Ch 54 — Ecology of Individuals and Populations

54.1

The Environmental Challenges

pp. 1216–1218

Ecology is the study of how organisms relate to one another and to their environments. The chapter starts with how an individual copes with the abiotic environment, then scales up to what that means for populations. Watch Population Ecology: How We Saved the Bald Eagle: Crash Course Biology #7 on YouTube, from CrashCourse
The nature of the physical environment largely determines which organisms live in a particular climate or region.
Temperature: most organisms are adapted to a relatively narrow temperature range and do not thrive outside it. Temperature strongly controls the length of a plant's growing season.
Water: every organism requires it. On land water is often scarce, so rainfall patterns have a major influence on which life is possible.
Sunlight: nearly all ecosystems run on energy captured by photosynthesis, so light availability sets how much life an ecosystem supports — especially with depth in marine communities.
Soil: physical consistency, pH, and mineral composition limit terrestrial plant growth, particularly the availability of nitrogen and phosphorus.
Homeostasis is maintaining a "steady-state" internal environment. Many animals and plants hold it with physiological, morphological, or behavioral mechanisms. Watch Thermoregulation on YouTube, from Bozeman Science
Conformers do not regulate — their bodies adopt the temperature, salinity, and other physical properties of their surroundings.
Responses come on two timescales: short term (minutes to a lifetime, within the individual) and long term (natural selection making the population better adapted across generations).
Physiological response, extreme version: at high elevation, lower atmospheric pressure and less available oxygen cause altitude sickness — heart palpitations, nausea, fatigue, headache, mental impairment, and in serious cases pulmonary edema.
After several days at altitude people feel fine because of acclimation: faster breathing, more erythrocytes (higher hemoglobin), decreased hemoglobin binding capacity so oxygen unloads into tissues faster, and denser mitochondria, capillaries, and muscle myoglobin.
Cold-weather physiology in insects: some avoid freezing by adding glycerol "antifreeze" to their blood; others tolerate freezing by converting glycogen into alcohols that protect cell membranes from freeze damage.
Morphological response: endotherms (animals that hold a constant internal temperature) in cold environments minimize energy loss — notably by growing thicker coats in winter. Watch Thermoregulation on YouTube, from Bozeman Science
In general, the thicker the fur, the greater the insulation. A wolf's fur is about three times thicker in winter than in summer and insulates more than twice as well (Fig 54.2, which also plots the polar bear).
Plants alter morphology too: more extensive roots when water is limited, larger leaves in shade.
Predator-induced morphology (predators usually detected by smell): snails grow thicker shells, Daphnia develop spines, tadpoles grow thicker tails for faster escape swimming.
Producing multiple phenotypes from a single genotype is a norm of reaction. It is most valuable to species that may end up in different environments — especially sedentary species that cannot move away.
Critical exam point: phenotypic differences produced by a norm of reaction are not genetic differences. Genetically identical individuals raised in different environments differ, and those nongenetic differences are not transmitted to the next generation.
Behavioral response: many animals simply move between habitat patches, avoiding unsuitable areas.
The Puerto Rican crested lizard Anolis cristatellus keeps a fairly uniform body temperature in open habitat by basking in sun patches and retreating to shade. In shaded forest it cannot thermoregulate behaviorally and becomes a conformer — same species, two strategies (Fig 54.3). Watch Thermoregulation on YouTube, from Bozeman Science
Extreme behavior: spadefoot toads (Scaphiophus) burrow nearly a meter down and stay there up to nine months a year with greatly reduced metabolic rates, living on fat reserves. When cool, moist conditions return they emerge and breed, and the young mature fast and burrow back down.
The ability to alter physiology, morphology, or behavior is itself an evolutionary adaptation produced by natural selection.
Humans can acclimate to altitude physiologically, and human populations can also evolve genetic solutions. Most Tibetan highlanders carry two mutations in a gene for oxygen uptake that are rare in nearby lowland populations — the signature of strong natural selection.
Those Tibetan mutations arose in only the few thousand years since Tibetans colonized the Himalayas, and other recently evolved oxygen-transport mutations have since been found. This is one of the best examples of natural selection acting on contemporary human populations.
Allen's rule: mammals from colder climates tend to have shorter ears and limbs, reducing the surface area across which heat is lost.
Climate shapes physiology between close relatives: desert lizards shrug off temperatures that would kill a northern European lizard, while northern lizards can run, capture prey, and digest food at cool temperatures that immobilize desert species.
Water-scarcity adaptations: camels and other desert animals go long periods without drinking. Most frogs have moist, permeable skin and could not survive arid climates, but some evolved greatly reduced skin water loss — one species secretes a waxy substance that waterproofs its skin and cuts water loss by 95%.
Adaptation can be watched in the lab: E. coli grown at 42 °C improved how fast they used resources, and after 2000 generations that ability was 30% better than at the start.

54.2

Populations: Groups of a Single Species in One Place

pp. 1218–1221

A population is a group of individuals of one species that occur together at one place and time. The term scales freely — the world's human population, the protists in one termite's gut, the deer in one forest.
Three properties matter most in population ecology: (1) the population range, the area throughout which the population occurs; (2) the spacing pattern of individuals within it; (3) how population size changes through time.
Most species have limited ranges. The Devil's Hole pupfish (Cyprinodon diabolis) lives in a single spring in southern Nevada — the smallest range of any vertebrate. The Socorro isopod is known from one spring system in New Mexico. At the other extreme, the common dolphin Delphinus delphis occurs in all the world's oceans.
Each population has its own requirements — temperature, humidity, food type — that set where it can live and reproduce. Polar bears are not found in tropical rainforest; near-boiling Yellowstone prokaryotes are absent from cooler streams nearby.
Even a physically suitable place can be unoccupied because of predators, competitors, or parasites — the biotic side, taken up in chapter 55.
Ranges are not static. They change for two reasons: the environment itself changes, and populations circumvent inhospitable habitat to colonize new suitable areas.
When glaciers retreated about 10,000 years ago, many North American plant and animal populations expanded northward. Warming also shifted species upward in elevation, since cold-adapted trees must climb to stay in the climate they are adapted to (Fig 54.5).
Cattle egret case study: native to Africa, it appeared in northern South America in the late 1800s after a ~3500-km transatlantic crossing, possibly wind-assisted, and has since spread through almost all of South and Central America and the United States.
The human effect: by altering the environment, humans let species such as coyotes expand into areas they never occupied.
Humans also act directly as dispersal agents. 100 starlings were released in New York City in 1896 to establish every bird Shakespeare mentioned; by 1980 starlings occupied the entire United States. Invader success often comes at the expense of native species.
Dispersal mechanisms explain odd distributions: lizards reach distant islands by floating or rafting on vegetation, and bats are the only mammals on many remote islands because they can fly there.
Seed dispersal falls into three designs (Fig 54.7): windblown fruits (milkweed, maple), adherent fruits that stick to fur or feathers (Bidens, Medicago), and fleshy fruits eaten by mammals or birds whose seeds germinate where defecated (nightshade, juniper, blackberry).
THE BIG ONE — spacing. Individuals are randomly, uniformly, or clumped in their distribution. Watch Population Dispersion: Uniform, Random, Clumped on YouTube, from JK Biology Class Watch Clumped, Random, and Uniform Population Distributions on YouTube, from HeyNowScience
Random spacing occurs only when individuals do not interact strongly and the environment is not patchy. It is uncommon in nature, though some Panamanian rainforest trees approach it.
Uniform spacing usually results from competition for resources. In animals it typically comes from territoriality — one or both sexes defend a territory, giving exclusive access to food, water, refuges, or mates. Even nonterritorial species often keep a defended personal space.
In plants, uniform spacing comes from competition for sunlight, nutrients, or water — directly (one plant shading another) or indirectly (drawing nutrients or water from a shared area).
Chemical version: the creosote bush releases compounds into the surrounding soil that are toxic to other members of its own species, so only adequately spaced plants coexist.
Clumped spacing arises when resources are unevenly distributed — organisms cluster in the soil type, moisture, or microhabitat they are best adapted to. This is why clumping is the most common pattern. Watch Clumped, Random, and Uniform Population Distributions on YouTube, from HeyNowScience
Social clumping: flocks, herds, and prides give increased predator awareness and defense, decreased energy cost of moving through air or water, and access to the knowledge of every group member.
On a broad scale, populations are densest in the interior of the range and thin toward the edges — conditions there suit them less, densities fall, and eventually individuals cannot persist at all, which is what defines the range edge.
A metapopulation is a network of distinct populations of one species that exchange individuals. Metapopulations form where suitable habitat is patchily distributed, separated by stretches of unsuitable habitat.
Exchange within a metapopulation is asymmetrical: growing populations send out many dispersers, while low-density populations receive more immigrants than they send. Isolated populations receive few arrivals.
Not every suitable patch is occupied at once. Individual populations wink out from epidemic disease, catastrophic fire, or loss of genetic variation after a bottleneck. Dispersal recolonizes them, so occupancy sits at an equilibrium between extinction rate and colonization rate.
Source–sink metapopulations: some habitats support long-term persistence (sources) and continually export dispersers into poorer habitats (sinks). Without that replenishment sink populations have a negative growth rate and go extinct.
Glanville fritillary butterfly (Melitaea cinxia), 1600 meadows in southwestern Finland: on average 200 populations went extinct each year while only 114 empty meadows were colonized. Extinction was likelier for populations that were small, isolated, low in resources (few flowers), or low in genetic variation.
Researchers blamed the extinction surplus on a string of very dry summers. No single Finnish population is large enough to survive alone, so the species persists only as a metapopulation — and continued bad weather could doom it there.
Two consequences for a species' range: continuous recolonization of empty patches prevents long-term extinction, and source–sink structure makes the occupied area larger than it otherwise would be, including marginal ground no population could hold on its own.
Because natural habitats are increasingly fragmented, metapopulation theory has become central to conservation biology.

54.3

Population Demography and Dynamics

pp. 1222–1224

Population dynamics is how a population changes through time. One of the strongest influences is the age distribution — what proportion are adults, juveniles, and young. Watch Population Ecology (Life Tables, Age Structure, Population Growth) on YouTube, from Professor Dave Explains
Demography is the quantitative study of populations.
The simplest rule: a population grows if births outnumber deaths and shrinks if deaths outnumber births (setting immigration and emigration aside for the moment).
Trends are easier to read if the population is broken into smaller same-age units (say, all 1-year-olds), with birth and death rates studied separately for each.
Sex ratio can matter. Births are usually tied to the number of females, and only loosely to males where one male can mate with several females.
Where males compete for mating opportunities, a few males mate a lot and many never mate. The effective sex ratio is female-biased, so removing males only changes which males reproduce — it does not lower births.
By contrast, in monogamous species with long-lasting pair bonds, reducing the number of males directly reduces the number of births. Exam trick: the answer depends on the mating system.
Generation time is the average interval between the birth of an individual and the birth of its offspring.
Generation time correlates with body size (Fig 54.9): mice pass through about 100 generations during one elephant generation. But the rule has exceptions — newts are smaller than mice yet have considerably longer generation times.
Populations with short generations can increase in size faster. Because generation time and life span are correlated, they can also shrink faster if birth rates drop suddenly.
A cohort is a group of individuals of the same age. In most species, the probability of reproducing or dying varies across the life span.
Each cohort has a characteristic fecundity rate (birth rate — offspring produced per standard time, e.g. per year) and mortality rate (deaths in that period).
Age structure is the relative number of individuals in each cohort. Because cohorts differ in fecundity and death rate, age structure has a critical effect on growth rate. Watch Population Ecology (Life Tables, Age Structure, Population Growth) on YouTube, from Professor Dave Explains
A population with a large proportion of young individuals tends to grow rapidly as more of them become reproductive — the situation in many developing countries (section 54.7). A population weighted toward the old declines, as now in Japan and parts of Europe.
A life table tabulates the fate of a cohort from birth to death: how many survive each interval and how many offspring each survivor produces. Watch Intro to Life Tables on YouTube, from Gregory Fulkerson
Table 54.2 follows 843 annual bluegrass (Poa annua) seedlings in 3-month intervals. Columns: age, number alive, proportion alive (survivorship), deaths, mortality rate, seeds produced, seeds per survivor (fecundity), and seeds per original cohort member (fecundity × survivorship). Watch Intro to Life Tables on YouTube, from Gregory Fulkerson
Read the mortality column carefully: it is the proportion of individuals that started an interval alive but died by its end — not a proportion of the original cohort. In bluegrass it climbs from 0.144 at age 0 to 1.000 at age 7.
In annual bluegrass both the probability of dying and the seeds produced per survivor rise with age — fecundity per survivor goes 0.42 → 1.18 → 1.36 → 1.46 → 1.11 → 2.00 → 3.33.
The number that matters: the last column sums to 1.98 offspring per original cohort member. 1.0 is the break-even value where the population is neither growing nor shrinking, so this population is growing rapidly.
Real life tables are messier. Except for very short-lived organisms it is hard to follow a cohort until the last individual dies, so ecologists substitute a cross-sectional study examining cohorts of different ages during one period.
Interpretation is complicated further because offspring may start reproducing before their parents' cohort has finished dying.
Survivorship is the percentage of an original population that survives to a given age; plotting it gives a survivorship curve. Watch AP Environmental Science Unit 3 – Topic 3.3: Survivorship Curves on YouTube, from Mr. Chipman - Biology
Survivorship curves are plotted with survival on a logarithmic axis, which turns a constant mortality rate into a straight line. That is the whole reason Type II is recognizable at a glance. Watch Decode Survivorship Graphs Before Your AP Exam on YouTube, from Ms. Parrott Science
Type III — oysters: vast numbers of offspring, almost all die young; once established and reproductive, mortality becomes extremely low. Curve plunges then flattens.
Type II — hydra: individuals are equally likely to die at any age, so the curve is a straight diagonal line.
Type I — humans (and many other animals and protists): mortality rates rise steeply late in life, so the curve stays high then drops off a cliff.
These three types are generalizations; many organisms show more complicated patterns. Annual bluegrass, for example, best resembles type II after its first few months (Fig 54.11). Watch AP Environmental Science Unit 3 – Topic 3.3: Survivorship Curves on YouTube, from Mr. Chipman - Biology

54.4

Life History and the Cost of Reproduction

pp. 1224–1226

Natural selection favors traits that maximize the number of surviving offspring left in the next generation. Two things determine that number: how long an individual lives and how many young it produces each year.
No organism can do everything at once — reproduce immediately after birth, produce large families, care for them intensively, and repeat that over a long life — because resources are finite.
So organisms must allocate: resources go either to current reproduction or to increasing survival and future reproduction.
The complete life cycle of an organism is its life history, and every life history involves significant trade-offs.
Because resources are limited, a change that increases reproduction may decrease survival and reduce future reproduction.
Plant example: a Douglas fir that produces more cones raises current reproductive success but grows more slowly. Since cone number depends on tree size, slower growth lowers future cone production.
Bird example: species that raise more offspring per year have a higher probability of dying that year — the pattern runs from the albatross (fewest offspring, lowest mortality) to the sparrow (most offspring, highest mortality) in Fig 54.12.
Conversely, individuals that delay reproduction can grow faster and larger, which enhances future reproduction.
THE experiment — collared flycatcher (Ficedula albicollis): researchers added or removed eggs from nests. Females given fewer eggs spent less energy and laid more the next year; females given more eggs worked harder and laid fewer the next year (Fig 54.13). This is manipulated cause, measured effect. Watch r and K selection on YouTube, from Bozeman Science
Cost of reproduction = the reduction in future reproductive potential resulting from current reproductive effort. This exact definition is a favorite exam question.
Natural selection favors the life history that maximizes lifetime reproductive success — not this year's output.
When the cost of reproduction is low, produce as many offspring as possible. Costs are low when resources are abundant, and also when overall mortality is high (you may not survive to breed again anyway).
When the cost of reproduction is high, lifetime success is maximized by deferring or minimizing current reproduction to boost growth and survival.
Second trade-off — number vs. investment per offspring. Energy for offspring is limited, so a balance must be struck between how many and how big. Watch r-selected and K-selected population growth strategies | High school biology | Khan Academy on YouTube, from Khan Academy
Great tits (Parus major): nestling size is inversely related to clutch size — the more mouths, the less each nestling gets (Fig 54.14).
Side-blotched lizard (Uta stansburiana) experiment: normally lays 4–5 eggs. Surgically removing some egg follicles early leaves the female producing only 1–3 eggs, but she supplies each with more yolk, producing much larger eggs and hatchlings (Fig 54.15). Removing yolk instead produced smaller lizards — direct causal proof that maternal yolk sets offspring size.
Offspring size is critical because larger offspring have a greater chance of survival.
But neither extreme wins: many tiny offspring with almost no survival chance is bad, and a single extraordinarily robust offspring is also bad. An intermediate number of fairly large offspring maximizes surviving offspring.
Third trade-off — reproductive events per lifetime. The trade-off between age and fecundity shapes many life histories. Watch Life History (Part 2): Iteroparity vs. Semelparity (Extra) on YouTube, from scienceclassisgreat
Semelparity: all reproductive resources go into one large event, then the organism dies — annual plants and most insects. Nicknamed "big bang" reproduction.
Iteroparity: offspring produced several times over many seasons. Iteroparous species must avoid overtaxing themselves in any one episode so they survive to breed again.
Semelparity is favored in short-lived species with a low probability of surviving to breed again — for example plants in harsh climates.
Semelparity is also favored when fecundity carries an enormous reproductive cost, exemplified by Pacific salmon migrating upriver to spawn. Betting everything on one event beats an unlikely bid to survive to the next season.
Fourth pattern — age at first reproduction correlates with life span. Among mammals and many other animals, longer-lived species delay reproduction longer relative to expected life span.
Why delay pays in long-lived species: juveniles gain experience and grow faster and larger before paying the high costs of reproduction, and those benefits outweigh the lost early breeding.
Why delay does not pay in short-lived animals: time is of the essence, so quick reproduction beats juvenile training and extra growth, and reproduction happens earlier.
Umbrella statement for the whole section: life history adaptations are a bundle of trade-offs — from laying fewer than the maximum possible eggs to putting everything into one bout — all selected to maximize lifetime reproductive success.

54.5

Environmental Limits to Population Growth

pp. 1227–1228

Populations often stay at a relatively constant size no matter how many offspring are born. Darwin built his argument for natural selection partly on this apparent contradiction. Watch Population Ecology: The Texas Mosquito Mystery - Crash Course Ecology #2 on YouTube, from CrashCourse
Natural selection follows from checks on reproduction: some individuals leave fewer surviving offspring than others.
The rate of population increase r = the difference between birth rate b and death rate d, corrected for movement: r = (b − d) + (i − e), where i is immigration into the area and e is emigration out of it.
Movement can dominate. The increase in the U.S. human population during the closing decades of the 20th century was mostly due to immigration.
The simplest growth model assumes no limits and equal immigration and emigration. The resulting maximal rate is the population's biotic potential.
Exponential growth equation: dN/dt = rᵢN, where N is the number of individuals, dN/dt the rate of change in numbers over time, and rᵢ the intrinsic rate of natural increase — the population's innate capacity for growth. Watch Exponential vs Logistic Growth on YouTube, from Baylor Tutoring Center
Even with r held constant, the actual number of individuals added accelerates as N grows. That is the whole point of exponential growth, and unchecked it produces a population explosion (red J-curve in Fig 54.16).
The textbook shock number: one pair of houseflies laying 120 eggs per generation could leave more than 5 trillion descendants in a year, and in 10 years the descendants would form a swarm more than 2 m thick over the entire surface of the Earth.
In practice, unrestrained growth lasts only briefly, usually when an organism reaches a new habitat with abundant resources: dandelions arriving in North America from Europe, algae colonizing a newly formed pond, cats introduced to an island of predator-naive birds.
Growth eventually runs into shortages of space, light, water, or nutrients.
Carrying capacity, K, is the maximum number of individuals the environment can support. A population may ultimately stabilize at this size. Watch Logistic Growth on YouTube, from Bozeman Science
The logistic growth model applies as a population approaches K: growth slows greatly because fewer resources remain for each new individual.
THE BIG ONE — logistic growth equation: dN/dt = rN[(K − N)/K]. Watch Logistic Growth on YouTube, from Bozeman Science
How to read that equation: rN is the unrestrained exponential part; the bracket [(K − N)/K] is the fraction of the carrying capacity still unused, and it scales growth down.
As N increases, the unused fraction shrinks, so the rate of increase declines. When N = K the bracket is 0 and dN/dt = 0.
Plotting N against time gives the sigmoidal growth curve — "sigmoidal" because its double curve resembles the letter S. Watch Exponential vs Logistic Growth on YouTube, from Baylor Tutoring Center
Fig 54.17 plots dN/dt against N instead of against time. Populations far from K have high growth rates; growth is positive below K, negative above K, and crosses zero exactly at N = K.
If a population exceeds K, then K − N is negative and the population has a negative growth rate. As it declines toward K, the magnitude of that negative rate shrinks until it reaches 0 at N = K.
The key conclusion: the population moves toward the carrying capacity whether it started above or below it, so logistic growth tends to return a population to the same size. Such populations are described as being in equilibrium at or near K.
Fig 54.16 uses concrete parameters worth remembering: exponential with r = 1.0 (dN/dt = 1.0N) versus logistic with r = 1.0 and K = 1000 (dN/dt = 1.0N[(1000 − N)/1000]).
Real populations often do fit the logistic curve — the fur seal (Callorhinus ursinus) population on St. Paul Island, Alaska, from about 1915 to 1945 (Fig 54.18a).
The fit is not always clean: laboratory populations of the cladoceran Bosmina longirostris first overshot the carrying capacity, then dropped back to a size that was maintained (Fig 54.18b). Overshoot-and-correct is a common real-world deviation.
K is not a constant of nature — it depends on the resources available in that environment, so anything that changes resources changes K, and the population then tracks toward the new K.
Bottom line for the exam: exponential growth describes the unlimited case and gives a J-curve; logistic growth describes the resource-limited case and gives an S-curve leveling off at K. Watch Exponential vs Logistic Growth on YouTube, from Baylor Tutoring Center

54.6

Factors That Regulate Populations

pp. 1229–1232

Regulating factors come in two flavors: density-dependent factors, whose effect depends on population size, and density-independent factors such as natural disasters, which act regardless of size. 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
Many populations also show cyclic fluctuations that arise from complex interactions among several factors.
The core density-dependent mechanism: as population size increases, reproductive rates decline, mortality rates increase, or both. This is negative feedback.
Fig 54.19 shows the three ways it can work — a density-dependent birth rate falling against a flat death rate, a density-dependent death rate rising against a flat birth rate, or both changing. The equilibrium population density is wherever the two lines cross.
Near the carrying capacity, competition for resources can be severe, producing both a decreased birth rate and an increased risk of death.
Song sparrow (Melospiza melodia) on Mandarte Island is the textbook data set: as breeding adults rise toward ~150, the number of young per female falls and juvenile mortality rises (Fig 54.20).
Predation is density-dependent because predators often focus on a particularly common prey species, so mortality rises as the prey population grows.
High density also causes an accumulation of toxic wastes in the environment.
Behavioral density dependence: some rodents become antisocial at high density — fighting more, breeding less, generally stressed. The changes are hormonal, but the ultimate cause is unclear; most likely they evolved as adaptive responses to resource scarcity.
Emigration is density-dependent too: in crowded populations growth slows because more individuals leave to find better conditions. Migratory locusts (Locusta migratoria) take this to the extreme — at high density they change hormonally and physically and take off as a swarm (Fig 54.21).
The exception — the Allee effect: sometimes growth rate increases with population size. Named for zoologist Warder Allee, it is an example of positive feedback. Watch Population regulation | Ecology | Khan Academy on YouTube, from Khan Academy
How the Allee effect works: in populations too sparsely distributed, individuals have trouble finding mates; some species need large groups to deter predators or to provide the social stimulation that triggers breeding.
The Allee effect is a major threat to endangered species, which may never recover from reduced population sizes caused by habitat destruction or overexploitation. Watch for it as an answer choice on questions about small populations.
Density-independent effects: growth at any instant is limited by something unrelated to population size. Watch Density Dependent & Density Independent Factors AP Biology 53.5 on YouTube, from ScienceCreAchins
Most density-independent factors are external environmental events — extremely cold winters, droughts, storms, volcanic eruptions. Individuals are hit regardless of how many there are.
Populations in places where such events are frequent show erratic growth: rapid increases when conditions are benign, large crashes when the environment turns hostile. They do not produce sigmoidal logistic curves (Fig 54.22).
Fig 54.22 tracks pupae of four German moth species (Panolis, Hyloicus, Dendrolimus, Bupalus) from about 1880 to 1940. The fact that all four fluctuate in concert is the evidence that a shared density-independent factor is driving them.
Population cycles: in some populations density-dependent effects lead not to a stable equilibrium but to repeating cycles of increase and decrease. Watch Predator prey cycle | Ecology | Khan Academy on YouTube, from Khan Academy
The classic case: the North American snowshoe hare (Lepus americanus) follows a "10-year cycle" — in reality 8 to 11 years. Hare numbers fall 10- to 30-fold in a typical cycle, and 100-fold crashes can occur. Ecologists have been studying it since the 1820s.
Hypothesis 1 — food plants: hares prefer willow and birch twigs. As hare density rises, those twigs vanish, forcing hares onto high-fiber, low-quality food, which lowers birth rates, juvenile survivorship, and growth rates. Hares also spend more time searching, which raises their exposure to predators. Twig abundance takes two to three years to recover.
Hypothesis 2 — predators: the Canada lynx (Lynx canadensis) shows a "10-year" abundance cycle remarkably synchronized with the hare cycle, lagging slightly behind it. Lynx rise when hares are plentiful and crash when hares crash (Fig 54.23, from Hudson Bay fur-trapping records, 1845–1935).
THE experiment — Krebs's Yukon plots. Add food and exclude predators → hare numbers rise 10-fold and stay there: the cycle is lost. Allow either factor alone (exclude predators without adding food, or add food with predators present) → the cycle is retained. Conclusion: both factors matter, and the cycle is generated by their interaction. Watch Predator prey cycle | Ecology | Khan Academy on YouTube, from Khan Academy
Cycles are commoner than once thought. A review of nearly 700 long-term (25+ year) population studies found that nearly 30% — birds, mammals, fish, crustaceans — showed some cyclic pattern, though most are far less dramatic in amplitude than hare–lynx.
Cycles can be driven by density-dependent factors (hare and lynx) or by density-independent ones such as cyclic climatic patterns.
Resource availability shapes life histories. Some species sit near K most of the time; others fluctuate wildly and spend most of their time far below K. The selection pressures on the two are opposite. Watch r and K selection on YouTube, from Bozeman Science
Near K, competition for limited resources is stiff and the cost of reproduction is high, so selection favors individuals that compete effectively and use resources efficiently — usually at the cost of lower reproductive rates, delayed reproduction, larger body size, and fewer, larger offspring. These are K-selected populations.
K-selected examples from the text: coconut palms, whooping cranes, whales, and humans.
Far below K, resources are abundant and the cost of reproduction is low, so selection favors the maximum number of offspring and the highest reproductive rates. These are r-selected populations. Watch r-selected and K-selected population growth strategies | High school biology | Khan Academy on YouTube, from Khan Academy
r-selected examples from the text: dandelions, aphids, mice, and cockroaches.
Important caveat: most natural populations fall on a continuum between purely r-selected and purely K-selected traits. Few species show every trait in table 54.3, so treat the list as generalities with many exceptions.

54.7

Human Population Growth

pp. 1232–1235

Humans show many K-selected traits: small brood size, late reproduction, and a high degree of parental care. These evolved early in hominin history when limited resources controlled population size. Watch Human Population Growth - Crash Course Ecology #3 on YouTube, from CrashCourse
For most of human history, populations were regulated by food availability, disease, and predators, with floods, plagues, and droughts as disturbances on top.
The numbers to remember: about 130 million people 2000 years ago; it took a thousand years to double that, and not until 1650 did it double again to about 500 million.
Then, starting in the early 1700s, technology gave humans more control over food supply, better weapons against predators, cures for many diseases, and improved shelter and storage against climatic uncertainty.
The key conceptual move: those changes expanded the carrying capacity of human habitats, letting humans escape the confines of logistic growth and re-enter the exponential phase of the sigmoidal curve. Watch Human Population Growth - Crash Course Ecology #3 on YouTube, from CrashCourse
The mechanism behind the explosion: the birth rate decreased over time, but the death rate decreased far more. The widening gap is what produced growth of up to 2% per year.
Growth has since slowed to 1.1% per year — but 1.1% of nearly 8 billion still adds nearly 80 million people in a year and would double the population in 63 years.
Fig 54.24 makes the shape visible: a flat line for millennia, temporary dips such as the bubonic plague "Black Death" of the 1300s that had little lasting effect, then a near-vertical spike beginning with the Industrial Revolution in the 1800s, which lowered the death rate permanently, plus later advances in public health.
Growth is not uniform across the planet. Most of it is occurring in Africa and Asia, while populations are actually decreasing in some European countries.
Fig 54.25 projects 2020 → 2050 → 2100: Africa rises from roughly 1300 to about 4300 million, while Asia peaks near 5300 million around 2050. Developed countries are predicted to grow little; almost all increase happens in less-developed countries.
Population pyramids are bar graphs of the number of people in each age category, with males conventionally to the left of the vertical age axis and females to the right. Watch Age Structure Diagram & Population Pyramids on YouTube, from Earth Science Classroom
In most human pyramids the number of older females is disproportionately large, because females in most regions have a longer life expectancy than males.
THE BIG ONE — pyramid shapes. Rectangular = stable, neither growing nor shrinking. Triangular (broad base) = rapid future growth, because most of the population has not yet entered childbearing years. Inverted triangle = shrinking, usually from sharply declining birth rates. Watch Age Structure Diagram & Population Pyramids on YouTube, from Earth Science Classroom
Sweden vs. Kenya (Fig 54.26) is the worked comparison. Sweden: nearly rectangular, birth rates down and life span up, only 18% under 15 years old, fertility rate 1.9, population will stay stable. It also shows a middle-aged bulge from the mid-20th-century baby boom and many postreproductive individuals.
Kenya: strongly triangular, high birth rates and shorter life spans, more than 40% under 15, fertility rate 3.5 — its population could double in 30 years.
Humanity is adding about 80 million people a year — over a million every 5 days, about 150 every minute.
India had 1.05 billion people in 2002; by 2050 it will likely exceed 1.6 billion.
Of the billion people added in the 1990s, 90% live in developing countries. In 1950 a third of the world lived in industrialized countries; by 1996 that was a quarter, and by 2020 a sixth.
The uncomfortable conclusion: future growth is centered in the parts of the world least equipped to absorb the pressures of rapid growth.
Rapid growth in developing countries has widened the gap between rich and poor: per capita income is $33,829 in the industrialized world versus $5,405 in developing countries, and about one-eighth of people in the developing world live on less than $2 per day.
Table 54.4 (2019 data) lines up the pattern: United States — fertility 1.8, growth 0.72%, doubling time 97 yr, 18% under 15; Brazil — 1.7, 0.67%, 104 yr, 21% under 15; Ethiopia — 4.1, 2.56%, 27 yr, 40% under 15.
Humans already use an estimated 45% of the total biological productivity of Earth's landmasses and more than half of all renewable freshwater. Between one-fourth and one-eighth of all people are malnourished.
The growth rate has been falling — from a high of 2.0% in 1965–1970 to 1.1% in 2015 — but because the base population is larger, that lower rate still adds 80 million per year, versus 53 million per year in the 1960s.
Family planning works: since 1984 programs in Kenya cut the fertility rate from 8.0 to 3.5 children per couple and the growth rate from 4.0% to 2.3% per year. Efforts like these may let global population stabilize around 8.9 billion by mid-century.
Population size is not the only variable — per capita consumption is the other half. The wealthiest 20% of humanity accounts for 86% of resource consumption and 53% of CO₂ emissions; the poorest 20% accounts for 1.3% of consumption and 3% of emissions. Watch Ecological Footprint on YouTube, from LMSD Science
An ecological footprint is the productive land required to support one individual at a given standard of living over a lifetime — food, forest products, housing, plus the forest area needed to absorb the CO₂ from that person's fossil fuel use. Watch Ecological Footprint on YouTube, from LMSD Science
Footprints in acres (Fig 54.28): USA 19.9, Germany 11.6, Brazil 6.9, Indonesia 4.1, India 2.9, Nigeria 2.5 — the U.S. figure is nearly eight times Nigeria's.
Total resource use by humans is now about one-third greater than what nature can sustainably replace. If everyone lived at the industrialized standard of living, two additional planet Earths would be needed.
The chapter's closing claim: building a sustainable world is the most important task facing humanity, and it requires limiting both population growth and per capita resource consumption.

54.8

Pandemics and Human Health

pp. 1236–1237

Disease outbreaks can play a major role in population dynamics — in some cases possibly even driving a species to extinction.
Chestnut blight, a pathogenic fungus accidentally introduced from Asia in 1904, destroyed the American chestnut, formerly a dominant tree of eastern forests. It has never recovered.
Rinderpest virus, introduced with a shipment of cattle from India, wiped out almost all cattle plus many wildlife species — Cape buffalo, giraffe, wildebeest — across eastern and southern Africa in the late nineteenth century.
Disease has also been introduced deliberately: European rabbits released in Australia in the eighteenth century exploded in the absence of predators, stripping vegetation and damaging native animal and sheep populations.
Wildlife managers responded with the myxoma virus, which killed more than 99% of the rabbits — though the population eventually rebounded, a fact that becomes important later in this section.
Vocabulary that gets tested: a pandemic is a widespread outbreak spanning multiple countries and even continents, usually said of human populations. An epidemic is a more localized outbreak.
The most notorious human pandemic is bubonic plague, caused by the bacterium Yersinia pestis, spread by fleas and by contact with infected individuals. It is one of a series of Y. pestis plagues over several thousand years, and the "Black Death" was the worst, killing a third of the population of Europe in the fourteenth century.
Two modern benchmarks: the Spanish flu killed more than 50 million people in 1918–19; COVID-19, which erupted in early 2020, had cost more than four million lives as of the end of July 2021, with the total still being counted.
The mechanism: a pandemic occurs when a pathogen becomes established in a population whose individuals lack adequate immune system defenses.
That immunological gap arises two ways. (1) The population has had no prior exposure, typically when a disease agent jumps from one species to another — a zoonotic disease, an increasing human health problem. (2) The pathogen evolves until the host immune system no longer recognizes it, as with influenza.
THE BIG ONE — the reproduction number R₀, pronounced "R naught": the average number of new individuals infected by each infected individual. It is the single measure that decides whether an outbreak grows or dies.
The three cases (Fig 54.29): R₀ > 1 → the disease increases, and the larger R₀ the faster it increases. R₀ = 1 → prevalence stays constant, each case replacing itself. R₀ < 1 → the disease eventually disappears from the population.
Numbers worth memorizing: SARS-CoV-2 was estimated at R₀ between 2 and 3 at the outset of the pandemic; the Delta variant of 2021 was estimated at R₀ between 6 and 9.
The counterexample that proves lethality is not the same as danger: Nipah virus, transmitted to humans from pigs and bats, kills 40% or more of those infected — but its R₀ is only about 0.48, so large outbreaks have never occurred.
The key to disease dynamics is the transmission rate. Pathogens that do not spread easily — not very "contagious" — are unlikely to cause an epidemic no matter how deadly they are.
Reducing transmission is how you stop a pandemic: fewer unprotected sexual interactions combats sexually transmitted diseases such as AIDS; mask-wearing and social distance lower the incidence of airborne diseases such as flu and COVID-19.
Averages can mislead: some individuals transmit far more than others. By one estimate, 10% of people infected with SARS-CoV-2 were responsible for 80% of subsequent infections.
Why some people are superspreaders is unclear, and probably involves both circumstances (attending a crowded indoor event) and biological differences among individuals. Identifying them and preventing the resulting clusters is a critical research area.
Herd immunity: as the immune proportion of a population rises, the transmission rate falls because there are fewer opportunities for spread. Eventually R₀ drops below 1 and the disease disappears — and even non-immune individuals are protected by everyone else's immunity.
The immune fraction needed for herd immunity depends on the pathogen's biology and circumstances, but is often at least 70%.
Herd immunity develops on its own given enough time (assuming individuals can become immune), but at the high cost of a large fraction of the population being exposed and possibly suffering. Note that 40 to 50% of people infected with the COVID-19 virus are asymptomatic.
Vaccines are the alternative: they make a large fraction of the population immune without being infected. Vaccines completely eliminated smallpox and rinderpest and brought many other diseases under control.
Pathogens evolve — even viruses. Any mutation that lets a pathogen infect new individuals more effectively, and thus be passed on, is favored by natural selection.
Evolutionary route 1 — become more contagious: a new SARS-CoV-2 variant appeared in England in late 2020 transmitting at a rate estimated 70% greater than the previously common strain, and the much more contagious Delta variant followed in 2021.
Evolutionary route 2 — escape acquired immunity: because of its high mutation rate, human influenza virus generates substantial variation in its surface HA and NA proteins. Mutations that by chance make the virus unrecognizable to human immune systems are favored — which is exactly why flu vaccines lose effectiveness and must be remade every year.
SARS-CoV-2 mutates at a slower rate than influenza, which gives hope that its vaccines will have longer-lasting effect.
Evolutionary route 3 — become less lethal: hosts that live longer have more opportunity to pass the disease along. After nearly wiping out Australia's rabbits, the myxoma virus evolved to be less lethal, and the rabbit population recovered. Higher virulence is not automatically favored.
Summary logic to carry into an exam question: outbreak size is governed by transmission, transmission is summarized by R₀, R₀ can be lowered by behavior, by herd immunity, or by vaccination — and the pathogen can evolve to push it back up.