Cram sheet
32 points- Ecology is the study of how organisms relate to one another and to their environments. This chapter works up one level at a time: individual → population. Chapter 55 does communities, 56–58 do ecosystems and the biosphere. Watch Population Ecology: How We Saved the Bald Eagle: Crash Course Biology #7 on YouTube, from CrashCourse
- Four physical factors set who can live where: temperature, water, sunlight, and soil (consistency, pH, minerals — especially nitrogen and phosphorus). Watch Population Ecology: How We Saved the Bald Eagle: Crash Course Biology #7 on YouTube, from CrashCourse
- Homeostasis = holding a steady-state internal environment. Conformers do the opposite — their bodies simply adopt the temperature and salinity of their surroundings. The same lizard can be a regulator in open sun and a conformer in shaded forest. Watch Thermoregulation on YouTube, from Bozeman Science
- Three ways an individual responds within its lifetime: physiological (sweating; acclimating to altitude), morphological (a wolf's winter fur is 3× thicker and insulates 2× better), and behavioral (basking, moving, burrowing). Watch Thermoregulation on YouTube, from Bozeman Science
- Norm of reaction = one genotype producing multiple phenotypes. These differences are not genetic and are not passed on — snails growing thicker shells near predators is plasticity, not evolution.
- A population = individuals of one species together at one place and time. Its three key properties: the range, the spacing pattern inside that range, and how size changes over time.
- THE BIG ONE — three dispersion patterns. Random (no strong interaction, no environmental patchiness; rare in nature). Uniform (usually competition — territories in animals, shading or allelopathic creosote-bush chemicals in plants). Clumped (patchy resources plus social grouping — the most common). Watch Population Dispersion: Uniform, Random, Clumped on YouTube, from JK Biology Class Watch Clumped, Random, and Uniform Population Distributions on YouTube, from HeyNowScience
- A metapopulation is a network of separate populations exchanging individuals across patchy habitat. Occupancy is a balance: extinction rate of existing patches = colonization rate of empty patches.
- Source–sink: good habitats (sources) export dispersers that prop up poor habitats (sinks). Sinks have negative growth and would go extinct without the continual influx — which is why the species occupies more area than it otherwise could.
- Demography is the quantitative study of populations. Growth is set by sex ratio, generation time, age structure, and cohort-specific fecundity and mortality rates. Watch Population Ecology (Life Tables, Age Structure, Population Growth) on YouTube, from Professor Dave Explains
- A cohort is a group of the same age. Populations with many young individuals grow fast; populations with many old individuals (Japan, parts of Europe) shrink. Watch Population Ecology (Life Tables, Age Structure, Population Growth) on YouTube, from Professor Dave Explains
- A life table follows one cohort from birth to death. In the annual bluegrass table the last column sums to 1.98 offspring per original cohort member; 1.0 is break-even, so this population is growing. Watch Intro to Life Tables on YouTube, from Gregory Fulkerson
- THE BIG ONE — survivorship curves. Type I = mortality rises steeply late (humans). Type II = constant death rate at every age, a straight line on a log plot (hydra; annual bluegrass fits here). Type III = enormous early mortality, then low mortality once established (oysters). Watch AP Environmental Science Unit 3 – Topic 3.3: Survivorship Curves on YouTube, from Mr. Chipman - Biology Watch Decode Survivorship Graphs Before Your AP Exam on YouTube, from Ms. Parrott Science
- Cost of reproduction = the reduction in future reproductive potential caused by reproducing now. Douglas firs that make more cones grow more slowly; birds that raise more young are more likely to die that year. Watch r and K selection on YouTube, from Bozeman Science
- The collared flycatcher experiment is the clean proof: add eggs to a nest → the female lays fewer eggs next year; remove eggs → she lays more. Manipulated cause, measured effect. Watch r and K selection on YouTube, from Bozeman Science
- Trade-off between number of offspring and size of each. Great tits: more eggs → smaller nestlings. Side-blotched lizards with follicles surgically removed lay 1–3 eggs instead of 4–5 but pack each with more yolk, producing much larger hatchlings. An intermediate number of fairly large offspring maximizes survivors. Watch r-selected and K-selected population growth strategies | High school biology | Khan Academy on YouTube, from Khan Academy
- Semelparity = one huge "big bang" reproductive event, then death (annual plants, most insects, Pacific salmon). Iteroparity = reproducing repeatedly over many seasons. Semelparity is favored when survival to another season is unlikely or the reproductive cost is enormous. Watch Life History (Part 2): Iteroparity vs. Semelparity (Extra) on YouTube, from scienceclassisgreat
- r = (b − d) + (i − e): birth rate minus death rate, corrected for immigration and emigration. Late-20th-century U.S. growth was mostly immigration. Watch Population Ecology: The Texas Mosquito Mystery - Crash Course Ecology #2 on YouTube, from CrashCourse
- Exponential growth: dN/dt = rᵢN, where rᵢ is the intrinsic rate of natural increase. This is biotic potential — the J-curve, a population explosion. One housefly pair could leave >5 trillion descendants in a year. Watch Population Ecology: The Texas Mosquito Mystery - Crash Course Ecology #2 on YouTube, from CrashCourse Watch Exponential vs Logistic Growth on YouTube, from Baylor Tutoring Center
- THE BIG ONE — logistic growth: dN/dt = rN[(K − N)/K]. The bracket is the fraction of K still unused. As N → K the bracket → 0, so dN/dt → 0. If N > K the bracket goes negative and the population shrinks back. Either way the population moves toward K. Plot gives the sigmoidal (S-shaped) curve. Watch Logistic Growth on YouTube, from Bozeman Science Watch Exponential vs Logistic Growth on YouTube, from Baylor Tutoring Center
- Density-dependent factors scale with population size — competition, predation focused on common prey, disease, toxic waste buildup, stress behavior, emigration. Classic negative feedback. Density-independent factors (cold winters, drought, storms, volcanoes) hit regardless of size and give erratic, non-sigmoidal growth. Watch Population regulation | Ecology | Khan Academy on YouTube, from Khan Academy Watch Density Dependent & Density Independent Factors AP Biology 53.5 on YouTube, from ScienceCreAchins
- The Allee effect is the exception: growth rate increases with density (mates are easier to find, groups deter predators). It is positive feedback and a major threat to endangered species that can never recover from a crash. Watch Population regulation | Ecology | Khan Academy on YouTube, from Khan Academy
- The snowshoe hare–lynx "10-year cycle" (really 8–11 years, 10- to 30-fold swings). Krebs's Yukon plots: add food and exclude predators → the cycle disappears and hares stay 10× high. Either factor alone keeps the cycle. So the cycle comes from the interaction of food and predation. Watch Predator prey cycle | Ecology | Khan Academy on YouTube, from Khan Academy
- r-selected = far below K, abundant resources, many small offspring, early reproduction, short life, no parental care (dandelions, aphids, mice, cockroaches). K-selected = near K, stiff competition, few large offspring, delayed reproduction, long life, extensive care (coconut palms, whooping cranes, whales, humans). Real species sit on a continuum. Watch r and K selection on YouTube, from Bozeman Science Watch r-selected and K-selected population growth strategies | High school biology | Khan Academy on YouTube, from Khan Academy
- Humans are K-selected by design but broke out of logistic growth around the early 1700s: technology raised the carrying capacity and the death rate fell far more than the birth rate, restarting exponential growth. Watch Human Population Growth - Crash Course Ecology #3 on YouTube, from CrashCourse
- Human numbers: ~130 million 2000 years ago → 500 million by 1650 → approaching 8 billion today. Peak growth 2.0% (1965–70) is now 1.1%, which still adds ~80 million people per year and would double the population in 63 years. Watch Human Population Growth - Crash Course Ecology #3 on YouTube, from CrashCourse
- Population pyramids: triangular (broad base) = rapid future growth (Kenya, >40% under 15, fertility 3.5); rectangular = stable (Sweden, 18% under 15, fertility 1.9); inverted = shrinking from sharply falling birth rates. Older females outnumber older males because of longer life expectancy. Watch Age Structure Diagram & Population Pyramids on YouTube, from Earth Science Classroom
- Consumption, not just headcount: the richest 20% of humanity consumes 86% of resources and emits 53% of CO₂; the poorest 20% consumes 1.3% and emits 3%. A U.S. ecological footprint (19.9 acres) is nearly 8× Nigeria's (2.5 acres). Watch Ecological Footprint on YouTube, from LMSD Science
- Pandemic = a disease outbreak spanning multiple countries or continents; epidemic = a localized outbreak. Pandemics start when a pathogen establishes in hosts with no adequate immune defense — a zoonotic jump between species, or a pathogen that evolves past existing immunity.
- THE BIG ONE — R₀ ("R naught") = the average number of new people infected by each infected person. R₀ > 1 → the disease spreads; R₀ = 1 → prevalence holds constant; R₀ < 1 → the disease dies out. SARS-CoV-2 started at 2–3; Delta was 6–9; Nipah virus is ~0.48, which is why it kills 40%+ of victims but never causes big outbreaks.
- Herd immunity: once enough of the population is immune (often at least 70%), R₀ drops below 1 and even non-immune people are protected. Reaching it by infection is costly; vaccines get there without the disease, and have eliminated smallpox and rinderpest.
- Pathogens evolve three ways: more contagious (the 2020 England variant spread ~70% faster), escaping immunity (influenza's high mutation rate changes its HA and NA surface proteins, so flu vaccines need annual replacement), and less lethal (myxoma virus in Australian rabbits — longer-lived hosts spread it more).