Ecological Tolerance & Disturbance
- Interpret a tolerance curve and identify the optimal and stress ranges
- Distinguish a generalist from a specialist and predict which tolerates change better
- Compare periodic, episodic, and random natural disruptions to ecosystems
The range of tolerance
Every species survives only within a range of tolerance for each environmental condition — temperature, pH, salinity, oxygen. Plot survival against the condition and you get a bell-shaped tolerance curve: a central optimal range where the species thrives, flanked by zones of physiological stress where individuals survive but reproduce poorly, and beyond those the zones of intolerance where the species cannot survive at all. The width of this curve determines how much environmental change a species can withstand.
Generalists vs. specialists
A generalist has a wide range of tolerance and a broad niche — it eats many foods and lives in many conditions (raccoons, cockroaches, humans). A specialist has a narrow range of tolerance and a specific niche (the panda, which eats almost only bamboo; the koala on eucalyptus). In a stable environment, specialists compete efficiently and can outperform generalists. But when the environment changes rapidly — the situation humans increasingly create — generalists tolerate the change far better, while specialists are the first to become threatened.
Natural disruptions to ecosystems
Even without humans, ecosystems face natural disruptions on different rhythms. Periodic disruptions occur on a regular cycle — seasons, annual flooding, tides. Episodic disruptions occur occasionally with some irregularity — hurricanes, wildfires, droughts. Random disruptions have no predictable pattern — a volcanic eruption, an asteroid impact. Many species are adapted to periodic and even episodic disturbance; some pine forests, for example, need periodic fire to release seeds. Trouble arises when human activity changes the frequency or intensity of these disturbances faster than species can adapt.
A trout species survives in water from 5°C to 20°C, reproduces well only from 10°C to 15°C, and thrives best near 12°C. A warming river now averages 18°C in summer. Explain, using tolerance zones, what happens to the population.
- 1.Identify the zones: optimal ≈ 10–15°C, physiological stress from 5–10°C and 15–20°C, intolerance below 5°C or above 20°C.
- 2.Locate 18°C on the curve: it lies within the survival range (5–20°C) but above the optimal reproductive range (10–15°C).
- 3.At 18°C the trout are in a zone of physiological stress: individuals survive but reproduction is impaired.
- 4.With poor reproduction sustained each summer, the population declines even though adults are not immediately killed.
Two species live in a forest facing rapid climate change. The generalist eats many foods and tolerates a wide temperature range; the specialist eats one plant and tolerates a narrow range. Which is more likely to survive, and why?
On a tolerance curve, read the y-axis as performance (survival or reproductive success). The peak is the optimal range; the tails sloping toward zero are the stress zones; where the curve hits zero are the limits of tolerance. Widen the curve and you have described a generalist.
A pine forest depends on periodic wildfires to open its cones and release seeds. Decades of human fire suppression have prevented these fires. What is the most likely long-term result?
Expect a scenario asking which species is "most vulnerable" to an environmental change. The answer is almost always the specialist with the narrow tolerance range or the single food source. Generalists are the survivors; specialists are the ones that end up on the endangered list.
Answer the 2 checkpoints as you read.
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