Demographic Transition Model: Stage 5 & Population Pyramids

Species

• A species comprises individuals of the same type capable of interbreeding and producing fertile offspring in natural conditions.

Naming Species

• Each species has a two-part scientific name; this system is called binomial nomenclature.
• Scientific names are treated grammatically as if they were Latin, always in italics if typed or underlined if handwritten.
• The first name always starts with a capital letter, and the second starts with a small letter.
• Examples of common and scientific names:
  • Human: Homo sapiens
  • House Cat: Felis domesticus
  • Chimpanzee: Pan troglodytes
  • Olive Tree: Olea europea
• Closely related species may share the first part of their scientific name, but the second part is unique.
• Carolus Linnaeus devised the naming system, mandating that new species be named in Latin, or a Latinized form.
• The species name is based on a feature, location, or biological aspect, but it can be named to honor someone, but not the discoverer.
• Scientific names facilitate global communication about species, avoiding language difficulties and misconceptions.

Hybrids

• Hybrids are offspring from interbreeding of different species.
• Hybrids are usually sterile due to an odd number of chromosomes, which prevents sex cell production and subsequent offspring.
• Hybridization is rare but can occur in regions where closely related species cohabitate.

Biological Populations

• A population refers to all organisms of the same species living in a defined area at a specific time that can interbreed to produce fertile offspring.

Population Size

• Population size is the total number of individuals from a species within a specific geographic area at a particular time.
• Four main factors affect population size, including Birth, Deaths, Immigration, and Emigration.
  • Births: New individuals being born into a population, increase the population size.
  • Deaths: Individuals dying within a population, reduce the population size.
  • Immigration: Individuals moving into a population from other areas, increase the population size.
  • Emigration: Individuals moving out of a population to other areas, reduce the population size.

Population Density

• Population density measures the number of individuals of a species within a specific area or volume, quantifying the level of crowdedness.
• Population density is expressed as the number of individuals per unit area or volume.
• Population density is calculated as Population Size / Area Inhabited by the Population.

Population Growth

• Biological population growth signifies an increase in the number of individuals of a particular species in a specific location over a period.
• Population growth is a pivotal aspect of ecology and is key in shaping ecosystems, understanding species dynamics, and managing natural resources.

Biotic Potential

• Biotic potential, refers to the maximum rate a species can grow under perfect conditions without any limiting factors.
• It represents a species' inherent capacity to reproduce and increase in numbers.
• Biotic potential is affected by the following factors:
  • Reproductive Output (Litter Size): Species producing many offspring have higher biotic potential.
  • Reproductive Frequency: Species with multiple yearly reproductive events have higher biotic potential.
  • Age at Reproductive Maturity: Species reaching reproductive maturity earlier have higher biotic potential.
  • Survivorship: Species with higher offspring survivorship have more opportunities to contribute to population growth.
  • Lifespan: Species with longer lifespans have more reproduction opportunities, contributing to population growth.
• Full biotic potential expression is unlikely as it is usually restricted by limiting factors.

Environmental Resistance

• Environmental resistance includes environmental factors that limit population growth, counterbalancing biotic potential by preventing indefinite growth.
• Two types of limiting factors make up environmental resistance:
  • Density Dependent Factors: Influenced by population density; more significant as density increases and includes biological (biotic) factors like competition, predation, disease, parasitism, and territoriality.
  • Density Independent Factors: Not influenced by population density; limit growth regardless of density and includes physical (abiotic) factors like natural disasters, extreme weather, climate change, fire, habitat destruction, and pollution.
• When a species population is small, there are many resources and the population can grow quickly as the environmental resistance is low.
• As population size increases, environmental resistance also increases, as resources decrease and competition increases.

Carrying Capacity

• Carrying capacity is the maximum number of individuals from a species an ecosystem or habitat can support over a sustained period.
• It represents balance between available resources, environmental conditions, and population needs, leading to stable population size.
• Carrying capacity is determined by interactions between biotic potential and environmental resistance.
• When a population is below carrying capacity, there are abundant resources and individuals can access what they need.
• Population can grow and expand under these conditions.
• As population size approaches carrying capacity, resource availability is limited; scarcity increases competition leads to reduced reproductive success, increased stress, and higher mortality rates, which regulate population growth.

Population Growth Models

• Exponential Growth: A population increases rapidly and continuously in size over time.
  • Growth occurs at an accelerating rate; starts slow but becomes rapid over time.
  • Characterized by a J-shaped curve on a population-time graph.
  • Assumes unlimited resources and absence of limiting factors like predators or disease but is common in abundant populations.
• Stabilization of population growth will lead to sigmoid or irruptuve growth.

Sigmoid Growth

• Sigmoid population growth starts exponentially and then slows because of resistance until it stabilizes at the carrying capacity.
  • This growth is typical in stable populations within a fixed geographic space.
  • The growth curve is S-shaped and stabilizes at carrying capacity.
• The Lag Phase shows slow growth due to a shortage of reproducing individuals that are widely dispersed.
• Log Phase shows exponential growth while resources are abundant; natality greatly exceeds mortality.
• Transitional Phase has declining population growth as resources diminish competition increases; natality reduces, and mortality increases.
• Stationary Phase occurs when population stabilizes at carrying capacity with biotic potential balanced by environmental resistance; natality and mortality rates are approximately equal, and growth is static.
• Species showing sigmoid growth are described as k-strategists who grow slowly, adapt, and live close to their habitat's carrying capacity.
• K-selected species typically includes long lifespan, slow maturity, large body size, produces few offspring, high parental care and high offspring survival rates.

Irruptive Growth

• Irruptive growth comes from species with irregular population outbreaks and is characterized by populations that experience occasional, rapid increases followed by sharp declines.
  • Species are described as having a boom-and-bust lifecycle that fluctuates above and below the carrying capacity.
• Begins exponentially and increases until it overshoots the carrying capacity.
• Following an overshoot, a dieback occurs because of limited resources and space.

Population Crashes

• A population crash is a sudden, significant decline in population size over a short period. It includes:
  • Overconsumption of Resources: A population rapidly exceeds the carrying capacity of the habit, depleting the habitat leading to a crash.
  • Disease: Rapid disease spread in high density populations can lead to widespread mortality.
  • Natural Disasters: Events like hurricanes, floods, volcanic eruptions, wildfires, and tsunamis can wipe out large populations.
  • Habitat Destruction: Human activities destroy habitats, such as deforestation, urbanization, and agriculture that can cause population crashes.
  • Overexploitation: Populations are decimated through Activities such as overfishing, overhunting, and overlogging.

Human Populations

• Demography studies human populations' size, structure, and movements over space and time.

Collecting Demographic Data

• Census: A national stocktaking that is a reliable source of information of a population.
• It captures a comprehensive snapshot of a country's population at a point in time. Usually done by a count, characteristics, composition and condition of a population.
• Censuses are usually taken every 10 years, mandatory by law. All residents must participate and provide accurate information.
• A specific reference date is chosen for accuracy, to minimize problems related to counting individuals who may be temporarily absent.
• Census data is usually collected through door-to-door surveys by trained workers, online forms, or phone interviews.
• Government Registries: A system used by government to record vital data from its citizens and residents like, births, deaths, adoptions, marriages, divorces, and residence permits.
• Other government departments document the movement of individuals from immigration visas, residence permits, and customs records.

Population Size & Density

• Population size refers to the number of people in a population at a given time
• Population density measures how crowded a population is within a given area, it's calculated by dividing population size by land area.

Crude Birth Rate

• Crude Birth Rate measures live births per 1,000 people in a population per year.

Fertility Rate

• Fertility rate refers to the average number of children that would be born to a female over their lifetime.
• Replacement level fertility rate is the number of children a woman needs to have to stabilize the population without growing or shrinking; typically 2.1 births per woman.
• A woman needs to have two children on average - one to replace herself and one to replace her husband.
• An extra 0.1 is needed to account for child mortality: The fertility rate is higher than 2 because not all children born will survive to adulthood due to factors like infant mortality, accidents, or illnesses. The additional 0.1 compensates for these losses.

Crude Death Rate

• Crude death rate measures the number of deaths in a population each year per 1,000 individuals.

Rate of Natural Increase

• The rate of natural increase indicates a population growth based on births and deaths, without considering migration.
• Rate of Natural Increase = Crude Birth Rate – Crude Death Rate
  • A positive rate of natural increase means that births exceed deaths, and the population grows.
  • A negative rate of natural increase means deaths exceed births, and the population declines.
  • A rate of natural increase that equals zero means the number of deaths and births are equal, and the population is stable.

Immigration Rate

• Immigration rate measures the number of immigrants who join a population annually per 1,000 residents.

Emigration Rate

• Emigration rate measures the number of emigrants who leave a population annually per 1,000 residents.

Net Migration Rate

• The net migration rate measures the difference between the immigration rate and the emigration rate in a population, indicating population growth excluding deaths and births.
  • Net Migration = Immigration Rate – Emigration Rate
  • A positive net migration rate indicates that the number of immigrants exceeds the number of emigrant.
  • A negative net migration rate indicates that the number of emigrants exceeds the number of immigrant.
  • A net migration rate that equals zero means the number of immigrants and emigrants are equal.

Growth Rate

• Growth rate is the overall change in a population per year as a percentage, considering natural increase and net migration.
  • Growth Rate = Natural Increase + Net Migration/ 10

Doubling Time

• Doubling time is how long it takes for a population to double in size if the current growth rate remains constant.
  • T=70/G

Life Expectancy

• Life expectancy is the expected average number of years a person is expected to live at a given area.
• High expectancy is an indicator of economic progress because Countries advance due to sanitation, medical care, and food storage, contribute to lower mortality and a higher life expectancy.

Life Expectancy & Gender

• Life expectancy varies by gender.
  • Female life expectancy is higher because, men suffer from greater complications.
  • Men tend to consume more tobacco, alcohol and drugs than women are likelier to die from accidents and diseases and men also tend to pursue high risk occupations

Sex Ratio

• Sex ratio measures the proportion of males to females in a population that is usually expressed as the number of males per 100 females.
• Sex Ratio = Number of Males/ Number of Females
• The sex ratio at birth refers to the ratio of male to female births in a population.

Sex Ratio in Different Countries

• Sex ratio is approximately one but can greatly influenced by various factors such as cultural practices, migration patterns and high mortality rates.

Are there more Males or Females in the World?

• The ratio of men to women is not exactly equal to about 1.01, for every 100 females.

Dependency Ratio

• Dependency ratio compares the proportion of dependent to the working-age population that provides insight into the economic pressure on the productive population to support those who are too old or too young to work.
  • Dependency Ratio = Number of Young Dependents + Number of Aged Dependents/ Working Population × 100

Demographic Transition Model

• (DTM) shows shifts in population trends as countries go through different stages of progress.
• It demonstrates how populations shift from high rates to low rates, due to progress.

Stage 1 (High Stationary/Fluctuating)

• High birth and death rates, balance leading to stagnant growth.
  • Typically associated with pre-industrial societies.
  • Factors resulting in high birth rates:
  • Need for Labour: Agriculture is the livelihood.
  • Lack of knowledge on contraceptives.
  • Due to high child mortality families have more children.
    • Early Marriage leads to childbearing for women leading the birthing rate to thrive. -Traditional beliefs prioritize having children.
  • Factors resulting in high death rates:
  • Poor medical knowledge, and vaccines.
  • Lack of sanitation.
  • Famines.
• With balance in birth rate and death rate the growth is slow or unmoving.

Stage 2 (Early Expanding)

• Is described as high and declining rates and the shifts from a pre-industrial with rapid growth.
  • Many under developed countries sit in stage 2 and industrialized nations occurred early 1800 and less begun in 1950s.
  • High birth rate Factors persist from stage 1.
  • Declining death rate Factors
  • Sanitation improves, food is plentiful.
  • Urbanization increases with access to healthcare increases while communication improve.
• As birth rates stay with death countries grow rapidly and births widen therefore there is a large increase of the size of the population.

Stage 3 (Late Expanding)

• Results when towns start to industrializing its characterized through death and birth rates.
  • Shift in growth due to early 1900's.

Stage 4 (Low Stationary/Fluctuating)

• Low birth and death rates that stabilize while countries have economies higher standards of living and healthcare system and potential labor force shortages.
• Factors resulting in Low birth rates :
  • Active women in the work force has impacted the workforce to be balance.
  • Abortion gets normalized.
    • Increase of costs of living discourages people from big families.
• Factors resulting in Low death rates:
• Improved Sanitation.
• Improved healthcare infrastructure.
• In stage 4, birth and death rates balance with low growth and small variations.

Stage 5 (Declining)

• DTM is fairly recent event with only a few countries at this stage as Europe is moving to this stage.
  • Factors resulting in low birth rates:
  • Economic Factors: Raising kids is expensive.
  • It is too hard to balance big families with demanding careers.
    • The rates decline despite healthcare systems and high standards of living due to aging population -Negative Population Growth
  • Aging and High-age dependency ratio.

Limitations of the Demographic Transition Model

• This may be due to Limited attention and political policies, with regional cultures of variations and factors that it does not depict.
• A population pyramid, displays number of the population's age and gender.

Population Pyramid Age Groups

• Pre - Productive: individuals who cannot bare offspring (0-14).
• Reproductive: those who contribute to production (15-44).
• Post - Reproductive: those who no longer bare children (45+). Growth population can indicate from the width of those individual's rate, with a wide base meaning that their birth rates are high.

Stable Population

A standard shaped which shows a balance of those living in all ages.

Declining Populations

• Declining is where a general base highlighting that there is the reduced amount to be used in generations.

Explore population growth, challenges, and pyramid characteristics in Stage 5 of the Demographic Transition Model. Understand factors contributing to negative growth, demographic challenges, and the interpretation of population pyramids. Includes matching characteristics to stages of the Demographic Transition Model.