Biology 100/101
Lecture 4: Populations in the Ecosystem
(Print Version)

Announcements &

Lecture Objectives

Web Resources

Population Definition

Application of
Population Information

Factors that affect
Population Growth

Biotic (Intrinsic)

Resistance Factors

Density Dependent

Density Independent

Lecture Syllabus

IB 100/101 Home Page


Text readings in Biology: Concepts and Investigations, 1st edition, by Marielle Hoefnagels

Chapter 39, Populations 786-800

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You may also ask questions and see answers to your classmates' questions in Moodle in the "Talk to Ed" forum.


The content of today's lecture will help you complete these assignments:

After studying this material you should be able to:

  1. Define the term biological population and provide examples.

  2. Draw a simple graph illustrating a population that is growing at an exponential rate of increase (J-shaped curve).

  3. Describe how population growth rate is calculated.

  4. Describe how and explain why each of the following factors affects the growth of a population:

    • birth rate

      death rate



      population growth rate


      biotic or intrinsic factors

      exponential population growth

      logistic population growth

      density dependent environmental resistance factors

      density independent environmental resistance factors

  5. Distinguish between density dependent and density independent environmental resistance factors that regulate population growth.

  6. Create a diagram, concept map, or text that represents the relationships among the concepts; environmental resistance factors (density dependent and density independent), birth rate, death rate, population growth rate, logistic population growth, and carrying capacity.

General Web Resources

What is a Population?

"A population is a group of organisms of the same species occupying a given geographic location at the same time." (Hoefnagels, text pg. 788)

The Glossary on pg. G-17 gives a slightly different wording of the same concept, "members of the same species occupying a region".

Understanding the demography of populations has many critical applications:

Factors that affect the growth of Populations

    What is population "growth"?

    • What we might talk about as population size is actually population density, the number of individuals per unit area (or unit volume).
    • Population growth is based on four fundamental factors: birth rate, death rate, immigration, and emigration.

    • Population growth rate = (birth rate + immigration) - (death rate + emigration)

      "Per capita rates" are calculated as the number of events (births, deaths, or growth) divided by the number of individuals in the population over a specific time period.

    Biotic or Intrinsic factors that affect population growth

    • Population growth is affected by biotic or intrinsic factors that are built into the genetic basis of each species.

    • Biotic or Intrinsic factors are specific to each species and include:

      • The age of reproductive maturity

      • The number of offspring produced per reproductive event

      • The number of reproductive events in an individual's lifetime

      • The three factors above are refered to together as fecundity, or the number of offspring an individual produces in its lifetime.

    • Exponential (J-Shaped) Population Growth

    Population Regulation & Environmental limits

    Environmental Resistance Factors that Affect Birth and Death Rates

    Obviously, populations cannot realistically grow exponentially. There are environmental limits, called environmental resistance factors, that affect the number of individuals that can survive and reproduce in a given habitat.

    "The power of population is so superior to the power of the earth to produce subsistence for man, that premature death must in some shape or other visit the human race. The vices of mankind are active and able ministers of depopulation. They are the precursors in the great army of destruction, and often finish the dreadful work themselves. But should they fail in this war of extermination, sickly seasons, epidemics, pestilence, and plague advance in terrific array, and sweep off their thousands and tens of thousands. Should success be still incomplete, gigantic inevitable famine stalks in the rear, and with one mighty blow levels the population with the food of the world." Thomas Malthus, 1798

    Environmental resistance factors fall into two categories: density dependent and density independent.

  • A population explosion and crash. Graph: Hoefnagels text, Fig. 39.7, pg. 793

    • Density dependent environmental resistance factors (See Hoefnagels text, Pg 792)

      • Density dependent factors include the environmental resources needed by the individuals of a population. Competition for food, water, shelter, etc., results as the population density increases. The survival, health, and reproduction of individuals will be affected if they cannot acquire the basic requirements of life.

      • Density dependent factors ALSO include environmental factors, such as predators, infectious disease organisms, and parasites that do not necessarily result in competition for needed resources, but do affect the health, survival, and reproduction of individuals in the population as population density increases. Individuals that are diseased may have a reduced ability to reproduce. Dead individuals cannot reproduce.

      • Density dependent factors are referred to as Environmental Resistance Factors that determine the Carrying Capacity of the environment for a population.

      • Carrying Capacity - "The theoretical maximum number of individuals that an environment can support for an indefinite time period is its carrying capacity." (Horfnagels. pg. 792)

      • Logistic Population Growth

        • In the presence of density dependent environmental factors, population growth is constrained at high population densities. This is because the impact of density dependent factors depends on the density of the population.

        • Logistic Population Growth Simulation
        • At low population densities, density dependent factors exert little influence on population growth, which initially grows rapidly.

        • This is to say that individuals have an abundance of resources so their health is good. They have a high capacity to reproduce and are less likely to die.

        • At high population densities, density dependent factors exert an increasing negative effect on population growth which slows and finally stops at the carrying capacity.

        • The health of individuals is stressed because of lack of resources, crowding, prevalent diseases, etc. Their reproductive capacity is reduced and their liklihood of dying is greater.

        • Density dependent factors therefore produce an S-shaped growth curve.

        • Graph: Hoefnagels text, Fig. 39.6, pg. 792

        • Population Reference Bureau 2009 World Population Data Sheet (Downloadable PDF)

        • Lecture Activity - Graph: Lewis et al., Life, Fig. 43.8, pg. 857
    • Density Independent environmental resistance Factors (See Hoefnagels Pg 793)

      • Density Independent factors are Environmental Resistance Factors that occur or have an effect on a population regardless of the density of the population.

      • Density independent factors include weather phenomena and natural disasters that affect the population, but the chance of their occurrence or level of severity is unrelated to the density of the population.

      • Density independent factors may affect the availability of resources that are required by the population (density dependent factors), indirectly affecting the carrying capacity of the environment.

      • A hurricane might destroy trees that are the required nesting site for a population of birds. (from