Species can be classified by the breadth of their ecological niche — the range of conditions, resources, and interactions they use. Generalist species have broad niches; specialist species have narrow niches. This distinction affects vulnerability to environmental change.
| Characteristic | Generalist | Specialist |
|---|---|---|
| Niche breadth | Wide — tolerates many conditions | Narrow — requires specific conditions |
| Diet | Varied, omnivorous or opportunistic | Restricted to specific food sources |
| Habitat | Multiple habitat types | One or few specific habitats |
| Adaptability | High — thrives in changing environments | Low — vulnerable to environmental change |
| Extinction risk | Lower | Higher |
| Examples | Raccoons, cockroaches, coyotes, rats, crows | Koalas (eucalyptus only), pandas (bamboo), spotted owls (old-growth forest) |
Specialist species are often used as indicator species because their narrow tolerances make them sensitive to environmental change. If specialists disappear, it signals ecosystem degradation.
Which species would most likely be negatively affected by deforestation of old-growth forests?
❌ Assuming generalist = "better." Specialists are highly competitive in their specific niche and outperform generalists there — until conditions change.
Species are categorized by their reproductive strategies along a continuum from r-selected (quantity) to K-selected (quality). The letters refer to variables in the logistic growth equation: r = intrinsic growth rate, K = carrying capacity.
| Trait | r-Selected | K-Selected |
|---|---|---|
| Offspring number | Many (hundreds–thousands) | Few (1–few) |
| Parental care | Little to none | Extensive |
| Body size | Small | Large |
| Lifespan | Short | Long |
| Maturity | Early sexual maturity | Late sexual maturity |
| Population size | Variable, boom-and-bust | Stable, near K |
| Survivorship | Type III curve | Type I curve |
| Examples | Insects, bacteria, mice, dandelions, salmon | Elephants, whales, humans, eagles, bears |
K-selected species are more vulnerable to extinction because: slow reproduction, long generation times, small populations, and high parental investment mean they cannot quickly recover from population decline. This is why most endangered species are K-selected.
A species has the following characteristics: long lifespan, few offspring per reproductive cycle, extensive parental care. This species is most likely
❌ Treating r/K as a binary. It is a CONTINUUM — most species fall somewhere in between.
❌ Saying r-selected species have "no chance" of survival. Their strategy is high reproduction to compensate for high mortality — it works in unstable environments.
Survivorship curves graph the proportion of individuals surviving at each age. They reveal the mortality patterns of a species across its lifespan. Three idealized types exist:
| Type | Mortality Pattern | Characteristics | Examples |
|---|---|---|---|
| Type I | Low mortality until old age | K-selected, few offspring, extensive parental care, long lifespan | Humans, elephants, whales, primates |
| Type II | Constant mortality at all ages | Equal chance of dying at any age | Songbirds, squirrels, coral, some lizards |
| Type III | Very high mortality early, low after | r-selected, many offspring, little/no parental care | Oysters, sea turtles, oak trees, most fish, insects |
On survivorship curves, the Y-axis is logarithmic (number of survivors) and X-axis is age. Type I is concave (curves down late); Type II is a straight diagonal line; Type III is convex (drops steeply early then flattens).
A marine fish species produces 2 million eggs per reproductive cycle, of which fewer than 10 survive to adulthood. This species most likely exhibits which survivorship curve?
❌ Mixing up Type I and Type III: Type I = low early mortality, high late mortality (humans, elephants). Type III = high early mortality, low late mortality (fish, insects). Remember: Type I looks like an "L" flipped horizontally; Type III drops steeply then flattens.
❌ Assuming Type II means "no deaths." Type II means CONSTANT mortality at every age — individuals have equal probability of dying at any point in their life.
A wildlife biologist is studying two animal species in the same ecosystem: Species A produces 2 offspring per year, provides extensive parental care, has a 30-year lifespan, and reaches sexual maturity at age 8. Species B produces 500 eggs per year, provides no parental care, has a 3-year lifespan, and reaches sexual maturity at 6 months.
(a) Identify the reproductive strategy (r-selected or K-selected) for each species. Justify your answer using TWO characteristics for each.
(b) Draw and label the expected survivorship curve for each species. Explain the shape of each curve.
(c) A severe drought reduces the habitat by 60%. Predict which species will recover faster and explain why, referencing their reproductive strategy.
Carrying capacity (K) is the maximum population size an environment can sustain indefinitely, given available resources (food, water, shelter, space). It is determined by limiting factors — resources in shortest supply.
| Type | Definition | Examples | Effect |
|---|---|---|---|
| Density-Dependent | Impact increases as population density increases | Competition, predation, disease, parasitism, waste accumulation | Regulates population around K |
| Density-Independent | Impact is the same regardless of population size | Natural disasters, extreme weather, wildfires, volcanic eruptions | Can crash population well below K |
K is NOT fixed — it changes with resource availability. Drought reduces K; nutrient enrichment increases K. When a population exceeds K (overshoot), resources are depleted, leading to a die-off or population crash.
In a deer population, disease spreads more rapidly when population density is high. This is an example of a
❌ Thinking carrying capacity is a permanent constant. K changes with environmental conditions — drought, pollution, or habitat loss all reduce K.
❌ Classifying natural disasters as density-dependent. A wildfire kills the same proportion regardless of population density → density-independent.
Population growth follows two mathematical models: exponential growth (J-curve, unlimited resources) and logistic growth (S-curve, limited resources approaching K).
| Model | Equation | Curve Shape | When It Occurs |
|---|---|---|---|
| Exponential | dN/dt = rN | J-curve (accelerating) | Unlimited resources, no competition, new habitat colonization |
| Logistic | dN/dt = rN(K-N)/K | S-curve (levels off at K) | Limited resources, density-dependent regulation |
N = population size, r = per capita growth rate, K = carrying capacity, dN/dt = change in population over time
In logistic growth, the population grows fastest at N = K/2 (half carrying capacity). This is the inflection point of the S-curve.
When population exceeds K, resources are depleted → population crashes below K. May oscillate before stabilizing. Reindeer on St. Matthew Island: classic example.
r = (births - deaths) / N. When r > 0, population grows. When r = 0, population is stable. When r < 0, population declines.
You MUST know that maximum growth rate occurs at K/2. This is the most frequently tested concept in population ecology. At K/2, the (K-N)/K term equals 0.5, balancing growth potential with remaining resource availability.
A population of rabbits has a carrying capacity of 1,000. At what population size will the growth rate be greatest?
❌ Thinking maximum growth rate is at the beginning. Early growth may seem fast proportionally, but ABSOLUTE growth rate (dN/dt) peaks at K/2.
❌ Confusing exponential and logistic models. Exponential has NO carrying capacity term. Real populations almost always follow logistic growth.
A population of elk is introduced to a new valley with abundant resources. The initial population is 40 elk, and the carrying capacity is estimated at 800.
(a) Describe the expected pattern of population growth over the first 20 years. Identify which growth model applies at the beginning versus later stages.
(b) Calculate the population size at which the elk population will experience the maximum growth rate. Show your work.
(c) After 15 years, a severe winter kills 70% of the elk. Explain what will happen to the growth rate immediately after this event, referencing the logistic growth equation.
(d) Identify ONE density-dependent and ONE density-independent factor that could affect the elk population. Explain how each operates.
Age structure diagrams (population pyramids) show the distribution of a population across age groups and sex. The shape predicts future population growth trends.
| Shape | Growth Trend | Characteristics | Country Examples |
|---|---|---|---|
| Broad Base (Triangle) | Rapid growth | High birth rate, many young, short life expectancy | Nigeria, Ethiopia, Afghanistan |
| Column (Rectangle) | Stable/slow growth | Birth rate ≈ death rate, even distribution | USA, France, Australia |
| Inverted Triangle | Declining population | Low birth rate, aging population, long life expectancy | Japan, Germany, Italy |
The proportion of pre-reproductive individuals (0-14 years) determines future growth potential. A country with 40% under 15 will continue growing for decades even if birth rates drop immediately — this is called population momentum.
A country has an age structure diagram with a very broad base and narrow top. This indicates the country most likely has
❌ Ignoring population momentum: Even if a broad-base country drops to replacement-level fertility TODAY, its population will keep growing for decades because the large youth cohort hasn't yet reproduced.
❌ Confusing age structure shape with current growth rate. The pyramid shows POTENTIAL — a column-shaped pyramid can still have positive growth if birth rate slightly exceeds death rate.
Total Fertility Rate (TFR) is the average number of children a woman will have during her reproductive years. It is the most important predictor of population growth trends.
| TFR Value | Meaning | Population Trend |
|---|---|---|
> 2.1 | Above replacement level | Population growing (assuming no migration) |
= 2.1 | Replacement level fertility | Population eventually stabilizes (zero population growth) |
< 2.1 | Below replacement level | Population eventually declines |
The extra 0.1 accounts for infant/child mortality. In developing nations with higher child mortality, replacement TFR may be 2.3-2.5.
Education (especially for women), access to contraception, urbanization, economic development, delayed marriage, higher cost of raising children.
World TFR dropped from 5.0 (1950) to ~2.3 (2024). Sub-Saharan Africa remains highest (~4.5). East Asia lowest (~1.2).
Even when TFR drops to replacement level, population continues growing for 1-2 generations due to population momentum — the large cohort of young people already born will still reproduce.
A country's TFR drops from 4.5 to 2.1 over 20 years. Which prediction is most accurate?
❌ Thinking TFR = 2.0 is replacement: Replacement is 2.1 (not 2.0) because it accounts for infant/child mortality. In developing nations with higher child mortality, replacement TFR may be even higher (2.3-2.5).
❌ Assuming low TFR means instant population decline. Due to population momentum, a country can have below-replacement TFR and STILL grow for decades.
Human population growth is shaped by crude birth rate (CBR), crude death rate (CDR), and migration. The population equation: Growth = (CBR - CDR) + (Immigration - Emigration).
| Milestone | Year | Time to Add 1 Billion |
|---|---|---|
| 1 billion | 1804 | All of human history |
| 2 billion | 1927 | 123 years |
| 3 billion | 1960 | 33 years |
| 4 billion | 1974 | 14 years |
| 5 billion | 1987 | 13 years |
| 6 billion | 1999 | 12 years |
| 7 billion | 2011 | 12 years |
| 8 billion | 2022 | 11 years |
Doubling time = 70 / growth rate (%). At 2% growth → 35 years to double. At 1% → 70 years. At 0.5% → 140 years.
World population ~8.1 billion. Growth rate ~0.9%/year (slowing). Projected peak: 10.4 billion around 2086 (UN median).
Impact = Population × Affluence × Technology. Environmental impact is a function of how many people, how much each consumes, and the environmental impact of that technology.
More people → more resource demand, more waste, more habitat conversion. But AFFLUENCE matters enormously: one American has ~16× the carbon footprint of one person in Sub-Saharan Africa.
A country has a population growth rate of 1.4%. Using the Rule of 70, approximately how many years will it take for the population to double?
❌ Using Rule of 70 with the wrong number: Use the PERCENTAGE growth rate, not a decimal. If growth rate is 1.4%, divide 70 by 1.4 (not 0.014). Answer: 50 years.
❌ Forgetting that the IPAT equation includes Technology. A country with small population but high affluence and dirty technology can have a larger environmental impact than a large, poor population.
The Demographic Transition Model (DTM) describes how countries move through four (or five) stages as they industrialize and develop economically. Birth rates and death rates change predictably.
| Stage | Birth Rate | Death Rate | Population Growth | Examples |
|---|---|---|---|---|
| Stage 1: Pre-industrial | High | High | Slow/stable | No modern countries (historical societies) |
| Stage 2: Transitional | High | Rapidly falling | Rapid growth | Many Sub-Saharan African nations |
| Stage 3: Industrial | Falling | Low | Slowing growth | India, Brazil, Mexico |
| Stage 4: Post-industrial | Low | Low | Stable (ZPG) | USA, France, UK, Australia |
| Stage 5: Decline | Very low | Low (rising slightly) | Negative growth | Japan, Germany, Italy, South Korea |
Improved sanitation, medicine, nutrition, and public health reduce CDR before cultural shifts reduce CBR. This gap causes rapid growth in Stage 2.
Urbanization (children = cost, not labor), women's education, access to contraception, delayed marriage, social security (no need for old-age support from children).
Aging population, shrinking workforce, pension/healthcare burden, economic stagnation. Japan's population shrinking by ~500,000/year.
Stage 2-3 countries: population pressure on resources. Stage 4-5 countries: high per-capita consumption pressure. Both have environmental impacts.
The AP exam loves asking: "In which stage does population grow FASTEST?" → Stage 2 (high birth rate + rapidly declining death rate = maximum gap). Also frequently tested: "What drives the transition from Stage 2 to 3?" → industrialization, urbanization, women's education.
During which stage of the demographic transition does population growth rate reach its maximum?
❌ Saying Stage 1 has fast growth. Stage 1 has high birth AND high death rates — they cancel out, resulting in slow/stable growth.
❌ Assuming all countries follow the same path. Some may skip stages, and the model doesn't account for war, famine, or policy interventions (China's one-child policy).
Country X has the following characteristics: CBR = 42 per 1,000; CDR = 10 per 1,000; TFR = 5.8; 45% of the population is under age 15; life expectancy = 58 years.
(a) Identify the stage of the Demographic Transition Model for Country X. Provide TWO pieces of evidence from the data.
(b) Describe the shape of Country X's age structure diagram and explain what it predicts about future population growth.
(c) Calculate the rate of natural increase (as a percentage) and the doubling time using the Rule of 70. Show your work.
(d) Propose TWO specific strategies that could help Country X transition to Stage 3. Explain the mechanism by which each strategy reduces birth rates.
Mixed MCQ and FRQ in AP APES exam style. Attempt each before revealing the answer.
A specialist bird species feeds exclusively on insects found in old-growth forest canopy. Which of the following would most directly reduce the carrying capacity for this species?
Country A has a population of 50 million, a growth rate of 2.8%, and an ecological footprint of 1.2 global hectares per capita. Country B has a population of 20 million, a growth rate of 0.3%, and an ecological footprint of 8.5 global hectares per capita. Which statement best compares their environmental impact?
A remote island ecosystem contains both r-selected rodent species and K-selected tortoise species. A new invasive predator is introduced to the island.
(a) Compare how the rodent and tortoise populations will likely respond to the new predator in the SHORT TERM (1-5 years).
(b) Explain how the introduction of the predator could change the carrying capacity for the tortoise population. Reference the logistic growth model in your answer.
(c) If the predator is eventually removed after 10 years, predict which species will recover to pre-invasion population levels first. Justify using reproductive strategy concepts.
The table below shows demographic data for three countries:
Country P: CBR = 38, CDR = 8, TFR = 5.2, % under 15 = 43%
Country Q: CBR = 12, CDR = 10, TFR = 1.8, % under 15 = 16%
Country R: CBR = 18, CDR = 7, TFR = 2.3, % under 15 = 28%
(a) Assign each country to a stage in the Demographic Transition Model. Justify each assignment with data from the table.
(b) Calculate the doubling time for Country P. Show your work.
(c) Explain why Country Q's population may decline even though its CDR (10) is lower than Country P's CDR (8).
(d) Country R is considering policies to reduce its environmental impact. Using the IPAT model, identify TWO approaches besides population reduction that could lower environmental impact.
Focus study time on logistic growth and K/2 (most tested), demographic transition stages (especially Stage 2), and r-selected vs. K-selected species. The AP exam frequently presents data tables requiring Rule of 70 calculations and asks you to interpret age structure diagrams. Practice identifying density-dependent vs. density-independent factors — this distinction appears in both MCQ and FRQ.