivorship Curve

• A plot of the number of individuals alive at each age.

• Type I: Low death rates during early and middle life, more death rates in late life.

  • Equilibrial Life History: The pattern of reaching sexual maturity slowly and producing few offspring but caring for the young; often seen in long-lived, large-bodied species.

• Type II: Mortality is constant over the life span of the individual.

• Type III: High death rates for the very young.

  • Opportunistic Life History: The pattern of reproducing when young and producing many offspring that receive little or no parental care; often seen in short-lived, small-bodied species.

Population Age Structure

• The relative proportion of individuals in different age groups.

• Gives information on the history of a population’s survival and reproduction.

• Helps predict future growth of populations.

Human Population

• Increase in population size brought about by an increase in birth rate and a decrease in death rate.

• Decrease in population size brought about by a decrease in birth rate and an increase in death rate.

• Population size is steady when the birth rate equals the death rate.

Age Structure

• The relative proportion of individuals in different age groups.

• Population Pyramid: A graphical illustration of the age structure of a population.

  • Expansive

  • Stable

  • Constrictive