AZSCI Stability and Change 2 — Questions and Answers
Question 1: A normally clear-water lake in Arizona rapidly becomes dominated by turbid algae. This sudden, drastic shift, even with only a small increase in nutrient runoff, is an example of a system crossing a critical threshold. Which concept best explains this phenomenon?
- Primary succession
- Climax community
- Catastrophic regime shift (Correct answer)
- Negative feedback loop
Correct answer: Catastrophic regime shift
A catastrophic regime shift describes a sudden, often irreversible change in an ecosystem's state when a critical threshold is passed. The shift from a clear lake to a turbid, algae-dominated one is a classic example. Primary succession is the colonization of a new, lifeless area. A climax community is a stable, mature stage of succession. A negative feedback loop would work to stabilize the clear-water state, not cause a sudden shift.
Question 2: In the context of geological stability, why might an excavation cut in a clay slope be most unstable long after the excavation is complete, rather than immediately after?
- The clay immediately expands, increasing pressure on the slope.
- The removal of material instantly reduces the cohesive strength of the remaining clay.
- The slope angle is at its steepest point immediately after cutting.
- Pore water pressure, initially decreased by the excavation, slowly increases over time, reducing the effective stress and shear strength. (Correct answer)
Correct answer: Pore water pressure, initially decreased by the excavation, slowly increases over time, reducing the effective stress and shear strength.
Excavating a cut in clay can cause a short-term decrease in pore water pressure (negative excess pore pressure). Over the long term, water flows back towards the cut, causing the pore pressure to gradually increase. This increase in water pressure reduces the effective stress within the soil, which in turn decreases its shear strength, making the slope less stable over time. The most critical point for stability is often long after construction, when these pressures have equilibrated to a new, higher steady-state flow.
Question 3: Which of the following scenarios in an ecosystem best illustrates the distinction between homeostasis and chemical equilibrium?
- A sealed terrarium where gases eventually reach a stable, unchanging concentration.
- A predator-prey population cycle that oscillates around a stable average over time.
- A cell actively using energy to pump ions across its membrane to maintain a specific concentration gradient, different from its surroundings. (Correct answer)
- A forest fire clearing a section of land, which then begins the process of secondary succession.
Correct answer: A cell actively using energy to pump ions across its membrane to maintain a specific concentration gradient, different from its surroundings.
Homeostasis is the maintenance of a stable internal environment through active, energy-consuming processes, resulting in a state that is often far from equilibrium with the surroundings. A cell using an ion pump to maintain a concentration gradient is a perfect example of this dynamic, steady state. In contrast, a sealed terrarium reaching stable gas concentrations is approaching chemical equilibrium, a static state with no net change and no energy input. Predator-prey cycles represent a dynamic stability at the ecosystem level, and succession is a process of change, not maintenance of a steady state.
Question 4: A grassland ecosystem is subjected to a severe, multi-year drought. While the overall biomass of the plant community is significantly reduced, the variety of grass species remains largely the same. How does this demonstrate the complexity of ecosystem stability?
- The ecosystem is unstable in all its properties.
- The ecosystem shows resistance to change in biodiversity but not in function (biomass production). (Correct answer)
- The ecosystem is in a state of primary succession.
- The ecosystem has low resilience and low resistance.
Correct answer: The ecosystem shows resistance to change in biodiversity but not in function (biomass production).
Ecosystem stability is not a single property; a system can be stable in one aspect and unstable in another. In this scenario, the ecosystem demonstrates resistance to change in its species composition (biodiversity) because the same species persist. However, it shows low resistance to change in its function, as evidenced by the significant loss of biomass. This illustrates that stability must be considered across different properties of an ecosystem.
Question 5: In many complex systems, such as social insect colonies or global climate, stability is maintained by a network of interactions where the output of a process influences its own input. What is the term for a mechanism that counteracts change to promote stability?
- Positive feedback loop
- Reinforcing loop
- Tipping point
- Negative feedback loop (Correct answer)
Correct answer: Negative feedback loop
A negative feedback loop (or balancing loop) is a self-regulating mechanism where a change in a variable triggers a response that counteracts the initial change, pushing the system back toward a set point or equilibrium. This promotes stability. A positive (or reinforcing) feedback loop amplifies change, leading to instability, while a tipping point is a threshold where the system shifts to a new state.
Question 6: Imagine a coastal cliff in Arizona's ancient past, when the area was covered by sea. Which of the following best describes the difference between its short-term and long-term stability?
- Short-term stability is determined by fossil content, while long-term stability is determined by rock type.
- Short-term stability involves resistance to a single storm, while long-term stability involves the cumulative effects of weathering, wave action, and sea-level change over centuries.
- Short-term stability is static, while long-term stability is always catastrophic. (Correct answer)
- Short-term stability depends on the season, while long-term stability depends on daily tidal cycles.
Correct answer: Short-term stability is static, while long-term stability is always catastrophic.
Short-term stability refers to a system's ability to resist immediate perturbations, like a single weather event. Long-term stability, however, considers the cumulative impact of persistent, often slower-acting forces over extended periods. For a coastal cliff, this means accounting for gradual weathering, erosion from continuous wave action, and changes in boundary conditions like sea level, all of which can weaken the structure over geological or human timescales.
AZSCI Stability and Change 2
A normally clear-water lake in Arizona rapidly becomes dominated by turbid algae.
This sudden, drastic shift, even with only a small increase in nutrient runoff, is an example of a system crossing a critical threshold.
Which concept best explains this phenomenon?