3 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 3
The table shows the number of individuals of each of four species in two communities, X and Y. Using Simpson's Diversity Index, what is the diversity index of community Y?
Answer and reasoning
A0.73 A student who leaves out the '1 −' picks this: Σ(n/N)² = 0.73. The index is 1 − 0.73 = 0.27.
B0.00 A student who adds the proportions without squaring them picks this: 1 − (0.85 + 0.05 + 0.05 + 0.05) = 0. Each proportion must be squared before adding.
C0.75 A student who thinks diversity depends only on the number of species picks this, giving Y the same value as X: 1 − 4 × 0.25² = 0.75. Y has the same four species, but species A makes up 85% of its individuals.
D0.27Correct N = 100. Σ(n/N)² = 0.85² + 3 × 0.05² = 0.7225 + 0.0075 = 0.73, so the index = 1 − 0.73 = 0.27. Community X, with the same four species in equal numbers, has an index of 0.75; community Y, dominated by species A, is much less diverse and so is likely to be less resilient.
Working Community Y: N = 85 + 5 + 5 + 5 = 100. Proportions n/N: 0.85, 0.05, 0.05, 0.05. Σ(n/N)² = 0.7225 + 0.0025 + 0.0025 + 0.0025 = 0.73. Diversity Index = 1 − 0.73 = 0.27. Distractors: reporting Σ(n/N)² without subtracting from 1 gives 0.73; adding the proportions without squaring gives 1 − 1.00 = 0.00; treating the four species as equally abundant, as in community X, gives 1 − 4 × 0.25² = 0.75.
Soil eroded from nearby farms makes the water of a lake cloudy for several years, so much less light reaches the plants and algae growing in the lake. The number of animal species in the lake then decreases. Which explanation best accounts for the decrease?
Answer and reasoning
ALess light harms only plants and algae, so only animals that eat them can decline A student who thinks a change affects only directly linked species picks this. Predators depend on herbivores, which depend on producers, so a loss of producers affects every trophic level.
BLess light reduces photosynthesis, so less energy is available at every trophic levelCorrect Light is an essential abiotic factor for the producers. With less light, plants and algae photosynthesize less, so less energy enters the food web; herbivores, and the predators that eat them, decline in turn, and fewer animal species can be supported.
CWithout light, the energy released by animals is not recycled back to the plants A student who thinks energy is recycled in ecosystems picks this. Energy enters as light captured by producers and leaves as heat; it is not passed back to plants.
DCloudy water upsets the natural balance, so animals decline until it restores itself A student who thinks ecosystems are held in a natural balance that is always restored picks this. Nothing guarantees a return: the animals decline because less light means less photosynthesis and less energy for every trophic level, and they recover only if the producers do.
Which statement best describes a keystone species?
Answer and reasoning
AIt is the most abundant species, so it has the largest effect A student who thinks a species' importance matches its abundance picks this. Keystone species are often rare; their effects are out of proportion to their numbers.
BIt is a top predator, which holds the most energy in the web A student who thinks top predators hold the most energy picks this. Top predators hold the least energy; some keystone species are predators, but that is not what defines them.
CIt is the producer that every other species uses for food A student who links 'keystone' with the base of a food web picks this. Producers support diversity, but a keystone species is defined by an effect out of proportion to its abundance.
DIt is a species whose effect is large relative to its abundanceCorrect A keystone species has an effect on its ecosystem that is disproportionately large relative to its abundance. When it is removed, the ecosystem often collapses.
In preparation: 0 of 3 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
8.6.A.1 Resilience of an ecosystem Fix
Resilience of an ecosystem
The ability of an ecosystem to keep its structure and functioning when the environment changes, or to recover them after a disturbance. Ecosystems with fewer component parts, and with little diversity among those parts, are often less resilient.
Natural and artificial ecosystems
Natural ecosystems form without human design; artificial ecosystems, such as crop fields, plantations and fish farms, are set up and managed by humans. The link between diversity and resilience applies to both.
Genetic uniformity
Little genetic diversity among the individuals of a population, as in a crop of one variety. Because the individuals share the same alleles, a disease or environmental change that harms one is likely to harm most of them.
Species richness and relative abundance
Species richness is the number of species in a community; relative abundance is the proportion of all individuals that belong to each species. A community dominated by one species is less diverse than one with the same species in more even numbers.
Simpson's Diversity Index
Diversity Index = 1 − Σ(n/N)², where n is the number of organisms of one species and N is the total number of organisms of all species. It takes into account both the number of species and their relative abundance; values closer to 1 indicate greater diversity.
Students often think Simpson's Diversity Index is Σ(n/N)²; the final step of subtracting from 1 can be left out. In fact No. The index is 1 − Σ(n/N)². The sum Σ(n/N)² is larger when one species dominates, so subtracting it from 1 gives a value that is larger when diversity is greater.
Students often think The proportions n/N are added directly, without squaring them. In fact No. Each proportion is squared before the proportions are added. The proportions themselves always add up to 1, so leaving out the squaring would give an index of 0 for every community.
8.6.A.2 Producers Fix
Producers
Organisms, such as plants and algae, that capture energy (for example, by photosynthesis) and make organic compounds. They are the source of energy and matter for every other trophic level, so their loss reduces the diversity an ecosystem can support.
Essential abiotic and biotic factors
Nonliving factors, such as light, water and temperature, and living factors, such as food sources and habitat-forming organisms, that many species in an ecosystem depend on. Changes in them affect the species that depend on them directly and, through feeding relationships, other species too.
Students often think Energy is recycled in an ecosystem: consumers pass energy back to producers, which use it again. In fact No. Energy enters ecosystems mainly as light captured by producers and leaves as heat; it flows through the ecosystem and is not recycled. Matter, such as carbon and nitrogen, is recycled.
Students often think Top predators hold the most energy in a food web, because they eat everything below them. In fact No. Only part of the energy at each trophic level is passed to the next, so the top trophic level has the least energy. Producers hold the most.
8.6.B.1 Keystone species Fix
Keystone species
A species whose effect on its ecosystem is disproportionately large relative to its abundance. Keystone species help maintain an ecosystem's diversity; when they are removed, the ecosystem often collapses.
Indirect effects in a food web
Effects of one species on species it does not eat or is not eaten by, passed through the species between them; for example, a predator that limits an herbivore can allow the herbivore's food plants, and the species that depend on them, to persist.
Ecosystem collapse
A large loss of the species and interactions that make up an ecosystem's structure, so that it changes to a much simpler state. The removal of a keystone species often leads to collapse.
Students often think A change in one species affects only the species directly linked to it by feeding; species with no direct link are unaffected. In fact No. Effects spread through a food web. A change in one species can affect species it has no direct feeding link with, through the species in between.
Students often think A species' importance to its ecosystem is proportional to its abundance, so rare species have little effect and the most abundant species matter most. In fact No. Abundant species can be important, but keystone species have effects on their ecosystems that are disproportionately large relative to their abundance; a rare predator can control the structure of a whole community.
6 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 6
A farmer plants one field with a single, genetically uniform variety of a hypothetical grain crop and a second field with a mixture of many genetically different varieties of the same crop. A new fungal disease that kills susceptible plants reaches both fields. Which prediction is best supported?
Answer and reasoning
APlants in the uniform field become resistant during the outbreak, as they adapt to the threat A student who thinks individuals adapt to a threat because they need to picks this. A plant does not acquire resistance because it needs it; a field of one susceptible variety has few or no resistant plants.
BNeither field loses many plants for long, as each field returns to its natural balance A student who thinks ecosystems always return to balance picks this. A field of one susceptible variety can lose most of its plants; nothing guarantees its return.
CThe uniform field is likely to lose more plants, as they share one susceptibilityCorrect In the uniform field, every plant carries the same alleles, so if the variety is susceptible, nearly all the plants are. In the mixed field, some varieties are likely to carry alleles that give resistance, so more plants survive. Less diversity among the parts makes the field less resilient.
DBoth fields lose about the same share of plants, as members of a species are genetically alike A student who thinks all members of a species are genetically alike picks this. Varieties of a crop differ in many alleles, including alleles that affect resistance to disease.
In a hypothetical community, researchers removed one species at a time from separate, similar plots. The table shows each species' share of the community's biomass and the number of other species lost from the plot within three years of its removal. Which claim is best supported by the data?
Answer and reasoning
ASpecies Y's effect on the community is far larger than its share of the biomassCorrect Species Y makes up only 4% of the biomass, yet removing it led to the loss of 9 other species, far more than for any other species. An effect disproportionate to abundance is the mark of a keystone species.
BSpecies W has the largest effect, as it makes up the largest share of the biomass A student who thinks a species' importance matches its abundance picks this. Removing W, the most abundant species, led to the loss of only 1 species, against 9 for Y.
CHaving a small share of the biomass causes a species to have a large effect A student who reads a pattern as cause and effect picks this. One species, Y, has both features; the data do not show that low abundance causes large effects, and they do not show it for species in general.
DRemoving any one species causes the community to lose many of its species A student who thinks removing any species makes an ecosystem collapse picks this. Removing X led to no losses and removing W to one; only Y's removal led to many.
On a rocky shore, a predatory sea star feeds mainly on mussels. Mussels are the strongest competitors for space on the rocks, where many species of invertebrates and algae must attach to live. Sea stars are much less abundant than mussels. Which prediction about the long-term effect of removing all the sea stars is best supported?
Answer and reasoning
ASpecies diversity rises, as the species the sea star ate are no longer being eaten A student who thinks removing a predator increases diversity picks this. Freed from predation, mussels outcompete other species for space, so diversity falls.
BMussels spread over the rock, and many species that need space there disappearCorrect With no sea stars eating them, mussels, the strongest competitors for space, cover more and more of the rock and crowd out the species that need to attach there. The community loses many species: the sea star is a keystone species whose effect is far larger than its abundance.
CMussels increase, but all the species that the sea star does not eat are unaffected A student who thinks a change affects only directly linked species picks this. The spreading mussels take the space that other species need, so they are affected indirectly.
DLittle changes, as sea stars are too few to affect the community as a whole A student who thinks rare species have little effect picks this. A predator that holds back the dominant competitor can have effects far larger than its abundance.
Researchers hypothesize that species K, a predatory fish in a hypothetical stream, is a keystone species. They remove all K from five stretches of the stream and leave five similar stretches untouched, then count the species in all ten stretches for several years. Which result, if observed, would best support the hypothesis?
Answer and reasoning
AThe number of species fell sharply in both the stretches without K and untouched ones A student who thinks any change after a removal shows its effect picks this. A fall in the untouched stretches too means that something other than the removal, such as weather, caused it.
BIn stretches without K, only the prey species of K changed in number A student who thinks a species affects only those it is directly linked to picks this. A keystone species' removal has effects that spread well beyond its prey, often collapsing the community.
CIn untouched stretches, K made up more of the biomass than any other species A student who thinks the most abundant species is the most important picks this. A keystone species is defined by an effect out of proportion to its abundance, not by being abundant.
DThe number of species fell sharply in stretches without K but not in untouched onesCorrect If K is a keystone species, removing it should cause a large change in the community, often a collapse, that does not occur where K remains. A sharp fall in species number only where K was removed shows a large effect of K.
The food web shows some feeding relationships in a kelp forest. Based on the food web, which statement best describes the role of sea otters in maintaining the diversity of this community?
Answer and reasoning
AOtters limit urchins, so kelp, the base of the food web, is not overgrazedCorrect Energy flows from kelp to urchins and from urchins to otters. By eating urchins, otters keep urchin numbers low enough that kelp is not overgrazed; kelp is the producer on which the invertebrates, and the fish that eat them, depend. Through this indirect effect, otters help maintain the community's diversity.
BOtters affect only urchins, as no arrow links them directly to other species A student who thinks a species affects only those it is directly linked to picks this. Through urchins, otters affect kelp and, through kelp, the invertebrates and fish.
COtters hold the most energy in the web, as they are at the top of the food chain A student who thinks top predators hold the most energy picks this. Energy decreases at each step up a food chain; kelp, the producer, holds the most.
DOtters return the energy they eat to the kelp, which then uses it to grow A student who thinks energy is recycled in ecosystems picks this. Energy flows one way, from kelp to consumers, and is lost as heat; kelp gets its energy from light.
The diagram shows food webs in two hypothetical grassland ecosystems. A disease kills all of herbivore H1 in both ecosystems. Based on the diagram, which statement about the predator is best supported?
Answer and reasoning
AIt is more likely to persist in ecosystem 1, as fewer species there can be harmed A student who thinks simple ecosystems are more stable picks this. With only one food source, the predator in ecosystem 1 has nothing left to eat once H1 is gone.
BIt is equally likely to persist in both, as it can switch to eating grass instead of H1 A student who thinks organisms change to meet their needs picks this. Nothing in the web shows the predator eating plants; an animal does not acquire a new diet because its food is gone.
CIt is more likely to persist in ecosystem 2, as it can still feed on H2 and H3Correct In ecosystem 1, H1 is the predator's only food, so losing H1 removes its energy source. In ecosystem 2, the predator also eats H2 and H3, which feed on grass and shrubs, so it can persist. The ecosystem with more component parts is more resilient to the loss of one.
DIt disappears from both, as the loss of any one species makes every ecosystem collapse A student who thinks removing any species makes an ecosystem collapse picks this. In ecosystem 2, the predator still has two food sources.
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
