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Difficulty: Very hardExplicit Causal Relationships

In examining the perturbation of high-alpine lentic ecosystems in the Sierra Nevada, biologists have elucidated a definitive trophic cascade triggered by the anthropogenic introduction of non-native salmonids. The primary mechanism of this disruption stems from the salmonids' selective predation on large cladoceran zooplankton, notably *Daphnia*. The extirpation of these robust grazers effectively abolishes the competitive suppression they typically exert on smaller, less efficient herbivorous zooplankton. Released from this constraint, the smaller zooplankton proliferate but fail to adequately control phytoplankton biomass. The subsequent exponential proliferation of phytoplankton engenders intense algal blooms that profoundly attenuate the penetration of solar radiation through the water column. This induced turbidity fundamentally alters the benthic habitat by inhibiting the photosynthesis of epilithic algae, thereby depleting the foundational nutritional resource upon which indigenous macroinvertebrate assemblages depend.

Based on the passage, arrange the following phenomena in the chronological order in which they occur as part of the causal chain initiated by the introduction of non-native salmonids.

  1. 1A precipitous reduction in the population of large cladocerans.
  2. 2A relaxation of competitive constraints on inefficient herbivorous zooplankton.
  3. 3An uncontrolled expansion of phytoplankton biomass.
  4. 4The attenuation of solar radiation reaching the benthic zone.
  5. 5A depletion of the primary nutritional resources for indigenous macroinvertebrates.

Answer

The correct chronological sequence is: 1) A precipitous reduction in the population of large cladocerans, 2) A relaxation of competitive constraints on inefficient herbivorous zooplankton, 3) An uncontrolled expansion of phytoplankton biomass, 4) The attenuation of solar radiation reaching the benthic zone, and 5) A depletion of the primary nutritional resources for indigenous macroinvertebrates.
The correct sequence maps directly to the explicit multi-step causal chain described in the text: the salmonids prey on large cladocerans (causing a precipitous reduction in their population), which releases smaller zooplankton from competition (a relaxation of competitive constraints). These smaller zooplankton are inefficient and cannot control the phytoplankton, causing its biomass to expand uncontrollably. This expansion creates intense blooms that block light (attenuation of solar radiation), which finally prevents benthic algae growth, thus starving the macroinvertebrates (depletion of nutritional resources).

Step-by-Step Solution

1
Identify the initiating event in the trophic cascade as described in the passage.
The passage states the cascade begins with 'selective predation on large cladoceran zooplankton', which matches 'A precipitous reduction in the population of large cladocerans.'
Causal chains must be ordered starting from the primary mechanism of disruption identified in the text.
2
Trace the immediate consequence of the reduction in large cladocerans.
The text notes that extirpating these grazers 'abolishes the competitive suppression they typically exert on smaller... zooplankton.' This corresponds to 'A relaxation of competitive constraints on inefficient herbivorous zooplankton.'
This establishes the second direct link in the chronological sequence.
3
Determine the subsequent effect of the smaller zooplankton's proliferation.
The passage explains that these smaller zooplankton 'fail to adequately control phytoplankton biomass,' leading to an 'exponential proliferation of phytoplankton.' This maps to 'An uncontrolled expansion of phytoplankton biomass.'
The inability of the smaller zooplankton to graze efficiently directly causes the phytoplankton expansion.
4
Identify the environmental impact caused by the phytoplankton proliferation.
The phytoplankton explosion 'engenders intense algal blooms that profoundly attenuate the penetration of solar radiation.' This matches 'The attenuation of solar radiation reaching the benthic zone.'
The algal blooms act as a physical barrier to light, constituting the fourth step.
5
Locate the final resulting impact on the ecosystem.
The induced turbidity inhibits algae photosynthesis, 'thereby depleting the foundational nutritional resource upon which indigenous macroinvertebrate assemblages depend.' This corresponds to 'A depletion of the primary nutritional resources for indigenous macroinvertebrates.'
This completes the explicit multi-step causal chain provided in the text.

Key Concept

Explicit Causal Relationships
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Explicit Causal Relationships

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