Populations

Objectives of Population Study

By the end of this lecture, you should be able to understand the following:

  • The definition of a ‘population’

  • The concepts of ‘life tables’, ‘age pyramids’, and their applications

  • ‘Survivorship curves’

  • Mechanisms of population growth, with practical examples

  • Key concepts like ‘carrying capacity’, ‘time lags’, and reasons for ‘population oscillations’

  • ‘r and K selection’ population strategies

  • Case study on Arctic reindeer populations

Population Concepts

Definition of a Population

  • A group of organisms of the same species occupying a particular space at a particular time. This concept can be applied to various species, including bryophytes and lichens in diverse environments.

Life Tables and Age Pyramids

Static Life Tables

  • Static life tables provide insights into the age structure and mortality rate of a population

  • Infant mortality has decreased with the developments of medicine

  • Mortality rate increases with age typically

Age Pyramids

  • Age pyramids are graphical representations that depict different age groups within a population:

    • Bottom portion: Pre-reproductive years (0-14)

    • Middle portion: Reproductive years (15-44)

    • Top portion: Post-reproductive years (45-85+)

Survivorship Curves

Types of Survivorship Curves

  • Survivorship curves demonstrate the mortality rates based on age and life history strategies

Population Growth Models

Exponential vs Logistic Growth

  • Populations can grow in two distinct ways:

    • Exponential Growth: Occurs in favorable conditions without resource limitations.

    • Logistic Growth: Growth that stabilizes as the population reaches the environment’s carrying capacity.

Carrying Capacity

  • Defined as the maximum population of a given organism that a particular environment can sustain.

Population Dynamics

Population Oscillations

  • Populations may experience fluctuations due to overshoot and dieback, reflecting the balance between environmental capacity and population size over time.

  • Eventually the population will crash

r and K Selection Theory

r-selected Species

  • Characteristics:

    • High variability and density independence

    • Unstable populations that do not stabilize and often ignore carrying capacity

    • Exhibits a Type III survivorship curve

    • E.g. Insects

K-selected Species

  • Characteristics:

    • More predictable with density dependence

    • Stabilized populations that remain near carrying capacity

    • Exhibits Type I and II survivorship curves

    • E.g. Large mammals and fish

Case Study: Arctic Reindeer Populations

  • Reindeer serve as a practical example of population growth dynamics.

  • Introduced to the two different Islands in Alaska in 1911

  • The population growth trends are documented over several decades showing significant increases in numbers.

  • Graph shows an exponential dynamic and one logistic population due to difference in location

Summary of Key Points

  • Populations exist in varying environments, including extreme conditions.

  • Life tables and age pyramids are essential for understanding age distributions and predicting future trends.

  • Three types of survivorship curves correspond to different life strategies in various species.

  • Population growth can be exponential or logistic depending on environmental factors.

  • Carrying capacity reflects the sustainable limits of a population in its environment.

  • Species can be classified as ‘r’ or ‘K’ selected based on their reproductive strategies.

  • Monitoring population dynamics over time, exemplified by reindeer in the Arctic, can provide valuable insights.