Natural Populations

  • The more genetically diverse a population is, the better it can respond to environmental stressors. Additionally, a population bottleneck can lead to a loss of genetic diversity.

  • Specialist species tend to be advantaged in habitats that remain constant, while generalist species tend to be advantaged in habitats that are changing.

  • Population growth is limited by environmental factors, especially by the available resources and space.

  • Resource availability and the total resource base are limited and finite over all scales of time.

  • When the resources needed by a population for growth are abundant, population growth usually accelerates.

  • When the resource base of a population shrinks, the increased potential for unequal distribution of resources will ultimately result in increased mortality, decreased fecundity, or both, resulting in population growth declining to, or below, carrying capacity.

K vs r-selected Species

  • K-selected species tend to be large, have few offspring per reproduction event, live in stable environments, expend significant energy for each offspring, mature after many years of extended youth and parental care, have long life spans/life expectancy, and reproduce more than once in their lifetime. Competition for resources in K-selected species’ habitats is usually relatively high.

    1. What are some examples of K-selected species?

    2. Are they likely to be endangered or not?

    3. Do they tend to be generalists or specialists?

  • r-selected species tend to be small, have many offspring, expend or invest minimal energy for each offspring, mature early, have short life spans, and may reproduce only once in their lifetime. Competition for resources in r-selected species’ habitats is typically relatively low.

    1. What are some examples of K-selected species?

    2. Are they likely to be endangered or not?

    3. Do they tend to be generalists or specialists?

  • Biotic potential refers to the maximum reproductive rate of a population in ideal conditions.

  • Many species have reproductive strategies that are not uniquely r-selected or K-selected, or they change in different conditions at different times.

  • K-selected species are typically more adversely affected by invasive species than r-selected species, which are minimally affected by invasive species. Most invasive species are r-selected species.

    1. Which species are more likely to be invasive?

    2. Which species are more likely to be affected by invasive species?

Survivorship Curves

  • A survivorship curve is a line that displays the relative survival rates of a cohort—a group of individuals of the same age—in a population, from birth to the maximum age reached by any one cohort member. There are Type I, Type II, and Type III curves.

  • Survivorship curves differ for K-selected and r-selected species, with K-selected species typically following a Type I or Type II curve and r-selected species following a Type III curve.

    1. What species are likely to be Type I? What type of curve will Type I species exhibit?

    2. What species are likely to be Type II? What type of curve will Type II species exhibit?

    3. What species are likely to be Type III? What type of curve will Type III species exhibit?

    4. Know how to draw a survivorship curve with type I, II, and III species.

Carrying Capacity

  • When a population exceeds its carrying capacity (carrying capacity can be denoted as K), overshoot occurs. There are environmental impacts of population overshoot, including resource depletion.

  • A major ecological effect of population overshoot is dieback of the population (often severe to catastrophic) because the lack of available resources leads to famine, disease, and/or conflict.

    1. What is carrying capacity?

    2. What happens when a species overshoots its carrying capacity?

Human Population Distribution

  • Birth rates, infant mortality rates, and overall death rates, access to family planning, access to good nutrition, access to education, and postponement of marriage all affect whether a human population is growing or declining.

  • Factors limiting global human population include the Earth’s carrying capacity and the basic factors that limit human population growth as set forth by Malthusian theory.

  • Population growth can be affected by both density-independent factors, such as major storms, fires, heat waves, or droughts, and density-dependent factors, such as access to clean water and air, food availability, disease transmission, or territory size.

  • The rule of 70 states that dividing the number 70 by the percentage population growth rate approximates the population’s doubling time.

    1. Can there be long-term solutions to environmental problems without a decrease in human population growth rate?

    2. What factors control the rate of human population growth?

    3. How many people can the earth sustain?

    4. What has been the impact of modern medical practices, improvements in sanitation, control of disease-spreading organisms, and supplies of human necessities, on the birth rates and death rates of human populations?

    5. Why have countries with a high standard of living moved more quickly to a lower birth rate than have countries with a low standard of living?

Age Structure Diagrams

  • Population growth rates can be interpreted from age structure diagrams by the shape of the structure.

  • A rapidly growing population will, as a rule, have a higher proportion of younger people compared to stable or declining populations.

    1. Know what an age-structure diagram looks like for a developing country, developed country, country with a "greying population", etc.

    2. Know how to tell what phase of the demographic transition a country is in by looking at the pyramid and what might cause the shape of the pyramid.

Total Fertility Rate

  • Total fertility rate (TFR) is affected by the age at which females have their first child, educational opportunities for females, access to family planning, and government acts and policies.

  • If fertility rate is at replacement levels, a population is considered relatively stable.

  • Factors associated with infant mortality rates include whether mothers have access to good healthcare and nutrition. Changes in these factors can lead to changes in infant mortality rates over time.

    1. Know which countries are likely to have high and low TFR (developed vs. developing).

Demographic Transition

  • The demographic transition refers to the transition from high to lower birth and death rates in a country or region as development occurs and that country moves from a pre- industrial to an industrialized economic system. This transition is typically demonstrated through a four-stage demographic transition model (DTM).

  • Characteristics of developing countries include higher infant mortality rates and more children in the workforce than developed countries.

    1. Know how to draw the demographic transition.

    2. Know what is happening in each phase of the demographic transition in regards to birth and death rates and WHY.