Questions: Disturbance Ecology and Succession Dynamics

Question 1

Fire has been actively suppressed for 70 years in a fire-adapted ponderosa pine forest. Which outcome is most ecologically likely?

• A. The forest reaches a stable climax state with maximum species diversity because disturbance is eliminated
• B. Fire-adapted species gradually expand their range as competition from fire-intolerant species is reduced
• C. Fuel accumulates and shade-tolerant species replace fire-adapted species, paradoxically increasing the risk and severity of future fires
• D. The forest becomes more resilient because species have more time to establish deep root systems without disturbance

Answer: (C)

Fire suppression in fire-adapted ecosystems does not eliminate fire — it delays it while radically changing what burns. Without regular low-intensity burns, leaf litter, dead wood, and shrubs accumulate over decades, creating heavy fuel loads. Meanwhile, shade-tolerant species (which would normally be killed by fire) establish themselves in the understory, replacing fire-adapted species. When fire finally occurs, it burns hotter and more extensively than it would have under a natural fire regime, often killing trees that would have survived low-intensity burns. This is a classic example of how removing a disturbance can create conditions that make future disturbances far more destructive.

Question 2

According to the intermediate disturbance hypothesis, which scenario would produce the LOWEST species diversity in a community?

• A. A coral reef experiencing storms every 3-5 years
• B. A grassland with a fire regime of every 15-20 years
• C. An intertidal zone that experiences wave disturbance every few days
• D. A forest with virtually no natural disturbance for centuries

Answer: (C)

The IDH predicts low diversity at both extremes: very high disturbance frequency (only fast-colonizing pioneers can persist) AND very low disturbance frequency (competitive dominants exclude most species as succession proceeds to late stages). A coral reef disturbed every few years sits at an intermediate frequency that typically supports high diversity. A grassland burning every 15-20 years may also be intermediate. An intertidal zone disturbed every few days is very high frequency — only disturbance-tolerant specialists persist. A forest with virtually no disturbance allows a few competitive dominant trees to monopolize light and resources, suppressing most other species — this is the low-frequency extreme that produces low diversity.

Question 3

Reducing disturbance frequency in any ecosystem will generally increase species diversity by allowing more species to establish and persist.

• T. True
• F. False

Answer: False

This is the intuition that the intermediate disturbance hypothesis directly contradicts. Reducing disturbance frequency from a very high level can increase diversity by allowing more species to persist. But reducing disturbance from an already-intermediate level can *decrease* diversity by allowing competitive dominants to monopolize resources and exclude weaker competitors as succession advances. In fire-adapted grasslands, reducing fire frequency allows woody shrubs and trees to outcompete the diverse prairie species, collapsing diversity. The relationship between disturbance and diversity is hump-shaped, not linear: diversity peaks at intermediate disturbance, not at minimum disturbance.

Question 4

Many species in fire-adapted ecosystems have traits — such as thick bark, serotinous cones, and rapid resprouting — that evolved in response to regular fire, not despite it.

• T. True
• F. False

Answer: True

This is a central insight of disturbance ecology: disturbance regimes are not merely something species must survive but are selective forces that shape species' traits over evolutionary time. Longleaf pines have bark several inches thick that insulates their cambium from low-intensity fires. Some Australian Banksia species have serotinous cones that require fire's heat to open and release seeds. Prairie grasses evolved extensive underground root and rhizome systems that allow rapid resprouting after burning. These traits reveal that for these species, fire is part of their adaptive environment — they are not merely tolerating disturbance but are evolutionarily dependent on it.

Question 5

Why does the intermediate disturbance hypothesis predict lower species diversity at both very low AND very high disturbance frequencies?

Model answer: At very low disturbance frequencies, succession proceeds to late stages where a few highly competitive dominant species monopolize resources (especially light and space) and outcompete most others — competitive exclusion reduces diversity. At very high disturbance frequencies, the community is constantly reset to early successional stages, and only fast-colonizing, disturbance-tolerant pioneer species can complete their life cycles and persist — all late-successional species are excluded. At intermediate frequencies, a landscape mosaic develops: some patches are recently disturbed with pioneer species, others are in mid-succession, others are approaching late stages. This spatial and temporal heterogeneity allows both early- and late-successional species to coexist regionally, maximizing total diversity.

The IDH is a spatial and temporal argument as much as a successional one. The key mechanism is heterogeneity: intermediate disturbance creates a patchwork of successional ages across the landscape, expanding the diversity of microhabitats that different species can exploit. The two low-diversity extremes are caused by opposite forms of competitive exclusion — dominants in the low-frequency case, pioneers in the high-frequency case.