Question

why are some systems stable and others are not?
dolly sods: an ecosystem forever changed

• dolly sods is a rocky high plateau area in the allegheny mountains of eastern west virginia, usa. originally the area was covered with spruce, hemlock, and black cherry. during the 1880s, logging began in the area and virtually all of the commercially viable trees were cut down. the logging caused the underlying humus and peat to dry out.
• sparks from locomotives and campfires frequently set fire to this dry peat, producing fires that destroyed almost all the remaining forest. in some areas, the fires were so intense that they burnt everything right down to the bedrock, destroying seed banks. one fire during the 1930s destroyed over 100 km² of forest. the forests have never recovered. what was once a forested landscape is now mostly open meadow.

right: the original dense forests of dolly sods have been replaced mostly by maple, birch, beech, and low growing scrub.
equilibrium over time - spruce-fir forests

• a case study of ecosystem resilience is provided by the spruce-fir forest community in northern north america. organisms in the community include the spruce budworm (right), and balsam fir, spruce, and birch trees. the community fluctuates between two extremes. during budworm outbreaks the environment favors the spruce and birch species, which are less susceptible to defoliation by spruce budworm. between spruce budworm outbreaks, the environment favors the balsam fir, which outcompetes spruce and birch.
• the spruce-fir ecosystem is resilient even though some of the interacting components have low resistance.

1 under certain environmental conditions, the spruce budworm population grows so rapidly it overwhelms the ability of predators and parasites to control it.
2 the budworm feeds on balsam fir (despite their name), killing many trees. the spruce and birch trees are left as the major species.
3 the budworm population collapses through lack of food.
4 balsam fir saplings grow back in thick stands, eventually outcompeting the spruce and birch. evidence suggests these cycles have been occurring for possibly thousands of years.

1. (a) describe the large scale ecosystem change that occurred at dolly sods:

(b) what caused this change in the ecosystem?
(c) explain why the ecosystem has not been able to recover quickly after the change:

1. in what way is the spruce-fir-budworm system resilient in the long term, despite fluctuations?

Explanation:

• 3(a): The text states that Dolly Sods was originally covered with spruce, hemlock, and black cherry. Logging began in the 1880s, and fires destroyed the remaining forest. Now it is mostly open meadow.
• 3(b): Logging caused the underlying humus and peat to dry out. Sparks from locomotives and campfires set fire to the dry peat, destroying the forest.
• 3(c): One fire in the 1930s destroyed over \(100\ km^{2}\) of forest, burning everything down to the bedrock and destroying seed banks.
• 4: The spruce - fir - budworm system is resilient because it has cyclical population changes. After budworm outbreaks (which favor spruce and birch), balsam fir saplings grow back and outcompete spruce and birch. These cycles have occurred for possibly thousands of years.

Answer:

• 3(a): The ecosystem at Dolly Sods changed from a forested landscape (with spruce, hemlock, black cherry) to mostly open meadow.
• 3(b): Logging (which dried out humus and peat) and subsequent fires (caused by sparks from locomotives and campfires) destroyed the forest.
• 3(c): Fires burned everything down to the bedrock, destroying seed banks.
• 4: The system has cyclical population changes (budworm outbreaks favor spruce and birch; then balsam fir outcompetes them), and these cycles have persisted for a long time (possibly thousands of years).