2 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 2
Which of the following is a population?
Answer and reasoning
AAll the plants and animals living together in one meadow A student who thinks a population includes every organism in an area picks this. Organisms of many species living together form a community.
BThe field mice of one species living in one meadowCorrect A population is made of individuals of the same species living in the same area, where they can interact.
CAll the mice, of several species, that share one meadow A student who groups similar organisms regardless of species picks this. Each mouse species in the meadow is a separate population.
DEvery field mouse of one species, wherever it lives on Earth A student who thinks a population is a whole species picks this. A population is the members of a species in one area, not everywhere.
A few cells of a hypothetical species of bacterium are placed in a large volume of fresh nutrient medium, where they reproduce without constraints for the first several hours. Which graph best represents the population size (N) during this period?
Answer and reasoning
AGraph 1 A student who thinks an unconstrained population adds the same number of individuals in each time interval picks the straight-line graph. Each new cell also divides, so the number added per hour increases.
BGraph 2 A student who thinks a population adds the most individuals when it is small picks the graph that rises steeply at first and then more slowly. Under exponential growth the slope increases as N increases.
CGraph 3Correct With unconstrained reproduction, dN/dt = rmax N, so the number of cells added per hour grows as N grows. The correct graph rises slowly at first and then more and more steeply: a J-shaped curve.
DGraph 4 A student who thinks every population levels off picks the S-shaped graph. During unconstrained reproduction the population does not level off; its growth keeps speeding up.
In preparation: 0 of 2 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
8.3.A.1 Population Fix
Population
A group of individual organisms of the same species living in the same area at the same time, which interact with one another and with their environment.
Students often think A population is all the organisms, of every species, living together in one place. In fact No. All the organisms of different species living and interacting in one area form a community. A population is the individuals of one species in an area.
Students often think A population is a group of similar organisms, such as all the mice in an area, even if they belong to different species. In fact No. A population is made of individuals of the same species. Several species of mouse in one field are several populations.
8.3.A.2 Adaptation for obtaining and using energy and matter Fix
Adaptation for obtaining and using energy and matter
An inherited characteristic that helps organisms obtain or use energy and matter in a particular environment, such as a low metabolic rate that reduces the energy an animal must obtain where food is scarce.
Birth rate (B) and death rate (D)
The number of individuals born into, and the number dying in, a population per unit time (for example, per year).
Population growth rate, dN/dt = B − D
The change in population size per unit time equals births minus deaths (when immigration and emigration are negligible). It is positive when births exceed deaths, zero when they are equal, and negative when deaths exceed births.
Population size (N) versus growth rate (dN/dt)
N is the number of individuals at a given time; dN/dt is how fast N is changing. A large population can be shrinking, and a population whose growth rate is falling but still positive is still getting larger.
Exponential growth, dN/dt = rmax N
When reproduction is without constraints, the population grows at its maximum per capita growth rate, rmax. The number of individuals added per unit time is proportional to N, so it increases as the population grows, and the population size curve is J-shaped.
Maximum per capita growth rate (rmax)
The largest possible growth rate per individual of a population, reached when reproduction is not constrained. During exponential growth rmax stays the same, so the population increases by the same factor in each equal time interval (for example, doubling every hour).
Students often think Energy is recycled within an organism, so the energy released in its cells can be used many times over. In fact No. Energy released in cells is used for work and is eventually lost as heat; it cannot be reused. Organisms must keep obtaining energy from food (or light, in producers).
Students often think Organisms convert the matter in their food into energy, so matter disappears as energy is made. In fact No. Organisms release energy from food molecules by breaking them down and rearranging their atoms into other molecules, such as CO₂ and H₂O, that have less chemical energy. The atoms are kept; matter is not converted into energy.
7 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 7
A student plans to investigate how soil moisture affects the survival of seedlings in a population of a hypothetical species of wildflower. She will sow 50 seeds in each of 12 plots, water 6 plots heavily and 6 plots lightly, and count the surviving seedlings after 6 weeks. Because of where her hose reaches, the 6 heavily watered plots are all in the shade of a building and the 6 lightly watered plots are all in full sun. Which evaluation of the plan is correct?
Answer and reasoning
AValid: changing water and light together makes the effect larger and easier to see. A student who thinks changing two factors at once helps show an effect picks this. When water and light change together, a difference cannot be assigned to water.
BValid: with 50 seeds in each plot, other differences between plots average out. A student who thinks a large sample cancels any difference between groups picks this. Every heavily watered plot is shaded, so the shade effect is the same in all of them and does not average out.
CInvalid: the effect of light on survival cannot be separated from that of water.Correct Light differs between the two groups of plots along with water, so any difference in seedling survival could be caused by either factor. Only the amount of water should differ between the groups.
DInvalid only because there is no plot left without any water to act as a control. A student who thinks an experiment is controlled as long as one group gets no treatment picks this. Adding an unwatered plot would not fix the problem that light differs between the two watering treatments.
Individuals of a hypothetical species of fish that lives in a cold, deep lake, where food is scarce, have a lower metabolic rate than individuals of a closely related species from a warm lake where food is plentiful, when both are measured at the same temperature. Which statement best explains how the low metabolic rate is an adaptation to the cold, deep lake?
Answer and reasoning
AIt reduces the energy each fish must obtain from scarce food in order to stay alive.Correct A lower metabolic rate means each fish uses less energy per unit time, so it needs less food to meet its energy needs. Where food is scarce, this helps individuals survive and reproduce.
BIt lets each fish reuse the energy released in its cells many times over. A student who thinks energy is recycled within an organism picks this. Energy released in cells is used and lost as heat; it must be replaced from food.
CIt lets each fish turn more of the matter in its food directly into energy. A student who thinks food matter is converted into energy picks this. Organisms release energy by breaking down food molecules, and the atoms are kept in other molecules, such as CO₂ and H₂O.
DIt leaves more food in the lake for other species, which benefits the ecosystem. A student who thinks adaptations exist to benefit the ecosystem picks this. An adaptation is favored because it helps the individuals that have it survive and reproduce.
The table shows data for a population of a hypothetical species of deer in a forest. Immigration and emigration were negligible. Using dN/dt = B − D, what was the rate of change in the size of the population during 2021?
Answer and reasoning
A210 deer per year A student who thinks births alone determine population growth picks this. The 90 deaths must be subtracted: 210 − 90 = 120 deer per year.
B300 deer per year A student who counts every birth and death as a change picks this: 210 + 90 = 300. Births add deer and deaths remove them, so the net change is 210 − 90.
C620 deer per year A student who confuses population size with its rate of change picks this. 620 is the number of deer at the start of 2021; the rate of change during 2021 is B − D = 120 deer per year.
D120 deer per yearCorrect dN/dt = B − D = 210 − 90 = 120 deer per year. This matches the change in N from 620 at the start of 2021 to 740 at the start of 2022.
Working For 2021: B = 210 deer, D = 90 deer. dN/dt = B − D = 210 − 90 = 120 deer per year. Check: N rises from 620 to 740, a change of 120.
For several years, a population of a hypothetical species of rabbit on a large island had the same number of births as deaths each year, and immigration and emigration were negligible. A new disease then increased the number of deaths each year, while the number of births each year stayed the same. Which prediction about the size of the population over the next few years is best supported?
Answer and reasoning
AIt will decrease, because the number of deaths each year will now exceed the births.Correct Before the disease, B = D, so dN/dt = 0. With more deaths and the same births, dN/dt = B − D is negative, so the population will decrease.
BIt will stay the same size, because the number of births each year has not changed. A student who thinks births alone determine population growth picks this. dN/dt = B − D, so a rise in deaths makes the change negative even though births are unchanged.
CIt will stay the same size, because populations stay balanced with their environment. A student who thinks populations always stay balanced picks this. A population stays the same size only when births equal deaths; here deaths now exceed births.
DIt will increase, because the total number of births and deaths has risen. A student who counts every birth and death as an increase in the population picks this: more deaths make B + D larger. Deaths remove individuals, so dN/dt = B − D is now negative and the population will decrease.
A small number of cells of a hypothetical species of bacterium were placed in a large volume of nutrient-rich medium, where they reproduced without constraints. The graph shows the population size over 5 hours. Which statement correctly describes the data?
Answer and reasoning
AThe same number of cells was added in each hour of the experiment. A student who expects unconstrained growth to be linear picks this. The numbers added were 1, 2, 4, 8 and 16 thousand, not the same each hour.
BThe number of cells added in each hour was smaller than in the hour before. A student who thinks a population adds the most individuals when it is small picks this. The graph shows the opposite: 1 thousand cells were added in the first hour and 16 thousand in the last.
CThe number of cells added in each hour was larger than in the hour before.Correct The population went 1, 2, 4, 8, 16, 32 thousand, so the number added each hour was 1, 2, 4, 8 and then 16 thousand: larger every hour. This is exponential growth.
DEach cell divided more often in each hour than it had in the hour before. A student who thinks individuals reproduce faster in larger populations picks this. The population doubled every hour, which means each cell divided once per hour throughout; the per capita rate stayed the same.
The graph shows the size of a population of a hypothetical species of beetle growing exponentially, with a constant rmax, on an island with abundant resources. The dashed lines show the slope of the curve at points P and Q. Which statement best explains why the slope is greater at Q than at P?
Answer and reasoning
AThe slope equals births plus deaths, and both of these are greater at Q than P. A student who counts every birth and death as a change picks this. Births add individuals and deaths remove them, so the slope is dN/dt = B − D, not B + D; with rmax constant, B − D = rmax N, which is greater at Q because N is greater.
BWith rmax constant, the slope is proportional to N, so it is greater at Q.Correct The slope of the curve is dN/dt. With rmax constant, dN/dt = rmax N is proportional to N, so where N is four times as large (800 at Q compared with 200 at P), the slope is four times as steep.
CThe slope at any point equals N, so it is greater wherever N is greater. A student who confuses population size with its rate of change picks this. The slope is dN/dt = rmax N, not N itself; at Q, N is 800 but the slope is far smaller than 800 beetles per year.
DMore beetles are born at Q than at P, and deaths do not affect the slope. A student who thinks births alone determine population growth picks this. The slope depends on births minus deaths; rmax is the per capita rate of births minus deaths.
A researcher claims that a population of a hypothetical species of lizard on a small island is decreasing in size. Immigration and emigration are negligible. Which observation would best support the researcher's claim?
Answer and reasoning
AFewer lizards were born this year than were born last year. A student who thinks births alone determine population change picks this. Fewer births than last year can still be more births than deaths, in which case the population is growing.
BThe population grew more slowly this year than it did last year. A student who thinks slower growth means a smaller population picks this. A population that grows more slowly is still growing; it shrinks only when deaths exceed births.
CThe island has fewer lizards than a similar island nearby has. A student who confuses population size with its rate of change picks this. A small population can be growing; size alone says nothing about whether N is increasing or decreasing.
DIn each of the last three years, more lizards died than were born.Correct When deaths exceed births, dN/dt = B − D is negative, so the population is getting smaller. This was true in each of three years.
Compiled from the AP Biology Course and Exam Description (effective Fall 2025) and our question bank · Specialist review in progress. How these pages are made · Free, no account
