C4.1 Populations and communities

1. Its members belong to one species, live in the same area and normally breed with one another. — A population is an interacting group of organisms of the same species living in an area, and its members normally breed with each other. Reproductive isolation is what distinguishes one population of a species from another.
2. It includes every member of a species, wherever in the world that species is found. — A student who uses the everyday sense of 'the elephant population' picks this. In fact a population is tied to an area; a species usually consists of many separate populations.
3. It includes all the organisms of every species that interact within one area. — A student who transfers the idea of a town's population to a habitat picks this. In fact that describes a community; a population contains only one species.
4. Its members cannot interbreed with members of other populations of the species. — A student who reads reproductive isolation as biological incompatibility picks this. In fact members of separate populations remain one species and could interbreed if brought together; they simply do not, being separated.

Syllabus statement C4.1.1 · Read this in Learn

1. A large enough random sample gives the true population size exactly, so a complete count is unnecessary. — A student who believes that good technique removes all error picks this. In fact any estimate from a sample carries a sampling error; estimation is a practical compromise, not a way of obtaining the exact size.
2. Sampling lets the investigator select the most typical parts of the habitat in which to count. — A student who thinks choosing representative sites is the point of sampling picks this. In fact sampling sites must be chosen at random, precisely to remove the investigator's choice and the bias it introduces.
3. Counting every individual is usually impractical and may damage the habitat or disturb the organisms. — Complete counts take too long over large areas, individuals may be hidden or mobile, and searching the whole habitat can damage it. A random sample gives a usable estimate at far lower cost.
4. The number counted in the sample quadrats is itself the population size, so no further counting is needed. — A student who treats the sample count as the result picks this. In fact the sample count is scaled up by the total area to give the estimate; it is not the population size.

Syllabus statement C4.1.2 · Read this in Learn

1. About 8 — A student who does not see that the estimate rests on the proportion marked, and so places R in the numerator (M × R ÷ N = 40 × 10 ÷ 50), picks this. In fact the estimate cannot be smaller than the 40 woodlice already marked; R is the denominator.
2. About 2000 — A student who thinks the two sample sizes alone give the estimate multiplies M by N and forgets to divide by R. In fact the proportion of marked woodlice recaptured (10 of 50) must be used to scale M up, giving 40 × 50 ÷ 10 = 200.
3. About 200 — Population estimate = M × N ÷ R = 40 × 50 ÷ 10 = 200. One fifth of the second sample was marked, so the 40 marked woodlice are estimated to be one fifth of the population.
4. About 80 — A student who adds up the distinct woodlice seen (40 marked plus 40 unmarked in the second sample) picks this. In fact the method estimates the individuals never caught; M × N ÷ R gives 200.

Syllabus statement C4.1.4 · Read this in Learn

1. Any factor that kills more individuals when the population is larger, including a flood or a frost, is density-dependent. — A student who counts deaths instead of comparing proportions picks this. In fact a flood affects a similar fraction at any density; it is density-independent even though more individuals die in a larger population.
2. Predation and disease are density-independent, since predators and pathogens come from outside the population. — A student who limits density dependence to competition picks this. In fact dense populations suffer a higher risk of predation and faster transfer of pathogens or pests, so both are density-dependent.
3. The proportion of individuals they kill or prevent from breeding rises as the population becomes more dense. — Density-dependent factors, which include competition for limited resources, the increased risk of predation and the transfer of pathogens or pests, affect a larger fraction of the population at higher density, so they push numbers back towards the carrying capacity.
4. They act as negative feedback, so they continue to reduce the population until it is very small. — A student who reads 'negative' as 'downward' picks this. In fact negative feedback opposes change in either direction: the same factors that reduce a large population relax and allow a small one to recover.

Syllabus statement C4.1.6 · Read this in Learn

1. The yeast cells have all stopped dividing, and none of them are dying, so the count stays the same. — A student who reads a flat line as inactivity picks this. In fact cells continue to divide and to die during the plateau; the count is constant because the two rates are equal.
2. The plateau is reached only at the moment when the glucose in the flask has been completely used up. — A student who treats food as the only limit and as a switch picks this. In fact growth slows progressively as glucose becomes scarcer, and the accumulation of ethanol limits the population at the same time.
3. Natality now equals mortality, because glucose has become scarce and ethanol has accumulated. — In a closed culture, density-dependent factors intensify as the population grows: glucose per cell falls, ethanol and other wastes build up and pH falls. Cell division slows and death rises until the two balance at the carrying capacity of the flask.
4. The flask has reached the carrying capacity that applies to yeast in any environment. — A student who thinks carrying capacity is a fixed property of yeast picks this. In fact the plateau is set by the resources and waste levels in this flask; a larger flask or more glucose would give a higher plateau.

Syllabus statement C4.1.8 · Read this in Learn

1. All the populations of plants, animals, fungi and bacteria that live and interact in an area. — A community is all of the interacting organisms in an ecosystem: every population in the area, including the plants, animals, fungi and bacteria, but not the abiotic environment.
2. All the populations of plants and animals, but not microorganisms, that live in an area. — A student whose mental picture comes from food-web diagrams picks this. In fact fungi and bacteria are populations too and are part of the community as decomposers, pathogens and mutualists.
3. All the organisms in an area together with the soil, water and climate they depend on. — A student who attaches the abiotic environment to the wrong term picks this. In fact this describes an ecosystem; the community is the living part only.
4. All the individuals of one species that live, interact and breed together in an area. — A student who has not separated 'population' from 'community' picks this. In fact this is the definition of a population; a community contains all the populations of every species in the area.

Syllabus statement C4.1.10 · Read this in Learn

1. Predation, because the tick is an animal feeding on the body of another animal. — A student who calls any animal that feeds on another a predator picks this. In fact a predator kills its prey and eats it; the deer survives the tick, so the relationship is parasitism.
2. Parasitism, because the tick takes nutrition from a living host without killing it. — A parasite lives on or in its host, obtains food from it over a period of time and harms it, but does not usually kill it. The tick feeding on the deer's blood fits this exactly.
3. Pathogenicity, because the tick is a small organism that harms a much larger one. — A student who stretches 'pathogen' to mean anything harmful picks this. In fact pathogenicity is disease caused by a microorganism or virus living in a host; the tick is a parasite taking food.
4. Mutualism, because the deer provides the tick with the food that it needs to survive. — A student who thinks mutualism means one species providing for another picks this. In fact mutualism requires both species to benefit, and the deer gains nothing from losing blood to the tick.

Syllabus statement C4.1.11 · Read this in Learn

1. Grey squirrels digest unripe acorns and store more fat, so they take a larger share of the shared food supply and out-reproduce red squirrels. — The basis of an introduced species becoming invasive is a competitive advantage in acquiring resources. Grey squirrels are larger, can eat acorns before they ripen and survive winter better, so where the two species share a wood the red squirrel population declines.
2. Grey squirrels kill red squirrels and their young in fights, so red squirrels are driven out of any wood the greys enter. — A student who pictures invasion as aggression picks this. In fact grey squirrels do not need to attack red squirrels; taking more of the shared food is enough to reduce red squirrel survival and breeding.
3. Red squirrels, being adapted to British woodland, hold their ground wherever food is scarce, so greys spread only where food is plentiful. — A student who assumes the endemic species must be better fitted to its habitat picks this. In fact the red squirrel is adapted to its former competitors, not to the grey squirrel, and loses out where food is limited.
4. The two species compete only where individuals meet and fight over the same acorn, so replacement happens only in crowded woods. — A student who thinks competition requires a direct contest picks this. In fact competition occurs whenever both species draw on the same limited resource, whether or not individuals ever meet.

Syllabus statement C4.1.13 · Read this in Learn

1. 16 quadrats — A student who does not separate the observed count from the expected value, and reads the number of quadrats that actually contain both species as the answer, picks this. In fact 16 is the observed frequency; the expected frequency under independence is 48 × 40 ÷ 80 = 24, and the test compares the two.
2. 24 quadrats — Species A is present in 16 + 32 = 48 quadrats and species B in 16 + 24 = 40. Expected frequency = (row total × column total) ÷ grand total = 48 × 40 ÷ 80 = 24. Both species were observed together in only 16 quadrats, fewer than expected, so the association is negative.
3. 20 quadrats — A student who assumes that independence makes the four combinations equally likely divides 80 by 4 and picks this. In fact the expected values depend on how common each species is: A occurs in 60% of quadrats and B in 50%, so both are expected in 0.6 × 0.5 × 80 = 24.
4. 30 quadrats — A student who multiplies the two percentages (60% of quadrats contain A, 50% contain B) and reports the resulting 30% as if it were a number of quadrats picks this. In fact 30% is the expected fraction; 30% of the 80 quadrats is 24.

Syllabus statement C4.1.15 · Read this in Learn

1. Neither type of control is shown, because otters do not eat kelp and so their loss cannot have affected it. — A student who considers only direct feeding links picks this. In fact the effect passed along the food chain: fewer otters meant more urchins, and more urchins meant less kelp.
2. Top-down control is dominant: the predator limited the herbivore, which in turn limited the producer below it. — Control originating from a higher trophic level and acting on the levels below is top-down. Otters limited urchins, which limited kelp; removing the otters released both lower levels in turn. Bottom-up control by nutrient supply still operates but is not dominant here.
3. Top-down control is shown, which demonstrates that nutrient supply plays no part in limiting this community. — A student who treats the two types of control as mutually exclusive picks this. In fact both are possible in any community; the evidence shows that top-down control is dominant, not that bottom-up control is absent.
4. Bottom-up control is dominant, because the effect of losing the otters travelled down to the kelp at the base of the food chain. — A student who names the control by where the effect ends up picks this. In fact the terms refer to where control originates: here a predator at the top drove the changes, so control is top-down.

Syllabus statement C4.1.17 · Read this in Learn

1. Two species, because groups that do not interbreed cannot belong to the same species. — A student who equates reproductive isolation between populations with the isolation that separates species picks this. In fact the newts could interbreed if brought together; they are one species in two populations.
2. Two populations of one species, because the groups are reproductively isolated from each other. — The newts are the same species, but because no individual moves between the ponds the two groups never interbreed. Reproductive isolation of this kind is used to distinguish one population from another.
3. One population, because all the newts belong to the same species, wherever they breed. — A student who thinks a population is the whole species picks this. In fact a population is an interacting group in one area whose members normally breed together, which these two groups never do.
4. Two communities, because each pond contains newts together with the other organisms living in it. — A student who merges 'population' with 'all the organisms in a place' picks this. In fact the question concerns the newts alone, one species, so the groups are populations, not communities.

Syllabus statement C4.1.1 · Read this in Learn

1. It is 40 limpets, and arose from mistakes the student made when counting inside the quadrats. — A student who thinks 'error' means a blunder picks this. In fact sampling error occurs even with perfect counting, because a random sample can by chance contain a lower or higher density than the shore as a whole.
2. It would have been zero if the student had placed enough quadrats to sample the shore properly. — A student who thinks enough quadrats guarantee the true value picks this. In fact more quadrats reduce the sampling error on average but cannot eliminate it; only a complete count has no sampling error.
3. It arose because the quadrats were placed at random instead of in areas typical of the whole shore. — A student who prefers chosen 'typical' sites to random ones picks this. In fact random placement is what makes the estimate unbiased; choosing sites would add bias on top of the unavoidable sampling error.
4. It is 40 limpets, and arose because only a sample of the shore was measured, not all of it. — Sampling error is the difference between the estimate (420) and the true size (460), which is 40. It is the inevitable consequence of measuring a random sample rather than the whole population.

Syllabus statement C4.1.2 · Read this in Learn

1. 2000 plants — Each quadrat is 0.25 m², so the sampled area is 10 × 0.25 = 2.5 m². Density = 20 ÷ 2.5 = 8 orchids per m², and 8 × 250 m² = 2000 orchids.
2. 20 plants — A student who takes the sample count as the population picks this. In fact the 20 orchids were counted in only 2.5 m² of a 250 m² meadow and must be scaled up by the total area.
3. 500 plants — A student who treats each quadrat as 1 m² picks this: a mean of 2 per quadrat × 250 m². In fact a 0.5 m × 0.5 m quadrat is 0.25 m², so the density is 8 per m², not 2.
4. 20000 plants — A student who divides the total count by the area of a single quadrat (20 ÷ 0.25 = 80 per m²) picks this. In fact the 20 orchids came from ten quadrats, a sampled area of 2.5 m², giving 8 per m².

Syllabus statement C4.1.3 · Read this in Learn

1. Shore B has a larger limpet population than shore A, because its standard deviation is higher. — A student who reads any larger statistic as 'more limpets' picks this. In fact population size is estimated from the mean, which is the same on both shores; the standard deviation describes spread, not number.
2. Limpets on shore B are distributed less evenly, because the counts varied much more from quadrat to quadrat. — The standard deviation of the mean number per quadrat measures variation between quadrat counts and so shows how evenly the population is spread. A larger standard deviation with the same mean means a patchier, more clumped distribution.
3. The sampling on shore B was carried out incorrectly, so its mean of 8 per quadrat is not valid. — A student who treats a large deviation as a sign of faulty work picks this. In fact a clumped population produces a large standard deviation however carefully the sampling is done; it is information about the shore, not the student.
4. Using more quadrats on shore B would have reduced its standard deviation to about that of shore A. — A student who thinks larger samples remove variation picks this. In fact the standard deviation reflects the real unevenness of the limpets on shore B; more quadrats would estimate that spread more precisely, not shrink it.

Syllabus statement C4.1.3 · Read this in Learn

1. Too low, because fewer marked beetles are recaptured and fewer recaptures mean a smaller population. — A student who reasons by direct proportion picks this. In fact R is the denominator of M × N ÷ R, so a smaller R makes the estimate larger, not smaller.
2. Unaffected, provided the second sample is taken at the release point soon after the beetles are released. — A student who thinks the aim is to recapture as many marked beetles as possible picks this. In fact sampling before the marked beetles mix in breaks a different assumption of the method, and does not cancel the effect of predation on marked beetles.
3. Too low, because fewer beetles in total are encountered across the two samples, so fewer are counted. — A student who thinks the estimate is the number of beetles seen picks this. In fact the estimate depends on the proportion marked in the second sample, and a reduced proportion inflates the estimate.
4. Too high, because fewer marked beetles survive to be recaptured, so R is too small. — The method assumes marking does not affect survival. Marked beetles are eaten preferentially, so the marked fraction in the second sample is lower than it should be; with R smaller, M × N ÷ R is larger than the true population.

Syllabus statement C4.1.4 · Read this in Learn

1. The maximum population size that the resources available in the environment can support sustainably over time. — Carrying capacity is set by the supply of limiting resources such as food, water, light, space, mineral nutrients or nesting sites; it is the largest population the environment can maintain over time.
2. The number of individuals that can physically fit into the area that the population occupies. — A student who uses 'capacity' in the sense of a bus or a stadium picks this. In fact resources, not room, set the limit, and populations stop growing long before the area is physically full.
3. The greatest number of offspring that each individual of the species is able to produce in its lifetime. — A student who thinks carrying capacity is a fixed property of the species picks this. In fact it belongs to the environment: the same species has different carrying capacities in habitats with different resource supplies.
4. The population size below which negative feedback stops reducing the numbers of a species in its environment. — A student who reads negative feedback as a one-way downward pressure picks this. In fact carrying capacity is the size the environment's resources can sustain, and density-dependent feedback acts in both directions around it: pushing numbers down when above it and allowing growth when below it.

Syllabus statement C4.1.5 · Read this in Learn

1. The storm was a density-dependent factor, because it killed so many birds in a colony as large as this one was. — A student who judges by the number killed rather than the proportion picks this. In fact a storm kills roughly the same fraction of a small or a large colony, which makes it density-independent.
2. The storm was density-independent; reduced competition afterwards let natality exceed mortality until carrying capacity was regained. — A storm kills a similar proportion of birds whatever the colony's density, so it is density-independent. Once numbers fell, density-dependent pressures such as competition for food and nest sites relaxed, and negative feedback returned the population towards the carrying capacity.
3. Density-dependent factors can only ever reduce populations, so the recovery afterwards must have been due to density-independent factors. — A student who sees density-dependent factors purely as brakes picks this. In fact their effect weakens at low density, and that relaxation is exactly what allows the population to grow back.
4. The recovery shows that the carrying capacity is fixed by the birds' breeding rate rather than by the environment. — A student who thinks carrying capacity is a property of the species picks this. In fact the colony returned to the size that the local food and nesting resources can support, which is what carrying capacity means.

Syllabus statement C4.1.6 · Read this in Learn

1. Each individual reproduces faster and faster as the phase continues. — A student who confuses the population's growth rate with the per-individual rate picks this. In fact each individual's reproductive rate stays roughly constant; the population accelerates because more individuals are reproducing.
2. Individuals reach maturity sooner than they will later at higher density. — A student who explains acceleration by faster individual development picks this. In fact the explanation is a constant proportional increase applied to a growing number of individuals, not a change in each one.
3. Growth continues at full speed until the food has been entirely used up. — A student who thinks food acts as a switch picks this. In fact growth slows through a transitional phase as resources become scarcer per individual, well before any resource is exhausted.
4. Resources are plentiful, so most individuals survive to reproduce. — With abundant resources and negligible competition, predation and disease, natality exceeds mortality by a roughly constant proportion. Each generation adds to the number of reproducing individuals, so the population increases by a constant factor per unit time.

Syllabus statement C4.1.7 · Read this in Learn

1. The points lie on a straight line, because the population multiplied by the same factor in each two-day interval. — On a logarithmic axis, equal multiples occupy equal distances. Exponential growth multiplies the population by a constant factor per interval, so it appears as a straight line; a line that bends downwards would show growth slowing.
2. The points form a curve that becomes steeper with time, as exponential growth does on ordinary axes. — A student who treats the J-shape as the definition of exponential growth picks this. In fact the logarithmic axis straightens a J-curve; a curve still steepening on a log scale would mean growth faster than exponential.
3. The points lie on a straight line, because a fixed number of plants was added in each two-day interval. — A student who thinks exponential growth means a constant number added per interval picks this. In fact that would be linear growth, which is straight on ordinary axes but bends over on a logarithmic axis.
4. The points trace an S-shaped curve, because all populations grow according to the sigmoid model. — A student who mistakes the model for reality picks this. In fact the sigmoid curve is an idealized model; on this graph the test for exponential growth is a straight line, and an S-shape would show that growth had slowed and stopped.

Syllabus statement C4.1.7 · Read this in Learn

1. The reindeer must have levelled off at a plateau at some point between 1938 and 1950 that the counts simply failed to record properly. — A student who believes the model must be obeyed picks this. In fact the data are the evidence, and they show an overshoot and crash; the model, not the data, is the simplification.
2. The model is a simplification: the reindeer overshot the carrying capacity because their food was destroyed faster than it regrew. — The sigmoid curve is an idealized model in which growth slows smoothly to a plateau. Real populations can grow past the carrying capacity if the limiting resource is depleted faster than the feedback acts; the lichen was destroyed, the carrying capacity fell, and the population crashed.
3. The crash shows that the carrying capacity of reindeer is fixed at about 2000 animals, wherever they live. — A student who treats carrying capacity as a species constant picks this. In fact carrying capacity belongs to the environment; on this island it was set by the lichen supply, and it fell when the lichen was destroyed.
4. Negative feedback pushed the population down and would continue to reduce it until the reindeer were extinct. — A student who reads negative feedback as a one-way decline picks this. In fact negative feedback returns a population towards the carrying capacity; the crash reflected a collapse in the carrying capacity itself.

Syllabus statement C4.1.7 · Read this in Learn

1. Grey squirrels taking acorns from red squirrels; wolves hunting together as a pack. — A student who thinks competition only happens between species picks this. In fact grey and red squirrels are different species, so that is interspecific competition; pack hunting is a correct example of cooperation.
2. Sardines swimming together in a shoal; emperor penguins huddling against the cold. — A student who reads any grouping as the whole story picks this. In fact both examples are cooperation; neither is intraspecific competition, so the pair does not match the question.
3. Lions and hyenas fighting over a carcass; ants guarding aphids in return for honeydew. — A student who treats all the animals in an area as one population picks this. In fact both examples involve two different species: an interspecific competition and a mutualism, not relationships within a species.
4. Oak seedlings shading each other; meerkats taking turns as sentinels. — Seedlings of one species in a dense stand compete for light because they have identical needs. Meerkat sentinels give a warning that benefits every member of the group, at a cost to the sentinel's own foraging time, which is cooperation within a species.

Syllabus statement C4.1.9 · Read this in Learn

1. The bacteria supply the plant with nitrogen gas from the soil; the plant supplies the bacteria with oxygen. — A student who uses 'nitrogen' loosely picks this. In fact plants cannot use nitrogen gas at all; the whole point of the relationship is that the bacteria fix the gas into ammonia the plant can use.
2. The plant gains fixed nitrogen from the bacteria; the bacteria gain nothing from the plant but are not harmed by living in the nodule. — A student who remembers only the plant's side picks this. In fact mutualism means both partners benefit: the bacteria receive sugars made by photosynthesis and a sheltered environment.
3. The bacteria supply the plant with fixed nitrogen as ammonia or amino acids; the plant supplies the bacteria with sugars. — Rhizobium bacteria in the nodules of Fabaceae convert nitrogen gas into ammonia, which the plant uses to make amino acids. In return the plant supplies sugars from photosynthesis and a protected environment inside the nodule, so both benefit.
4. The bacteria take sugars from the roots and give the plant nothing, so the nodules are a mild infection the plant tolerates. — A student who assumes microorganisms inside roots must be harmful picks this. In fact legumes with nodules grow better in nitrogen-poor soil than legumes without them, because they receive fixed nitrogen.

Syllabus statement C4.1.12 · Read this in Learn

1. The coral can no longer photosynthesize, because the algae carried its chlorophyll for it. — A student who thinks the coral is itself photosynthetic picks this. In fact the coral polyp is an animal and has no chlorophyll of its own; photosynthesis is done entirely by the zooxanthellae.
2. The coral loses the shelter and the carbon dioxide that the algae had been providing for it. — A student who has the direction of the benefits reversed picks this. In fact the coral provides the algae with shelter, a sunlit position and carbon dioxide; the algae provide the coral with carbon compounds and oxygen.
3. The algae were parasites, so the coral is weakened by the damage they caused before being expelled. — A student who assumes an organism living inside another must be harming it picks this. In fact the relationship is mutualism: the coral starves after the expulsion because it has lost a partner, not because of earlier damage.
4. The coral loses the sugars and other carbon compounds the algae made by photosynthesis. — Zooxanthellae living inside the polyp's cells photosynthesize and pass most of the carbon compounds they make to the coral, along with oxygen. Without them the coral polyp, an animal, cannot obtain enough food from the plankton it catches.

Syllabus statement C4.1.12 · Read this in Learn

1. Interspecific competition between A and B is proven, because A colonises and grows well in every single plot from which B was removed. — A student who treats a pattern that fits the hypothesis as proof picks this. In fact the guide states that greater success in the absence of another species indicates but does not prove competition.
2. Competition is indicated but not proven, because A's success could depend on other factors that differ where B is absent. — One species being more successful in the absence of another indicates competition but does not prove it. The removal experiment strengthens the case by manipulating one variable, but other differences between plots, and between the two species' tolerances, are not ruled out.
3. The initial survey of where A and B occur was an experiment, so by itself it establishes that they compete. — A student who calls any planned data collection an experiment picks this. In fact the survey was an observation, because nothing was manipulated; only the scraping of B from plots was an experiment.
4. The two species cannot be competing, because barnacles are fixed in place and were not seen to fight each other. — A student who thinks competition needs a contest picks this. In fact sessile organisms compete by occupying space and taking resources; no contact or fighting is required.

Syllabus statement C4.1.14 · Read this in Learn

1. Recording the abundance of both limpet species in quadrats placed at random positions along the shore. — A student who thinks planned data collection makes an experiment picks this. In fact random sampling records what is already there without altering it, so it is a field observation.
2. Measuring the shell sizes of both limpet species in laboratory tanks of seawater collected from the shore. — A student who uses the laboratory as the cue for 'experiment' picks this. In fact nothing is manipulated; measuring the limpets indoors is still an observation.
3. Removing one limpet species from marked plots and recording the growth of the other over a year. — An experiment deliberately changes a variable and records the effect. Field manipulation by removal of one species is one of the experimental approaches the guide names; the untouched plots act as the control.
4. Comparing where each limpet species occurred in shore surveys made over the past twenty years. — A student who equates careful analysis of data with experimentation picks this. In fact survey records are observations; hypotheses can be tested with them, but no variable was changed.

Syllabus statement C4.1.14 · Read this in Learn

1. No significant association, because the chi-squared value of 13.7 is far above the critical value. — A student who reads a large statistic as 'not significant', confusing it with a p-value, picks this. In fact a calculated value larger than the critical value means the departure from independence is significant.
2. A significant negative association, which proves that the two species compete for the same resources. — A student who treats a significant association as proof of its cause picks this. In fact the two species may avoid each other for other reasons, such as different soil requirements; competition is indicated, not proven.
3. A significant positive association, because the two species were found growing together in 8 of the quadrats. — A student who judges association from the raw co-occurrence count picks this. In fact 8 is well below the 14.4 expected under independence, so the species occur together less often than chance predicts.
4. A significant negative association, consistent with competition but not a proof of it. — 13.7 exceeds 3.84, so the null hypothesis of no association is rejected. Both species occurred together in fewer quadrats (8) than expected (14.4), so the association is negative. This may provide evidence for interspecific competition but does not prove it.

Syllabus statement C4.1.15 · Read this in Learn

1. Abundant hares let more lynx survive and breed, so lynx rise later; more lynx then reduce hares, after which lynx decline and hares recover. — This is density-dependent control in both directions: predation intensifies as hares become dense, and the predator population is itself limited by prey. The delay arises because lynx take time to breed in response to food and time to starve when it is scarce.
2. Lynx and hare numbers should peak at the same time, so the delay of one to two years shows that the fur records were unreliable. — A student who expects an immediate response picks this. In fact the lag is the expected signature of a predator–prey cycle, because predator reproduction responds to prey abundance with a delay.
3. The lynx eat nearly all the hares, so each crash is followed by a period with no lynx at all until new ones immigrate into the area. — A student who pictures predators as unstoppable picks this. In fact lynx starve and breed less as hares become scarce, so their numbers fall before the hares are eliminated and the cycle continues.
4. Predation is density-independent, so the cycles must instead be driven by alternating runs of harsh winters and milder winters. — A student who thinks only competition is density-dependent picks this. In fact the risk of predation increases with prey density, and the predator–prey relationship is the guide's example of density-dependent control.

Syllabus statement C4.1.16 · Read this in Learn

1. In both, a chemical is released to destroy pathogens that would otherwise cause disease in the producer. — A student who knows antibiotics only as medicines picks this. In fact the antibiotic targets microorganisms competing for the same resources, and allelopathic chemicals target competing plants, not pathogens.
2. In both, a chemical is released as a defence against the animals that would otherwise eat the producer. — A student who assumes every plant toxin is aimed at herbivores picks this. In fact allelopathy acts on other plants that compete for light, water and mineral ions, and antibiotics act on other microorganisms.
3. In both, a chemical substance is released into the surroundings to deter potential competitors. — Allelopathic plants such as black walnut release chemicals that inhibit neighbouring plants, and microorganisms such as Penicillium secrete antibiotics that inhibit competing bacteria. In each case the chemical reduces competition for limited resources.
4. In both, the producer must make direct contact with a competitor before the chemical acts. — A student who thinks competition requires a confrontation picks this. In fact the chemical diffuses into the soil or the surrounding medium and acts on competitors that the producer never touches.

Syllabus statement C4.1.18 · Read this in Learn

1. Defence against herbivores by the walnut; antibiotic secretion by the mould. — A student who assumes plant chemicals target animals picks this. In fact juglone suppresses other plants competing with the walnut for resources; it is not a defence against being eaten.
2. Allelopathy by the walnut; treatment of the bread's bacterial disease by the mould. — A student who thinks of antibiotics as cures picks this. In fact the mould secretes penicillin to deter bacteria competing for the bread, the same competitive purpose as allelopathy.
3. Pathogenicity by the walnut towards plants; pathogenicity by the mould towards bacteria. — A student who uses 'pathogen' for anything that harms picks this. In fact pathogenicity is disease caused by a microorganism or virus living in a host; neither the walnut nor the mould lives inside the organisms it inhibits.
4. Allelopathy by the walnut tree; secretion of an antibiotic by the mould. — Black walnut releases juglone into the soil, which inhibits the growth of competing plants beneath it, an example of allelopathy. Penicillium secretes penicillin, which prevents bacteria from growing on the same food source, an example of antibiotic secretion.

Syllabus statement C4.1.18 · Read this in Learn

1. The fungus supplies mineral ions and, to the seedling, carbon compounds; the photosynthesizing orchid supplies sugars. — Orchid seeds are minute and have almost no food reserves, so the seedling depends on the fungus for carbon compounds as well as water and mineral ions absorbed by the hyphae. Once the orchid has leaves it supplies sugars to the fungus, so both partners benefit.
2. The fungus fixes nitrogen gas into ammonia for the orchid; the orchid supplies the fungus with sugars made in photosynthesis. — A student who merges mycorrhizae with root nodules picks this. In fact nitrogen fixation is carried out by bacteria such as Rhizobium; mycorrhizal fungi absorb mineral ions that are already in the soil.
3. The fungus takes sugars from the orchid's roots and the orchid gains nothing in return, so the fungus is really a parasite. — A student who assumes a fungus inside roots must be an infection picks this. In fact orchid seeds cannot germinate without the fungus, which supplies the seedling with the carbon compounds and mineral ions it lacks.
4. The fungus supplies the orchid with mineral ions and water but receives nothing in return from the orchid. — A student who remembers only the plant's benefit picks this. In fact mutualism requires both to benefit: the fungus receives sugars from the orchid once the orchid photosynthesizes.

Syllabus statement C4.1.12 · Read this in Learn

1. It does not occur: members of one species share their resources, and competition arises only between different species. — A student whose picture of one species is a herd or flock picks this. In fact intraspecific competition is often the most intense competition of all: trees in a stand compete for light and male red deer compete for mates, precisely because they need the same things.
2. It occurs only when two individuals confront each other over the same item of food, the same mate or the same nesting site. — A student who thinks competition means a contest picks this. In fact competition occurs whenever a limited resource is shared: seedlings shading one another and animals feeding from the same depleted food supply compete without any confrontation.
3. Individuals of one species have identical needs, so when a resource is scarce what one uses leaves less for others. — Members of a population need exactly the same food, water, light, space, mates or nest sites. Whenever such a resource is in limited supply, each individual's consumption reduces the amount available to the rest, which is why intraspecific competition arises.
4. It occurs only in solitary species, because individuals living together in a group are by definition cooperating. — A student who equates grouping with cooperation picks this. In fact members of a grazing herd or a nesting colony still compete for grass, mates and nest sites; cooperation and competition can both occur within the same group.

Syllabus statement C4.1.9 · Read this in Learn