- Scientists measure the concentration of a pollutant in four species from a food web: Species A (0.05 ppm), Species B (1.5 ppm), Species C (25.0 ppm), and Species D (1.7 ppm). Given that species C preys on both B and D, and that both B and D prey on A, which species occupies the highest trophic level?
Answer: Species C
Persistent pollutants biomagnify, meaning their concentration increases at successively higher trophic levels. Species A has the lowest concentration, suggesting it is the producer. Species B and D feed on A, making them primary consumers. Species C feeds on B and D, making it a secondary consumer. Consistent with this, Species C has the highest concentration of the pollutant, indicating it is at the highest trophic level in this food web.
- A secondary consumer eats a primary consumer. The primary consumer's body contained 1000 kJ of energy. Due to metabolic inefficiencies, the secondary consumer only manages to convert 120 kJ of that energy into its own new biomass. What happened to the remaining 880 kJ of energy?
Answer: It was converted into a larger amount of lower-quality energy (heat) and lost to the environment.
The question specifies that the 1000 kJ was in the primary consumer's body, meaning it was already assimilated. The 120 kJ converted to new biomass is the net secondary production. The vast majority of the difference (assimilated energy minus production) is lost as heat during cellular respiration, a consequence of the second law of thermodynamics. While some energy might be egested if the 1000kJ was consumed not assimilated, the most significant loss from assimilated energy is respiratory heat.
- An ecologist observes an open ocean ecosystem where the standing crop of phytoplankton (producers) is consistently lower than the standing crop of zooplankton (primary consumers), resulting in an inverted pyramid of biomass. What is the best explanation for this observation?
Answer: The phytoplankton have an extremely high rate of reproduction and are consumed rapidly.
An inverted pyramid of biomass is possible when the producers have a very short lifespan and high reproductive rate (high turnover). Even though the phytoplankton biomass at any single moment is low, they reproduce so quickly that they can support a larger standing crop of longer-lived primary consumers. The pyramid of energy for this ecosystem would still be upright.
- A forest ecosystem has the following annual carbon fluxes: absorption by photosynthesis is 15 GtC, release by plant respiration is 8 GtC, and release by soil and decomposer respiration is 6 GtC. Assuming no other major fluxes such as fire, what is the net ecosystem production (NEP) for this forest?
Answer: A net sink of 1 GtC per year
Net ecosystem production (NEP) is the total carbon influx minus the total carbon outflux. Influx = Photosynthesis = 15 GtC. Outflux = Plant Respiration + Decomposer Respiration = 8 GtC + 6 GtC = 14 GtC. Therefore, NEP = 15 GtC - 14 GtC = +1 GtC. A positive value indicates that the ecosystem is accumulating carbon, acting as a net sink.
- In an aquatic food chain, the concentration of a persistent toxin is 0.04 ppm in phytoplankton. Zooplankton that feed on phytoplankton have a concentration of 0.60 ppm. Small fish that feed on zooplankton have a concentration of 7.2 ppm. What is the biomagnification factor from the zooplankton to the small fish?
Answer: 12
The question asks for the biomagnification factor specifically between zooplankton and small fish. This is calculated by dividing the concentration in the higher trophic level (small fish) by the concentration in the lower trophic level (zooplankton). Calculation: 7.2 ppm / 0.60 ppm = 12.
- In an effort to sequester atmospheric carbon, a large-scale reforestation project is undertaken on land previously used for grazing. Assuming other factors remain constant, which change in carbon fluxes is most likely to occur in this area during the first few decades of forest growth?
Answer: The flux of carbon from the atmosphere to biomass will increase significantly.
Reforestation involves planting trees, which have a high rate of photosynthesis. This process fixes atmospheric carbon dioxide (CO₂) into organic compounds (biomass). Therefore, the most significant and direct change will be an increased flux of carbon from the atmosphere into the growing plant biomass.
- In a marine food web, sea otters prey on sea urchins, which in turn graze on large kelp forests. If a disease epidemic causes a drastic decline in the sea otter population, what is the most probable indirect effect on the transfer of energy within this ecosystem?
Answer: Energy transfer to all other primary consumers that feed on kelp will be reduced.
This describes a trophic cascade. The decline in sea otters (the keystone predator) will lead to a rapid increase in the sea urchin population. The large number of sea urchins will overgraze the kelp forests, drastically reducing the total producer biomass. This loss of kelp means there is less energy available for all primary consumers that rely on it, not just the urchins.
- The Haber-Bosch process, used for industrial fertilizer production, converts atmospheric N₂ to ammonia (NH₃). What is the most direct and significant impact of the widespread use of these fertilizers on the global nitrogen cycle?
Answer: It has dramatically increased the rate of nitrogen fixation, adding a massive new input of reactive nitrogen into terrestrial and aquatic ecosystems.
The Haber-Bosch process is an artificial method of nitrogen fixation. Its industrial scale is so vast that it is estimated to have doubled the rate at which nitrogen is removed from the atmosphere and converted into reactive forms available to organisms. This massive influx of new nitrogen into the biosphere, far exceeding natural fixation rates, has profound effects on ecosystems, including eutrophication.
- An ecosystem's producers have a Gross Primary Production (GPP) of 20,000 kJ m⁻² yr⁻¹ and lose 12,000 kJ m⁻² yr⁻¹ through cellular respiration. Primary consumers in this ecosystem assimilate 800 kJ m⁻² yr⁻¹ from consuming the producers. What is the approximate trophic efficiency of energy transfer from the producers to the primary consumers?
Answer: 10%
Trophic efficiency is the percentage of energy from one trophic level that is incorporated into the biomass of the next. First, calculate the Net Primary Production (NPP), which is the energy available to primary consumers: NPP = GPP - Respiration = 20,000 - 12,000 = 8,000 kJ m⁻² yr⁻¹. Then, calculate the efficiency: (Energy assimilated by primary consumers / NPP) 100 = (800 / 8,000) 100 = 10%.
- The conversion of atmospheric nitrogen gas (N₂) into ammonia (NH₃) or ammonium (NH₄⁺) is an essential step in the nitrogen cycle. What is this process called, and why is it a limiting factor in many ecosystems?
Answer: Nitrogen fixation; it is an energetically expensive process carried out by only a few types of microorganisms.
The process of converting N₂ gas into ammonia/ammonium is called nitrogen fixation. The triple bond in N₂ is extremely strong, and breaking it requires a large amount of energy. Only a limited number of specialized prokaryotes (e.g., Rhizobium, cyanobacteria) can perform this conversion, making the availability of fixed nitrogen a major limiting factor for plant growth in many environments.
- Which statement best distinguishes the movement of energy from the movement of matter in ecosystems?
Answer: Energy flows unidirectionally and is dissipated as heat, whereas matter is cycled between biotic and abiotic pools.
The fundamental difference is that energy flows through an ecosystem, typically entering as light and exiting as heat at each trophic level transfer, following the laws of thermodynamics. It does not cycle. In contrast, chemical elements (matter) like carbon, nitrogen, and phosphorus are finite and are continuously recycled between living organisms (biotic) and the non-living environment (abiotic).
- In aquatic ecosystems, carbon dioxide dissolves in water to form carbonic acid (H₂CO₃), which can then dissociate into hydrogen carbonate ions (HCO₃⁻). What is the primary significance of this chemical equilibrium for aquatic food webs?
Answer: It provides an alternative source of inorganic carbon that aquatic producers can use for photosynthesis.
Many aquatic producers, including algae and submerged plants, have adaptations (like the enzyme carbonic anhydrase) that allow them to utilize hydrogen carbonate ions (HCO₃⁻) as a source of carbon for photosynthesis, in addition to dissolved CO₂. This equilibrium makes a larger pool of inorganic carbon available for fixation into organic matter at the base of the aquatic food web.
- Peat bogs are characterized by acidic, waterlogged, and anaerobic conditions that inhibit the activity of decomposers. How does the formation of peat affect the global carbon cycle?
Answer: It functions as a net carbon sink by significantly slowing the flux of carbon from dead biomass to the atmosphere.
The primary effect of peat bog conditions is the incomplete decomposition of organic matter. Plant biomass, which contains fixed carbon, accumulates over thousands of years instead of being respired by decomposers. This traps carbon that would otherwise be released as CO₂, making peat bogs a significant long-term carbon sink.
- Food chains are generally limited to four or five trophic levels. Which hypothetical change to an ecosystem would most likely allow for the support of a viable population at a sixth trophic level?
Answer: A significant increase in the net primary production of the ecosystem.
The length of a food chain is limited by the amount of energy available at its base. Due to the ~90% energy loss at each trophic transfer, there is insufficient energy to support a viable population at higher levels. If the net primary production (the energy captured by producers) were to increase significantly, there would be more energy available at the base of the food web, which could potentially be enough to be transferred up through the trophic levels to support a sixth level.
- Based on the data provided, what can be deduced about the energy allocation in these two ecosystems?
Answer: The rainforest allocates a greater proportion of its gross production to respiration.
Respiration (R) can be calculated as GPP - NPP. For the rainforest, R = 2200 - 1000 = 1200. The proportion of GPP used for respiration is 1200/2200 ≈ 54.5%. For the grassland, R = 600 - 300 = 300. The proportion is 300/600 = 50%. Therefore, the rainforest allocates a greater proportion of its GPP to respiration.
- A farmer observes that corn plants are showing signs of chlorosis (yellowing leaves) despite the application of a fertilizer rich in ammonium (NH₄⁺). The field is heavily compacted and waterlogged. Which process in the nitrogen cycle is most likely inhibited, directly causing these symptoms?
Answer: Nitrification
Nitrification is the aerobic process where bacteria convert ammonium (NH₄⁺) into nitrates (NO₃⁻), the form of nitrogen most readily absorbed by corn plants. In waterlogged, anaerobic soil, the bacteria responsible for nitrification cannot function effectively. Although ammonium was added, it is not being converted to the usable nitrate form, leading to nitrogen deficiency and chlorosis.
- What is the Net Secondary Production (NSP) for this insect?
Answer: 300 J
First, calculate the assimilated energy, or Gross Secondary Production (GSP). GSP = Energy Consumed - Energy Egested = 2500 J - 1200 J = 1300 J. Next, calculate the Net Secondary Production (NSP), which is the energy converted to new biomass. NSP = GSP - Respiration = 1300 J - 1000 J = 300 J.
- Which statement provides the most fundamental, universal reason for the progressive loss of energy at each successive trophic level in an ecosystem?
Answer: Energy is converted into heat during metabolic activities in accordance with the second law of thermodynamics.
While choices A, B, and D describe valid ways in which energy is lost from a trophic pathway, the most fundamental reason is rooted in physics. The second law of thermodynamics states that in any energy conversion, some energy is lost as unusable heat. Cellular respiration, the process by which all organisms release energy from food, is an energy conversion that releases a significant amount of heat. This is a universal and unavoidable loss at every trophic level.
- Which is the most likely outcome regarding their net secondary production (NSP)?
Answer: The caterpillar will have a higher NSP because it allocates less assimilated energy to maintaining body temperature.
Net secondary production (NSP) is the assimilated energy minus the energy lost to respiration. Endotherms like shrews have high metabolic rates and expend a large proportion of their assimilated energy on respiration to maintain a constant internal body temperature. Ectotherms like caterpillars do not, so a much smaller proportion of their assimilated energy is lost to respiration. Consequently, the caterpillar will have more energy left over to allocate to growth and reproduction (NSP).
