Free Unit Notes

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I. Generalist and Specialist Species (Topic 3.1)

  A. Populations and niches

1. A population includes individuals of the same species living in the same area at the same time.

2. An ecological niche describes a species’ resource use, environmental needs, and interactions with other organisms.

  B. Broad and narrow niches

1. Generalist species use many resources or tolerate a wide range of conditions. Raccoons eat many foods, helping them survive habitat changes.

2. Specialist species depend on a narrow range of resources or conditions. Giant pandas rely mainly on bamboo.

3. Specialists tend to do well in stable habitats. Generalists often have an advantage when habitats change because they have more alternatives.

4. A species can specialize in one way, such as diet, while tolerating a broad range of other conditions.

II. K-Selected r-Selected Species (Topic 3.2)

  A. Reproductive strategies

1. Life history includes when an organism matures, how it reproduces, and how long it lives. Energy spent on many offspring leaves less energy for each one.

2. K-selected species tend to be large, mature late, live longer, and reproduce repeatedly. They produce few young and invest heavily in each offspring, often through parental care.

   a. Elephants and humans show these traits. Their habitats are often stable, with strong competition for limited resources.

3. r-selected species tend to be small, mature early, and live shorter lives. They produce many offspring with little investment in each and may reproduce only once.

   a. Mosquitoes and many annual weeds show these traits. They can increase rapidly after disturbance, when resources are available and competition is relatively low.

4. These strategies form a continuum. Many species combine traits or change reproductive investment as conditions change.

5. Reproductive strategy differs from niche breadth. Being a generalist does not automatically make a species r-selected.

  B. Growth potential and vulnerability

1. Biotic potential is a population’s maximum reproductive rate under ideal conditions. Early maturity, frequent reproduction, and many offspring increase it.

2. r represents the per-capita population growth rate. K represents carrying capacity, the population an environment can support over time.

3. An invasive species is a nonnative species that spreads and causes harm. Many invaders reproduce rapidly and have r-selected traits.

4. K-selected populations often recover slowly from losses because they mature late and produce few young. This can make them especially vulnerable to invasive competitors or predators.

III. Survivorship Curves (Topic 3.3)

  A. Reading the curves

1. A cohort is a group born during the same period. A survivorship curve shows the number or proportion of that cohort still alive at each age.

2. Age is on the horizontal axis; survivors are on the vertical axis. A steep decline shows ages when many cohort members die.

3. The survivor count cannot increase because the graph follows the original cohort. It does not show growth of the whole population.

  B. Three patterns

1. Type I: Most individuals survive early and middle life, followed by high mortality in old age. Humans and many large mammals often show this K-selected pattern.

2. Type II: Individuals have a roughly constant chance of dying during each equal time interval. Many birds and some rodents show this pattern.

   a. The same fraction, rather than the same number, dies per interval. The curve is straight when survivors are plotted on a logarithmic scale.

3. Type III: Many individuals die young. Those that survive early life have a lower risk of death afterward. Oysters, many fish, and many plants show this pattern, often associated with r-selection.

4. K-selected species commonly show Type I or Type II survivorship; r-selected species commonly show Type III. Actual patterns vary with environmental conditions.

IV. Carrying Capacity (Topic 3.4)

  A. Resource limits

1. Carrying capacity (K) is the population size an environment can support over time with its resources and conditions.

2. Food, water, shelter, space, light, or dissolved oxygen can limit a population. K depends on the species’ needs, not just available physical space.

3. K can change. Drought may reduce food and lower K; habitat restoration may increase resources and raise K.

  B. Overshoot and dieback

1. Overshoot occurs when a population exceeds K. Reproduction may continue before shortages take effect, or K may suddenly fall.

2. Resource depletion occurs when resources are used faster than they recover. Overshoot can worsen shortages and damage habitat.

3. A dieback is a major population decline when conditions cannot support the existing population. Starvation, disease, and conflict can increase deaths.

4. Habitat damage during overshoot can lower future K. For example, overgrazing may remove vegetation and increase soil erosion.

5. Real populations may fluctuate above and below K because resources vary and births or deaths respond with a delay.

V. Population Growth and Resource Availability (Topic 3.5)

  A. Measuring population change

1. Births (B) and immigration (I, movement into an area) add individuals. Deaths (D) and emigration (E, movement out) remove individuals.

2. Change in population size: ΔN = (B + I) − (D + E). Use counts for the same population and time period.

3. Percent growth = (ΔN ÷ starting population) × 100. The ending population equals the starting population plus ΔN.

   a. Starting with 800 individuals, 160 births, 40 arrivals, 100 deaths, and 20 departures give ΔN = 80, an ending population of 880, and 10% growth.

4. Population density = number of individuals ÷ area or volume. Include units, such as individuals/km².

5. Dispersion describes spacing: clumped individuals gather in groups, uniform individuals are evenly spaced, and random individuals have no clear spacing pattern.

  B. Exponential and logistic growth

1. Exponential growth occurs when a population increases by a constant percentage per time interval. A constant positive rate produces a J-shaped curve, with more individuals added as the population grows.

2. Abundant resources allow rapid growth, but finite resources prevent indefinite exponential growth. Linear growth instead adds a constant number per interval.

3. In the continuous exponential model, dN/dt = rN. N is population size, r is the per-capita growth rate per unit time, and dN/dt is the change in population size per unit time.

   a. With constant r, N(t) = N₀e^(rt). N₀ is starting population, t is elapsed time, and e is about 2.718. Use matching time units for r and t.

4. Logistic growth slows as a population approaches K, producing an S-shaped curve. Competition reduces resource availability per individual.

5. The logistic model is dN/dt = rN(1 − N/K). At N = K, net growth is zero; births and deaths can still occur and balance.

6. In this ideal model, total growth is fastest at N = K/2. The model assumes constant K and an immediate response to crowding; real populations may overshoot.

  C. Limiting factors

1. A limiting factor is a resource or condition that restricts population growth. Shortages can increase deaths and reduce fecundity, the ability to produce offspring.

2. Scarce resources may be distributed unequally. Individuals with less access may die or reproduce less, slowing growth toward or below K.

3. Density-dependent factors generally have stronger effects per individual as crowding increases. Examples include competition, disease transmission, and predation.

4. Density-independent factors can affect populations regardless of density. Examples include storms, freezes, droughts, heat waves, and fires.

5. These factors can interact. A drought reduces water supply, while crowding increases competition for the remaining water.

VI. Age Structure Diagrams (Topic 3.6)

  A. Reading age groups

1. An age structure diagram shows the number or percentage of people in each age group, often with males on the left and females on the right.

2. Younger ages appear at the bottom; older ages appear at the top. Wider bars represent more people or a larger percentage in that group.

3. Broad groups are prereproductive (roughly 0–14), reproductive (roughly 15–44), and postreproductive (roughly 45 and older). These boundaries are approximate.

4. Check whether the axis shows counts or percentages. Percentages alone do not reveal the total population size.

  B. Interpreting shapes

1. A wide base indicates many young people and potential for rapid future growth as they reach reproductive ages.

2. A column-like shape has similar-sized younger and middle age groups, often indicating slow growth or relative stability.

3. A narrow base indicates smaller younger generations, often suggesting population aging and possible decline.

4. Population momentum is continued change caused by the existing age structure. A large young generation can keep births high after fertility falls to replacement level.

5. Migration and changes in fertility or mortality can alter these predictions. Large child populations increase demand for schools; aging populations increase demand for elder care.

VII. Total Fertility Rate (Topic 3.7)

  A. Fertility and replacement

1. Total fertility rate (TFR) estimates children per woman over a lifetime if current age-specific fertility rates continue. It differs from annual births per 1,000 people.

2. Replacement-level fertility is the average number of children per woman needed for a generation to replace itself without migration.

3. Replacement is about 2.1 children per woman where childhood survival is high. It is higher where more children die before reproducing.

4. Sustained replacement fertility can support long-term stability, but population momentum and migration can delay or prevent immediate stability.

  B. Factors affecting fertility

1. Later first childbirth often reduces the number of children born over a lifetime.

2. Education and employment opportunities for women can support later childbearing and smaller families.

3. Family planning provides information, contraception, and services that help people choose whether and when to have children.

4. Higher child survival, urban living costs, and reliable old-age support can reduce incentives for large families. Children’s farm labor may encourage larger families in some settings.

5. Government policies can affect fertility. Childcare benefits may encourage births; access to voluntary family planning can reduce unintended births. Effects vary among populations.

  C. Infant mortality

1. Infant mortality rate (IMR) counts deaths before age one per 1,000 live births in a year.

2. IMR = (infant deaths ÷ live births) × 1,000. The denominator is live births, rather than total population.

3. Maternal nutrition, prenatal care, safe delivery, clean water, sanitation, vaccines, and infant healthcare improve survival and can lower IMR.

VIII. Human Population Dynamics (Topic 3.8)

  A. Births, deaths, and growth

1. Agriculture increased food supplies. Later improvements in sanitation, nutrition, and medicine lowered deaths. Population grew rapidly when death rates fell before birth rates.

2. Crude birth rate (CBR) = (annual births ÷ total population) × 1,000. Its unit is births per 1,000 people per year.

3. Crude death rate (CDR) = (annual deaths ÷ total population) × 1,000. Its unit is deaths per 1,000 people per year.

4. Crude rates combine all ages. A population with many older adults can have a high CDR despite good healthcare.

5. Rate of natural increase (RNI), % per year = (CBR − CDR) ÷ 10, when both crude rates are per 1,000 people per year. RNI excludes migration.

   a. If CBR = 28 and CDR = 8, RNI = (28 − 8) ÷ 10 = 2% per year. If rates are already percentages, subtract without dividing by 10.

6. Total annual growth, % = [(B − D + I − E) ÷ starting population] × 100. Migration changes a country’s population but cancels out globally.

7. Zero population growth occurs when births plus immigration equal deaths plus emigration. A declining percentage growth rate can still mean the population is increasing.

  B. Doubling time

1. The rule of 70 estimates doubling time: years to double ≈ 70 ÷ annual percentage growth rate.

2. At 2% annual growth, doubling takes about 35 years. Enter 2, rather than 0.02, in the formula.

3. This estimate assumes a roughly constant positive growth rate. Zero or negative growth does not produce doubling under those conditions.

  C. Human resource limits and environmental effects

1. Malthusian theory warns that population can grow geometrically while food supply grows arithmetically, eventually producing shortages.

   a. Geometric growth multiplies repeatedly, such as 2, 4, 8, 16. Arithmetic growth adds a fixed amount, such as 2, 4, 6, 8.

2. Food shortages can increase deaths through famine, disease, and conflict. Lower fertility can reduce pressure on resources.

3. Agricultural technology and trade have increased food availability beyond Malthus’s simple model. They do not remove all environmental limits.

4. Human carrying capacity depends on resource use per person, technology, resource distribution, and environmental damage. There is no single agreed fixed value.

5. Density-dependent pressures include competition for food, clean water, clean air, and living space, plus disease transmission. Density-independent hazards include storms, fires, heat waves, and droughts.

6. More people generally increase demand for food, water, energy, housing, and waste disposal. Consumption per person and production methods also affect environmental impact.

7. Resource efficiency and cleaner production reduce resource use and pollution. Education, healthcare, and voluntary family planning support informed choices and can reduce unintended births.

IX. Demographic Transition (Topic 3.9)

  A. Four-stage model

1. The demographic transition is a shift from high to lower birth and death rates as a country or region develops economically.

2. Stage 1: Preindustrial. Birth and death rates are high, so growth is usually low or variable. Unsafe water, food shortages, and limited healthcare keep mortality high.

3. Stage 2: Transitional. Death rates fall as sanitation, nutrition, and healthcare improve, while birth rates remain high. The population grows rapidly.

4. Stage 3: Industrial. Birth rates fall as education, employment opportunities, urbanization, and family planning increase. Death rates remain low, so growth slows but may remain positive.

5. Stage 4: Postindustrial. Birth and death rates are low and close together. Natural increase is small or near zero; births below deaths cause natural decrease.

  B. Applying the model

1. On a graph with matching rate units, the gap between birth and death rates shows natural increase, rather than total population size.

2. Lower-income countries have historically tended to have higher infant mortality and more children in the workforce. Improved survival, schooling, and family security can support smaller families.

3. Countries do not follow identical paths or spend equal time in each stage. War, inequality, and healthcare access can change the pattern.

4. The four-stage model leaves out migration. Immigration can increase a population even when deaths exceed births.

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