- Farmers often use crop rotation, planting a field with a leguminous crop like soybeans one year and a non-leguminous crop like corn the next. What is the primary benefit of this practice related to nutrient cycling?
Answer: Soybeans host nitrogen-fixing bacteria (Rhizobium), which convert atmospheric nitrogen into a form usable by plants, enriching the soil.
Leguminous plants like soybeans form mutualistic relationships with Rhizobium bacteria in their root nodules. These bacteria perform nitrogen fixation, converting atmospheric N₂ gas into ammonia (NH₃), which can then be used by the plant. When the soybean crop is harvested or tilled into the soil, this fixed nitrogen becomes available, enriching the soil and reducing the need for synthetic fertilizers for the subsequent corn crop.
- [HL] Comparing a tropical rainforest and an arctic tundra ecosystem, which statement correctly describes their productivity?
Answer: The NPP of the rainforest is much higher due to greater light intensity, temperature, and precipitation, leading to a high GPP and high biomass.
Tropical rainforests have optimal conditions for photosynthesis year-round (high temperature, high light intensity, abundant water). This leads to a very high gross primary productivity (GPP). While autotrophic respiration is also high, the GPP is so massive that the net primary productivity (NPP = GPP - R) is the highest of any terrestrial ecosystem. Tundra has low temperature, a short growing season, and often frozen water, all of which severely limit GPP and consequently NPP. The ratio of NPP to GPP is not necessarily higher in the tundra; while respiration is low, GPP is extremely low.
- What is the primary role of the decomposers in maintaining the long-term stability of this sealed mesocosm?
Answer: To release inorganic nutrients from dead organic matter, making them available for re-use by the plants.
In a sealed mesocosm, nutrients are finite. The primary role of decomposers (like bacteria and fungi) is to break down dead organic matter (dead plants, waste) and mineralize it, releasing essential inorganic nutrients (like nitrates, phosphates) back into the soil. These nutrients can then be taken up by the plants, allowing the cycle of matter to continue, which is crucial for long-term stability.
- Both chemoautotrophs at hydrothermal vents and photoautotrophs in terrestrial ecosystems are producers. What is the fundamental difference in how they acquire energy to synthesize organic molecules?
Answer: Chemoautotrophs oxidize inorganic chemicals, while photoautotrophs capture light energy.
The defining characteristic of an autotroph is its ability to produce its own food, typically by fixing carbon. The prefix indicates the energy source used for this process. Photoautotrophs ('photo' = light) use light energy from the sun via photosynthesis. Chemoautotrophs ('chemo' = chemical) derive energy from the oxidation of inorganic substances, such as hydrogen sulfide (H₂S) common at hydrothermal vents, to fuel the process of chemosynthesis.
- Why do food chains in most ecosystems rarely extend beyond four or five trophic levels?
Answer: The cumulative loss of energy at each trophic transfer is so large that there is insufficient energy to support populations at higher levels.
The length of food chains is limited by the efficiency of energy transfer. Only about 10% of the energy from one trophic level is converted into biomass at the next level; the rest is lost, primarily as heat during metabolic processes. After three or four transfers (four or five levels), the amount of available energy is too low to sustain a viable population of consumers.
- Which of the following describes the process of nitrification in the nitrogen cycle?
Answer: The conversion of ammonia (NH₃) into nitrites (NO₂⁻) and then into nitrates (NO₃⁻) by bacteria.
Nitrification is a two-step process carried out by specific soil bacteria. First, bacteria like Nitrosomonas oxidize ammonia or ammonium ions to nitrites (NO₂⁻). Then, other bacteria like Nitrobacter oxidize the nitrites to nitrates (NO₃⁻). Nitrates are the primary form of nitrogen taken up by plant roots. The other options describe nitrogen fixation (A), denitrification (B), and ammonification (D).
- The concentration of a persistent, fat-soluble pesticide was measured at 0.04 ppm in the phytoplankton of a lake. If there is an average of 90% energy loss at each trophic transfer, what is the most probable concentration of the pesticide in a fish that primarily eats zooplankton?
Answer: 4.0 ppm
This question links biomagnification with energy loss. A 90% energy loss means only 10% of the biomass is transferred, but the persistent pesticide is retained and accumulates. This leads to an approximate 10-fold increase in concentration at each trophic level. Phytoplankton (producer) -> Zooplankton (primary consumer) -> Fish (secondary consumer). The concentration in zooplankton would be 0.04 ppm 10 = 0.4 ppm. The concentration in the fish that eats zooplankton would be 0.4 ppm 10 = 4.0 ppm.
- Peat forms in anaerobic, acidic waterlogged areas called bogs. How does the formation and harvesting of peat for fuel affect the carbon cycle?
Answer: Peat forms when decomposition is incomplete, sequestering carbon. Burning it releases this stored carbon as CO₂.
Peat is partially decomposed organic matter that accumulates in anaerobic and acidic conditions where decomposers like bacteria and fungi cannot function effectively. This process sequesters, or locks away, carbon that would otherwise have been released back into the atmosphere through full decomposition. When humans harvest and burn peat as fuel, this long-stored carbon is rapidly oxidized and released as carbon dioxide, contributing to the greenhouse effect.
- What is a direct consequence of the low efficiency of energy transfer between trophic levels?
Answer: The biomass of top carnivores is very small compared to the biomass of herbivores in a given ecosystem.
Because only about 10% of the energy (and thus potential biomass) is transferred from one level to the next, each successive trophic level can only support a fraction of the biomass of the level below it. This results in a pyramid of biomass (and energy) where the total mass of top carnivores (tertiary or quaternary consumers) is significantly smaller than the biomass of herbivores (primary consumers) they feed on.
- The burning of fossil fuels and deforestation both contribute to an increase in atmospheric CO₂. How do their impacts on the carbon cycle differ?
Answer: Fossil fuel combustion releases carbon from the lithosphere, while deforestation reduces the capacity of the biosphere to assimilate atmospheric carbon.
The key difference lies in the source and mechanism. Burning fossil fuels releases vast quantities of carbon that were stored in the lithosphere for millions of years, adding it to the active cycle. Deforestation has a dual effect: the burning or decomposition of trees releases stored carbon, and more importantly, it removes a major sink for atmospheric CO₂, reducing the planet's overall capacity for photosynthesis to draw down CO₂ from the atmosphere.
- The biomass of primary consumers in an ecosystem is 500 kg. The biomass of secondary consumers is 60 kg. The energy value of the biomass is 20 kJ g⁻¹. What is the approximate efficiency of energy transfer between these two trophic levels?
Answer: 12.0%
First, calculate the total energy at each trophic level. The energy value is given per gram, so convert kg to g. Primary consumer energy = 500 kg 1000 g/kg 20 kJ/g = 10,000,000 kJ. Secondary consumer energy = 60 kg 1000 g/kg 20 kJ/g = 1,200,000 kJ. Efficiency = (energy at higher level / energy at lower level) 100 = (1,200,000 kJ / 10,000,000 kJ) 100 = 0.12 100 = 12.0%. The energy value per gram is the same for both levels and cancels out, so the calculation can be simplified to (60 kg / 500 kg) 100.
- Which comparison of carbon fluxes is most accurate?
Answer: The flux of carbon from terrestrial respiration to the atmosphere is of a similar magnitude to the flux from the atmosphere via photosynthesis.
On a global scale, the amount of carbon taken up by terrestrial photosynthesis and the amount released by terrestrial respiration (from plants, animals, and decomposers) are very large fluxes that are roughly in balance. Fossil fuel combustion, while critically important for the net increase in atmospheric CO₂, is a smaller flux than either gross photosynthesis or respiration. Limestone formation is a very slow, long-term process.
- [HL] A scientist is studying energy flow. Gross production of producers is 10,000 kJ m⁻². Respiration of producers is 6,000 kJ m⁻². Gross production of herbivores is 500 kJ m⁻². Respiration of herbivores is 300 kJ m⁻². What is the net production of the herbivores?
Answer: 200 kJ m⁻²
Net production (NP) at any trophic level is calculated as the gross production (GP) minus the energy lost through respiration (R) at that same level. For the herbivores: NP = GP - R = 500 kJ m⁻² - 300 kJ m⁻² = 200 kJ m⁻². The data for the producers is extra information designed to distract the test-taker. The question specifically asks for the net production of the herbivores.
- Waterlogged soils in marshes and swamps are anaerobic. Which process in the nitrogen cycle is most likely to be accelerated in these conditions, leading to a net loss of nitrogen from the soil?
Answer: Denitrification
Denitrification is the process where bacteria convert nitrates (NO₃⁻) back into gaseous nitrogen (N₂), which then escapes to the atmosphere. This process is carried out by anaerobic bacteria that use nitrate as a final electron acceptor in the absence of oxygen. Therefore, waterlogged, anaerobic soils promote denitrification, leading to a loss of usable nitrogen from the ecosystem.
- In some marine ecosystems, the pyramid of biomass can be inverted, with a smaller biomass of producers (phytoplankton) than primary consumers (zooplankton) at any given time. How can the pyramid of energy for this same ecosystem be explained?
Answer: The pyramid of energy is upright because the high turnover rate of phytoplankton means their rate of energy production is greater than that of the zooplankton.
Pyramids of energy are always upright due to the second law of thermodynamics; energy is always lost at each trophic transfer. An inverted pyramid of biomass occurs when the producers have a very high turnover rate (i.e., they reproduce and are consumed very quickly). Although their standing biomass at any one time is low, their rate of energy production (productivity) over time is high enough to support a larger biomass of primary consumers.
- Runoff from an agricultural field treated with excess ammonium-based fertilizer enters a nearby lake. Which sequence of events is most likely to occur, leading to a fish kill?
Answer: Nitrifying bacteria convert ammonia to nitrates -> An algal bloom occurs -> Decomposers break down dead algae -> Oxygen is depleted.
This describes eutrophication. 1. The excess ammonium is oxidized to nitrates by nitrifying bacteria. 2. Nitrates are a limiting nutrient for algae, so their population explodes (an algal bloom). 3. The algae have short lifespans and die, sinking to the bottom. 4. Aerobic decomposers break down the vast amount of dead algae. 5. This decomposition consumes large amounts of dissolved oxygen from the water, leading to hypoxia or anoxia, which kills the fish.
- Methanogenic archaea play a role in the carbon cycle. Under what conditions are they most active, and what is their metabolic process?
Answer: Anaerobic conditions; they convert organic acids and carbon dioxide into methane.
Methanogenic archaea are obligate anaerobes, meaning they thrive in oxygen-free environments like waterlogged soils, ruminant guts, and landfill sites. They carry out methanogenesis by reducing carbon dioxide with H₂ or by cleaving acetate (an organic acid) to produce methane (CH₄) and CO₂. This is a form of anaerobic respiration.
- In a temperate forest ecosystem, the gross primary productivity (GPP) is measured at 20,000 kJ m⁻² yr⁻¹. Autotrophic respiration accounts for 60% of the GPP. If herbivores consume 15% of the net primary productivity (NPP), what is the amount of energy that becomes available to decomposers from dead primary producer biomass annually?
Answer: 6,800 kJ m⁻² yr⁻¹
This is a multi-step calculation. First, calculate autotrophic respiration (R): R = 0.60 20,000 = 12,000 kJ m⁻² yr⁻¹. Second, calculate net primary productivity (NPP): NPP = GPP - R = 20,000 - 12,000 = 8,000 kJ m⁻² yr⁻¹. Third, calculate the energy consumed by herbivores: 0.15 8,000 = 1,200 kJ m⁻² yr⁻¹. Finally, calculate the energy from dead primary producers available to decomposers, which is the NPP not consumed by herbivores: 8,000 - 1,200 = 6,800 kJ m⁻² yr⁻¹.
- Which statement best distinguishes the flow of energy from the cycling of inorganic nutrients in an ecosystem?
Answer: Energy flows unidirectionally and is dissipated as heat, while nutrients are cycled between abiotic and biotic components.
The fundamental difference is that energy flows in one direction through an ecosystem—from the sun, to producers, to consumers, with a significant loss as heat at each transfer. It is not recycled. Inorganic nutrients, however, are finite and are continuously cycled between living (biotic) and non-living (abiotic) parts of the ecosystem, largely through the action of decomposers.
