Unit 3 - Populations

3.2 K-Selected and R-Selected Species

Learning Objective

• Identify differences between k- and r-selected species

Quality vs Quantity

K-selected - “quality”

• few offspring, heavy parental care
• high reproduction rates (reproduce many times)
  • ex. most mammals, birds
• long lifespan
  • long time to sexual maturity = low biotic potential = slow population growth rate
  • more likely to be disrupted by environmental change or invasives

R-selected - “quantity”

• many offspring, little to no care

• low reproduction rates (may reproduce only once)

  • ex. insects, fish, plants

• shorter lifespan

  • quick to sexual maturity = high biotic potential = high population growth rate
  • more likely to be invasive
  • better suited for rapidly changing environmental conditions

  

Invasiveness and Disturbances

K-selected

• low biotic potential (rep. rate) = hard for population to recover after a disturbance (env. change)
• high parental care means death of parent = death of offspring
• invasives (usually r) outcompete for resources with high biotic potential and rapid population growth
• less likely to adapt and more likely to go extinct

R-selected

• high biotic potential (rep. rate) = more rapid population recovery after disturbance
• low parental care means death of parent doesn’t impact offspring
• not as impacted by invasive species since their population grows quickly
  • more likely to be invasive
• larger population and faster generation time = higher chance of adaptation and lower chance of extinction

3.3 Survivorship curves

Learning Objective

• Explain survivorship curves

Survivorship Curves

A survivorship curve is a line that shows survival rate of a cohort (group of same aged individuals) in a population from birth to death

• Faster drop in line = quicker die-off of individuals
• Slower drop in line = longer average lifespan

Type I, II, and III

Type 1 (mostly K-selected)

• high survivorship early in life due to high parental care
• high survivorship in mid life due to large size and defensive behavior
• rapid decrease in survivorship in late life as old age sets in
  • ex. most mammals

Type 2 (in between r and K)

• steadily decreasing survivorship throughout life

Type 3 (mostly r-selected)

• high mortality (low survivorship) early in life due to little to no parental care
• few make it to midlife; slow, steady decline in survivorship in mid life
• even fewer make it to adulthood; slow decline in survivorship in old age
  • ex. insects, fish plants

3.4 Carrying Capacity

Learning Objective

• Describe carrying capacity
• Describe the impact of carrying capacity on ecosystems

Carrying Capacity (k)

Population briefly “overshoots” (k) and then die-off happens

• the highest population size an ecosystem can support based on limiting resources:
  • food
  • water
  • habitat (nesting sites, space)

Overshoot:

• when a population briefly exceeds carrying capacity
  • ex. deer breed in fall, give birth all at once in spring; sudden spike in population = overshoot

Consequence of overshoot:

• resource depletion ex: overgrazing in deer

Die-off:

• sharp decrease in population size when resource depletion (overshoot) leads to many individuals dying
  • ex. many deer starve with too many new fawns feeding in spring

Die-off Example

• Reindeer of St. Paul Island

  • 25 introduced in 1910
  • Growth was gradual (10’ - 30’) then exponential (30’-37’)
  • Carrying capacity was overshot
  • Sharp die-off lead to population crash as food resource (lichen) were severely depleted

  

• Real populations don’t always fluctuate around carrying capacity. If resource depletion is severe enough, total population crash can occur

Predator-Prey

• Hare population increase due to low predator population (lynx)

• Lynx population increase due to increase in food (hare)

• Increasing lynx population limits hare population; leads to die-off

• Hare die-off decreases lynx food source, leading to die-off

• Hare population increase due to low predator population (lynx)

  \n

3.5 Population Growth and Resource Availability

Learning Objective

• Explain how resource availability affects population growth

Population Growth

Size (n)

• total number of individuals in a given area at a given time
  • larger = safer from population decline

Density

• of individuals/area
  • ex : (12 panthers/km^2)
  • high density = higher competition, possibility for disease outbreak, possibility of depleting food sources

Distribution

• how individuals in population in are spaced out compared to each other
  • random (trees)
  • uniform (territorial animals)
  • clumped (herd/group animals)

Growth Factors

Sex ratio

• ratio of males to females. the closer to 50:50, the more ideal for breeding (usually)
  • die-off or bottleneck effect can lead to skewed sex ratio (not enough females) limiting population growth

Density-Dependent Factors

• factors that influence population growth based on size
  • ex. food, competition for habitat, water, light, even disease
  • ex. cont. when twice as much food was added, carrying capacity increased by about 2x
  • all of these things limit population growth based on their size; aka - small population

Density-Independent

• factors that influence population growth independent of their size
  • ex. natural disasters (flood, hurricane, tornado, fire)
  • it doesn’t matter how big or small population is, natural disasters limit them both.

Biotic Potential

Biotic potential

• exponential growth

Logistic growth

• initial rapid growth, then limiting factors limit pop. to K

Biotic Potential

• max. potential growth rate, with no limiting resources
• May occur initially, but limiting resources (competition, food, disease, predators) slow growth, & eventually limit pop. to carrying capacity (k)

Calculating Population Change

Population Size = (Immigrations + births) - (immigrations + deaths)

• Ex: An elk pop. of 52 elk has 19 births and 6 deaths in a season, and 5 new \n elk immigrate to the herd and 0 elk emigrate from the heart

3.6 Age Structure Diagrams

Learning Objective

• Explain age structure diagrams

Age Cohorts

Age cohorts & growth

• groups of similarly aged individuals

0-14 = prereproductive

15 - 44 = reproductive age

45 + = post reproductive

• Size difference between 0-14 & 15-44 indicates growth rate
  • Larger 0-14 cohort = current & future growth
  • Roughly equal 0-14 & 15-44 = slight growth/stable
  • Larger 15-44 = pop. decline

Pyramid Shapes

• Extreme Pyramid shape = rapid growth
• Less extreme pyramid = slow, stable growth
• House = stable, little to no growth
• Narrowest @ base = declining pop.

3.7 Total Fertility Rate

Learning Objective

• Explain factors that affect total fertility rate in human populations

TFR & Infant Mortality

Total Fertility Rate

• average number of children a woman in a population will bear throughout her lifetime
  • higher TFR = higher birth rate, higher population growth rate (generally)

Factors that affect TFR

• development: more developed or wealthy nations have a lower TFR than less developed nations
  • more educational access for women
  • more economic opportunity for women
  • higher access to family planning education and contraceptives
  • later age of first pregnancy
  • less need for children to provide income through agricultural labor
  • educational and economic opportunities require more time and leave less time for raising children
• government policy can play a huge role in fertility by coercive (forceful) or non-coercive (encouraging) policies
  • forced or voluntary sterilization
  • china’s 1 (now 2) child policy
  • tax incentives to have fewer children
  • microcredits or loans to women without children to start businesses

More education = fewer unplanned pregnancies

More education = more job opportunities for women

• alternative to marrying young

Replacement Level Fertility

• the TFR required to offset deaths in a population and keep population size stable
  • about 2.1 in developed countries
  • higher in less developed countries due to higher infant mortality

Infant Mortality Rate (IMR)

• number of deaths of children under 1 year per 1,000 people in a population
  • higher in less developed countries due to lack of access to health care, clean water, enough food, etc.
• Higher IMR = higher TFR, due to families having replacement children
• Lower IMR = Lower TMR

Factors in IMR decline

• access to clean water
• access to healthcare (hospitals, vaccines, vitamins and supplements for moms and babies)

3.8 Human Population Dynamics

Learning Objective

• Explain how human populations experience growth and decline

Does Earth Have a Human Carrying Capacity?

Malthusian theory (what Malthus theorized):

• Earth has a human carrying capacity, probably based on food production
• Human population growth is happening faster than growth of food production
• Humans will reach a carrying capacity limited by food

Technological Advancement

• Humans can alter earth’s carrying capacity with tech.
  Innovation
• Ex: synthetic fixation of Nitrogen in 1918 leads to
  synthetic fertilizer, dramatically increasing food
  supply

Birth Rate, Death Rate, and Growth

Growth Rate (r) = % increase in a population (usually per year)

• Ex: a growth rate of 5% for a population of 100 means they grow to 105

Crude Birth Rate & Crude Death Rate (CBR & CDR)

• Births & deaths per 1,000 people in a pop.
  • Ex: Global CBR = 20 & CDR = 8

Calculating Growth Rate (r)

• {{Growth Rate = (CBR - CDR )/ 10{{

Rule of 70

The time it takes (in years) for a population to double is equal to 70 divided by the growth rate

Factors Affecting Human Population Growth

Factors that increase population growth

• higher TFR = higher birth rate
• high IMR can drive up TFR
• high immigration level
• increased access to clean water and healthcare (decrease death rate)

Factors that decrease population growth rate

• high death rate
• high IMR
• increased development (education and affluence)
• increased education for women
• delayed age of first child
• postponement of marriage age

Standard of Living Indicators

Standard of Living

• what the quality of life is like for people of a country based

Gross Domestic Product (GDP)

• key economic indicator of standard of living
  • total value of the goods and services produced
  • per capita GDP is total GDP/total population

Life Expectancy

• key health indicator of standard of living
  • average age a person will live to in a given country
  • increases with access to clean water, health care, and stable food sources

NOTE: High GDP and life expectancy are both indicators of developments and low population growth

3.9 Demographic Transition

Learning Objective

• Define the demographic transition

Industrialization

Industrialization

• the process of economic and social transition from an agrarian (farming) economy to an \n industrial one (manufacturing based)

Pre-industrialized/Less developed

• A country that has not yet made the \n agrarian to industrial transition
• Typically very poor (low GDP)
• Typically high death rate & high infant mortality
• High TFR for replacement children & agricultural labor

Industrializing/developing

• part way through this transition
• Decreasing death rate & IMR
• Rising GDP

Industrialized/developed

• completed the transition
• Very low DR & IMR
• Very High GDP
• Low TFR

Stage 1 - Preindustrial

• High IMR and high death rate due to lack of access to clean water, stable food supply, and healthcare
• high TFR due to lack of access to
  • education for women
  • contraceptives/family planning
• need for child agricultural labor
• little to no growth due to high CBR and CDR balancing each other out
  • ex. virtually no country is in phase 1

Stage 2 - Industrializing/Developing

• Modernizations brings access to clean water, healthcare, stable food supply
  • IMR and CDR decline
• TFR remains high due to
  • lack of education for women and contraceptives/family planning
  • need for child agricultural labor
  • generational lag (takes time for education and societal change to spread)
• Economic and societal indicators
  • low per capita GDP
  • shorter life expectancy
  • high infant mortality
  • high TFR
  • low literacy rate and school expectancy for girls

NOTE: rapid growth due to high CBR and declining CDR

Stage 3 - Developed/Industrialized

• modernized economy and society increase family income, so TFR declines significantly due to
  • more education opportunities for women
  • delayed age of marriage and first child to focus on education and career
  • access to family planning and contraceptives
• economical and social indicators
  • high per capita GDP
  • long life expectancy
  • low infant mortality
  • TFR near replacement level
  • high literacy rate and school life expectancy for all

Stage 4 - Post-industrialized/Highly Developed

• highly modernized countries that are very affluent
  • TFR declines even further as families become more wealthy and spend even more time on educational and career pursuits
  • increased wealth and education brings even more prevalent use of family planning and contraception
• economical and social indicators
  • very high per capita GDP
  • longest life expectancy
  • TFR below replacement level
  • highest contraceptive use rates

NOTE: CBR drops lower that CDR and growth becomes negative (population decline)