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 by leaf surfaces of the producers
• CEnergy that is the wrong wavelength for photosynthesis
• DEnergy converted into light by the consumers

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
• BDenitrification, by anaerobic denitrifying bacteria
• CNitrification, by nitrifying bacteria such as Nitrosomonas and Nitrobacter✓
• DAmmonification, by saprobiotic (decomposer) microorganisms

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
• BThey release carbon dioxide back into the atmosphere by respiring dead organic matter✓
• CThey convert carbon dioxide into carbonate in animal shells
• DThey prevent the return of carbon to the atmosphere by locking it in 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 possibly reproduce
• BThe total number of different species the environment contains
• CThe maximum population size that the environment can support sustainably given available resources✓
• DThe minimum population size needed to avoid extinction

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 are the climax community that persists indefinitely
• BThey reduce biodiversity by preventing any further colonisation
• CThey are the largest organisms present and shade out all competitors
• 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
• BThe predator population always peaks before the prey population
• CNeither population changes once a stable community is 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.