Campbell Biology Chapter 53 Review: Population Ecology

53.1 Counting populations and tracking who survives

pp. 1170–1175

On the AP exam? Background

Topic 8.3 tests the idea that births, deaths and movement in or out set a population's size. Dispersion patterns, mark-recapture, life tables and survivorship curves aren't named in the current course, but they show up in labs and in data questions.

In the course: Topic 8.3 Population Ecology (notes, videos and more questions)

Key points

A population is every member of one species in a defined area, close enough to meet, compete and breed. Where its edges lie is often the researcher's call, depending on the species and the study.
Density means individuals per unit of area or volume. Counting everyone is rarely possible, so ecologists estimate it: they count randomly placed sample plots and scale up, count signs like nests or droppings, or use mark-recapture.
In mark-recapture you mark a first catch (s), let them go, then take a second catch (n) and count the marked ones in it (x). The estimate is N = s × n ÷ x. It only works if marked and unmarked animals are equally easy to catch, the marked ones mix back in, and few are born, die, arrive or leave between catches.
Only four things change a population's size: births and immigration add individuals, while deaths and emigration take them away.
Dispersion is how individuals are spaced. Clumped spacing, the most common, comes from patchy resources or living in groups. Uniform spacing usually comes from competition or defended territories. Random spacing appears when individuals neither attract nor avoid each other and conditions are even.
Demography is the study of a population's vital statistics. A life table follows a cohort and records how many survive to each birthday; a reproductive table (fertility schedule) records how many offspring females of each age produce.
A survivorship curve plots survivors (usually on a log scale) against age. Type I: most live into old age. Type II: about the same fraction dies at every age. Type III: most die very young, but those that get through the early losses face much lower death rates afterward.

Key terms (14)

population: All the individuals of one species living in the same area at the same time, sharing resources and able to breed with each other.
population density: How many individuals there are per unit of space, such as snails per square meter or algae cells per milliliter.
dispersion: The spacing pattern of individuals inside a population's range: clumped, uniform or random.
mark-recapture method: A way to estimate population size: mark a first catch, release it, catch again later, and use the share of marked animals in the second catch to work out the total.
immigration: Individuals moving into a population from somewhere else. It raises the population's size.
emigration: Individuals leaving a population for somewhere else. It lowers the population's size.
clumped dispersion: Individuals bunched into groups, usually around patchy resources or because living together helps them.
uniform dispersion: Individuals spaced about evenly, usually because they push each other away by competing or defending space.
random dispersion: Spacing with no pattern: where one individual sits tells you nothing about where its neighbors are.
demography: The study of a population's vital statistics, like birth rates, death rates and age makeup, and how they change.
cohort: A group of individuals all born in the same time period, followed together through their lives.
life table: A table that tracks a cohort from birth and shows how many are alive at each age and what fraction die in each age interval.
survivorship curve: A graph of how many members of a cohort survive to each age. Its shape shows when in life most deaths happen.
reproductive table: A table of how many offspring a female of each age produces on average. It's also called a fertility schedule.

Check yourself: 53.1 Counting populations and tracking who survives

4 questions on 53.1 Counting populations and tracking who survives. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

Biologists net 60 painted turtles in a pond, mark each shell with a small dab of paint and release them. Two weeks later they net 75 turtles, and 15 of them carry paint. Assuming the method's conditions are met, what is the best estimate of the pond's turtle population?

Question 2 of 4

A team estimates a lizard population by mark-recapture, using colored ink dots on the lizards' backs. They later learn that about a third of the lizards shed their skin between the two captures, losing the dot. How does this affect their estimate?

Question 3 of 4

A species of lichen grows only on bare rock outcrops, and the outcrops are scattered across a large grassy meadow. If ecologists mapped every lichen colony in the meadow, which dispersion pattern would they most likely find, and why?

Question 4 of 4

Researchers follow a cohort of 1,000 newly hatched larvae of a marine snail. Age (years) | Number still alive 0 | 1,000 1 | 50 2 | 30 3 | 20 4 | 14 5 | 10 Which survivorship curve do these data fit, and what life history does that suggest?

0 of 4 answered

53.2 Exponential growth with no limits

pp. 1175–1177

On the AP exam? Yes

This is Topic 8.3. $\frac{dN}{dt} = B - D$ and $\frac{dN}{dt} = r_{\max}N$ are both on the AP formula sheet, and you should be able to calculate with them and recognize a J-shaped curve.

In the course: Topic 8.3 Population Ecology (notes, videos and more questions)

Key points

Over any time interval, the change in population size equals births plus immigrants minus deaths minus emigrants. Leaving migration out gives the simple model $\frac{dN}{dt} = B - D$ .
Dividing births and deaths by population size gives per capita rates: b is births per individual per unit of time and m is deaths per individual. Subtracting gives r = b − m, the per capita growth rate.
If r is positive the population grows, if it's negative the population shrinks, and if it's zero you have zero population growth (ZPG). In ZPG, births and deaths still happen; they just cancel out.
With unlimited food, space and other resources, every individual reproduces as fast as the species can, so r reaches its maximum, rₘₐₓ. Growth then follows $\frac{dN}{dt} = r_{\max}N$ .
rₘₐₓ stays the same, but each step adds more individuals than the last, because more individuals are breeding. That's why a plot of N against time makes a J shape that keeps getting steeper.
A larger rₘₐₓ makes the J-curve climb faster. Species that mature quickly and have many young tend to have a high rₘₐₓ.
Exponential growth never lasts long in nature. You see it when a species reaches a new habitat with plenty of resources, or when a population rebounds after a disaster or after hunting stops.

Key terms (8)

per capita birth rate (b): The average number of offspring produced per individual in a set time. 50 births in a year among 1,000 animals gives b = 0.05 per year.
per capita death rate (m): The average number of deaths per individual in a set time, written as a fraction of the population.
r (per capita rate of increase): Births per individual minus deaths per individual (r = b − m). It tells you how fast a population changes relative to its size.
zero population growth (ZPG): When births per individual exactly match deaths per individual, so the population size holds steady even though births and deaths go on.
exponential population growth: Growth with no limits, where the per capita rate stays at its maximum and the number added each period keeps rising.
rₘₐₓ: The highest per capita growth rate a species can reach when nothing holds it back. It's set by traits like age at first breeding and number of young.
J-shaped curve: The shape of exponential growth on a graph of population size against time: flat at first, then rising more and more steeply.
dN/dt: The rate of change of population size, N, with time, t: how many individuals the population gains (or loses) per unit of time.

Check yourself: 53.2 Exponential growth with no limits

4 questions on 53.2 Exponential growth with no limits. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

A population of 2,500 tree frogs has 400 births and 150 deaths in one year. Ignoring migration, what is r, the population's per capita growth rate, for that year?

Question 2 of 4Calculator allowed

A population of algae in a nutrient-rich pond is growing exponentially with rₘₐₓ = 0.3 per day. If the current density is 800 cells per milliliter, what is dN/dt?

Question 3 of 4Calculator allowed

A bacterial culture in fresh medium is counted every hour. Time (h) | Cells per mL 0 | 1,000 1 | 1,500 2 | 2,250 3 | 3,375 4 | 5,063 Which statement best describes this growth?

Question 4 of 4

In a lake population of perch, births per individual and deaths per individual are both 0.045 per year, and no fish enter or leave. Which statement is correct?

0 of 4 answered

53.3 Logistic growth and carrying capacity

pp. 1177–1179

On the AP exam? Yes

This is Topic 8.4. The logistic equation $\frac{dN}{dt} = r_{\max}N\frac{K - N}{K}$ is on the AP formula sheet; expect to calculate dN/dt and read S-shaped curves. The term "Allee effect" isn't required.

In the course: Topic 8.4 Effect of Density on Populations (notes, videos and more questions)

Key points

Carrying capacity (K) is the largest population an environment can support over the long run. Limiting resources like food, water, nest sites or shelter set it, so K differs from place to place and can change from year to year.
As a population gets crowded, each individual gets less, so births per individual drop, deaths per individual climb, or both. Either way, r shrinks.
The logistic model adds a brake to exponential growth: $\frac{dN}{dt} = r_{\max}N\frac{K - N}{K}$ . The fraction (K − N)/K is the share of the carrying capacity that's still unused.
When N is small, (K − N)/K is nearly 1 and growth is almost exponential. When N reaches K, growth stops. If N goes above K, the fraction turns negative and the population shrinks.
Plotted against time, logistic growth makes an S-shaped (sigmoid) curve. The most individuals are added per unit of time at N = K/2, where there are plenty of breeders and still plenty of room.
Real populations often don't follow the curve. Effects of crowding can arrive late, so a population overshoots K and swings around it. And in some species, individuals in very small populations do worse, for example because mates are hard to find (the Allee effect).
Even so, the model is a useful tool in conservation and wildlife management: for predicting how fast a depleted population can recover, setting sustainable harvests, and judging how small a population can get before it's in trouble.

Key terms (8)

carrying capacity (K): The largest population of a species that an environment can support over time with the resources it has. It can rise or fall as conditions change.
limiting resource: A resource in short enough supply that it caps a population's growth, like nest holes for cavity-nesting birds.
logistic population growth: Growth that slows as a population nears its carrying capacity, because the per capita rate drops as N rises.
sigmoid curve: The S-shaped graph of logistic growth: slow at first, steepest in the middle, then flattening at K.
(K − N)/K: The part of the logistic equation that measures unused carrying capacity. It's near 1 when the population is small and 0 when N = K.
overshoot: When a population climbs above its carrying capacity, usually because crowding takes time to affect births and deaths.
time lag: A delay between a change in population size and its effect on birth or death rates. Lags make populations swing above and below K.
Allee effect: When individuals in a very small population survive or reproduce worse than in a larger one, for example because mates are hard to find.

Check yourself: 53.3 Logistic growth and carrying capacity

4 questions on 53.3 Logistic growth and carrying capacity. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

A rabbit population on a ranch has rₘₐₓ = 0.5 per year and a carrying capacity of 800. Using $\frac{dN}{dt} = r_{\max}N\frac{K - N}{K}$ , what is the population's growth rate when N = 200?

Question 2 of 4Calculator allowed

A student grows duckweed (a tiny floating plant) in a tank and counts the plants every two days. Day | Number of plants 0 | 20 2 | 60 4 | 170 6 | 380 8 | 560 10 | 640 12 | 660 14 | 665 During which interval was the population adding plants fastest?

Question 3 of 4

Hermit crabs must live in empty snail shells, and a crab without one is soon eaten. Two rocky coves have the same amount of crab food, but cove 1 has about 300 usable empty shells and cove 2 has about 900. Once both crab populations stop growing, which outcome is most likely?

Question 4 of 4

A rare orchid depends on bees that visit only patches with many flowers. Patches with fewer than 20 orchids set almost no seed, while larger patches set plenty. Which assumption of the logistic model does this violate?

0 of 4 answered

53.4 How selection shapes life histories

pp. 1179–1181

On the AP exam? Background

Semelparity, iteroparity and r- and K-selection aren't in the current course. The tested idea underneath is Topic 7.2: natural selection shapes traits, including how organisms split limited energy between surviving and reproducing.

In the course: Topic 7.2 Natural Selection, Topic 8.3 Population Ecology (notes, videos and more questions)

Key points

Life history means the timing of an organism's growth, breeding and death. Three numbers capture much of it: the age when breeding starts, how many times it breeds, and how many young it makes each time.
Life histories aren't choices. They evolve by natural selection: the schedule that leaves the most surviving offspring in a given environment becomes common.
Semelparous organisms breed once, often producing a huge number of offspring, and then die. Iteroparous organisms breed again and again over their lives.
One leading hypothesis: breeding once tends to be favored when adults rarely live to another breeding season and offspring survival is low or unpredictable. Breeding repeatedly tends to be favored when adults usually survive from one season to the next.
Energy and nutrients are limited, so traits trade off. Putting more into reproduction now can lower a parent's survival or future breeding, and making more offspring usually means putting less into each one.
When young face heavy, unavoidable losses, selection often favors many small offspring. When competition is intense, fewer, larger, better-provisioned offspring tend to do better.
r-selection (favoring fast reproduction when the population is far below K) and K-selection (favoring competitive ability near K) describe two ends of a range. Ecologists now treat them as a rough sketch; modern life history theory focuses on how survival at different ages shapes these trade-offs.

Key terms (9)

life history: The pattern of when an organism matures, how often and how heavily it breeds, and how long it lives, all shaped by natural selection.
semelparity: Reproducing only once in a lifetime, usually with a large burst of offspring, followed by death. It's also called big-bang reproduction.
iteroparity: Reproducing several or many times over a lifetime.
trade-off: A situation where putting more energy into one trait (like making offspring) leaves less for another (like the parent's own survival).
parental investment: The time, energy and resources a parent puts into each offspring, such as yolk in an egg or weeks of feeding.
age at first reproduction: How old an organism is when it first breeds. Breeding earlier speeds up population growth but can cost the parent.
clutch size: The number of eggs laid in one breeding attempt. For mammals, the matching idea is litter size.
r-selection: Selection favoring fast reproduction when a population is far below carrying capacity, such as early breeding and many offspring.
K-selection: Selection for traits that help individuals compete and survive when a population is near carrying capacity, such as fewer, well-provisioned offspring.

Check yourself: 53.4 How selection shapes life histories

4 questions on 53.4 How selection shapes life histories. Pick an answer to see if you got it, and why.

Question 1 of 4

A species of bamboo grows by spreading underground stems for several decades. Then all the plants in a stand flower at once, release seeds and die. Which term best describes this bamboo's reproduction?

Question 2 of 4

In a laboratory, researchers bred one set of fruit fly lines only from eggs laid in the first week of adult life ("early" lines) and kept matched control lines. After many generations (invented data): Lines | Eggs per female in week 1 | Average adult life span (days) Early | 95 | 31 Control | 60 | 44 Which explanation best accounts for these data?

Question 3 of 4

A small fish lives in many separate streams. Where the main predator eats mostly adult fish, the fish mature young and put a lot of energy into each early spawning. Where the main predator eats only juveniles, they mature later and spawn smaller batches over more years. The differences persist when fish from both kinds of stream are raised side by side in the lab. Which explanation fits best?

Question 4 of 4

Two related marine snails live on the same coast. Species 1 releases about 50,000 tiny eggs that drift in the plankton for weeks. Species 2 lays 30 large, yolk-rich eggs in a tough capsule, and they hatch as crawling juveniles. Which prediction is most reasonable?

0 of 4 answered

53.5 What holds populations in check

pp. 1182–1187

On the AP exam? Yes

Density-dependent and density-independent factors are Topic 8.4, and predators' effects on prey numbers fit Topic 8.5. You don't need the term "metapopulation" or the details of any particular population cycle.

In the course: Topic 8.4 Effect of Density on Populations, Topic 8.5 Community Ecology (notes, videos and more questions)

Key points

A birth or death rate that stays the same whatever the density is density independent. A death rate that climbs with density, or a birth rate that drops with it, is density dependent.
Density-independent factors, such as a flood, a hard frost or a heat wave, hit sparse and crowded populations about equally hard, removing roughly the same share either way. They can cause crashes, but on their own they can't hold a population at a steady size.
Density-dependent factors work as negative feedback. More crowding means fewer births or more deaths, which slows growth; a smaller population has more to go around, so it grows again. The population tends to settle where births per individual equal deaths per individual.
Common density-dependent factors: competition for food, water or nutrients; territoriality, where space itself is limited and surplus individuals can't breed; disease and parasites, which spread faster in crowds; predators that catch more of a prey as it becomes common; buildup of toxic wastes; and stress-related changes in the body that cut reproduction.
Every population fluctuates. Weather, food, parasites and predators combine, and a population may be limited by different factors in different years.
Some populations rise and fall in regular cycles. Field experiments that add food or keep predators out help separate causes. In predator-prey cycles, predator peaks usually come a little after prey peaks.
Movement matters too. A metapopulation is a network of local populations in separate habitat patches linked by individuals moving between them. Patches can lose their population and later be recolonized, so the species survives across the network.

Key terms (9)

density-independent factor: Something that changes birth or death rates by the same proportion no matter how crowded the population is, like a sudden freeze.
density-dependent factor: Something whose effect on birth or death rates grows stronger as a population gets more crowded, like disease or competition.
negative feedback: A response that pushes a change back the other way. Crowding lowers births and raises deaths, so the population stops growing.
equilibrium density: The population density at which births per individual equal deaths per individual, so the population stops changing.
intraspecific competition: Competition among members of the same species for the same limited resources.
territoriality: Defending a space against others of your species. When territories run out, extra individuals can't breed, which caps the population.
population dynamics: The ups and downs in a population's size over time and from place to place, and the factors behind them.
population cycle: A fairly regular rise and fall in population size that repeats every few years.
metapopulation: A group of local populations in separate habitat patches, linked by individuals moving between patches.

Check yourself: 53.5 What holds populations in check

4 questions on 53.5 What holds populations in check. Pick an answer to see if you got it, and why.

Question 1 of 4

Ecologists sample a rodent population at sites with different densities and record the percentage infected by a parasitic worm. Rodents per hectare | Percent infected 5 | 4 20 | 15 60 | 41 What kind of factor is this parasite, and why?

Question 2 of 4Calculator allowed

In a model of a pond snail population, the per capita birth rate stays at b = 0.5 per year at every density, while the per capita death rate rises with crowding: m = 0.1 + 0.001N (per year). Ignoring migration, at what population size does the population stop growing?

Question 3 of 4

In a woodland songbird population, researchers removed the males holding breeding territories from one area. Within days, other males that hadn't been breeding moved in, set up territories and attracted mates. What does this result most strongly suggest?

Question 4 of 4

Researchers counted aphids (prey) and ladybird beetles (predators) on greenhouse plants each week. Week | Aphids per plant | Ladybirds per plant 1 | 20 | 0.5 2 | 60 | 0.6 3 | 150 | 1.0 4 | 210 | 2.2 5 | 120 | 3.5 6 | 40 | 3.0 7 | 15 | 1.6 8 | 25 | 0.8 What best explains the fall in ladybirds from week 5 to week 7?

0 of 4 answered

53.6 Human population growth

pp. 1187–1191

On the AP exam? Background

Human demography (the demographic transition, age-structure diagrams, ecological footprints) belongs to AP Environmental Science, not AP Biology. It's still good practice in reading growth data for Topics 8.3 and 8.4, and food supply and human resource use tie to Topics 8.2 and 8.7. The book's numbers are from 2009; updated figures are given here.

In the course: Topic 8.2 Energy Flow Through Ecosystems, Topic 8.3 Population Ecology, Topic 8.4 Effect of Density on Populations, Topic 8.7 Disruptions in Ecosystems (notes, videos and more questions)

Key points

For most of history, human numbers grew slowly. From about the 1600s, and much faster after the Industrial Revolution, growth was roughly exponential, and the world passed 8 billion people in 2022.
The global growth rate peaked at a little over 2% a year in the 1960s and has since fallen below 1%, so human growth is no longer exponential. Because the population is so large, it still adds about 70 million people a year.
The demographic transition is a country's shift from many births and many deaths each year to few of both as medicine, clean water and schooling improve, with girls' and women's education playing an especially big part. Death rates usually fall first, so the population grows fast until birth rates catch up.
Total fertility rate is the average number of children per woman. About 2.1 is replacement level where few children die young. Many countries are now below replacement and will shrink unless immigration makes up the difference.
An age-structure diagram shows the share of people in each age group. A wide base means many young people about to reach childbearing age, so the population keeps growing for decades even if families get smaller (population momentum). A narrow base points to slowing growth or decline.
Infant mortality and life expectancy differ hugely among countries and track living conditions. Where many babies die, families tend to have more children.
Earth's carrying capacity for people can't be pinned to one number, because it depends on technology, diet and how much each person uses. An ecological footprint, the land and water needed to supply someone's resources and absorb their wastes, is one way to compare demands.

Key terms (9)

demographic transition: A country's shift toward fewer births and fewer deaths as living conditions improve. Death rates usually drop first.
total fertility rate: The average number of children a woman would have over her lifetime at current birth rates.
replacement fertility: The fertility rate at which each generation just replaces itself, about 2.1 children per woman where child deaths are rare.
age structure: How a population is divided among age groups, from babies to the elderly.
age-structure diagram: A chart, often shaped like a pyramid, with bars showing the share of males and females in each age group.
population momentum: The tendency of a young population to keep growing for years after fertility falls, because so many people are entering their childbearing years.
infant mortality: The number of babies who die before their first birthday for every 1,000 live births.
life expectancy at birth: The average number of years a newborn is expected to live if current death rates stay the same.
ecological footprint: An estimate of how much biologically productive land and sea it takes to keep up with what a person or group consumes and to absorb the waste, especially CO₂, it gives off.

Check yourself: 53.6 Human population growth

4 questions on 53.6 Human population growth. Pick an answer to see if you got it, and why.

Question 1 of 4

The table shows the age makeup of two countries (percent of population). Age group | Country P | Country Q 0–14 | 42 | 13 15–44 | 44 | 36 45–64 | 10 | 29 65 and older | 4 | 22 Suppose both countries reach replacement fertility next year and have no migration. Which prediction is most reasonable?

Question 2 of 4Calculator allowed

Birth and death rates for a country over a century, per 1,000 people per year: Year | Births | Deaths 1900 | 40 | 35 1940 | 41 | 22 1970 | 33 | 12 2000 | 16 | 9 2020 | 11 | 10 Ignoring migration, in which year was the population growing fastest in percentage terms?

Question 3 of 4

Replacement-level fertility in countries with low child mortality is about 2.1 children per woman rather than exactly 2.0. Why?

Question 4 of 4Calculator allowed

An island has 6,000 hectares of productive land and water and 2,000 residents. The average resident's ecological footprint is 4.5 hectares. Which conclusion follows?

0 of 4 answered