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Difficulty: HardEcological Functions, Nutrient Cycles and Ecological Succession

Regarding ecological functions, nutrient cycling, and biological succession in natural ecosystems, evaluate the following statements:

I. Sedimentary nutrient cycles have their main reservoir pool in Earth's crust, whereas gaseous cycles rely primarily on atmospheric or hydrospheric reservoirs.
II. During ecological succession, net community productivity typically increases as the climax community approaches, despite total biomass remaining constant.
III. In primary xerarch succession, pioneer organisms such as lichens facilitate pedogenesis by releasing organic acids that weather the rocky substrate.
IV. Phosphorus cycling relies predominantly on rock weathering and biological uptake, lacking a substantial gaseous phase under normal environmental conditions.

Which of the statements given above are correct?

  1. I, III and IV onlyAnswer
  2. B
    I and II only
  3. C
    II, III and IV only
  4. D
    I, II, III and IV

Answer

Statements I, III, and IV are correct (I, III and IV only).
The combination specifying statements I, III, and IV only is correct. Statement I accurately defines the reservoir distinction between gaseous and sedimentary cycles. Statement III correctly describes how pioneer lichens weather rocks via organic acid secretion during xerarch succession. Statement IV accurately notes that phosphorus cycles sedimentarily without a significant atmospheric gaseous phase. Statement II is false because total biomass increases (rather than staying constant) and net community productivity decreases (rather than increases) as succession approaches climax.

Step-by-Step Solution

1
Analyze Statement I regarding reservoir pools of biogeochemical cycles.
Gaseous cycles (nitrogen, carbon) reserve primarily in the atmosphere/hydrosphere, while sedimentary cycles (phosphorus, sulfur) reserve in the lithosphere (Earth's crust). Statement I is correct.
Classification of biogeochemical cycles is determined by the location of their primary reservoir pool.
2
Analyze Statement II regarding ecosystem parameters during ecological succession.
As ecological succession progresses toward a climax community, total organic biomass increases significantly (not constant). Meanwhile, net community productivity decreases as total respiration increases to match gross primary productivity. Statement II is incorrect.
Climax ecosystems reach a steady state with high total biomass and near-zero net community productivity (P/R≈1P/R \approx 1).
3
Analyze Statement III regarding pioneer species in xerarch succession.
Lichens act as pioneers on bare rock, secreting carbonic and organic acids that erode rock surfaces and initiate soil formation (pedogenesis). Statement III is correct.
Chemical weathering by lichen acid secretion is a fundamental mechanism enabling subsequent moss and plant colonization.
4
Analyze Statement IV regarding the phosphorus cycle.
Phosphorus is a classic sedimentary cycle without any major gaseous component in atmospheric circulation. Statement IV is correct.
Phosphates circulate through weathering of rock deposits into soil/water and subsequent biological uptake.

Key Concept

Reservoirs of biogeochemical cycles and structural/functional parameter shifts during ecological succession.
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Topics

Ecological Functions, Nutrient Cycles and Ecological Succession