Unit 3: Populations Topic Summary

Essential Skills

You should be able to understand and describe:

• 3.1 Identify differences between generalists and specialist  

  1. Specialists are species that have a specific habitat and diet - they tend to do better in consistent environments, while generalists do better in changing habitats 

• 3.2 Identify differences between r-selected species and K-selected species and their reproductive strategies 

  1. K selected species are large, few offspring, living in stable environments, expend large amounts of energy in offspring, take a long time to reach sexual maturity, receive parental care, have long life spans, and reproduce throughout their lifespan.  They face large amounts of competition for resources in their habitat. They are found late in succession.  

  2. r-selected species are small, have many offspring, don’t invest a lot of energy in offspring, mature quickly, have short life spans, and may reproduce only once in their lifespan (they may also .  They typically don’t have competition for resources in their habitat.  They are found early in succession.  

  3. Biotic potential refers to the maximum reproductive rate of a population when nothing holds it back (ideal conditions). 

  4. Some species have reproductive strategies that aren’t r or K, or they change depending on conditions. 

  5. K selected species are more adversely affected by invasive species, while r-selected species aren’t impacted.  Most invasive species are r-selected species.  Endangered species can be both. 

• 3.3 Explain survivorship curves 

  1. Survivorship curve is a line that displays the survival rates of a cohort (group of individuals of the same age) in a population from birth to death.  There are Type I, Type II, Type III. 

  2. Survivorship curves differ for K and r selected species - K selected are typically Type I or Type II, while r are Type III. 

•  3.4 & 3.5 Describe carrying capacity and how the amount of resources present will impact the carrying capacity 

  1. When a population exceeds carrying capacity (max number of individuals an ecosystem can support, K), overshoot occurs.  There are environmental impacts of population overshoot, including resource depletion.  

  2. A major impact of population overshoot is dieback of the population (often severe) because of lack of resources leading to famine, disease, and conflict. 

  3. Population growth is limited by environmental factors such as resources and space 

  4. Resource availability and total amount of resources are limited and finite 

  5. When the resources needed by a population for growth are abundant, then growth accelerates. 

  6. When resources shrink, there is the possibility of unequal distribution of resources, resulting in increased mortality, decreased number of offspring (or both), and population growth declining to below the carrying capacity. 

• 3.6 Explain age structure diagrams 

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

  2. Rapidly growing populations have a higher proportion of younger people compared with stable or declining populations. 

• 3.7 Explain factors that affect total fertility rate in human populations  

  1. 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 policies. 

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

  3. Factors associated with infant mortality rates include whether mothers have access to good healthcare and nutrition.  Changes in these factors lead to changes in IMR. 

• 3.8 Explain how human populations experience growth and decline 

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

  2. Factors limiting human population growth include the Earth’s carrying capacity and basic factors that limit human population growth described by Malthusian theory. 

  3. Population growth can be affected by both density-independent factors  (storms, fires, heat waves, droughts), and density dependent factors (predation, access to resources, disease, territory size)

  4. Rule of 70 states that dividing the number 70 by the percentage population growth rate gives you the approximate number of years it takes the population to double. 

• 3.9 Define the demographic transition

  1. The demographic transition refers to the transition from high to lower birth and death rates in a country/region as development occurs and that country moves from a preindustrial to an industrialized economic system.  This is seen in the four stage model.  

  2. Characteristics of developing countries include high IMR and more children in the workforce.  

Vocab to Know

Generalist species

Specialist species

K-selected species

 r-selected species

survivorship curve

carrying capacity

density-independent factor

density-dependent factor

Population Growth rate 

Immigration 

Emmigration 

Biotic Potential 

Exponential Growth 

Logistic Growth 

Limiting Factor 

Survivorship: Type I, Type II, Type III 

Age Structure diagram 

Doubling Time/Rule of 70

crude birth rate

crude death rate

Infant Mortality Rates (IMR)

Replacement Level Fertility  (RLF)

Total Fertility Rate (TFR)

Demographic Transition 

Diagrams/Equations to Know

Global Population Growth Rate (r%) = (CBR - CDR)10 

Doubling Time = 70r%

National Population Growth Rate (r%)=(B+I) - (D+E)total population100