Stability and change practice questions.

Define the term homeostasis as used in physiology.

• Maintaining a stable internal environment within narrow limits
• The permanent, unchanging equilibrium of all cellular reactions in an organism
• The gradual directional change of a population over successive generations
• The tendency of an ecosystem to return to a fixed climax community after disturbance

Answer: Maintaining a stable internal environment within narrow limits

Homeostasis is the maintenance of internal conditions (e.g. temperature, pH, blood glucose) within narrow limits around a set point, achieved dynamically rather than as a fixed unchanging state.

In a negative feedback control system, a rise in the controlled variable above the set point triggers a response that:

• Amplifies the rise, moving the variable further from the set point
• Reduces the variable back towards the set point
• Resets the set point to the new higher value
• Has no effect until the variable doubles

Answer: Reduces the variable back towards the set point

Negative feedback counteracts the change: a deviation above the set point activates a corrective response that reduces the variable back towards the set point, restoring stability.

Which process is an example of positive feedback rather than negative feedback?

• Insulin secretion lowering elevated blood glucose
• Sweating in response to a rise in core body temperature
• Oxytocin intensifying contractions in childbirth
• Vasoconstriction in the skin when body temperature falls

Answer: Oxytocin intensifying contractions in childbirth

In positive feedback the response amplifies the original stimulus: oxytocin causes contractions that stretch the cervix, stimulating more oxytocin release until birth occurs. The others are negative feedback that restore a set point.

A tundra ecosystem is described as fragile because it has low resilience. Which statement best explains low resilience?

• It contains very high species diversity that buffers any disturbance
• It recovers slowly and may not return to its original state after disturbance
• It changes its set point rapidly to match any new conditions
• It is completely resistant to all forms of external disturbance

Answer: It recovers slowly and may not return to its original state after disturbance

Resilience is the capacity to recover after disturbance. Low-resilience systems like tundra, with slow growth and low diversity, recover slowly and may tip into an alternative stable state.

Keystone species help maintain the stability of an ecosystem. Which description best fits a keystone species?

• The most abundant species, contributing most of the community biomass
• A species whose removal causes a disproportionately large change in community structure
• Any predator at the top of the food chain regardless of its ecological role
• A species that is unaffected by the removal of any other organism

Answer: A species whose removal causes a disproportionately large change in community structure

A keystone species has an effect on community structure far greater than its abundance would suggest; its removal triggers major, often cascading, changes even though it may not be numerically dominant.

Tipping points in ecosystems are significant because beyond them the change is typically:

• Immediately and easily reversed by removing the disturbance
• Abrupt and hard to reverse, shifting to a new stable state
• Always beneficial to biodiversity
• Restricted to aquatic ecosystems only

Answer: Abrupt and hard to reverse, shifting to a new stable state

At a tipping point a small additional change triggers a rapid shift to a different stable state (e.g. coral reef to algal-dominated). Such shifts often show hysteresis and are hard to reverse.