The phrase “alternative stable state” in ecology refers to the tendency of many ecosystems to have different, stable configurations of biotic and abiotic conditions across large time scales separated by what are called regime or phase shifts. Alternative stable state theory claims that instead of a forest transitioning slowly along a gradient toward a different stable state, that forest will reach a crucial tipping point (known as an ecological threshold) as deforestation occurs. Any change beyond this threshold will lead to a rapid change towards the second stable state of the biome in question, in this case a grassland.
Ecologists typically describe this theory with an analogy: Picture a set of three hills, between which are two valleys with a ball sitting in one of them that you want to push into the other. If you don’t push enough, the ball just rolls back down to where it started, but if you give the ball a big enough push, it will roll all the way into the valley on the other side of the hill. From there, it would require a similarly big push to get the ball back to where it started. Stable states are the valleys – where the balls want to stay if no outside forces are involved. However, if there is a big enough change in the environment to cause the ball to roll all the way up the hill to its highest point (the ecological threshold), the ball can be forced from one valley into another relatively quickly – this is a phase shift.
Until recently, most work discussing alternative stable states was theoretical – the idea of purposefully changing an environment to this extent was considered unthinkable – but several cases of confirmed alternative stable states have been reported. For example, disrupting the balance of phosphorous in a clearwater lake can lead to out-of-control phytoplankton blooms. Reducing this addition of phosphorous has so far not been an effective way of stopping the phytoplankton blooms, leading researchers to think that the ecosystem has been pushed into a new local equilibrium. This idea of hysteresis – that the state of an environment depends at least in part on its history and not just its current state – is at the core of many debates surrounding alternative stable state theory. However, whether most ecosystems that can exist under multiple stable states will readily convert between the two is still an open debate.
1. Which one of the following scenarios is most analogous to the type of ecological change predicted by alternative stable state theory?
A. After 30 years working at a bank, a man quits suddenly after a severe illness and then spends the rest of his life working as a patient advocate.
B. The stock market slowly rises over a 10-year period and then crashes precipitously after unexpected news regarding the health of the economy.
C. Over a period of a few years, a company switches from manufacturing a particular software product to consulting on that product after sales slowed.
D. The price of a certain stock that has been constant for months spikes dramatically on rumors of a takeover bid but then quickly returns to the old price after the rumor is debunked.
E. The ecology of a lake slowly changes as silt deposits increase from frequent flooding, but the lake returns to its original state as floodwaters recede.
2. Which of the following is confirmed in the passage as causing a phase shift in the environment?
A. the addition of phosphorous to a lake
B. the blooming of phytoplankton in a lake
C. the movement of balls over a hill
D. the presence of drought conditions in a certain region
E. the repeated logging of certain forests
3. The primary purpose of the passage is to
A. highlight the uncertainties and debates relating to a particular theory.
B. suggest that the alternative stable state theory is incomplete.
C. argue that the alternative stable state theory is primarily correct.
D. provide a thorough understanding of an important ecological theory.
E. give several detailed examples of the alternative stable state theory.
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