B4.2 Ecological niches

1. The role of the species in its ecosystem, including its biotic and abiotic interactions — This is the guide's definition. A niche is a role, described by the biotic and abiotic interactions that influence the species' growth, survival and reproduction, including how it obtains food.
2. The place where the species lives, described by its location and ecosystem type — A student who equates niche with habitat picks this. The place in which a species lives is its habitat; the niche is what the species does in that place.
3. The position of the species in a food chain, defined by the food it eats — A student who thinks a niche is just a trophic level picks this. Obtaining food is one part of the niche, but predators, competitors and abiotic tolerances are also part of it.
4. The interactions of the species with other organisms, leaving out abiotic factors — A student who files abiotic conditions under 'habitat' picks this. The guidance includes abiotic interactions, such as tolerance of temperature or oxygen, in the niche.

Syllabus statement B4.2.1 · Read this in Learn

1. An organism that does not use oxygen but is unharmed when oxygen gas is present — A student who pictures anaerobes as simply ignoring oxygen picks this. Oxygen is toxic to obligate anaerobes; they cannot tolerate it and grow only where it is absent.
2. An organism that normally respires anaerobically but can use oxygen if needed — A student who thinks the anaerobic mode is the organism's normal mode and oxygen is merely tolerated picks this. An organism that can use oxygen at all is a facultative anaerobe; an obligate anaerobe cannot tolerate oxygen.
3. An organism that can grow only where oxygen gas is absent from its environment — Obligate anaerobes cannot tolerate oxygen gas. They are found only in oxygen-free environments, such as deep sediments or the gut, because oxygen prevents their growth.
4. An organism that switches to anaerobic respiration when its oxygen supply runs out — A student who treats any organism that can respire without oxygen as an obligate anaerobe picks this. Switching between modes describes a facultative anaerobe; an obligate anaerobe never uses oxygen and cannot tolerate it.

Syllabus statement B4.2.2 · Read this in Learn

1. Plants, algae and several groups of photosynthetic prokaryotes such as cyanobacteria — Photosynthesis is the mode of nutrition of plants, of algae and of several groups of photosynthetic prokaryotes. All synthesize their own carbon compounds from carbon dioxide using light energy.
2. Plants and algae only, because prokaryotes are unable to photosynthesize — A student who has met bacteria only as pathogens and decomposers picks this. Several groups of prokaryotes photosynthesize; the details of their types of photosynthesis are not required.
3. Plants, algae and any prokaryotes whose cells happen to contain chloroplasts — A student who ties photosynthesis to the chloroplast picks this. Prokaryotic cells have no chloroplasts, yet several groups of prokaryotes are photosynthetic.
4. Plants, algae and photosynthetic prokaryotes, though plants also feed from the soil — A student who thinks plants take in food through their roots picks this. Plants absorb only water and mineral ions from the soil; all their carbon compounds are synthesized in photosynthesis, so this is not a second mode of nutrition.

Syllabus statement B4.2.3 · Read this in Learn

1. Animals that host photosynthetic algae inside their cells are autotrophic — A student who locates the mode of nutrition in the body rather than the organism picks this. The algae are autotrophs; the animal obtains carbon compounds from them and is heterotrophic.
2. Animals assimilate their food by absorbing it across the lining of the gut — A student who merges the two stages picks this. Passage across the gut wall is absorption; assimilation is the later use of the absorbed molecules in the animal's cells.
3. Animals are heterotrophs that ingest, digest, absorb and assimilate food — All animals are heterotrophic. Their holozoic nutrition consists of ingestion of food, internal digestion, absorption of the products and their assimilation into the animal's tissues.
4. Animals ingest food when digested molecules pass into their cells — A student who uses 'ingest' for uptake by cells picks this. Ingestion is taking food into the body at the start of holozoic nutrition, before digestion.

Syllabus statement B4.2.4 · Read this in Learn

1. Secretion of enzymes onto dead organic matter, followed by absorption of the products of digestion — Saprotrophs such as many fungi and bacteria digest dead organic matter externally by secreting enzymes onto it and then absorb the soluble products. Organisms with this mode of nutrition can be referred to as decomposers.
2. Ingestion of dead organic matter, followed by internal digestion and absorption of products — A student who applies the animal model of digestion to fungi picks this. Saprotrophs do not ingest; they digest externally and absorb the products.
3. Any feeding on dead organic matter, whether by fungi and bacteria or by earthworms and woodlice — A student who uses 'decomposer' loosely picks this. Earthworms and woodlice ingest dead matter and digest it internally; saprotrophic nutrition is specifically external digestion by fungi and bacteria.
4. Synthesis of carbon compounds by fungi and bacteria that grow on dead organic matter — A student who thinks of fungi as plant-like picks this. Fungi and saprotrophic bacteria are heterotrophs; they obtain carbon compounds from the dead matter rather than synthesizing them.

Syllabus statement B4.2.6 · Read this in Learn

1. Archaea are a group of bacteria, so they show the same modes of nutrition as other bacteria — A student who still thinks of 'archaebacteria' picks this. Archaea are a separate domain of life from Bacteria; the observations illustrate diversity within that domain, not bacterial nutrition.
2. Archaea are metabolically diverse, using light, inorganic chemicals or carbon compounds — The three species use the three energy sources named in the guidance: oxidation of an inorganic chemical (species 1), oxidation of a carbon compound (species 2) and light (species 3). Archaea are metabolically very diverse.
3. Archaea are chemosynthetic extremophiles, so only species 1 is a typical archaeon — A student who generalises from hot-spring examples picks this. All three are archaea; there is no single 'typical' archaeal metabolism, and many archaea live in ordinary soils and waters rather than extreme environments.
4. Archaea are prokaryotes, so all three must be heterotrophs oxidizing carbon compounds — A student who thinks all prokaryotes are heterotrophic decomposers picks this. Only species 2 oxidizes carbon compounds; species 1 and 3 obtain energy from an inorganic chemical and from light.

Syllabus statement B4.2.7 · Read this in Learn

1. By developing theories relating dentition to diet from the fossils themselves, then confirming them with living mammals — A student who assumes the fossils supplied the theory picks this. The diets of fossil hominids were not observable, so the theory had to come from living mammals and was then applied to the fossils.
2. By comparing the size of the fossil jaws with those of living carnivores, since powerful jaws indicate meat-eating — A student who equates a heavy jaw with carnivory picks this. In living mammals it is herbivores grinding tough plants that have massive jaws and large flat molars, so the comparison would be with herbivores.
3. By treating the question as unanswerable, since the diet of an extinct animal is a guess that cannot be tested — A student who equates science with direct observation picks this. Deductions can be made from theories; the deduction can also be tested against other evidence such as tooth wear.
4. By applying theories relating dentition to diet, developed from observing living mammals, to the fossil teeth — This is the NOS point: observation of living mammals led to theories relating dentition to herbivorous or carnivorous diets, and these theories allowed the diets of extinct organisms to be deduced.

Syllabus statement B4.2.8 · Read this in Learn

1. A rattlesnake's venom is a physical adaptation of a predator, because it is injected through the fangs — A student who classifies by the visible body part picks this. The fangs are a physical adaptation, but the venom itself is a chemical adaptation for killing prey.
2. Zebras living in herds is not an adaptation, because this behaviour is learned rather than inherited — A student who counts only anatomical features picks this. Herding is a behavioural adaptation of prey animals for resisting predation; it has a genetic basis and is shaped by natural selection.
3. A skunk spraying a foul-smelling secretion at an attacker is a chemical adaptation of a prey animal — The secretion is a chemical, and the skunk uses it to resist predation, so this is a chemical adaptation of a prey animal.
4. A leopard's spotted coat cannot be a predator adaptation, because camouflage protects prey animals — A student who has met camouflage only in prey picks this. Camouflage is also a physical adaptation of predators, allowing them to stalk or ambush prey unseen.

Syllabus statement B4.2.10 · Read this in Learn

1. They are shade tolerant, able to photosynthesize at the low light intensities reaching the floor — Shade-tolerant shrubs and herbs are one of the strategies for harvesting light in forests: they are adapted to photosynthesize and complete their life cycle under the low light that reaches the forest floor.
2. They obtain carbon compounds from the decaying leaf litter, so they need little light — A student who thinks plants cannot manage in shade picks this. Forest-floor herbs are autotrophs; they photosynthesize at low light intensity rather than feeding on litter.
3. They receive sugar from the canopy trees through underground fungal networks — A student influenced by popular accounts of the 'wood wide web' picks this. Shade-tolerant plants make their own carbon compounds by photosynthesis at low light intensities.
4. They cannot; plants that fail to reach the canopy survive only until they are shaded — A student who sees a single race to the canopy picks this. Shade tolerance is a distinct strategy; these plants live their whole lives on the shaded forest floor.

Syllabus statement B4.2.11 · Read this in Learn

1. Chthamalus is absent from the lower shore because the physical conditions there are beyond its tolerance limits — A student who explains distribution by abiotic factors alone picks this. Chthamalus survived on the lower shore once Balanus was removed, so those conditions are within its tolerance.
2. The fundamental niche of Chthamalus includes the lower shore, but competition restricts its realized niche — The removal experiment shows Chthamalus can tolerate the lower shore, so it lies within its fundamental niche. In the presence of Balanus its realized niche, the extent actually occupied, is restricted to the upper shore by competition.
3. The realized niche of Chthamalus is the whole shore and its fundamental niche is the upper shore — A student who inverts the two terms picks this. The fundamental niche is the potential (whole shore); the realized niche is the actual extent in competition (upper shore only).
4. Removing Balanus caused Chthamalus to adapt so that it could tolerate lower-shore conditions — A student who thinks tolerance changes in response to circumstances picks this. The tolerance limits of Chthamalus were unchanged; it occupied more of a fundamental niche it already had.

Syllabus statement B4.2.12 · Read this in Learn

1. Its niche is the eucalypt woodland of eastern Australia in which it lives and feeds — A student who treats the niche as a place picks this. The woodland is the koala's habitat; the niche is its role in that woodland.
2. Its niche includes its Eucalyptus diet and its tolerance of rainfall and temperature — The niche is the role of the species, made up of biotic interactions such as feeding on Eucalyptus and abiotic interactions such as the ranges of rainfall and temperature it tolerates.
3. Its niche is fully described by its position as a primary consumer of leaves — A student who thinks the niche is only the trophic level picks this. Diet is part of the niche, but the abiotic conditions tolerated and interactions with predators and competitors are part of it too.
4. Its niche includes its diet and its predators, but not the rainfall and temperature it tolerates — A student who keeps abiotic factors out of the niche picks this. The guidance includes abiotic interactions that influence growth, survival and reproduction in the niche.

Syllabus statement B4.2.1 · Read this in Learn

1. X obligate aerobe; Y obligate aerobe; Z obligate anaerobe, as Y grew better with oxygen — A student who reads 'obligate' as 'does best' picks this. Y grew without oxygen as well, so it does not require oxygen; growth in both conditions makes it a facultative anaerobe.
2. X obligate aerobe; Y obligate anaerobe; Z obligate anaerobe, as Y can grow without oxygen — A student who thinks any organism able to grow without oxygen is an obligate anaerobe picks this. Y also grew with oxygen present, which an obligate anaerobe cannot do.
3. X facultative anaerobe; Y facultative anaerobe; Z obligate anaerobe, as X could respire anaerobically — A student who assumes every organism can fall back on anaerobic respiration doubts that X truly needs oxygen. The data show X grew only with oxygen present, so it is an obligate aerobe.
4. X obligate aerobe; Y facultative anaerobe; Z obligate anaerobe, by oxygen tolerance — Classification depends on tolerance of the presence or absence of oxygen. X tolerates only oxygen present (obligate aerobe), Y tolerates both (facultative anaerobe) and Z tolerates only oxygen absent (obligate anaerobe).

Syllabus statement B4.2.2 · Read this in Learn

1. Absorption, because the amino acids are being taken into the dog's tissues — A student who treats absorption and assimilation as the same stage picks this. Absorption was the passage of amino acids across the gut wall into the blood; their use to build protein is a later stage.
2. Assimilation, because the products of digestion become part of the dog — Holozoic nutrition ends with assimilation: absorbed products of digestion are taken up by cells and built into the animal's own molecules and tissues.
3. Digestion, because it is the last step in the processing of the meat — A student who uses 'digestion' for everything the body does with food picks this. Digestion is the hydrolysis of proteins to amino acids, which happened in the gut before absorption.
4. Ingestion, because the amino acids are taken into the muscle cells — A student who applies 'ingest' to uptake by cells picks this. Ingestion is the first stage, taking food into the body through the mouth; uptake by cells is part of assimilation.

Syllabus statement B4.2.4 · Read this in Learn

1. It is an autotroph, because an organism with chloroplasts cannot also be a heterotroph — A student who thinks producer and consumer are exclusive categories picks this. Growth in the dark on organic compounds shows heterotrophic nutrition; Euglena uses both modes and is a mixotroph.
2. It is an obligate mixotroph, because it must photosynthesize whenever light is available — A student who attaches 'obligate' to photosynthesis picks this. An obligate mixotroph needs both modes at once; Euglena survives on either mode alone, so it is facultative.
3. It is one of very few mixotrophs, because mixotrophy is confined to fresh water — A student who knows only the textbook example picks this. Euglena is the well-known freshwater example, but many mixotrophic species are part of oceanic plankton.
4. It is a facultative mixotroph, because either mode alone can sustain it — Euglena grows by photosynthesis alone in the light and by heterotrophic nutrition alone in the dark. A mixotroph that can survive on one mode alone is facultative rather than obligate.

Syllabus statement B4.2.5 · Read this in Learn

1. A diet of meat, since large jaw muscles indicate a powerful bite for seizing and killing prey — A student who reads a robust skull as a predator's picks this. Meat is sheared with cutting teeth; large flat molars, thick enamel and heavy chewing muscles are for grinding tough plant material.
2. An omnivorous diet, since the presence of canines shows that some meat was eaten — A student who treats canines as 'meat teeth' picks this. The canines here are small, and dentition must be read as a whole; the dominant features are the grinding molars and heavy jaw.
3. A diet of tough plant material, since the teeth and jaw are adapted for prolonged grinding — Large flat molars with thick enamel, a massive jaw and a sagittal crest for big chewing muscles match the dentition of living herbivorous mammals that grind fibrous plant food, so a herbivorous diet is deduced.
4. Nothing reliable, since the diet of an extinct species cannot be inferred from its bones alone — A student who accepts only direct observation picks this. Theories relating dentition to diet, built from living mammals, allow the diet of extinct species to be deduced from their skulls.

Syllabus statement B4.2.8 · Read this in Learn

1. The aphid has piercing mouthparts that reach the sap, whereas the caterpillar has chewing mouthparts that bite off leaf tissue — Leaf-eating insects have either piercing or chewing mouthparts. An aphid's stylets penetrate the leaf to feed on sap without removing tissue; a caterpillar's mandibles cut and chew pieces of leaf.
2. Both insects have chewing mouthparts, but the aphid is so small that the holes it bites are too tiny to be seen — A student who assumes all leaf-eating insects chew picks this. Aphids do not bite; they have piercing mouthparts and feed on sap, which is why no tissue is removed.
3. The aphid pierces the leaf to inject a toxin that digests the tissue, whereas the caterpillar chews the tissue — A student who links piercing insects with injecting picks this. The aphid's piercing mouthparts are a feeding adaptation for reaching sap, not for injecting a toxin.
4. Leaves are soft and undefended, so neither insect needs special mouthparts; the aphid is too small to remove visible tissue — A student who thinks plants offer no resistance picks this. Leaves are tough and often toxic, and the two insects differ because they have different mouthparts, piercing stylets in the aphid and chewing mandibles in the caterpillar, not because of size.

Syllabus statement B4.2.9 · Read this in Learn

1. Koalas have become immune to the terpenes, which no longer have any effect on their cells — A student who applies the everyday idea of immunity picks this. The terpenes still enter the koala's body; what differs is that the koala's metabolism chemically breaks them down.
2. Koalas have a metabolic adaptation: liver enzymes break the terpenes down for excretion — Some animals have metabolic adaptations for detoxifying plant toxins. Koalas have enzymes that chemically alter the terpenes into products that can be excreted, allowing them to eat a plant other mammals cannot.
3. The terpenes are waste the leaf cannot excrete, so they have no real effect on animals — A student who thinks secondary compounds are stored waste picks this. Terpenes are toxic secondary compounds that resist herbivory; they poison most mammals that eat the leaves.
4. Terpenes deter insects but do not harm mammals, so koalas need no special adaptation — A student who generalises from grazing livestock picks this. Terpenes are toxic to most mammals; the koala is exceptional because of its detoxifying enzymes.

Syllabus statement B4.2.9 · Read this in Learn

1. Liana: germinates on a high branch and grows roots down the trunk until they reach the soil — A student who confuses stranglers with climbers picks this. A liana is rooted in the soil and climbs up the tree; germinating high and rooting downwards is the strategy of a strangler epiphyte.
2. Epiphyte: absorbs water and carbon compounds from the xylem and phloem of the tree it grows on — A student who thinks epiphytes are parasites picks this. Epiphytes use the branch only as a support to reach light; they photosynthesize for themselves and take water from rain.
3. Forest-floor herb: absorbs carbon compounds from decaying leaves instead of photosynthesizing — A student who thinks plants cannot live in shade picks this. Shade-tolerant herbs photosynthesize at the low light intensities of the forest floor; they are not heterotrophs.
4. Strangler epiphyte: germinates high on a branch and later sends roots down to the soil — A strangler epiphyte starts life as an epiphyte, high up where light is available, and then grows roots down to the soil; as they thicken around the trunk and its crown shades the host, the host tree may die.

Syllabus statement B4.2.11 · Read this in Learn

1. The two species would have reached a stable balance, sharing the bacteria, if the culture had been continued — A student who expects similar species to share picks this. The data show P. caudatum declining steadily to almost nothing, not levelling off; two species with the same niche cannot coexist indefinitely.
2. P. aurelia eliminated P. caudatum by feeding on it, which is why the P. aurelia population stayed high — A student who imagines competition as a fight picks this. Both species feed on bacteria and neither eats the other; P. caudatum declined because P. aurelia used the shared food more effectively.
3. P. caudatum was excluded from the culture because the two species occupied exactly the same niche — With identical food and conditions the two species had the same niche, and competitive exclusion followed: the species that used the bacteria more effectively persisted and the other was eliminated from the culture.
4. P. caudatum became extinct as a species because P. aurelia is better adapted in all conditions — A student who reads elimination as extinction picks this. P. caudatum was eliminated only from this culture; it persists wherever its realized niche does not overlap completely with a superior competitor.

Syllabus statement B4.2.13 · Read this in Learn

1. Both species continue to occupy their whole fundamental niches, sharing the resources between them — A student who expects coexisting species to share picks this. Where niches overlap, competition either eliminates one species or restricts both to part of their fundamental niches.
2. The species that loses is driven to extinction throughout its range, wherever it occurs — A student who equates exclusion with extinction picks this. Elimination is from the area of competition; the species can persist elsewhere or in parts of its fundamental niche the competitor does not use.
3. The stronger species eliminates the weaker by preying on it until none remains in the area — A student who confuses competition with predation picks this. Competitors do not eat each other; one species is eliminated because the other uses the shared resource more effectively.
4. One species is eliminated, or both are restricted to part of their fundamental niche — The guidance gives exactly these two outcomes. Either competitive exclusion removes one species from the area, or each species is confined to a realized niche that is only part of its fundamental niche.

Syllabus statement B4.2.13 · Read this in Learn

1. A was herbivorous, grinding tough plant material; B was omnivorous, with unspecialised teeth suited to a mixed diet — Large flat molars, a massive jaw and a crest match living herbivores that grind fibrous plants; small unspecialised teeth with reduced canines match living omnivores such as Homo sapiens, so A is deduced to be herbivorous and B omnivorous.
2. A was carnivorous, because a massive jaw and crest indicate a powerful killing bite; B was herbivorous — A student who reads a massive jaw and crest as a predator's picks this. Meat is sheared by cutting teeth; heavy chewing muscles and large flat molars are for grinding tough plant food, so A was herbivorous and B, with unspecialised teeth, omnivorous.
3. Both were carnivorous, because both skulls have canine teeth, which are the teeth used for tearing meat — A student who treats canines as 'meat teeth' picks this. Almost all mammals have canines; dentition must be read as a whole, and neither skull has the large pointed canines and shearing teeth of a carnivore.
4. Nothing can be deduced about the diet of either species, because neither of them can be observed feeding — A student who accepts only direct observation picks this. Theories relating dentition to diet, developed from living mammals, allow the diets of both extinct species to be deduced from their skulls.

Syllabus statement B4.2.8 · Read this in Learn