Ecology & Ecosystems

A pyramid of energy for a typical ecosystem shows roughly 10% of energy transferred between successive trophic levels. Which factor most directly explains energy 'lost' between a primary consumer and a secondary consumer?

• AEnergy used in respiration and lost as heat by the primary consumer✓
• BEnergy reflected from the leaf surfaces of the producers rather than absorbed
• CEnergy arriving in wavelengths of light that chlorophyll is unable to absorb
• DEnergy released as heat by the decomposers living in the soil

Answer: Much of the energy assimilated by a consumer is released in respiration and ultimately lost as heat to the surroundings; together with losses in faeces and excretion, this means only a small proportion is available to the next trophic level.

Net primary productivity (NPP) of a producer is best defined as:

• AThe total quantity of energy fixed by photosynthesis per unit area per unit time
• BThe gross primary productivity minus the energy lost in respiration by the producer✓
• CThe energy transferred to primary consumers per unit area per unit time
• DThe energy lost as heat by producers per unit area per unit time

Answer: Gross primary productivity (GPP) is the total energy fixed by photosynthesis. Net primary productivity is GPP minus respiratory losses by the producer (NPP = GPP − R); this remaining chemical energy is available for growth and to consumers.

In the nitrogen cycle, which process converts ammonium ions into nitrite and then nitrate ions, and which type of organism carries it out?

• ANitrogen fixation, by Rhizobium bacteria living in the root nodules of legumes
• BDenitrification, by anaerobic denitrifying bacteria living in waterlogged soils
• CNitrification, by nitrifying bacteria such as Nitrosomonas and Nitrobacter✓
• DAmmonification, by saprobiotic (decomposer) microorganisms feeding on dead matter

Answer: Nitrification is the oxidation of ammonium ions to nitrite (by Nitrosomonas) and then to nitrate (by Nitrobacter). These free-living nitrifying bacteria are aerobic, so well-aerated soils favour nitrate production.

Which of the following best describes the role of decomposers (saprobionts) in the carbon cycle?

• AThey fix atmospheric carbon dioxide into organic compounds by photosynthesising in the soil
• BThey release carbon dioxide back into the atmosphere by respiring dead organic matter✓
• CThey convert dissolved carbon dioxide into the carbonate of animal shells
• DThey prevent the return of carbon to the atmosphere by locking it into their own biomass

Answer: Saprobionts feed on dead organic matter and respire it, releasing carbon dioxide back to the atmosphere. This decomposition is a key route returning carbon from dead organisms to the atmospheric/abiotic pool.

The carrying capacity of an environment for a particular species is best described as:

• AThe fastest rate at which a population can reproduce when food, water and space are unlimited
• BThe total number of different species that the environment contains
• CThe maximum population size that the environment can support sustainably given available resources✓
• DThe minimum population size needed to avoid extinction through inbreeding

Answer: Carrying capacity is the maximum population size that the available resources (food, water, space, etc.) can sustain over time. As a population approaches it, density-dependent factors slow growth so numbers level off.

During primary succession on bare rock, what is the typical role of the pioneer species, such as lichens?

• AThey form the stable climax community that persists indefinitely once the bare rock surface has been fully colonised by them
• BThey reduce biodiversity by preventing any further species from colonising the rock
• CThey are the largest organisms present and shade out their competitors for light
• DThey colonise the inhospitable surface and, on death, contribute to soil formation, enabling other species to establish✓

Answer: Pioneer species are adapted to harsh abiotic conditions. By colonising bare rock and then dying and decomposing, they help form a thin soil and change conditions, allowing less tolerant species to colonise in later seral stages.

In a classic predator-prey relationship, which statement best describes the typical pattern of the two population sizes over time?

• APredator and prey populations rise and fall together in perfect synchrony each year
• BThe predator population always peaks shortly before the prey population does
• CNeither population changes in size once a stable community has been reached
• DThe predator population peaks slightly after the prey population, and the cycles are out of phase✓

Answer: Predator and prey populations oscillate out of phase: a rise in prey provides more food, so predators increase; the larger predator population then reduces prey numbers, after which predators decline. The predator peak lags slightly behind the prey peak.

What is a niche?

• AThe physical place in which a species lives
• BThe total number of individuals of one species
• CThe role and position of a species within its ecosystem✓
• DAll the species living together in one area

Answer: A niche covers everything about how a species lives — what it eats, when it is active, what it tolerates. Two species cannot occupy the same niche indefinitely; one will outcompete the other. The physical place is the habitat.

What is carrying capacity?

• AThe minimum population an environment can sustain
• BThe rate at which a population is currently growing
• CThe maximum population an environment can sustain✓
• DThe total biomass of producers in the ecosystem

Answer: Carrying capacity is set by limiting factors — food, water, space, predation, disease. Populations tend to level off around it, and may oscillate above and below as those factors bite.

Why is energy transfer between trophic levels typically only about 10% efficient?

• AEnergy is lost through respiration, movement and excretion✓
• BEnergy is lost through photosynthesis, movement and excretion
• CMost energy is stored permanently within the producer's biomass
• DMost energy is converted into new mineral ions in the soil

Answer: Much of the energy consumed is used in respiration and lost as heat, some is never eaten, and some leaves in faeces and urine. Only the fraction incorporated into new biomass is available to the next level — which is why food chains are short.

What is net primary production?

• AGross production plus the energy gained by plant respiration
• BThe total energy captured by producers from sunlight
• CGross production minus the energy lost in plant respiration✓
• DThe total energy transferred to the primary consumers

Answer: NPP = GPP − R, and it is the energy actually available to the next trophic level as new plant biomass. GPP is the total captured before the plant's own respiratory losses.

What is the role of saprobionts in the nitrogen cycle?

• ADecomposing organic matter and releasing nitrate ions
• BConverting nitrogen gas into ammonia within root nodules
• CConverting nitrate ions back into nitrogen gas in the soil
• DDecomposing organic matter and releasing ammonium ions✓

Answer: Saprobionts break down proteins in dead matter and excreta, releasing ammonium — ammonification. Nitrifying bacteria then oxidise that to nitrite and nitrate; fixation and denitrification are separate processes.

What do nitrifying bacteria do?

• AReduce nitrate to nitrite and then to nitrogen gas
• BConvert atmospheric nitrogen directly into ammonia
• COxidise ammonium to nitrite and then to nitrate✓
• DBreak down dead organisms to release ammonium ions

Answer: Nitrification is an oxidation carried out in two steps by different genera, and it requires oxygen — which is why waterlogged soils are poor in nitrate. Denitrifying bacteria carry out the reverse in anaerobic conditions.

What is primary succession?

• ARecolonisation of ground where soil already remains
• BThe gradual replacement of a climax community
• CColonisation of bare ground with no existing soil✓
• DThe seasonal change in species present in an area

Answer: Primary succession starts on bare rock or sand, where pioneer species must create soil as they die and decompose. Secondary succession begins where soil survives, as after a fire, and is therefore much faster.

Why does species diversity generally increase during succession?

• APioneer species prevent other species from establishing
• BThe abiotic environment becomes progressively harsher
• CCompetition between species steadily decreases over time
• DPioneer species create conditions allowing others to establish✓

Answer: Pioneers weather rock and add organic matter, building soil that supports larger plants and more animals, and each stage makes conditions less hostile. Diversity usually peaks before the climax community, when a few dominant species take over.

How is the mark-release-recapture method used to estimate population size?

• ATotal is first sample plus second sample divided by the marked recaptures
• BTotal is the number of marked individuals recaptured in the second sample
• CTotal is first sample times second sample divided by the marked recaptures✓
• DTotal is the second sample size divided by the first sample size

Answer: The Lincoln index assumes marked individuals mix randomly and are neither harmed nor made more conspicuous, and that the population is closed. Breaking those assumptions is the usual source of error.

In a mark–release–recapture study of a snail population, 35 snails were marked with paint and released. A week later, 42 snails were caught, and 6 of them were marked. What is the estimated population size?

• A245✓
• B1470
• C83
• D7

Answer: Population = (number marked × number in second sample) ÷ number recaptured marked = (35 × 42) ÷ 6 = 1470 ÷ 6 = 245. The tempting 1470 forgets to divide by the recaptures; 83 adds the three numbers, and 7 is 42 ÷ 6.

In a mark–release–recapture study of snails, the paint used made marked snails easier for song thrushes to see, so marked snails were eaten more often than unmarked ones between the two samples.

How would this affect the estimate of population size?

• AIt would be an underestimate, as fewer marked snails would be recaptured
• BIt would be an underestimate, as more unmarked snails would be caught later
• CIt would be an overestimate, as fewer marked snails would be recaptured✓
• DIt would be unaffected, as predators remove marked and unmarked snails alike

Answer: Estimate = (marked × second sample) ÷ marked recaptured. Selective predation removes marked snails, so the number recaptured falls; dividing by a smaller number gives a larger estimate, so the population is overestimated. Fewer recaptures cannot give an underestimate, and the stem says predation is not random, which breaks an assumption of the method.

The net production of the primary consumers in a pond is 1450 kJ m⁻² year⁻¹, and that of the secondary consumers is 116 kJ m⁻² year⁻¹. What is the percentage efficiency of energy transfer between these two trophic levels?

• A12.5%
• B8.0%✓
• C0.08%
• D92%

Answer: Efficiency = 116 ÷ 1450 × 100 = 8.0%. The tempting 12.5% divides the larger value by the smaller (1450 ÷ 116) and is a ratio, not a percentage; 0.08% forgets to multiply by 100, and 92% is the percentage NOT transferred.

A student estimated the mean percentage cover of clover on a lawn using randomly placed quadrats, recording a running mean as more quadrats were added. After 4 quadrats: 31% After 8 quadrats: 24% After 12 quadrats: 27% After 16 quadrats: 26% After 20 quadrats: 26%

What do these results suggest about the number of quadrats needed?

• AAbout 4 are enough, because the first running mean is already near the final mean
• BAbout 16 are enough, because the running mean changes very little after that point✓
• CAbout 8 are enough, because the running mean reaches its lowest value at that point
• DAt least 40 are needed, because a running mean keeps changing as quadrats are added

Answer: A running mean levels off once enough quadrats have been sampled to give a representative mean. Here it changes by 1% between 12 and 16 quadrats and not at all between 16 and 20, so about 16 is sufficient. After 4 quadrats the mean (31%) is still 5% away from the final value, and the lowest value at 8 quadrats is a fluctuation, not a stable mean.

Soil samples were incubated with compound N. After 14 days, compared with untreated soil: Ammonium ion concentration: 310% Nitrite ion concentration: 8% Nitrate ion concentration: 21%

Which conclusion is best supported?

• AN stops saprobionts releasing ammonium, so less nitrite and nitrate are formed
• BN speeds up denitrification, so more nitrate is converted into nitrogen gas
• CN kills nitrogen-fixing bacteria, so less ammonium is made from nitrogen gas
• DN stops bacteria oxidising ammonium, so ammonium builds up and less nitrate forms✓

Answer: Ammonium has more than trebled while nitrite and nitrate have fallen sharply, so the step converting ammonium to nitrite (and then nitrate) is blocked: N inhibits the nitrifying bacteria. If saprobionts or nitrogen fixers were inhibited, ammonium would fall, not rise. Faster denitrification would lower nitrate but would not explain the build-up of ammonium.