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Quiz: Ecosystem Ecology, Biogeochemical Cycles, and Conservation

Test your understanding of energy flow, biogeochemical cycles, biomes, and conservation biology with these review questions.

1. According to the ten percent energy rule, approximately what percentage of energy at one trophic level is transferred to the next trophic level?

  1. 1%
  2. 10%
  3. 50%
  4. 90%
Show Answer

The correct answer is B. On average, only about 10% of the energy available at one trophic level is transferred to the next; the remaining ~90% is lost as metabolic heat during cellular respiration, used for life processes, or contained in indigestible material. This is why food chains rarely extend beyond four or five trophic levels — there is simply not enough energy remaining to support additional levels.

Concept Tested: Ten Percent Energy Rule

2. Which statement correctly distinguishes energy flow from chemical cycling in an ecosystem?

  1. Both energy and matter flow unidirectionally and exit the ecosystem as heat
  2. Matter flows unidirectionally and is lost as heat, while energy is recycled between organisms and the environment
  3. Neither energy nor matter can be reused once it enters an ecosystem
  4. Energy flows unidirectionally and is ultimately lost as heat, while matter is recycled repeatedly between organisms and the abiotic environment
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The correct answer is D. Energy enters an ecosystem (typically as sunlight), passes through trophic levels, and is ultimately dissipated as heat, consistent with the second law of thermodynamics — it cannot be recycled. Matter, in contrast, is conserved and continuously cycles between living organisms and the abiotic environment through biogeochemical cycles such as the carbon, nitrogen, phosphorus, and water cycles.

Concept Tested: Energy Flow in Ecosystems

3. Why is the phosphorus cycle often described as the slowest of the major biogeochemical cycles?

  1. Phosphorus has no atmospheric gas phase, so it moves only through the slow processes of rock weathering, soil uptake, and sedimentation
  2. Phosphorus has an atmospheric gas phase that circulates extremely slowly around the globe
  3. Phosphorus cycles only through living organisms and never interacts with rock or soil
  4. Phosphorus is used up completely by producers and does not return to the environment
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The correct answer is A. Unlike the carbon, nitrogen, and water cycles, the phosphorus cycle has no significant atmospheric gas phase. Phosphorus enters ecosystems primarily through the slow geological process of rock weathering, moves through soil and organisms, and eventually is deposited in ocean sediments, making the overall cycle far slower than cycles that include rapid atmospheric transport.

Concept Tested: Phosphorus Cycle

4. A wetland ecosystem receives 50,000 kcal/m²/yr of sunlight, and producers capture 2% of this energy through photosynthesis. Assuming the ten percent rule applies at each subsequent transfer, approximately how much energy is available to secondary consumers?

  1. 1 kcal
  2. 100 kcal
  3. 10 kcal
  4. 1,000 kcal
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The correct answer is C. Producers capture \(50{,}000 \times 0.02 = 1{,}000\) kcal. Applying the ten percent rule, primary consumers receive \(1{,}000 \times 0.10 = 100\) kcal, and secondary consumers receive \(100 \times 0.10 = 10\) kcal. Each trophic transfer reduces available energy by roughly 90%, which is why so little energy remains by the second consumer level.

Concept Tested: Ten Percent Energy Rule (Quantitative Application)

5. A region has warm temperatures year-round, receives more than 200 cm of precipitation annually, and supports a tall, dense canopy of broadleaf trees with the highest biodiversity of any terrestrial biome. Which biome is being described?

  1. Temperate deciduous forest
  2. Tropical rainforest
  3. Tropical savanna
  4. Coniferous forest (taiga)
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The correct answer is B. Tropical rainforests are defined by warm year-round temperatures, very high annual precipitation exceeding 200 cm, and a tall, dense broadleaf canopy, and they support the highest biodiversity of any terrestrial biome. Temperate deciduous forests and taiga experience seasonal temperature swings, and tropical savannas have distinct wet and dry seasons with grasses and scattered trees rather than a dense forest canopy.

Concept Tested: Terrestrial Biomes

6. Certain soil bacteria convert ammonia (NH3) into nitrite and then nitrate, forms that plants can readily absorb. Which nitrogen cycle process is being described?

  1. Nitrification
  2. Nitrogen fixation
  3. Denitrification
  4. Ammonification
Show Answer

The correct answer is A. Nitrification is the conversion of ammonia into nitrite and then nitrate by nitrifying bacteria, producing the form of nitrogen most readily absorbed by plant roots. This is distinct from nitrogen fixation (converting atmospheric N₂ into ammonia), denitrification (converting nitrate back into atmospheric N₂), and ammonification (decomposers releasing ammonia from organic nitrogen).

Concept Tested: Nitrogen Cycle

7. Agricultural fertilizer containing phosphate runs off into a nearby lake, and within weeks the lake experiences a massive algal bloom followed by a fish kill. Which sequence of events best explains this outcome?

  1. Excess phosphorus limits algal growth, causing algae to die off and release oxygen, which paradoxically kills fish
  2. Excess phosphorus removes nitrogen from the water, and nitrogen deficiency directly kills fish
  3. Phosphorus is toxic to fish directly upon contact, independent of any effect on algae
  4. Excess phosphorus relieves a natural nutrient limitation on algal growth, causing an algal population boom; when the algae die and decompose, decomposer respiration consumes dissolved oxygen, creating hypoxic conditions that kill fish
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The correct answer is D. Phosphorus is typically the limiting nutrient in freshwater ecosystems, so fertilizer runoff removes that limitation and triggers explosive algal growth. When the short-lived algae die, decomposer bacteria break down the organic matter and consume large amounts of dissolved oxygen during cellular respiration, producing hypoxic (oxygen-depleted) conditions that suffocate fish — the classic eutrophication sequence.

Concept Tested: Phosphorus Cycle (Human Impact and Eutrophication)

8. Community X has 5 species with individuals distributed as 20% each. Community Y has the same 5 species, but one species makes up 92% of all individuals. Both communities have identical species richness. Which community has greater species diversity, and why?

  1. Community Y, because having one highly dominant species indicates a healthier ecosystem
  2. Both communities have identical diversity because species richness is the only factor that determines diversity
  3. Community X, because its individuals are distributed more evenly among the five species, giving it higher species evenness and therefore greater overall diversity
  4. Community Y, because a single dominant species always increases genetic diversity
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The correct answer is C. Species diversity depends on both species richness (the number of species present) and species evenness (how equally individuals are distributed among those species). Although Community X and Community Y have identical richness, Community X's even 20% distribution across all five species gives it far higher evenness than Community Y, where one species dominates at 92%, making Community X the more diverse community overall.

Concept Tested: Species Richness and Evenness

9. A national park reports that a region's average temperature has risen 2°C over 30 years, coral cover has declined 40%, and three native fish species have shifted their range further north. Which evaluation of the ecological data is most scientifically sound?

  1. The data is coincidental and does not support any conclusion about climate change affecting this ecosystem
  2. The combination of rising temperature, coral decline (consistent with ocean acidification and thermal stress), and poleward range shifts of fish together provide converging evidence that climate change is measurably altering this ecosystem's biodiversity and species distributions
  3. Coral decline is entirely unrelated to temperature and must be caused by an unrelated, unidentified factor
  4. Range shifts in fish species indicate that climate change is beneficial for regional biodiversity
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The correct answer is B. Scientific evaluation should weigh whether multiple independent lines of evidence converge on the same explanation. Rising temperature, coral decline consistent with known thermal stress and acidification effects, and poleward fish range shifts consistent with species tracking suitable thermal habitat all point toward the same underlying cause — climate change — making this the most scientifically sound interpretation rather than dismissing the pattern as coincidence or unrelated factors.

Concept Tested: Climate Change and Ecology (Evaluating Evidence)

10. Using a simplified estuary food web (phytoplankton → zooplankton → small fish → herons), a class is asked to predict the cascading effects of a chemical spill that eliminates most of the zooplankton population. Which prediction best synthesizes energy flow and trophic cascade principles?

  1. Small fish populations would decline due to lost food supply, which would then reduce heron populations relying on small fish; phytoplankton, released from zooplankton grazing pressure, would likely increase in the short term
  2. All trophic levels would be completely unaffected, since energy flow is independent of any single trophic level
  3. Heron populations would increase immediately, since fewer zooplankton means more available energy at the top of the food web
  4. Phytoplankton would decline because they depend directly on zooplankton for nutrients
Show Answer

The correct answer is A. Removing zooplankton cuts off the main energy pathway from phytoplankton to higher trophic levels, so small fish that depend on zooplankton for food would decline, and herons that depend on small fish would decline as a downstream consequence — a bottom-up disruption propagating upward. Simultaneously, phytoplankton would likely increase in the short term because their primary grazer has been removed, illustrating how a single trophic-level disturbance ripples through an entire food web.

Concept Tested: Energy Flow in Ecosystems (Food Web Prediction)