Why can a cockroach live almost anywhere while a giant panda needs a specific bamboo forest or it starves? The difference is tolerance — the range of conditions a species can survive — and how broad or narrow that range is. Broad-tolerance generalists (raccoons, dandelions, humans) shrug off change; narrow-tolerance specialists (pandas, coral, salamanders) thrive in a sweet spot and collapse when it shifts. This lesson closes Unit 2 by explaining how organisms come to fit their environment (adaptation), how we measure the limits of that fit (tolerance curves), and how nature itself keeps shuffling the deck through disturbances that range from clockwork-regular to totally random.
An adaptation is a heritable trait that improves an organism's survival and reproduction in its environment. Adaptations arise through natural selection: individuals with traits better suited to current conditions survive and reproduce more, passing those traits on. Over generations, the population's traits shift.
Requirements for natural selection: heritable variation in a trait, differential reproductive success based on that trait, and time. Environmental change alters which traits are favored — so adaptation is always relative to current conditions. High genetic diversity (Lesson 4) supplies the variation natural selection acts on; low diversity limits a population's ability to adapt.
Evolution by natural selection is the engine behind resistance and resilience: a population that can adapt survives change; one that can't may go extinct.
Every species has a range of tolerance — the span of an environmental variable (temperature, pH, salinity, dissolved oxygen) within which it can survive. Performance is highest in the optimal range, declines in zones of physiological stress on either side, and drops to zero in the zones of intolerance beyond the limits.
[DIAGRAM: Tolerance curve — bell-shaped population/performance curve vs. an environmental variable (e.g., temperature) on the x-axis. Center labeled "optimal range" (highest abundance); shoulders labeled "zones of physiological stress"; tails labeled "zones of intolerance" (no organisms survive). Vertical lines mark lower and upper tolerance limits.]
A species can also express tolerance through acclimation — a reversible physiological adjustment within its genetic limits (e.g., producing more red blood cells at altitude). Acclimation is individual and temporary; adaptation is genetic and multigenerational.
Ecosystems are disturbed not only by humans but by natural events. APES classifies these by their timing:
Effects depend on scale and the community's resilience. Moderate, periodic disturbance often maintains diversity (the intermediate disturbance idea: neither too much nor too little disturbance maximizes diversity). Large episodic/random events can reset succession (Lesson 5) or drive extinctions.
Natural disruptions also drive migration and range shifts — organisms move to stay within their tolerance ranges. Climate change (Unit 9) is now shifting many species' ranges poleward and upslope as conditions exceed local tolerance limits.
Tolerance curves are prime data-interpretation items (identify the optimal range from a graph). The generalist/specialist distinction predicts extinction risk (Units 2 and 9). The periodic/episodic/random classification is a quick vocabulary payoff, and it connects to succession and to why specialists are the first casualties of environmental change.
A fish's population is highest at 18 °C, drops sharply below 8 °C and above 26 °C, and is zero beyond those. Identify its optimal range and its zones of intolerance.
Solution: Optimal range is around 18 °C (peak); zones of intolerance are below 8 °C and above 26 °C (population = 0). Between the optimum and each limit lie the zones of physiological stress.
Interpretation: Peak = optimal; zero = intolerance; slopes in between = stress.
Two species live in a forest: a raccoon (eats almost anything, tolerates wide temperatures) and a salamander (needs cool, moist, narrow conditions). Predict which is more threatened by climate warming and habitat change.
Solution: The salamander — a specialist with narrow tolerance — is far more threatened. Warming and drying can push conditions outside its narrow range, while the generalist raccoon tolerates change and exploits new food sources.
Interpretation: Narrow tolerance = high vulnerability; broad tolerance = buffered.
Classify each as periodic, episodic, or random and give a likely ecological effect: (i) annual monsoon flooding, (ii) a Category 5 hurricane, (iii) an asteroid impact.
Solution: - (i) Periodic — regular seasonal flooding; species are adapted to it (some depend on it for reproduction/nutrient delivery). - (ii) Episodic — irregular timing; can flatten forests and trigger secondary succession. - (iii) Random — no predictable pattern; rare but potentially causes mass extinction and global disruption.
Interpretation: Timing/predictability is the sorting key: regular = periodic, irregular-but-recurring = episodic, patternless-and-rare = random.
A person moves to high altitude and, over weeks, produces more red blood cells. Is this adaptation or acclimation? Explain.
Solution: Acclimation — it's a reversible physiological adjustment within the individual's genetic limits, not a heritable genetic change passed to offspring. Adaptation would be a heritable trait spread through a population over generations by natural selection.
Interpretation: Individual + reversible = acclimation; population + heritable + multigenerational = adaptation.
(B) Species shift ranges to stay in their tolerance range.
Specialists have a narrow tolerance range and often depend on a single resource or habitat. As climate change pushes temperature/moisture outside that narrow range, specialists cannot survive locally and cannot easily switch resources, so their populations crash. Generalists' broad tolerance lets them persist through the same change, so specialists are far more likely to become endangered.
(a) Optimal range ≈ 26 °C (peak; roughly up to ~29 °C before decline). (b) From 28 °C, a 2 °C rise to 30 °C pushes the coral into its stress zone above 29 °C, causing reduced growth and bleaching; sustained warming toward 31 °C would kill it.
FRQ rubric (10 pts): - (a) 1 pt species has a narrow tolerance range for temperature/moisture; 1 pt warming pushes conditions beyond its upper limit into stress/intolerance, reducing survival. (2) - (b) 1 pt range shifts upslope (to higher elevation) and/or poleward; 1 pt because the species moves to track its tolerable (cooler) conditions. (2) - (c) 1 pt generalist rodent has broad tolerance and varied diet; 1 pt so it can persist or exploit new conditions, making it far less vulnerable than the specialist. (2) - (d) For each of two strategies: 1 pt name + 1 pt justification. Acceptable: protect/restore high-elevation refugia (cooler habitat within tolerance); maintain riparian shade/streamflow (keeps water cool); establish corridors to higher elevations (allow range shift); captive breeding/assisted migration; reduce other stressors (pollution, sediment) to raise resilience. (4)
A mountain salamander is a moisture- and temperature-specialist found only in cool, high-elevation streams. Regional temperatures are projected to rise 2–3 °C over the century.
(a) Explain, using tolerance, why this species is at high risk from warming. (2 pts) (b) Predict how the salamander's geographic range is likely to shift and why. (2 pts) (c) Contrast the salamander's vulnerability with that of a nearby generalist rodent. (2 pts) (d) Propose two conservation strategies to protect the salamander, and justify each. (4 pts)
MC: 1. (B) Adaptation — heritable trait improving fitness. 2. (C) Zone of intolerance — population reaches zero. 3. (B) Broad tolerance + varied diet = generalist. 4. (C) Giant panda (specialist). The others are generalists. 5. (A) Periodic — regular, predictable. 6. (B) Episodic — irregular timing. 7. (C) Selection has no conscious goal. 8. (B) Acclimation — reversible, individual, non-heritable. 9. (B) Low genetic diversity limits adaptation. 10. (B) Species shift ranges to stay in their tolerance range.
Specialists have a narrow tolerance range and often depend on a single resource or habitat. As climate change pushes temperature/moisture outside that narrow range, specialists cannot survive locally and cannot easily switch resources, so their populations crash. Generalists' broad tolerance lets them persist through the same change, so specialists are far more likely to become endangered.
(a) Optimal range ≈ 26 °C (peak; roughly up to ~29 °C before decline). (b) From 28 °C, a 2 °C rise to 30 °C pushes the coral into its stress zone above 29 °C, causing reduced growth and bleaching; sustained warming toward 31 °C would kill it.
FRQ rubric (10 pts): - (a) 1 pt species has a narrow tolerance range for temperature/moisture; 1 pt warming pushes conditions beyond its upper limit into stress/intolerance, reducing survival. (2) - (b) 1 pt range shifts upslope (to higher elevation) and/or poleward; 1 pt because the species moves to track its tolerable (cooler) conditions. (2) - (c) 1 pt generalist rodent has broad tolerance and varied diet; 1 pt so it can persist or exploit new conditions, making it far less vulnerable than the specialist. (2) - (d) For each of two strategies: 1 pt name + 1 pt justification. Acceptable: protect/restore high-elevation refugia (cooler habitat within tolerance); maintain riparian shade/streamflow (keeps water cool); establish corridors to higher elevations (allow range shift); captive breeding/assisted migration; reduce other stressors (pollution, sediment) to raise resilience. (4)
⭐ Exam strategy: If a data question shows a bell-shaped curve of "abundance vs. some condition," it's a tolerance curve — the peak is the optimum and the zeros are intolerance limits. And whenever a question mentions a "specialist," expect the answer to involve high vulnerability to change.
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